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  • 51.
    Chung, Sunjae
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Univ Gothenburg, Sweden.
    Eklund, Anders
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Iacocca, Ezio
    Mohseni, Seyed Majid
    Sani, Sohrab R.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Bookman, Lake
    Hoefer, Mark A.
    Dumas, Randy K.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. Univ Gothenburg, Sweden.
    Magnetic droplet nucleation boundary in orthogonal spin-torque nano-oscillators2016Inngår i: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 7, artikkel-id 11209Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Static and dynamic magnetic solitons play a critical role in applied nanomagnetism. Magnetic droplets, a type of non-topological dissipative soliton, can be nucleated and sustained in nanocontact spin-torque oscillators with perpendicular magnetic anisotropy free layers. Here, we perform a detailed experimental determination of the full droplet nucleation boundary in the current-field plane for a wide range of nanocontact sizes and demonstrate its excellent agreement with an analytical expression originating from a stability analysis. Our results reconcile recent contradicting reports of the field dependence of the droplet nucleation. Furthermore, our analytical model both highlights the relation between the fixed layer material and the droplet nucleation current magnitude, and provides an accurate method to experimentally determine the spin transfer torque asymmetry of each device.

  • 52.
    Chung, Sunjae
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Univ Gothenburg, Sweden.
    Le, Quang Tuan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Ahlberg, Martina
    Department of Physics, University of Gothenburg.
    Awad, Ahmad A.
    Department of Physics, University of Gothenburg.
    Mazraati, Hamid
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. NanOsc AB.
    Houshang, Afshin
    Department of Physics, University of Gothenburg.
    Jiang, Sheng
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Nguyen, Thi Ngoc Anh
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Institute of Materials Science, Vietnam Academy of Science and Technology.
    Dumas, Randy K.
    Department of Physics, University of Gothenburg.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Univ Gothenburg, Sweden; NanOsc AB, Sweden.
    Magnetic droplet solitons in all-perpendicular nano-contact spin torque oscillatorsManuskript (preprint) (Annet (populærvitenskap, debatt, mm))
    Abstract [en]

    Spin-torque-generated magnetic droplets are nontopological solitons that have both dynamic and static characteristics. Although   theoretical studies of these droplets originally dealt with  an all-perpendicular magnetic system, all experimental demonstrations have so far relied on orthogonal spin valve structures that require a rather strong magnetic field to nucleate the droplet. Here, for the first time, we show the nucleation and sustained operation of magnetic droplets under a low magnetic field using nanocontact spin-torque oscillators (NC-STO),  both the free Co/Ni and fixed Co/Pd multilayers of which have strong perpendicular magnetic anisotropy. Droplet nucleation is observed as a change in the NC-STO resistance and the appearance of significant broadband microwave signal generation below 1 GHz. We also observe another important phenomenon in which the nucleated magnetic droplet can transform into a skyrmionic nanobubble in the low-field regime. Both magnetic droplet solitons and skyrmionic nanobubbles have been studied in detail using micromagnetic simulation.

  • 53.
    Chung, Sunjae
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. University of Gothenburg, Sweden.
    Majid Mohseni, S.
    Eklund, Anders
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Dürrenfeld, P.
    Ranjbar, M.
    Redjai Sani, Sohrab
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Anh Nguyen, T. N.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. Vietnam Academy of Science and Technology, Viet Nam.
    Dumas, R. K.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. University of Gothenburg, Sweden.
    Magnetic droplet solitons in orthogonal spin valves2015Inngår i: Fizika Nizkih Temperatur, ISSN 0132-6414, E-ISSN 1816-0328, Vol. 41, nr 10, s. 1063-1068Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We review the recent experimental advancements in the realization and understanding of magnetic droplet solitons generated by spin transfer torque in orthogonal nanocontact based spin torque nanooscillators (STNOs) fabricated on extended spin valves and spin valve nanowires. The magnetic droplets are detected and studied using the STNO microwave signal and its resistance, the latter both quasistatically and time-resolved. The droplet nucleation current is found to have a minimum at intermediate magnetic field strengths and the nature of the nucleation changes gradually from a single sharp step well above this field, mode-hopping around the minimum, and continuous at low fields. The mode-hopping and continuous transitions are ascribed to droplet drift instability and re-nucleation at different time scales, which is corroborated by time-resolved measurements. We argue that the use of tilted anisotropy fixed layers could reduce the nucleation current further, move the nucleation current minimum to lower fields, and potentially remove the need for an applied magnetic field altogether. Finally, evidence of an edge mode droplet in a nanowire is presented.

  • 54.
    Chung, Sunjae
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. University of Gothenburg, Sweden.
    Mohseni, S. M.
    Eklund, Anders
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Dürrenfeld, P.
    Ranjbar, M.
    Sani, Redjai Sohrab
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Anh Nguyen, T. N.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. Vietnam Academy of Science and Technology, Viet Nam.
    Dumas, R. K.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. University of Gothenburg, Sweden.
    Magnetic droplet solitons in orthogonal spin valves2015Inngår i: Low temperature physics (Woodbury, N.Y., Print), ISSN 1063-777X, E-ISSN 1090-6517, Vol. 41, nr 10, s. 833-837Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We review the recent experimental advancements in the realization and understanding of magnetic droplet solitons generated by spin transfer torque in orthogonal nanocontact based spin torque nanooscillators (STNOs) fabricated on extended spin valves and spin valve nanowires. The magnetic droplets are detected and studied using the STNO microwave signal and its resistance, the latter both quasistatically and time-resolved. The droplet nucleation current is found to have a minimum at intermediate magnetic field strengths and the nature of the nucleation changes gradually from a single sharp step well above this field, mode-hopping around the minimum, and continuous at low fields. The mode-hopping and continuous transitions are ascribed to droplet drift instability and re-nucleation at different time scales, which is corroborated by time-resolved measurements. We argue that the use of tilted anisotropy fixed layers could reduce the nucleation current further, move the nucleation current minimum to lower fields, and potentially remove the need for an applied magnetic field altogether. Finally, evidence of an edge mode droplet in a nanowire is presented.

  • 55.
    Chung, Sunjae
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Mohseni, Seyed Majid
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Fallahi, Vahid
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Nguyen, T. N. Anh
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Benatmane, Nadjib
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Dumas, R. K.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Tunable spin configuration in [Co/Ni]-NiFe spring magnets2013Inngår i: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 46, nr 12, s. 125004-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We investigate a tunable spin configuration in [Co/Ni](4)/Co-NiFe exchange spring magnets. The strong perpendicular magnetic anisotropy of the Co/Ni multilayer, which competes with the in-plane shape anisotropy of the Ni81Fe19 (Py) layer, allows for unique magnetic configurations. By varying the NiFe thickness (t(NiFe)) from 2.6 to 3.0 nm, we show that the magnetization tilt angle can be easily tuned from completely out-of-plane (0 degrees) to in-plane (90 degrees). This transition, which occurs for a small range of t(NiFe), can be estimated from the major loop remanence and one-dimensional micromagnetic calculations. These tunable magnetization tilt spring magnets are highly promising for future applications in spin-transfer torque-based devices.

  • 56.
    Chung, Sunjae
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Mohseni, Seyed Majid
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Sani, Sohrab Redjai
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Iacocca, E.
    Dumas, R. K.
    Nguyen, Thi Ngooc Anh
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Pogoryelov, Ye
    Muduli, P. K.
    Eklund, Anders
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Hoefer, M.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Spin transfer torque generated magnetic droplet solitons (invited)2014Inngår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 115, nr 17, s. 172612-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present recent experimental and numerical advancements in the understanding of spin transfer torque generated magnetic droplet solitons. The experimental work focuses on nano-contact spin torque oscillators (NC-STOs) based on orthogonal (pseudo) spin valves where the Co fixed layer has an easy-plane anisotropy, and the [Co/Ni] free layer has a strong perpendicular magnetic anisotropy. The NC-STO resistance and microwave signal generation are measured simultaneously as a function of drive current and applied perpendicular magnetic field. Both exhibit dramatic transitions at a certain current dependent critical field value, where the microwave frequency drops 10 GHz, modulation sidebands appear, and the resistance exhibits a jump, while the magnetoresistance changes sign. We interpret these observations as the nucleation of a magnetic droplet soliton with a large fraction of its magnetization processing with an angle greater than 90 degrees, i.e., around a direction opposite that of the applied field. This interpretation is corroborated by numerical simulations. When the field is further increased, we find that the droplet eventually collapses under the pressure from the Zeeman energy.

