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• 1.
KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik. Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
Microwave probe stations with throw-dimensional control of the magnetic field to study high-frequency dynamic in nanoscale devices2018Inngår i: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 89, nr 6, artikkel-id 064701Artikkel i tidsskrift (Fagfellevurdert)

We present two microwave probe stations with motorized rotary stages for adjusting the magnitude and angle of the applied magnetic field. In the first system, the magnetic field is provided by an electromagnet and can be adjusted from 0 to similar to 1.4 T while its polar angle (theta) can be varied from 0 degrees to 360 degrees. In the second system the magnetic field is provided by a Halbach array permanent magnet, which can be rotated and translated to cover the full range of polar (theta) and azimuthal (phi) angles with a tunable field magnitude up to similar to 1 T. Both systems are equipped with microwave probes, bias-Ts, amplifiers, and spectrum analyzers, to allow for microwave characterization up to 40 GHz, as well as software to automatically perform continuous large sets of electrical and microwave measurements.

• 2.
KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
Autonomous Systems for Characterization of Spin Torque Oscillators: Design, Production, Optimization and Measurement2013Independent thesis Advanced level (degree of Master (Two Years)), 30 poäng / 45 hpOppgave

The Spin Torque Oscillator (STO) is a nano-scale electrical device, with a wide current and field tunability, highly promising for applications in next generation wide band microwave frequency generators, multifunction microwave components, ultra-fast microwave sensors, etc. For a better fundamental understanding of the functional properties of STOs it is important to develop flexible and easy-to-use characterization tools, in particular for routine test and characterization in preparation for a successful commercial applications. Most present measurement systems do not fulfill these qualities and have very low through-put. Therefore, an automated system including all capabilities for characterization of STO is indeed necessary in laboratories. In this work, two different setups for characterization of STO are proposed, designed and built. To increase measurement performance a high frequency (up to 60 GHz) measurement setup was designed and built based on the rotation of a large field electromagnet (up to 2 T), instead of rotating the sample as in older system. A second high frequency measurement setup utilizes a total of 5 degrees of freedom to rotate and position a permanent magnet with a magnetic field of 1 T. Moreover, as preliminary experimental investigation of STOs, the resistance of nanocantact (NC) STOs with different NC size and variation of the thickness of the Cu seed layer, was studied to find the real NC size based on Sharvin-Maxwell methods. The study resolves how the real resistance value of the NC and the resistance of the mesa varies. This will help to understand the microwave power delivery issues between the mesa and the NC and has direct applicability to the problem of impedance matching between these two sub-elements. This study will be finally useful to find a criteria for seed layer thickness and necessary NC size in order to get a high output power from STOs and will assist to design novel geometries of high power STO for commercial applications.

• 3.
KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
Determining and Optimizing the Current and Magnetic Field Dependence of Spin-Torque and Spin Hall Nano-Oscillators: Toward Next-Generation Nanoelectronic Devices and Systems2018Doktoravhandling, med artikler (Annet vitenskapelig)

Spin-torque and spin Hall nano-oscillators are nanoscale devices (about 100 nm) capable of producing tunable broadband high-frequency microwave signals ranging from 0.1 GHz to over 65 GHz that several research groups trying to reach up to 200 - 300 GHz. Their development is ongoing for applications in high-frequency nanoelectronic devices and systems, such as mobile phones, wireless networks, base stations, vehicle radars, and even medical applications.

This thesis covers a wide range of characterizations of spin-torque and spin Hall nano-oscillator devices that aim to investigate their current and magnetic field dependency, as well as to suggest improvements in these devices to optimize their application in spintronics and magnonics. The work is primarily based on experimental methods for characterizing these devices by building up new measurement systems, but it also includes numerical and micromagnetic simulations.

Experimental techniques: In order to characterize the fabricated nanodevices in a detailed and accurate manner through their electrical and microwave responses, new measurement systems capable of full 3D control over the external magnetic fields will be described. In addition, a new method of probing an operational device using magnetic force microscopy (MFM) will be presented.

