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  • 1.
    Bonanni, Valentina
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Fang, Yeyu
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Dumas, Randy K.
    Department of Physics, University of Gothenburg, Sweden.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Nogués, Josep
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    First-order reversal curve analysis of graded anisotropy FePtCu films2010In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 97, no 20, p. 202501-Article in journal (Refereed)
    Abstract [en]

    The reversal mechanisms of graded anisotropy FePtCu films have been investigated by alternating gradient magnetometer (AGM) and magneto-optical Kerr effect (MOKE) measurements with first-order reversal curve (FORC) techniques. The AGM-FORC analysis, which clearly shows the presence of soft and hard components, is unable to resolve how these phases are distributed throughout the film thickness. MOKE-FORC measurements, which preferentially probe the surface of the film, reveal that the soft components are indeed located toward the top surface. Combining AGM-FORC with the inherent surface sensitivity of MOKE-FORC analysis allows for a comprehensive analysis of heterogeneous systems such as graded materials.

  • 2. Dumas, Randy K.
    et al.
    Fang, Yeyu
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Kirby, B. J.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Bonanni, Valentina
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Nogues, Josep
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Probing vertically graded anisotropy in FePtCu films2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 5, p. 054434-Article in journal (Refereed)
    Abstract [en]

    Field-dependent polarized neutron reflectivity (PNR) and magnetometry are employed to study the magnetic properties of compositionally uniform and graded FePtCu films as a function of annealing temperature (T(A)). The PNR results are able to directly probe the compositional and anisotropy variations through the film thickness. Further details about how the reversal mechanisms evolve are then elucidated by using a first-order reversal curve technique. The reversal of the graded sample annealed at 300 degrees C occurs by an initial rapid switching of the dominant soft A1 phase toward the surface of the film, followed by the gradual reversal of the residual hard phase components toward the bottom. This indicates that the anisotropy gradient is not well established at this low T(A). A fundamentally different mechanism is found after annealing at 400 degrees C, where the rapid switching of the entire film is preceded by a gradual reversal of the soft layers. This suggests that the anisotropy gradient has become better established through the film thickness. The field-dependent PNR measurements confirm the existence of an anisotropy gradient, where the lower (higher) anisotropy portions are now toward the bottom (top) of the film because of the Cu compositional gradient. However, after annealing at 500 degrees C, a single rapid reversal is found, indicating the formation of a uniform hard film. In this case, PNR demonstrates a more uniform magnetic depth profile that is consistent with a uniform reference sample, suggesting significant interdiffusion of the Cu is degrading the compositional and induced anisotropy gradient at this elevated T(A).

  • 3. Dumas, Randy K.
    et al.
    Greene, Peter K.
    Gilbert, Dustin A.
    Ye, Li
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. Univ Gothenburg, Dept Phys, Sweden.
    Liu, Kai
    Accessing different spin-disordered states using first-order reversal curves2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 10, p. 104410-Article in journal (Refereed)
    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.

  • 4. Dumas, Randy K.
    et al.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Fang, Yeyu
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Bonanni, Valentina
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Lau, June W.
    Nogués, Josep
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Graded Anisotropy FePtCu Films2011In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 47, no 6, p. 1580-1586Article in journal (Refereed)
    Abstract [en]

    The fabrication and subsequent analysis of continuously graded anisotropy films are discussed. During deposition, a compositional gradient is first achieved by varying the Cu concentration from Cu-rich (Fe53Pt47)(70)Cu-30 to Cu-free Fe53Pt47. The anisotropy gradient is then realized after thermal post-annealing, and by utilizing the strong composition dependence of the low-anisotropy (A1) to high-anisotropy (L1(0)) ordering temperature. The magnetic properties are investigated by surface sensitive magneto-optical Kerr effect and alternating gradient magnetometer (AGM) measurements. AGM first-order reversal curve (FORC) measurements are employed in order to provide a detailed analysis of the reversal mechanisms, and therefore the induced anisotropy gradient. At low annealing temperatures, the FORC measurements clearly indicate the highly coupled reversal of soft and hard phases. However, significant interdiffusion results in virtually uniform films at elevated annealing temperatures. Additionally, the A1 to L1(0) ordering process is found to depend on the film thickness.