  • 57. Consolo, G.
    et al.
    Finocchio, G.
    Siracusano, G.
    Bonetti, S.
    Eklund, Anders
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Azzerboni, B.
    Non-stationary excitation of two localized spin-wave modes in a nano-contact spin torque oscillator2013Inngår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 114, nr 15, s. 153906-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We measure and simulate micromagnetically a framework based upon a nano-contact spin torque oscillator where two distinct localized evanescent spin-wave modes can be detected. The resulting frequency spectrum is composed by two peaks, corresponding to the excited modes, which lie below the ferromagnetic resonance frequency, and a low-frequency tail, which we attribute to the non-stationary switching between these modes. By using Fourier, wavelet, and Hilbert-Huang transforms, we investigate the properties of these modes in time and spatial domains, together with their spatial distribution. The existence of an additional localized mode (which was neither predicted by theory nor by previous numerical and experimental findings) has to be attributed to the large influence of the current-induced Oersted field strength which, in the present setup, is of the same order of magnitude as the external field. As a further consequence, the excited spin-waves, contrarily to what usually assumed, do not possess cylindrical symmetry: the Oersted field induces these modes to be excited at the two opposite sides of the region beneath the nano-contact.

  • 58. Davies, J. E.
    et al.
    Gilbert, D. A.
    Mohseni, Seyed Majid
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Dumas, R. K.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Liu, Kai
    Reversal mode instability and magnetoresistance in perpendicular (Co/Pd)/Cu/(Co/Ni) pseudo-spin-valves2013Inngår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 103, nr 2, s. 022409-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We have observed distinct temperature-dependent magnetization reversal modes in a perpendicular (Co/Pd)(4)/Co/Cu/(Co/Ni)(4)/Co pseudo-spin-valve, which are correlated with spin-transport properties. At 300 K, magnetization reversal occurs by vertically correlated domains. Below 200 K the hysteresis loop becomes bifurcated due to laterally correlated reversal of the individual stacks. The magnetic configuration change also leads to higher spin disorders and a significant increase in the giant magnetoresistance effect. First order reversal curve measurements reveal that the coupled state can be re-established through field cycling and allow direct determination of the interlayer coupling strength as a function of temperature.

  • 59. Delczeg-Czirjak, E. K.
    et al.
    Pereiro, M.
    Bergqvist, Lars
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Kvashnin, Y. O.
    Di Marco, I
    Li, Guijiang
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Vitos, Levente
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik. Uppsala Univ, Div Mat Theory, Dept Phys & Astron, Sweden.
    Eriksson, O.
    Origin of the magnetostructural coupling in FeMnP0.75Si0.252014Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, nr 21, s. 214436-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The strong coupling between the crystal structure and magnetic state (ferromagnetic or helical antiferromagnetic) of FeMnP0.75Si0.25 is investigated using density functional theory in combination with atomistic spin dynamics. We find many competing energy minima for drastically different ferromagnetic and noncollinear magnetic configurations. We also find that the appearance of a helical spin-spiral magnetic structure at finite temperature is strongly related to one of the crystal structures reported for this material. Shorter Fe-Fe distances are found to lead to a destabilized ferromagnetic coupling, while out-of-plane Mn-Mn exchange interactions become negative with the shortening of the interatomic distances along the c axis, implying an antiferromagnetic coupling for the nearest-neighbor Mn-Mn interactions. The impact of the local dynamical correlations is also discussed.

  • 60.
    Dev, Apurba
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. Uppsala University, Sweden.
    Horak, J.
    Kaiser, A.
    Yuan, X.
    Perols, A.
    Björk, P.
    Karlström, A. E.
    Kleimann, P.
    Linnros, Jan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Electrokinetic effect for molecular recognition: A label-free approach for real-time biosensing2016Inngår i: Biosensors & bioelectronics, ISSN 0956-5663, E-ISSN 1873-4235, Vol. 82, s. 55-63Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present a simple and inexpensive method for label-free detection of biomolecules. The method monitors the changes in streaming current in a fused silica capillary as target biomolecules bind to immobilized receptors on the inner surface of the capillary. To validate the concept, we show detection and time response of different protein-ligand and protein-protein systems: biotin-avidin and biotin-streptavidin, barstar-dibarnase and Z domain-immunoglobulin G (IgG). We show that specific binding of these biomolecules can be reliably monitored using a very simple setup. Using sequential injections of various proteins at a diverse concentration range and as well as diluted human serum we further investigate the capacity of the proposed technique to perform specific target detection from a complex sample. We also investigate the time for the signal to reach equilibrium and its dependence on analyte concentration and demonstrate that the current setup can be used to detect biomolecules at a concentration as low as 100 pM without requiring any advanced device fabrication procedures. Finally, an analytical model based on diffusion theory has been presented to explain the dependence of the saturation time on the analyte concentration and capillary dimensions and how reducing length and inner diameter of the capillary is predicted to give faster detection and in practice also lower limit of detection. © 2016 Elsevier B.V.

  • 61. Dumas, Randy K.
    et al.
    Greene, Peter K.
    Gilbert, Dustin A.
    Ye, Li
    Zha, Chaolin
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Univ Gothenburg, Dept Phys, Sweden.
    Liu, Kai
    Accessing different spin-disordered states using first-order reversal curves2014Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, nr 10, s. 104410-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Combined first-order reversal curve (FORC) analyses of the magnetization (M-FORC) and magnetoresistance (MR-FORC) have been employed to provide a comprehensive study of the M-MR correlation in two canonical systems: a NiFe/Cu/FePt pseudo spin valve (PSV) and a [Co/Cu](8) multilayer. In the PSV, due to the large difference in switching fields and minimal interactions between the NiFe and the FePt layers, the M and MR show a simple one-to-one relationship during reversal. In the [Co/Cu](8) multilayer, the correlation between the magnetization reversal and the MR evolution is more complex. This is primarily due to the similar switching fields of, and interactions between, the constituent Co layers. The FORC protocol accesses states with much higher spin disorders and larger MRs than those found along the conventional major loop field cycle. Unlike the M-FORC measurements, which only probe changes in the macroscopic magnetization, the MR-FORCs are more sensitive to the microscopic domain configurations as those are most important in determining the resultant MR effect size. This approach is generally applicable to spintronic systems to realize the maximum spin disorder and the largest MR.

  • 62. Dumas, Randy K.
    et al.
    Iacocca, E.
    Bonetti, S.
    Redjai Sani, Sohrab
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Mohseni, Seyed Majid
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Eklund, Anders
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Persson, J.
    Heinonen, O.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Spin-Wave-Mode Coexistence on the Nanoscale: A Consequence of the Oersted-Field-Induced Asymmetric Energy Landscape2013Inngår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 110, nr 25, s. 257202-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    It has been argued that if multiple spin wave modes are competing for the same centrally located energy source, as in a nanocontact spin torque oscillator, that only one mode should survive in the steady state. Here, the experimental conditions necessary for mode coexistence are explored. Mode coexistence is facilitated by the local field asymmetries induced by the spatially inhomogeneous Oersted field, which leads to a physical separation of the modes, and is further promoted by spin wave localization at reduced applied field angles. Finally, both simulation and experiment reveal a low frequency signal consistent with the intermodulation of two coexistent modes.