Spin-torque nano-oscillators: We will describe remarkable improvements in the performance of spin-torque nano-oscillators (STNOs) that enhance their integration capability with applications in microwave systems. In nanocontact (NC-)STNOs made from a conventional spin-valve stack, though with thicker bottom electrodes, it is found the auto-oscillations can be excited with higher frequencies at lower threshold currents, and with higher output powers. We also find that this idea is useful for tuning spin-wave resonance and also controlling the thermal budget. Furthermore, a detailed study of magnetic droplet solitons and spin-wave dynamics in NC-STNOs will be described. Finally, we demonstrate ultra-high frequency tunability in low-current STNOs based on perpendicular magnetic tunnel junctions(p-MTJs).

Spin Hall nano-oscillators: Characterizations of spin Hall nano-oscillator(SHNO) devices based on different structures and materials with both conventional and novel methods will be described. A detailed study of the current, temperature, and magnetic field profiles of nanogap SHNOs will be presented. In addition, we show the current and magnetic field dependence of nanoconstriction-based SHNOs.Moreover, it is shown that multiple SHNOs can be serially synchronized, thereby increasing their output power and enhancing the usage of these devices in applications such as neuromorphic computing. We show synchronization of multiple nanoconstriction SHNOs in the presence of a low in-plane magnetic field. Finally, there is a demonstration of the results of a novel method for probing an operationalSHNO using MFM.

• 4.
KTH, Skolan för teknikvetenskap (SCI).
Current, temperature, and magnetic field profiles in nanogap spin Hall nano-oscillatorsManuskript (preprint) (Annet vitenskapelig)

We carry out a detailed experimental and numerical study of nanogap spin Hall nano-oscillators (SHNOs) to determine the current distribution, the associated Oersted field, and the possible effects of the local temperature rise. We find substantial heating in the center of the SHNOs, leading to a nonuniform device resistance which redistributes the current in a nontrivial way. As a consequence, both the Oe field magnitude and its spatial profile are nonlinear functions of the current magnitude. Our results have direct consequences for spin-wave generation in these devices.

• 5.
KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
Magnetic force microscopy of an operational nanodeviceManuskript (preprint) (Annet vitenskapelig)

We present a new method for probing the spatial profile of an operational magnetic nanodevice using magnetic force microscopy (MFM). We have developed an MFM system by adding a microwave probe station equipped with microwave probe, bias-T, and amplifier to allow electrical and microwave characterization up to 40 GHz during the MFM process. The nanoscale spintronic devices---spin Hall nano-oscillators (SHNOs) based on Pt/NiFe bilayers with a specific design compatible with the developed system---were fabricated and scanned using a Co magnetic force microscopy tip with 10 nm spatial resolution, while a DC current sufficient to exert auto-oscillation flowed. Our results show that this method of developed provides a promising path for the characterization of the spatial profiles of operational nano-oscillators.

• 6.
KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
Control of thermal budget in nanocontact spin-torque nano-oscillatorsManuskript (preprint) (Annet vitenskapelig)

We investigate the influence of the bottom Cu electrode thickness (tCu) in nanocontact spin-torque nano-oscillators (NC-STNOs) based on Si/SiO2/Pd(8)/Cu(tCu)/Co(8)/Cu(7)/NiFe(4.5)/Cu(3)/Pd(3) GMR stacks on the thermal budget of the magnetodynamically active region. Increasing tCu from 10 to 70 nm results in a ~50% reduction in Joule heating in both the Co and NiFe layers, which directly improves the microwave output stability and linewidth. Numerical simulations of the NC-STNO current distribution suggest that this improvement originates from a strongly reduced lateral current spread in the top ferromagnetic layer and a reduction in the device's resistance.

• 7.
KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
KTH, Skolan för informations- och kommunikationsteknik (ICT), Elektronik, Integrerade komponenter och kretsar. KTH, Skolan för informations- och kommunikationsteknik (ICT), Elektronik, Integrerade komponenter och kretsar. KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. Univ Gothenburg, Sweden. KTH, Skolan för informations- och kommunikationsteknik (ICT), Elektronik, Integrerade komponenter och kretsar. KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. Univ Gothenburg, Sweden.
Order of magnitude improvement of nano-contact spin torque nano-oscillator performance2017Inngår i: Nanoscale, ISSN 2040-3364, E-ISSN 2040-3372, Vol. 9, nr 5, s. 1896-1900Artikkel i tidsskrift (Fagfellevurdert)