  • 5.
    Fang, Yeyu
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Dumas, Randy K.
    Department of Physics, University of Gothenburg, Sweden.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    An In Situ anneal study of graded anisotropy FePtCu films2011In: IEEE Magnetics Letters, ISSN 1949-307X, E-ISSN 1949-3088, Vol. 2Article in journal (Refereed)
    Abstract [en]

    We provide a detailed study of how the anisotropy (Ku) gradient in a compositionally graded FePtCu film gradually develops as a function of the postannealing temperature (TA). By utilizing the in situ annealing and magnetic characterization capabilities of a physical property measurement system, the evolution of the induced Ku gradient is elucidated. For low TA, the sample primarily remains in the low-Ku A1 phase. At intermediate TA, the gradual development of an A1 to L10 anisotropy gradient occurs. As TA is further increased, a well-developed L10 gradient is realized. Finally, annealing temperatures greater than 475 ◦C reduce the gradient until the film is effectively uniform for TA ≥ 525 ◦C and higher, presumably due to interdiffusion of the Cu through the film thickness. The resulting coercivity shows a nonmonotonic dependence on TA with an initial steep increase as the L10 fraction of the sample increases, a local minimum at TA = 525 ◦C where the gradient vanishes, and a final increase as the uniform L10 film orders completely.

  • 6. Fang, Yeyu
    et al.
    Persson, J.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Willman, J.
    Miller, Casey W.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Utility of reactively sputtered CuNx films in spintronics devices2012In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 111, no 7, p. 073912-Article in journal (Refereed)
    Abstract [en]

    We have studied nitrified copper (CuNx) thin films grown by reactive sputtering in the context of spintronic devices. The Ar-to-N-2 flow ratio enables tunability of the electrical resistivity and surface roughness of the CuNx films, with the former increasing to nearly 20 times that of Cu, and the latter reduced to the atomic scale. Incorporating this into a Ta/CuNx/Ta seed stack for spin valves improves the current-in-plane (CIP) magnetoresistance; maximum magnetoresistance results with CuNx seed layer and Cu interlayer. Finally, finite element modeling results are presented that suggest the use of CuNx in nanocontact spin torque oscillators can enhance current densities by limiting the current spread through the device. This may positively impact threshold currents, power requirements, and device reliability.

  • 7.
    Fang, Yeyu
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    FORC studies of exchange biased NiFe in L1(0)(111) FePt-based spin valve2010In: INTERNATIONAL CONFERENCE ON MAGNETISM (ICM 2009) / [ed] Goll, G; Lohneysen, HV; Loidl, A; Pruschke, T; Richter, M; Schultz, L; Surgers, C; Wosnitza, J, BRISTOL: IOP PUBLISHING LTD , 2010, Vol. 200, p. 072002-Conference paper (Refereed)
    Abstract [en]

    We use First-Order Reversal Curves (FORC) to study the switching distribution and exchange bias in L1(0) (111) FePt-based spin valves with a layer structure of Ta (6)/Pt (3)/L1(0) (111) Fe(53)Pt(47) (20)/Co(50)Fe(50) (1.5)/Cu (4.5)/Co(50)Fe(50) (2)/Ni(80)Fe(20)(3) (in nm). We find that the switching of the free layer magnetization is strongly influence by the magnetic state of the FePt/CoFe fixed layer, as evidenced by tunable coercivity and exchange bias fields.

  • 8. Kamali, S.
    et al.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Yoda, Y.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Oxidation states and quality of upper interfaces in magnetic tunnel junctions: oxygen effect on crystallization of interfaces2014In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 26, no 2, p. 026004-Article in journal (Refereed)
    Abstract [en]

    In this study, we have deposited an Fe-57 sensor layer at the upper interface, i.e. the interface between the oxide barrier and the upper electrode in selected magnetic tunnel junctions (MTJs), in order to perform nuclear resonant scattering with the aim of obtaining direct information on the magnetic properties and quality of this interface. This is a unique approach as it makes use of this powerful technique to give information at the atomic level, and specifically from the interface where the sensor layer is deposited. By varying sample tunnel barrier thicknesses and oxidation times in the preparation of this barrier, we have observed that longer oxidation time results in not only an increase of the magnetic hyperfine fields, but also causes an interesting crystallization and smoothing of the interface. We also observed that boron atoms diffuse away from the lower part of the upper FeCoB electrode toward the capping layer. An important observation, which has a crucial effect in tunnel magnetoresistance values, is the absence of any magnetically dead FeO layer at the interface. Another finding is that the deposition of Fe on MgO is much smoother than the deposition of MgO on Fe.