  • 63. Dumas, Randy K.
    et al.
    Redjai Sani, Sohrab
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Mohseni, S. Majid
    Iacocca, Ezio
    Pogoryelov, Yevgen
    Muduli, Pranaba K.
    Chung, Sunjae
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Durrenfeld, Philipp
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Univ Gothenburg, Sweden.
    Recent Advances in Nanocontact Spin-Torque Oscillators2014Inngår i: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 50, nr 6, s. 4100107-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present a comprehensive review of the most recent advances in nanocontact spin torque oscillators (NC-STOs). NC-STOs are highly tunable, with both applied magnetic field and dc, broadband microwave signal generators. As opposed to the nanopillar geometry, where the lateral cross section of the entire device has been confined to a typically <100 nm diameter, in NC-STOs, it is only the current injection site that has been laterally confined on top of an extended magnetic film stack. Three distinct material combinations will be discussed: 1) a Co/Cu/NiFe pseudospin valve (PSV) where both the Co and NiFe have a dominant in-plane anisotropy; 2) a Co/Cu/[Co/Ni](4) orthogonal PSV where the Co/Ni multilayer has a strong perpendicular anisotropy; and 3) a single NiFe layer with asymmetric non-magnetic Cu leads. We explore the rich and diverse magnetodynamic modes that can be generated in these three distinct sample geometries.

  • 64. Durrenfeld, P.
    et al.
    Gerhard, F.
    Chico, J.
    Dumas, R. K.
    Ranjbar, M.
    Bergman, A.
    Bergqvist, Lars
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Delin, Anna
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. KTH, Centra, SeRC - Swedish e-Science Research Centre. Uppsala University, Sweden.
    Gould, C.
    Molenkamp, L. W.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. University of Gothenburg, Sweden; NanOsc AB, Sweden.
    Tunable damping, saturation magnetization, and exchange stiffness of half-Heusler NiMnSb thin films2015Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, nr 21, artikkel-id 214424Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The half-metallic half-Heusler alloy NiMnSb is a promising candidate for applications in spintronic devices due to its low magnetic damping and its rich anisotropies. Here we use ferromagnetic resonance (FMR) measurements and calculations from first principles to investigate how the composition of the epitaxially grown NiMnSb influences the magnetodynamic properties of saturation magnetization M-S, Gilbert damping alpha, and exchange stiffness A. M-S and A are shown to have a maximum for stoichiometric composition, while the Gilbert damping is minimum. We find excellent quantitative agreement between theory and experiment for M-S and alpha. The calculated A shows the same trend as the experimental data but has a larger magnitude. In addition to the unique in-plane anisotropy of the material, these tunabilities of the magnetodynamic properties can be taken advantage of when employing NiMnSb films in magnonic devices.

  • 65. Durrenfeld, P.
    et al.
    Iacocca, E.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Muduli, P. K.
    Parametric excitation in a magnetic tunnel junction-based spin torque oscillator2014Inngår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 104, nr 5, s. 052410-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Using microwave current injection at room temperature, we demonstrate parametric excitation of a magnetic tunnel junction (MTJ)-based spin-torque oscillator (STO). Parametric excitation is observed for currents below the auto-oscillation threshold, when the microwave current frequency f(e) is twice the STO free-running frequency f(0). Above threshold, the MTJ becomes parametrically synchronized. In the synchronized state, the STO exhibits an integrated power up to 5 times higher and a linewidth reduction of two orders of magnitude, compared to free-running conditions. We also show that the parametric synchronization favors single mode oscillations in the case of multimode excitation.

  • 66. Dürrenfeld, P.
    et al.
    Gerhard, F.
    Ranjbar, M.
    Gould, C.
    Molenkamp, L. W.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. University of Gothenburg, Sweden.
    Spin Hall effect-controlled magnetization dynamics in NiMnSb2015Inngår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 117, nr 17, artikkel-id 17E103Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We investigate the influence of a spin current generated from a platinum layer on the ferromagnetic resonance (FMR) properties of an adjacent ferromagnetic layer composed of the halfmetallic half-Heusler material NiMnSb. Spin Hall nano-oscillator devices are fabricated, and the technique of spin torque FMR is used to locally study the magnetic properties as in-plane anisotropies and resonance fields. A change in the FMR linewidth, in accordance with the additional spin torque produced by the spin Hall effect, is present for an applied dc current. For sufficiently large currents, this should yield auto-oscillations, which however are not achievable in the present device geometry.

  • 67. Dürrenfeld, P.
    et al.
    Iacocca, E.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. University of Gothenburg, Sweden.
    Muduli, P. K.
    Modulation-mediated unlocking of a parametrically phase-locked spin torque oscillator2014Inngår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 105, nr 25, s. 252404-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Modulation of an oscillator is crucial for its application in communication devices. While the output power and linewidth of single magnetic tunnel junction-based spin-torque oscillators (MTJ-STO) are not yet adequate for practical uses, the synchronization of such devices can overcome these limitations. Here, we investigate the modulation behavior of a parametrically synchronized MTJ-STO and show experimentally that modulation of the synchronized state preserves synchronization as long as the modulation frequency, integral(mod), is above a characteristic frequency, integral(unlock). The unlocking frequency increases with the modulation amplitude in agreement with analytical estimates and numerical simulations. These phenomena are described as a non-resonant unlocking mechanism, whose characteristics are directly related to inherent parameters of the oscillator.

  • 68.
    Eklund, Anders
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Bonetti, Stefano
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Sani, Sohrab R.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Mohseni, Seyed Majid
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Persson, Johan
    Chung, Sunjae
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Banuazizi, S. Amir Hossein
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Iacocca, Ezio
    Östling, Mikael
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Malm, B. Gunnar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Dependence of the colored frequency noise in spin torque oscillators on current and magnetic field2014Inngår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 104, nr 9, s. 092405-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The nano-scale spin torque oscillator (STO) is a compelling device for on-chip, highly tunable microwave frequency signal generation. Currently, one of the most important challenges for the STO is to increase its longer-time frequency stability by decreasing the 1/f frequency noise, but its high level makes even its measurement impossible using the phase noise mode of spectrum analyzers. Here, we present a custom made time-domain measurement system with 150MHz measurement bandwidth making possible the investigation of the variation of the 1/f as well as the white frequency noise in a STO over a large set of operating points covering 18-25GHz. The 1/f level is found to be highly dependent on the oscillation amplitude-frequency non-linearity and the vicinity of unexcited oscillation modes. These findings elucidate the need for a quantitative theoretical treatment of the low-frequency, colored frequency noise in STOs. Based on the results, we suggest that the 1/f frequency noise possibly can be decreased by improving the microstructural quality of the metallic thin films.

  • 69.
    Eklund, Anders
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Dvornik, Mykola
    Göteborgs universitet.
    Qejvanaj, Fatjon
    NanOsc AB.
    Jiang, Sheng
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Chung, Sunjae
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Göteborgs universitet.
    Dumas, Randy K.
    Göteborgs universitet.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Göteborgs universitet.
    Malm, B. Gunnar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Nonlinearity, frequency stability and device-to-device variability in nano-contact spin torque oscillators with grainy thin filmsManuskript (preprint) (Annet vitenskapelig)
    Abstract [en]

    In nano-contact spin torque oscillators with a frequency range of 10-65 GHz, the propagating spin wave mode attracts interest due both to its high frequency stability and prospective use in magnonic devices. Its dependence of the frequency on the bias current however displays device-to-device variability on the order of several hundred MHz, with device specific nonlinearities that can be either continuous or discontinuous and have negative impact on the frequency stability. A model for this behavior is however still lacking. By using micromagnetic simulations, we investigate the impact of imperfections in the spin wave-carrying free magnetic layer and find that nonlinearities can be created when the propagating spin wave is reflected back to the active region. The oscillation then self-locks at the frequency of the resonant wavelength, resulting in a standing spin wave pattern. Simulations including nine randomly generated film structures with 30 nm-sized grains and exchange-reduced inter-grain boundaries give qualitative and partially quantitative agreement with experimental measurements. The results point out the spin wave-reflecting grain boundaries as a source of device nonlinearity, manufacturing variability and frequency destabilization.

  • 70.
    Eklund, Anders J.
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Redjai Sani, Sohrab
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Mohseni, Seyed Majid
    NanOsc AB.
    Persson, Johan
    NanOsc AB.
    Malm, B. Gunnar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Triple mode-jumping in a spin torque oscillator2013Inngår i: 2013 22nd International Conference on Noise and Fluctuations, ICNF 2013, New York: IEEE conference proceedings, 2013, s. 6578965-Konferansepaper (Fagfellevurdert)
    Abstract [en]

    In a nano-contact Co/Cu/NiFe spin torque oscillator, mode-jumping between up to three frequencies within 22.5-24.0 GHz is electrically observed in the time domain. The measurements reveal toggling between two states with differing oscillation amplitude, of which the low-amplitude state is further divided into two rapidly alternating modes. Analysis of the mode dwell time statistics and the total time spent in each mode is carried out, and it is found that in both aspects the balance between the modes is greatly altered with the DC drive current.

  • 71.
    Elgammal, Karim
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Density Functional Theory Calculations of Graphene based Humidity and Carbon Dioxide Sensors2016Licentiatavhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Graphene has many interesting physical properties which makes it useful for plenty of applications. In this work we investigate the possibility of using graphene as a carbon dioxide and humidity sensor. Carbon dioxide and water adsorbates are modeled on top of the surface of a graphene sheet, which themselves lie on one of two types of silica substrates or sapphire substrate. We evaluate the changes in the electronic and structural properties of the graphene sheet in the presence of the described adsorbates as well as the accompanying substrate. We perform the study using ab-initio calculations based on density functional theory (DFT), that allows fast, accurate and efficient investigations. In particular, we focus our attention on investigating the effects of defects in the substrate and how it influences the properties of the graphene sheet. The defects of the substrate contribute with impurity bands leading to doping effects on the graphene sheet, which in turn together with the presence of the adsorbates result in changes of the electronic charge distribution in the system. We provide charge density difference plots to visualize these changes and also determine the relaxed minimum distances of the adsorbates from the graphene sheet together with the respective minimum energy configurations. We also include the density of states, Löwdin charges and work functions for further investigations.

  • 72.
    Elgammal, Karim
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Hugosson, Håkan W.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Smith, Anderson D.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Råsander, Mikael
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Bergqvist, Lars
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Delin, Anna
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. KTH, Centra, SeRC - Swedish e-Science Research Centre. Uppsala University.
    Density functional theory calculations of graphene-based humidity and carbon dioxide sensors: effect of silica and sapphire substratesManuskript (preprint) (Annet vitenskapelig)
  • 73. Emelchenko, G.
    et al.
    Steinman, E.
    Masalov, V.
    Tereshchenko, A.
    Bazhenov, A.
    Grishin, Alex M.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    1.5 μm photoluminescence of Er3+ in opal based photonic crystals2008Inngår i: Proc SPIE Int Soc Opt Eng, 2008Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The study of the emission properties of opal-erbium oxide nanocomposites in the wide range of erbium concentrations was carried out. Erbium oxide concentration was varied from 0.25 to 16%wt. Maximal output of the photoluminescence (PL) took place at 1%wt of erbium oxide concentration. It was shown that the annealing temperatures from 600 to 900°C were too low to exhibit sufficient emission properties of the erbium-opal composites. The presence of the erbium silicates Er2SiO5 and Er 2Si2O7 in the opal-erbium nanocomposites was revealed by X-ray phase analysis. Amorphous silica in opal matrix was not crystallized at the annealing during a few hours at 1000 - 1200°C. The case of the tens hours of annealing the crystoballite phase occurred. No angle dependence of the PL intensity was observed as a result of degradation of the photonic band gap (PBG) at the annealing of the opal-erbium oxide nanocomposites. Further modification of the material processing to achieve a strong photonic band gap reflection peak near 1550 nm with high PL intensity in the opal-Er2O3 composite is running.

  • 74. Escobar, Roberto A.
    et al.
    Tryputen, Larysa
    Castillo-Sepulveda, Sebastian
    Altbir, Dora
    Chung, Sunjae
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Univ Gothenburg, Dept Phys, Sweden.
    Nguyen, Anh Tuan
    Vietnam Acad Sci & Technol, Inst Mat Sci, Hanoi, Vietnam.
    Mohseni, M.
    Akerman, Johan
    Ross, Caroline A.
    Monte Carlo Modeling of Mixed-Anisotropy [Co/Ni](2)/NiFe Multilayers2016Inngår i: IEEE Magnetics Letters, ISSN 1949-307X, E-ISSN 1949-3088, Vol. 7, artikkel-id UNSP 4101205Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The magnetic properties of a thin film consisting of an exchange-coupled [Co/Ni](2)/NiFe multilayer have been studied as a function of the NiFe thickness by using Monte Carlo modeling and compared with experimental results of [Co/Ni](4)/Co/NiFe multilayers. Both modeling and experiment showed that the NiFe thickness controls the effective anisotropy. The direction of the easy axis is determined by a competition between the perpendicular crystalline anisotropy of the Co/Ni and the shape anisotropy of the multilayer. As the thickness of the NiFe layer increases, the reversal mechanism of the thin film changes from the nucleation of reverse domains to vortex propagation. Therefore, our results reveal the magnetic configurations and the easy axis reorientation of mixed-anisotropy multilayers.

  • 75. Etz, Corina
    et al.
    Bergqvist, Lars
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Bergman, Anders
    Taroni, Andrea
    Eriksson, Olle
    Atomistic spin dynamics and surface magnons2015Inngår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 27, nr 24, artikkel-id 243202Artikkel, forskningsoversikt (Fagfellevurdert)
    Abstract [en]

    Atomistic spin dynamics simulations have evolved to become a powerful and versatile tool for simulating dynamic properties of magnetic materials. It has a wide range of applications, for instance switching of magnetic states in bulk and nano-magnets, dynamics of topological magnets, such as skyrmions and vortices and domain wall motion. In this review, after a brief summary of the existing investigation tools for the study of magnons, we focus on calculations of spin-wave excitations in low-dimensional magnets and the effect of relativistic and temperature effects in such structures. In general, we find a good agreement between our results and the experimental values. For material specific studies, the atomistic spin dynamics is combined with electronic structure calculations within the density functional theory from which the required parameters are calculated, such as magnetic exchange interactions, magnetocrystalline anisotropy, and Dzyaloshinskii-Moriya vectors.

  • 76. Evertsson, J.
    et al.
    Bertram, F.
    Zhang, Fan
    KTH, Skolan för kemivetenskap (CHE), Kemi, Yt- och korrosionsvetenskap.
    Rullik, L.
    Merte, L. R.
    Shipilin, M.
    Soldemo, Markus
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Ahmadi, Sareh
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Vinogradov, N.
    Carla, F.
    Weissenrieder, Jonas
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Götelid, Mats
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Pan, Jinshan
    KTH, Skolan för kemivetenskap (CHE), Kemi, Yt- och korrosionsvetenskap.
    Mikkelsen, A.
    Nilsson, J. -O
    Lundgren, E.
    The thickness of native oxides on aluminum alloys and single crystals2015Inngår i: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 349, s. 826-832Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present results from measurements of the native oxide film thickness on four different industrial aluminum alloys and three different aluminum single crystals. The thicknesses were determined using X-ray reflectivity, X-ray photoelectron spectroscopy, and electrochemical impedance spectroscopy. In addition, atomic force microscopy was used for micro-structural studies of the oxide surfaces. The reflectivity measurements were performed in ultra-high vacuum, vacuum, ambient, nitrogen and liquid water conditions. The results obtained using X-ray reflectivity and X-ray photoelectron spectroscopy demonstrate good agreement. However, the oxide thicknesses determined from the electrochemical impedance spectroscopy show a larger discrepancy from the above two methods. In the present contribution the reasons for this discrepancy are discussed. We also address the effect of the substrate type and the presence of water on the resultant oxide thickness.