  • 9. Kamali, S.
    et al.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Yoda, Y.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Oxidation states and the quality of lower interfaces in magnetic tunnel junctions: oxygen effect on crystallization of interfaces2013In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 25, no 13, p. 135302-Article in journal (Refereed)
    Abstract [en]

    Lower interfaces in magnetic tunnel junctions (MTJs), which are the basic components in many spintronic devices such as magnetoresistive random access memories, have crucial effects on the performance of these devices. To obtain more insight into such interfaces, we have introduced an ultrathin sensor layer of Fe-57 at the interface between the lower electrode and the oxide barrier in selected MTJs. This allowed us to perform nuclear resonant scattering measurements, which provide direct information on the magnetic properties and quality of the interfaces. The application of nuclear resonant scattering to study interfaces in MTJs is a unique approach in the sense that it gives information at the atomic level, and specifically from the interface where the sensor layer is deposited. Samples with different tunnel barrier thicknesses and varied oxidation times in the preparation of this barrier have been studied. These show that oxidation can not only increase the magnetic hyperfine fields but also cause an interesting smoothing and crystallizing of the interface. Another interesting finding is the observation of boron diffusion into the lower part of the FeCoB lower electrode towards the Ta seed layer.

  • 10.
    Luo, Jun
    et al.
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Qiu, Zhijun
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Zhang, Zhen
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Wu, Dongping
    Lu, Jun
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Östling, Mikael
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Hultman, Lars
    Zhang, Shi-Li
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Surface-energy triggered phase formation and epitaxy in nanometer-thick Ni1-xPtx silicide films2010In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 96, no 3Article in journal (Refereed)
    Abstract [en]

    The formation of ultrathin silicide films of Ni1-xPtx at 450-850 degrees C is reported. Without Pt (x=0) and for t(Ni)< 4 nm, epitaxially aligned NiSi2-y films readily grow and exhibit extraordinary morphological stability up to 800 degrees C. For t(Ni)>= 4 nm, polycrystalline NiSi films form and agglomerate at lower temperatures for thinner films. Without Ni (x=1) and for t(Pt)=1-20 nm, the annealing behavior of the resulting PtSi films follows that for the NiSi films. The results for Ni1-xPtx of other compositions support the above observations. Surface energy is discussed as the cause responsible for the distinct behavior in phase formation and morphological stability.

  • 11.
    Tian, Jie
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Photonics.
    Ma, Zhe
    Li, Qiang
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Photonics.
    Song, Yi
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Quantum Electronics and Quantum Optics, QEO.
    Liu, Zhihong
    Yang, Qing
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Tong, Limin
    Qiu, Min
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Photonics.
    Nanowaveguides and couplers based on hybrid plasmonic modes2010In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 97, no 23, p. 231121-Article in journal (Refereed)
    Abstract [en]

    Experimental demonstration of silicon nanowires based hybrid plasmonic waveguides and couplers with subwavelength mode confinement at the near infrared wavelength lambda = 980 nm are presented. By measuring the radiating light from the discontinuities in a nanowire, the estimated propagation length of the hybrid plasmonic waveguide is about 30 mu m (corresponding to a propagation loss of similar to 0.14 dB/mu m). For the coupler, the experimental results show that the hybrid plasmonic modes can be efficiently coupled between two overlapping nanowires only with a 1.9 mu m long coupling length.

  • 12.
    Zha, Chaolin
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Persson, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Zhou, Yan
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Pseudo-spin-valve with L1(0) (111)-oriented FePt fixed layer2009In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 105, no 7, p. 07E910-Article in journal (Refereed)
    Abstract [en]

    Recently we proposed a spin torque oscillator where the fixed layer has its magnetization easy-axis tilted with respect to the film plane to simultaneously achieve zero-field operation and high output power [Y. Zhou, C. L. Zha, S. Bonetti, J. Persson, and J. angstrom kerman, Appl. Phys. Lett. 92, 262508 (2008)]. Here we take the first step toward the realization of this device and fabricate successfully a pseudo-spin-valve using an L1(0) (111)-oriented FePt fixed layer with tilted magnetocrystalline anisotropy. A total magnetoresistance (MR) of 0.86% is experimentally observed for the standard pseudo-spin-valve of the L1(0) FePt/Cu/NiFe structure in applied fields up to 1.5 T. While part of the MR may originate from the FePt film alone, the dominating part of MR correlates with switching of the NiFe free layer.