  • 77. Fashandi, Hossein
    et al.
    Soldemo, Markus
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Weissenrieder, Jonas
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Gothelid, Mats
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Materialteknologi.
    Eriksson, Jens
    Eklund, Per
    Spetz, Anita Lloyd
    Andersson, Mike
    Applicability of MOS structures in monitoring catalytic properties, as exemplified for monolayer-iron-oxide-coated porous platinum films2016Inngår i: Journal of Catalysis, ISSN 0021-9517, E-ISSN 1090-2694, Vol. 344, s. 583-590Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Metal Oxide Semiconductor (MOS) capacitor devices comprised of monolayer iron oxide-coated as well as non-coated polycrystalline Pt deposited on oxidized silicon carbide substrates have been fabricated and their usefulness as realistic model systems in catalyst studies development was evaluated. The CO oxidation characteristics of both iron oxide- and non-coated Pt catalysts were investigated using mass spectrometry, monitoring the carbon dioxide production rate for different combinations of carbon monoxide (CO) and oxygen concentrations at various temperatures. Additionally, the output capacitance of the MOS model catalysts was recorded for each individual CO oxidation activity. A low-temperature shift in CO oxidation characteristics for the monolayer-coated compared to the non-coated Pt catalysts was observed, similar to that previously reported for monolayer iron oxide grown on single-crystalline Pt substrates. A strong correlation between the output capacitance of the MOS structures and the CO oxidation characteristics was found for both monolayer- and non-coated model catalysts. Furthermore, the devices exhibit retained MOS electrical output and CO oxidation characteristics as well as an unaffected catalyst surface composition, as confirmed by photoelectron spectroscopy, even after 200 h of continuous model catalyst operation. In addition to the implications on practical applicability of monolayer iron oxide coating on widely used polycrystalline Pt films in real-world catalysts and sensors, the findings also point to new possibilities regarding the use of MOS model systems for in situ characterization, high throughput screening, and tailoring of e.g. catalyst- and fuel-cell-electrode materials for specific applications.

  • 78. Fatuzzo, C. G.
    et al.
    Sassa, Y.
    Månsson, Martin
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Pailhes, S.
    Lipscombe, O. J.
    Hayden, S. M.
    Patthey, L.
    Shi, M.
    Grioni, M.
    Rønnow, H. M.
    Mesot, J.
    Tjernberg, Oscar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Chang, J.
    Nodal Landau Fermi-liquid quasiparticles in overdoped La1.77Sr0.23CuO42014Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, nr 20, s. 205104-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Nodal angle-resolved photoemission spectra taken on overdoped La1.77Sr0.23CuO4 are presented and analyzed. It is proven that the low-energy excitations are true Landau Fermi-liquid quasiparticles. We show that momentum and energy distribution curves can be analyzed self-consistently without quantitative knowledge of the bare band dispersion. Finally, by imposing Kramers-Kronig consistency on the self-energy Sigma, insight into the quasiparticle residue is gained. We conclude by comparing our results to quasiparticle properties extracted from thermodynamic, magnetoresistance, and high-field quantum oscillation experiments on overdoped Tl2Ba2CuO6+delta.

  • 79. Fazlali, Masoumeh
    et al.
    Dvornik, Mykola
    Iacocca, Ezio
    Durrenfeld, Philipp
    Haidar, Mohammad
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Chalmers University of Technology, Sweden.
    Dumas, Randy K.
    Homodyne-detected ferromagnetic resonance of in-plane magnetized nanocontacts: Composite spin-wave resonances and their excitation mechanism2016Inngår i: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, nr 13, artikkel-id 134427Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This work provides a detailed investigation of the measured in-plane field-swept homodyne-detected ferromagnetic resonance (FMR) spectra of an extended Co/Cu/NiFe pseudo-spin-valve stack using a nanocontact (NC) geometry. The magnetodynamics are generated by a pulse-modulated microwave current, and the resulting rectified dc mixing voltage, which appears across the NC at resonance, is detected using a lock-in amplifier. Most notably, we find that the measured spectra of the NiFe layer are composite in nature and highly asymmetric, consistent with the broadband excitation of multiple modes. Additionally, the data must be fit with two Lorentzian functions in order to extract a reasonable value for the Gilbert damping of the NiFe. Aided by micromagnetic simulations, we conclude that (i) for in-plane fields the rf Oersted field in the vicinity of the NC plays the dominant role in generating the observed spectra, (ii) in addition to the FMR mode, exchange-dominated spin waves are also generated, and (iii) the NC diameter sets the mean wave vector of the exchange-dominated spin wave, in good agreement with the dispersion relation.

  • 80. Fonseca, R. D.
    et al.
    Correa, D. S.
    Paris, E. C.
    Tribuzi, V.
    Dev, Apurba
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Voss, T.
    Aoki, P. H. B.
    Constantino, C. J. L.
    Mendonca, C. R.
    Fabrication of zinc oxide nanowires/polymer composites by two-photon polymerization2014Inngår i: Journal of Polymer Science Part B: Polymer Physics, ISSN 0887-6266, E-ISSN 1099-0488, Vol. 52, nr 4, s. 333-337Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present an approach to fabricate ZnO nanowires/polymer composite into three-dimensional microstructures, based on two-photon polymerization direct laser writing, a fabrication method that allows submicrometric spatial resolution. The structural integrity of the structures was inferred by scanning electron microscopy, while the presence and distribution of ZnO nanowires was investigated by energy dispersive X-ray, Raman spectroscopy, and X-ray diffraction. The optical properties of the produced composite microstructures were verified by imaging the characteristic ZnO emission using a fluorescence microscope. Hence, such approach can be used to develop composite microstructures containing ZnO nanowires aiming at technological applications. © 2013 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2014, 52, 333-337 Zinc oxide (ZnO) has proven to be a promising material for optoelectronic devices operating in the blue to near-UV spectral region. While ZnO/polymer composite films have been readily produced via cast and spin coating, these standard approaches do not allow the fabrication of three-dimensional (3D) microstructures due to the lack of spatial resolution. In this work, 3D microstructures of composites made up of acrylic resin and ZnO nanowires are created by using two-photon polymerization.

  • 81. Franco, A. F.
    et al.
    Gonzalez-Fuentes, C.
    Morales, R.
    Ross, C. A.
    Dumas, R.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. University of Gothenburg, Sweden.
    Garcia, C.
    Variable variance Preisach model for multilayers with perpendicular magnetic anisotropy2016Inngår i: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 94, nr 6, artikkel-id 064431Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present a variable variance Preisach model that fully accounts for the different magnetization processes of a multilayer structure with perpendicular magnetic anisotropy by adjusting the evolution of the interaction variance as the magnetization changes. We successfully compare in a quantitative manner the results obtained with this model to experimental hysteresis loops of several [CoFeB/Pd](n) multilayers. The effect of the number of repetitions and the thicknesses of the CoFeB and Pd layers on the magnetization reversal of the multilayer structure is studied, and it is found that many of the observed phenomena can be attributed to an increase of the magnetostatic interactions and subsequent decrease of the size of the magnetic domains. Increasing the CoFeB thickness leads to the disappearance of the perpendicular anisotropy, and such a minimum thickness of the Pd layer is necessary to achieve an out-of-plane magnetization.

  • 82.
    Grinter, David
    et al.
    Brookhaven Natl Lab, Chem, Ridge, NY USA..
    Luo, Si
    Brookhaven Natl Lab, Chem, Ridge, NY USA.;SUNY Stony Brook, Chem, Stony Brook, NY 11794 USA..
    Soldemo, Markus
    KTH.
    Piazza, Luca
    KTH.
    Weissenrieder, Jonas
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Senanayake, Sanjaya
    Brookhaven Natl Lab, Chem, Ridge, NY USA..
    Stacchiola, Dario
    Brookhaven Natl Lab, Chem, Ridge, NY USA..
    Rodriguez, Jose
    Brookhaven Natl Lab, Chem, Ridge, NY USA.;SUNY Stony Brook, Chem, Stony Brook, NY 11794 USA..
    Potassium promotion of a model Au/TiO2 catalyst2016Inngår i: Abstracts of Papers of the American Chemical Society, ISSN 0065-7727, Vol. 252Artikkel i tidsskrift (Annet vitenskapelig)
  • 83.
    Grishin, Alexander
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Petrozavodsk State University, Petrozavodsk.
    Markova, Nadezhda P.
    Broadband luminescent ferroelectric biocompatible Er:(Na,K)NbO3 nanofibers2016Inngår i: Journal of The American Ceramic Society, ISSN 0002-7820, E-ISSN 1551-2916Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Dense homogeneous fabric composed from continuous bead-free erbium-doped sodium potassium niobate (Er:NKN) 100 lm long and 100-200 nm in diameter nanofibers was sintered by sol-gel calcination assisted electrospinning technique. X-ray diffraction revealed preferential cube-on-cube [001]-directional growth of fibers containing predominantly monoclinic Na0.35K0.65NbO3-type phase and significantly less of tetragonal NbO2, cubic Er2O3, and monoclinic ErNbO4 phases. Er doping with the concentration of 2 at.% provides readily detectable room-temperature broad-band photoluminescence (PL) centered at kPL = 0.55 and 0.98 lm being pumped, respectively, with 532 and 785 nm lasers. Impedance spectroscopy and static electrical tests revealed ferroelectric properties, electric field induced resistance switching and strong rectification effect in nanoporous sandwich Au/Er: NKN/Pt capacitive cell. Memristor-type current-voltage (I-V) characteristics originate from the electrochemical migration of oxygen vacancies at the n-type NKN oxide/high work function Pt cathode junction interface.

  • 84. Gulbinas, K.
    et al.
    Grivickas, V.
    Grivickas, P.
    Linnros, Jan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Conversion of laser pulse optical energy to photo-acoustic wave in nm-scale layered tlgase2 crystals2014Inngår i: IOP Conference Series: Materials Science and Engineering, 2014, Vol. 68, nr 1Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Experiments are presented that reveal an efficient optical energy conversion from the visible to the infrared wavelengths range as a result of photo-acoustic response (PAR) after light pulse incites onto the free surface of Tlgase2 crystal. Excitation was carried out with a tunable wavelength of ns-pulse laser and the PAR was detected laterally with a focused cw- probe. The observed properties can be related to variety of successive factors: high electron- hole-phonon deformation potential, a high factor of refraction coefficient dependency on pressure, the absence of surface recombination and the band filling effect, in relation with low absorption coefficient due to the forbidden direct-band optical transition in Tlgase2. All these ensure that the acoustic energy remain well confined under a wide pulse power and energy range suggesting that Tlgase2 is a promising material for dynamic optical energy conversion.

  • 85. Haidar, M.
    et al.
    Durrenfeld, P.
    Ranjbar, M.
    Balinsky, M.
    Fazlali, M.
    Dvornik, M.
    Dumas, R. K.
    Khartsev, Sergiy
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. University of Gothenburg, Sweden.
    Controlling Gilbert damping in a YIG film using nonlocal spin currents2016Inngår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, nr 18, artikkel-id 180409Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We demonstrate the control of Gilbert damping in 65-nm-thick yttrium iron garnet (YIG) films using a spin-polarized current generated by a direct current through a nanocontact, spin filtered by a thin Co layer. The magnetodynamics of both the YIG and the Co layers can be excited by a pulse-modulated microwave current injected through the nanocontact and the response detected as a lock-in amplified voltage over the device. The spectra show three clear peaks, two associated with the ferromagnetic resonance (FMR) in each layer, and an additional Co mode with a higher wave vector proportional to the inverse of the nanocontact diameter. By varying the sign and magnitude of the direct nanocontact current, we can either increase or decrease the linewidth of the YIG FMR peak consistent with additional positive or negative damping being exerted by the nonlocal spin current injected into the YIG film. Our nanocontact approach thus offers an alternative route in the search for auto-oscillations in YIG films.

  • 86. Haidar, M.
    et al.
    Ranjbar, M.
    Balinsky, M.
    Dumas, R. K.
    Khartsev, Sergiy
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. University of Gothenburg, Sweden.
    Thickness- and temperature-dependent magnetodynamic properties of yttrium iron garnet thin films2015Inngår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 117, nr 17, artikkel-id 17D119Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The magnetodynamical properties of nanometer-thick yttrium iron garnet films are studied using ferromagnetic resonance as a function of temperature. The films were grown on gadolinium gallium garnet substrates by pulsed laser deposition. First, we found that the damping coefficient increases as the temperature increases for different film thicknesses. Second, we found two different dependencies of the damping on film thickness: at room temperature, the damping coefficient increases as the film thickness decreases, while at T = 8 K, we find the damping to depend only weakly on the thickness. We attribute this behavior to an enhancement of the relaxation of the magnetization by impurities or defects at the surfaces.

  • 87.
    Halldin Stenlid, Joakim
    et al.
    KTH, Skolan för kemivetenskap (CHE), Kemi, Tillämpad fysikalisk kemi.
    Soldemo, Markus
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Johansson, A. J.
    Leygraf, Christofer
    KTH, Skolan för kemivetenskap (CHE), Kemi, Yt- och korrosionsvetenskap.
    Götelid, Mats
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Weissenrieder, Jonas
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Brinck, Tore
    KTH, Skolan för kemivetenskap (CHE), Kemi, Tillämpad fysikalisk kemi.
    Reactivity at the Cu2O(100):Cu-H2O interface: a combined DFT and PES study2016Inngår i: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 18, nr 44, s. 30570-30584Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The water-cuprite interface plays an important role in dictating surface related properties. This not only applies to the oxide, but also to metallic copper, which is covered by an oxide film under typical operational conditions. In order to extend the currently scarce knowledge of the details of the water-oxide interplay, water interactions and reactions on a common Cu2O(100):Cu surface have been studied using high-resolution photoelectron spectroscopy (PES) as well as Hubbard U and dispersion corrected density functional theory (PBE-D3+U) calculations up to a bilayer water coverage. The PBE-D3+U results are compared with PBE, PBE-D3 and hybrid HSE06-D3 calculation results. Both computational and experimental results support a thermodynamically favored, and H2O coverage independent, surface OH coverage of 0.25-0.5 ML, which is larger than the previously reported value. The computations indicate that the results are consistent also for ambient temperatures under wet/humid and oxygen lean conditions. In addition, both DFT and PES results indicate that the initial (3,0; 1,1) surface reconstruction is lifted upon water adsorption to form an unreconstructed (1 x 1) Cu2O(100) structure.