  • 13.
    Zha, Chaolin
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Dumas, R. K.
    Department of Physics, University of Gothenburg, Sweden.
    Fang, Yeyu
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Bonanni, Valentina
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Nogués, Josep
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Continuously graded anisotropy in single (Fe53Pt47)(100-x)Cu-x films2010In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 97, no 18, p. 182504-Article in journal (Refereed)
    Abstract [en]

    We report on continuously graded anisotropy. During deposition, a compositional gradient is achieved by varying the Cu concentration from Cu-rich (Fe53Pt47)(70)Cu-30 to Cu-free Fe53Pt47. The anisotropy gradient is then realized after annealing using the composition dependence of the low-anisotropy (A1) to high-anisotropy (L1(0)) ordering temperature. The critical role of the annealing temperature on the resultant anisotropy gradient is investigated. Magnetic measurements support the creation of an anisotropy gradient in properly annealed films which exhibit both a reduced coercivity and moderate thermal stability. These results demonstrate that an anisotropy gradient can be realized, and tailored, in single continuous films without the need for multilayers.

  • 14.
    Zha, Chaolin
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Dumas, R. K.
    Lau, J. W.
    Mohseni, Seyed Majid
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Redjai Sani, Sohrab
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Golosovsky, I. V.
    Monsen, Á. F.
    Nogués, Josep
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Nanostructured MnGa films on Si/SiO(2) with 20.5 kOe room temperature coercivity2011In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 110, no 9, p. 093902-Article in journal (Refereed)
    Abstract [en]

    Nanostructured Mn(67)Ga(33) films exhibiting high room temperature coercivity (H(C) = 20.5 kOe) have been prepared by sputtering onto thermally oxidized Si substrates. Both the morphology and the coercivity of the films can be tuned by varying the growth parameters. The low deposition rate film, sputtered at a reduced power and working pressure, demonstrates a discontinuous island-like growth and the highest H(C). The large H(C) is linked to the presence of the high anisotropy DO(22) Mn(3)Ga phase and the single domain character of the exchange isolated, dipolar interacting, single crystal islands.

  • 15.
    Zha, Chaolin
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics (Closed 20120101).
    Dumas, Randy K.
    KTH, School of Information and Communication Technology (ICT), Material Physics (Closed 20120101).
    Persson, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics (Closed 20120101).
    Mohseni, Seyed Majid
    KTH, School of Information and Communication Technology (ICT), Material Physics (Closed 20120101).
    Nogues, Josep
    KTH, School of Information and Communication Technology (ICT), Material Physics (Closed 20120101). Universitat Autònoma de Barcelona, Spain .
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics (Closed 20120101). University of Gothenburg, Sweden .
    Pseudo spin valves using a (112)-Textured D022 Mn2.3-2.4Ga fixed layer2010In: IEEE Magnetics Letters, ISSN 1949-307X, E-ISSN 1949-3088, Vol. 1, p. 2500104-Article in journal (Refereed)
    Abstract [en]

    We demonstrate pseudo spin valves (PSVs) with a (112)-textured D022 Mn2.3-2.4Ga (MnGa) tilted magnetization fixed layer and an in-plane CoFe free layer. Single D022 MnGa films exhibit a small magnetoresistance (MR) typically observed in metals. In MnGa/Cu/CoFe PSVs, a transition from a negative (-0.08%) to positive (3.88%) MR is realized by introducing a thin spin polarizing CoFe insertion layer at the MnGa/Cu interface and tailoring the MnGa thickness. Finally, the exchange coupling between the MnGa and CoFe insertion layer is studied using a first-order reversal curve technique.

  • 16.
    Zha, Chaolin
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Fang, Yeyu
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Nogués, Josep
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Improved magnetoresistance through spacer thickness optimization in tilted pseudo spin valves based on L10 (111)-oriented FePtCu fixed layers2009In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 106, no 5Article in journal (Refereed)
    Abstract [en]

    Through optimization of the Cu spacer thickness, we demonstrate magnetoresistance (MR) up to 5% in FePtCu/CoFe/Cu/CoFe/NiFe pseudo spin valves based on L10 (111) FePtCu fixed layers with a tilted magnetization. We find an optimum spacer thickness of about 2.4 nm which correlates with a clear onset of strong interlayer exchange coupling below 2.4 nm and spin-independent current shunting in the spacer above 2.4 nm. We argue that yet higher MR should be possible through further reduction in the interlayer exchange coupling.