  • 88. Heinonen, O. G.
    et al.
    Muduli, P. K.
    Iacocca, E.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Decoherence, Mode Hopping, and Mode Coupling in Spin Torque Oscillators2013Inngår i: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 49, nr 7, s. 4398-4404Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Spin torque oscillators (STOs) often exhibit multiple modes, leading to complex behavior. One example is mode hopping between different eigenmodes of a magnetic tunnel junction (MTJ) STO. This mode hopping is a strong function of current and angle between the magnetization in the free and fixed layers, and away from anti-parallel configuration, mode hopping can be the dominant decoherence process. Another example is the linewidth of a nanocontact STO that can be a complex non-monotonic function of temperature in regions where two or more modes are excited by the oscillators. These phenomena require a generalization of the single-mode nonlinear STO theory to include mode coupling. We derive equations describing the slow time evolution of the coupled system and show they describe a dynamically driven system, similar to other systems that exhibit mode hopping in the presence of thermal fluctuations. In our description, mode coupling also leads to additional coupling between power and phase fluctuations, which can in certain limited cases lead to longer relaxation times for power fluctuations, and consequently to larger linewidths through the nonlinear frequency shift.

  • 89. Hellsvik, J.
    et al.
    Balestieri, M.
    Usui, T.
    Stroppa, A.
    Bergman, A.
    Bergqvist, Lars
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Prabhakaran, D.
    Eriksson, O.
    Picozzi, S.
    Kimura, T.
    Lorenzana, J.
    Tuning order-by-disorder multiferroicity in CuO by doping2014Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, nr 1, s. 014437-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The high Curie temperature multiferroic compound CuO has a quasidegenerate magnetic ground state that makes it prone to manipulation by the so-called "order-by-disorder" mechanism. First principle computations supplemented with Monte Carlo simulations and experiments show that isovalent doping allows us to stabilize the multiferroic phase in nonferroelectric regions of the pristine material phase diagram with experiments reaching a 250% widening of the ferroelectric temperature window with 5% of Zn doping. Our results allow us to validate the importance of a quasidegenerate ground state on promoting multiferroicity on CuO at high temperatures and open a path to the material engineering of multiferroic materials. In addition we present a complete explanation of the CuO phase diagram and a computation on the incommensurability in excellent agreement with experiment without free parameters.

  • 90. Hoglin, Viktor
    et al.
    Hudl, Matthias
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Uppsala University, Sweden .
    Caron, Luana
    Beran, Premysl
    Solby, Magnus H.
    Nordblad, Per
    Andersson, Yvonne
    Sahlberg, Martin
    Detailed study of the magnetic ordering in FeMnP0.75Si0.252015Inngår i: Journal of Solid State Chemistry, ISSN 0022-4596, E-ISSN 1095-726X, Vol. 221, s. 240-246Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Magnetic and crystallographic properties of FeMnP0.75Si0.25 in the hexagonal Fe2P-type structure have been investigated by X-ray powder diffraction, neutron powder diffraction and magnetic measurements. The room temperature diffractograms reveal co-existence of two distinct structural phases in the samples with small, but significant, differences only in the unit cell dimensions. The volume ratio between the two phases is governed by the annealing conditions. One of the phases orders ferromagnetically (T-C=250 K) and the other in an incommensurate antiferromagnetic structure at low temperatures (q(x) = 0.363(1), T-N=150 K).

  • 91.
    Hormozan, Yashar
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Sychugov, Ilya
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Linnros, Jan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    High-resolution x-ray imaging using a structured scintillator2016Inngår i: Medical physics (Lancaster), ISSN 0094-2405, Vol. 43, nr 2, s. 696-701Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Purpose: In this study, the authors introduce a new generation of finely structured scintillators with a very high spatial resolution (a few micrometers) compared to conventional scintillators, yet maintaining a thick absorbing layer for improved detectivity. Methods: Their concept is based on a 2D array of high aspect ratio pores which are fabricated by ICP etching, with spacings (pitches) of a few micrometers, on silicon and oxidation of the pore walls. The pores were subsequently filled by melting of powdered CsI(Tl), as the scintillating agent. In order to couple the secondary emitted photons of the back of the scintillator array to a CCD device, having a larger pixel size than the pore pitch, an open optical microscope with adjustable magnification was designed and implemented. By imaging a sharp edge, the authors were able to calculate the modulation transfer function (MTF) of this finely structured scintillator. Results: The x-ray images of individually resolved pores suggest that they have been almost uniformly filled, and the MTF measurements show the feasibility of a few microns spatial resolution imaging, as set by the scintillator pore size. Compared to existing techniques utilizing CsI needles as a structured scintillator, their results imply an almost sevenfold improvement in resolution. Finally, high resolution images, taken by their detector, are presented. Conclusions: The presented work successfully shows the functionality of their detector concept for high resolution imaging and further fabrication developments are most likely to result in higher quantum efficiencies.

  • 92.
    Hosseinpour, Saman
    et al.
    KTH, Skolan för kemivetenskap (CHE), Kemi, Yt- och korrosionsvetenskap.
    Götelid, Mats
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Leygraf, Christofer
    KTH, Skolan för kemivetenskap (CHE), Kemi, Yt- och korrosionsvetenskap.
    Johnson, C. Magnus
    KTH, Skolan för kemivetenskap (CHE), Kemi, Yt- och korrosionsvetenskap.
    Self-Assembled Monolayers as Inhibitors for the Atmospheric Corrosion of Copper Induced by Formic Acid: A Comparison between Hexanethiol and Hexaneselenol2014Inngår i: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 161, nr 1, s. C50-C56Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The corrosion protection by self-assembled monolayers (SAMs) of hexanethiol and hexaneselenol has been explored on copper exposed to humid air containing formic acid, a corrosive environment relevant for indoor atmospheric corrosion. The kinetics of the formation of corrosion products on SAM covered copper was followed using in-situ infrared reflection/absorption spectroscopy (IRAS) and compared with that of bare copper. Both SAMs initially showed a corrosion protection ability. The prolonged exposure of hexanethiol copper resulted in a reduced formation rate of copper formate and copper hydroxide compared to bare copper, while on bare and hexaneselenol covered samples copper (I) oxide, copper formate, and copper hydroxide were observed. To assess the quality of the SAMs during the sample exposure, vibrational sum frequency spectroscopy (VSFS) was used. It was found that hexaneselenol molecules are locally removed from the surface during corrosion in contrast to their thiol counterparts. This created localized galvanic effects which resulted in an accelerated corrosion of selenol covered copper. X-ray photoelectron spectroscopy at elevated pressure demonstrated that the hexanethiol removed thin oxide layers upon adsorption. A contributing parameter to the enhanced corrosion inhibiting ability of hexanethiol could be its greater ability than hexaneselenol to bind to the copper surface during its deposition and sample exposure.

  • 93. Houshang, A.
    et al.
    Fazlali, M.
    Redjai Sani, Sohrab
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. NanOsc AB, S-16440 Kista, Sweden.
    Durrenfeld, P.
    Iacocca, E.
    Akerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Univ Gothenburg, Dept Phys, Gothenburg, Sweden.
    Dumas, R. K.
    Effect of Excitation Fatigue on the Synchronization of Multiple Nanocontact Spin-Torque Oscillators2014Inngår i: IEEE Magnetics Letters, ISSN 1949-307X, E-ISSN 1949-3088, Vol. 5, s. 3000404-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Nanocontact spin-torque oscillators (NC-STOs) act as intrinsically nanoscale and highly current and magnetic field tunable, ultrawide band microwave signal generators. However, their low output power and high phase noise remain critical obstacles toward actual applications. Mutual synchronization of multiple NCs is one possibility to overcome these shortcomings. This letter presents a detailed study of the mutual synchronization in a NC-STO with two NCs. In particular, the effect of repeated measurements on the synchronization behavior is explored. Repeated measurements at high drive currents are shown to significantly degrade the performance of the devices with the most striking consequence being that the devices can no longer be synchronized. Ferromagnetic resonance measurements reveal a decrease in the saturation magnetization and an increase in the damping coefficient in annealed NiFe films, consistent with Cu diffusion into the NiFe from the adjacent Cu layers. This increase in damping will act to sever the spin wave-mediated communication channel between the NCs necessary for synchronization. These results highlight an important consideration when studying the synchronization behavior of multi-NC devices where Joule heating is expected to scale unfavorably with the number of NCs.

  • 94. Houshang, A.
    et al.
    Iacocca, E.
    Durrenfeld, P.
    Sani, S. R.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. University of Gothenburg, Sweden; NanOsc AB, Sweden.
    Dumas, R. K.
    Spin-wave-beam driven synchronization of nanocontact spin-torque oscillators2016Inngår i: Nature Nanotechnology, ISSN 1748-3387, E-ISSN 1748-3395, Vol. 11, nr 3, s. 280-+Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The synchronization of multiple nanocontact spin-torque oscillators (NC-STOs) is mediated by propagating spin waves (SWs). Although it has been shown that the Oersted field generated in the vicinity of the NC can dramatically alter the emission pattern of SWs, its role in the synchronization behaviour of multiple NCs has not been considered so far. Here we investigate the synchronization behaviour in multiple NC-STOs oriented either vertically or horizontally, with respect to the in-plane component of the external field. Synchronization is promoted (impeded) by the Oersted field landscape when the NCs are oriented vertically (horizontally) due to the highly anisotropic SW propagation. Not only is robust synchronization between two oscillators observed for separations larger than 1,000 nm, but synchronization of up to five oscillators, a new record, has been observed in the vertical array geometry. Furthermore, the synchronization can no longer be considered mutual in nature.

  • 95.
    Huang, Shuo
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Li, Wei
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Li, Xiaoqing
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Schönecker, Stephan
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Bergqvist, Lars
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Holmström, E.
    Varga, L. K.
    Vitos, Levente
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik. Wigner Research Centre for Physics, Hungary; Uppsala University, Sweden.
    Mechanism of magnetic transition in FeCrCoNi-based high entropy alloys2016Inngår i: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 103, s. 71-74Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    First-principles alloy theory and Monte-Carlo simulations are performed to investigate the magnetic properties of FeCrCoNiAlx high entropy alloys. Results show that face-centered-cubic (fcc) and body-centered-cubic (bcc) structures possess significantly different magnetic behaviors uncovering that the alloy's Curie temperature is controlled by the stability of the Al-induced single phase or fcc-bcc dual-phase. We show that the appearance of the bcc phase with increasing Al content brings about the observed transition from the paramagnetic state for FeCrCoNi to the ferromagnetic state for FeCrCoNiAl at room-temperature. Similar mechanism is predicted to give rise to room-temperature ferromagnetism in FeCrCoNiGa high entropy alloy.

  • 96.
    Hudl, Matthias
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Campanini, D.
    Caron, L.
    Hoglin, V.
    Sahlberg, M.
    Nordblad, P.
    Rydh, A.
    Thermodynamics around the first-order ferromagnetic phase transition of Fe2P single crystals2014Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, nr 14, s. 144432-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The specific heat and thermodynamics of Fe2P single crystals around the first-order paramagnetic to ferromagnetic (FM) phase transition at T-C similar or equal to 217 K are empirically investigated. The magnitude and direction of the magnetic field relative to the crystal axes govern the derived H-T phase diagram. Strikingly different phase contours are obtained for fields applied parallel and perpendicular to the c axis of the crystal. In parallel fields, the FM state is stabilized, while in perpendicular fields the phase transition is split into two sections, with an intermediate FM phase where there is no spontaneous magnetization along the c axis. The zero-field transition displays a textbook example of a first-order transition with different phase stability limits on heating and cooling. The results have special significance since Fe2P is the parent material to a family of compounds with outstanding magnetocaloric properties.

  • 97.
    Hudl, Matthias
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. ETH Zürich, Switzerland.
    Lazor, Peter
    Mathieu, Roland
    Gavriliuk, Alexander
    Struzhkin, Viktor
    PPMS-based set-up for Raman and luminescence spectroscopy at high magnetic field, high pressure and low temperature2015Inngår i: EPJ Techniques and Instrumentation, ISSN 2195-7045, Vol. 2, nr 1Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present an experimental set-up permitting Raman and luminescence spectroscopy studies in a commercial Physical Properties Measurement System (PPMS) from Quantum Design. Using this experimental set-up, gaseous, liquid and solid materials, in bulk or thin film form, may be investigated. The set-up is particularly suitable for the study of the spin-lattice coupling in strongly correlated oxide materials utilizing several different stimuli, e.g. magnetic and electric fields, high pressure and low temperatures. Details for the Raman extension, sample holder assembly and optical design, as well as data acquisition and measurement routine are described. Finally, we present exemplary results collected using the set-up, measured on reference materials, as well as on a correlated transition metal oxide.

  • 98. Höglin, Viktor
    et al.
    Cedervall, Johan
    Andersson, Mikael Svante
    Sarkar, Tapati
    Hudl, Matthias
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Uppsala University, Uppsala, Sweden.
    Nordblad, Per
    Andersson, Yvonne
    Sahlberg, Martin
    Phase diagram, structures and magnetism of the FeMnP1-xSix-system2015Inngår i: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 5, nr 11, s. 8278-8284Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The magnetic properties of the (Fe,Mn)(2)(P,Si)-system have been shown to be readily manipulated by small changes in composition. This study surveys the FeMnP1-xSix-system (0.00 <= x <= 1.00) reporting sample syntheses and investigations of crystallographic and magnetic properties using X-ray powder diffraction and magnetic measurements. Two single phase regions exist: the orthorhombic Co2P-type structure (x < 0.15) and the Fe2P-type structure (0.24 <= x < 0.50). Certain compositions have potential for use in magnetocaloric applications.

  • 99. Iacocca, Ezio
    et al.
    Dumas, Randy K.
    Bookman, Lake
    Mohseni, Majid
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Chung, Sunjae
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Hoefer, Mark A.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Confined Dissipative Droplet Solitons in Spin-Valve Nanowires with Perpendicular Magnetic Anisotropy2014Inngår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 112, nr 4, s. 047201-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Magnetic dissipative droplets are localized, strongly nonlinear dynamical modes excited in nanocontact spin valves with perpendicular magnetic anisotropy. These modes find potential application in nanoscale structures for magnetic storage and computation, but dissipative droplet studies have so far been limited to extended thin films. Here, numerical and asymptotic analyses are used to demonstrate the existence and properties of novel solitons in confined structures. As a nanowire's width is decreased with a nanocontact of fixed size at its center, the observed modes undergo transitions from a fully localized two-dimensional droplet into a two-dimensional droplet edge mode and then a pulsating one-dimensional droplet. These solitons are interpreted as dissipative versions of classical, conservative solitons, allowing for an analytical description of the modes and the mechanisms of bifurcation. The presented results open up new possibilities for the study of low-dimensional solitons and droplet applications in nanostructures.

  • 100. Iacocca, Ezio
    et al.
    Durrenfeld, Philipp
    Heinonen, Olle
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Dumas, Randy K.
    Mode-coupling mechanisms in nanocontact spin-torque oscillators2015Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, nr 10, artikkel-id 104405Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Spin-torque oscillators (STOs) are devices that allow for the excitation of a variety of magnetodynamical modes at the nanoscale. Depending on both external conditions and intrinsic magnetic properties, STOs can exhibit regimes of mode hopping and even mode coexistence. Whereas mode hopping has been extensively studied in STOs patterned as nanopillars, coexistence has been only recently observed for localized modes in nanocontact STOs (NC-STOs), where the current is confined to flow through a NC fabricated on an extended pseudo spin valve. By means of electrical characterization and a multimode STO theory, we investigate the physical origin of the mode-couplingmechanisms favoring coexistence. Two couplingmechanisms are identified: (i) magnon-mediated scattering and (ii) intermode interactions. These mechanisms can be physically disentangled by fabricating devices where the NCs have an elliptical cross section. The generation power and linewidth from such devices are found to be in good qualitative agreement with the theoretical predictions, as well as provide evidence of the dominant mode-coupling mechanisms.

12345 51 - 100 of 232
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