  • 17.
    Zha, Chaolin
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    He, S. H.
    Ma, B.
    Zhang, Z. Z.
    Gan, F. X.
    Jin, Q. Y.
    Dependence of Ordering Kinetics of FePt Thin Films on Different Substrates2008In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 44, no 11, p. 3539-3542Article in journal (Refereed)
    Abstract [en]

    FePt thin films are deposited on SMO3, MgO, and a 2 nm-FeOx underlayer on an Si substrate at room temperature and then annealed at elevated temperatures. Studies of the L1(0) ordering process in each case show that the ordering temperature for the FePt film on the nonepitaxial Si/FeOx substrate is similar to 150 degrees C lower than the epitaxial FePt films deposited on MgO and SrTiO3 substrates. We argue that internal stresses arising from lattice defects and a recrystallizing process as well as thermal strain from differences in thermal expansion between substrate and film are responsible for the differences in ordering kinetics from the A1 to L1(0) phase of FePt on the various substrates.

  • 18.
    Zha, Chaolin
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Muduli, Pranaba
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Nogues, Josep
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Exchange-bias-like effect in L1(0) (111) FePt based pseudo spin valves2010In: INTERNATIONAL CONFERENCE ON MAGNETISM (ICM 2009) / [ed] Goll, G; Lohneysen, HV; Loidl, A; Pruschke, T; Richter, M; Schultz, L; Surgers, C; Wosnitza, J, BRISTOL: IOP PUBLISHING LTD, DIRAC HOUSE , 2010, Vol. 200, p. UNSP 072110-Conference paper (Refereed)
    Abstract [en]

    The coupling between hard FePt/CoFe and soft CoFe/NiFe layers through a Cu spacer has been studied in L1(0) (111) FePt(20)/CoFe(1.5)/Cu(3.5or4.5)/CoFe(2)/NiFe(3) (in nm) pseudo spin valves. The soft layer hysteresis loops exhibit clear shifts in the field axis (exchange bias-like) and a marked coercivity enhancement. The thickness of the Cu layer or the interface roughness influence the exchange bias properties of the systems. This interlayer coupling arises from the competition between dipolar and RKKY interactions between the layers.

  • 19.
    Zha, Chaolin
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Nogués, Josep
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Exchange Bias in L1(0) (111)-Oriented FePt-Based Pseudo Spin Valves2009In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 45, no 10, p. 3881-3884Article in journal (Refereed)
    Abstract [en]

    The coupling between hard FePt and soft NiFe layers through a Cu interlayer has been studied in L1(0) (111) FePt(20)/CoFe(1.5)/Cu(3.5 or 4.5)/CoFe(2)/NiFe(3) (in nanometers) pseudo spin valves. The soft-layer hysteresis loops exhibit clear shifts in the field axis (exchange-bias-like) and a marked coercivity enhancement. It is found that several parameters such as the thickness of the Cu layer or the interface roughness influence the exchange bias properties of the systems, while others (e. g., temperature) are less important. This interlayer coupling arises from the competition between dipolar and Ruderman-Kittel-Kasuya-Yoshida (RKKY) interactions between the layers.

  • 20.
    Zha, Chaolin
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Persson, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Fang, Yeyu
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Pseudo spin valves based on L10 (111)-oriented FePt fixed layers with tilted anisotropy2009In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 94, no 16, p. 163108-Article in journal (Refereed)
    Abstract [en]

    We demonstrate magnetoresistance (MR) in excess of 4% in FePt/CoFe/Cu/CoFe/NiFe pseudo spin valves based on L10 (111)-oriented FePt fixed layers with a 36 degrees out-of-plane tilted magnetization. The high MR is achieved by increasing the spin polarization at the Cu interfaces, using thin CoFe, and optimizing the FePt growth and Cu interface quality using Ta and Ta/Pt underlayers. We observe well-separated switching of the FePt/CoFe fixed layer and the CoFe/NiFe free layer, suggesting that CoFe is rigidly exchange coupled to FePt and NiFe in the respective layers.

  • 21.
    Zha, Chaolin
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Pseudo spin valves based on L1(0) (111)-oriented FePt and FePtCu fixed layer with tilted anisotropy2010In: INTERNATIONAL CONFERENCE ON MAGNETISM (ICM 2009) / [ed] Goll, G; Lohneysen, HV; Loidl, A; Pruschke, T; Richter, M; Schultz, L; Surgers, C; Wosnitza, J, BRISTOL: IOP PUBLISHING LTD , 2010, Vol. 200, p. UNSP 052036-Conference paper (Refereed)
    Abstract [en]

    We demonstrate a series of pseudo-spin-valve structures based on L1(0) (111)-oriented FePt and FePtCu with titled magnetocrystalline anisotropy. Highly ordered (111)-oriented L1(0) FePtCu with large anisotropy is achieved by optimizing the Cu content. Magnetoresistance (MR) up to 5% has been obtained by i) optimizing the fixed-layer growth using different underlayers, ii) enhancing the interface spin polarization using thin CoFe at the Cu interfaces, and iii) adjusting the Cu spacer thickness. The substantial MR realized with tilted fixed layer magnetization is an important prerequisite for the realization of tilted polarizer Spin Torque Oscillators (STO) or Spin-Transfer Torque Magnetoresistive Random Access Memories (STT-MRAM).

  • 22.
    Zha, Chaolin
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Study of Pseudo Spin Valves Based on L1(0) (111)-Oriented FePt and FePtCu Fixed Layer With Tilted Magnetocrystalline Anisotropy2009In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 45, no 10, p. 3491-3494Article in journal (Refereed)
    Abstract [en]

    In this paper, we demonstrate a series of pseudo-spin-valve structures based on L1(0) (111)-oriented FePt and FePtCu with titled magnetocrystalline anisotropy. Highly ordered (111)-oriented L1(0) FePtCu with large anisotropy is achieved by optimizing the Cu content. Magnetoresistance (MR) up to 5% has been obtained by 1) optimizing the FePtCu growth using different underlayers, 2) enhancing the interface spin polarization using thin CoFe at the Cu interfaces, and 3) adjusting the Cu spacer thickness. The substantial MR realized with tilted fixed layer magnetization is an important prerequisite for the realization of tilted polarizer spin torque oscillators (STOs) or spin-transfer torque magnetoresistive random access memories (STT-MRAMs).

  • 23.
    Zhou, Yan
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Zero field precession and hysteretic critical currents in spin torque oscillators with tilted polarizerIn: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630Article in journal (Other academic)
  • 24.
    Zhou, Yan
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Zero-field precession and hysteretic threshold currents in a spin torque nano device with tilted polarizer2009In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 11Article in journal (Refereed)
    Abstract [en]

    Using nonlinear system theory and numerical simulations, we map out the static and dynamic phase diagrams in the zero applied field of a spin torque nano device with a tilted polarizer (TP). We find that for sufficiently large currents, even very small tilt angles (beta > 1 degrees) will lead to steady free layer precession in zero field. Within a rather large range of tilt angles, 1 degrees < beta < 19 degrees, we find coexisting static states and hysteretic switching between these using only current. In a more narrow window (1 degrees < beta < 5 degrees) one of the static states turns into a limit cycle (precession). The coexistence of current-driven static and dynamic states in the zero magnetic field is unique to the TP device and leads to large hysteresis in the upper and lower threshold currents for its operation. The nano device with TP can facilitate the generation of large amplitude mode of spin torque signals without the need for cumbersome magnetic field sources and thus should be very important for future telecommunication applications based on spin transfer torque effects.

  • 25.
    Zhou, Yan
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Persson, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Microwave generation of tilted-polarizer spin torque oscillator2009In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 105, no 7, p. 07D116-Article in journal (Refereed)
    Abstract [en]

    Microwave frequency generation in a spin torque oscillator (STO) with a tilted fixed layer magnetization is studied using numerical simulation of the Landau-Lifshitz-Gilbert-Slonczewski equation. The dependence of the STO free layer precession frequency on drive current is determined as a function of fixed layer tilt angle. We find that zero-field STO operation is possible for almost all tilt angles, which allow for great freedom in choosing the detailed layer structure of the STO.

  • 26.
    Zhou, Yan
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Persson, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Spin-torque oscillator with tilted fixed layer magnetization2008In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 92, no 26, p. 262508-Article in journal (Refereed)
    Abstract [en]

    A spin-torque oscillator with a fixed layer magnetization tilted out of the film plane is capable of strong microwave signal generation in zero magnetic field. Through numerical simulations, we study the microwave signal generation as a function of drive current for two realistic tilt angles. The tilted magnetization of the fixed layer can be achieved by using a material with high out-of-plane magnetocrystalline anisotropy, such as L1(0) FePt.

1 - 26 of 26
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