Spin torque nano-oscillators (STNO) represent a unique class of nano-scale microwave signal generators and offer a combination of intriguing properties, such as nano sized footprint, ultrafast modulation rates, and highly tunable microwave frequencies from 100 MHz to close to 100 GHz. However, their low output power and relatively high threshold current still limit their applicability and must be improved. In this study, we investigate the influence of the bottom Cu electrode thickness (t(Cu)) in nano-contact STNOs based on Co/Cu/NiFe GMR stacks and with nano-contact diameters ranging from 60 to 500 nm. Increasing t(Cu) from 10 to 70 nm results in a 40% reduction of the threshold current, an order of magnitude higher microwave output power, and close to two orders of magnitude better power conversion efficiency. Numerical simulations of the current distribution suggest that these dramatic improvements originate from a strongly reduced lateral current spread in the magneto-dynamically active region.

• 8.
KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
Microwave probe stations with three-dimensional control of the magnetic field to study high frequency dynamics in nanoscale devices2018Inngår i: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623Artikkel i tidsskrift (Fagfellevurdert)

We present two microwave probe stations with motorized rotary stages for adjusting the magnitude and angle of the applied magnetic field. In the first system, the magnetic field is provided by an electromagnet and can be adjusted from 0 to ~ 1.4 T while its polar angle (θ) can be varied from 0o to 360o. In the second system the magnetic field is provided by a Halbach array permanent magnet, which can be rotated and translated to cover the full range of polar (θ) and azimuthal (φ) angles with a tunable field magnitude up to ~ 1 T. Both systems are equipped with microwave probes, bias-Ts, amplifiers, and spectrum analyzers, to allow for microwave characterization up to 40 GHz, as well as software to automatically perform continuous large sets of electrical and microwave measurements.

• 9.
KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar. KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. 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)

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.

• 10.
Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik. Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden.. Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden.. MIT, Dept Mat Sci & Engn, Cambridge, MA 02139 USA.. Shahid Beheshti Univ, Dept Phys, Tehran 19839, Iran.. KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik. Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
Tuning exchange-dominated spin-waves using lateral current spread in nanocontact spin-torque nano-oscillators2019Inngår i: Journal of Magnetism and Magnetic Materials, ISSN 0304-8853, E-ISSN 1873-4766, Vol. 492, artikkel-id UNSP 165503Artikkel i tidsskrift (Fagfellevurdert)

We present an efficient method to tailor propagating spin waves in quasi-confined systems. We use nanocontact spin-torque nano-oscillators based on NiFe/Cu/Co spin-valves and study the ferromagnetic and spin-wave resonances (FMR and SWR) of both layers. We employ homodyne-detected ferromagnetic resonance spectroscopy, resembling spin-torque FMR, to detect the magnetodynamics. The external field is applied in-plane, giving a parallel configuration of the magnetic layers, which do not provide any spin-transfer torque. Instead, the excitation is caused by the Oersted field. By varying the thickness of the bottom Cu electrode (t(Cu)) of the devices, we tune the current distribution in the samples, and thereby the Oersted field, which governs the spin wave characteristics. Both the average k-vector and the bandwidth of the SWR increases as t(Cu) increases.

• 11. Fazlali, Masoumeh
KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
Tuning exchange-dominated spin-waves using lateral current spread in nanocontact spin-torque nano-oscillators2018Inngår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118Artikkel i tidsskrift (Fagfellevurdert)

We present an efficient method to tailor propagating spin waves in quasi-confined systems. We use nanocontact spin-torque nano-oscillators based on NiFe/Cu/Co spin-valves and study the ferromagnetic and spin-wave resonances (FMR and SWR) of both layers. We employ homodyne-detected ferromagnetic resonance spectroscopy, resembling spin-torque FMR, to detect the magnetodynamics. The external field is applied in-plane, giving a parallel configuration of the magnetic layers, which do not provide any spin-transfer torque. Instead, the excitation is caused by the Oersted-field. By varying the thickness of the bottom Cu electrode of the devices, we tune the current distribution in the samples, and thereby the Oersted field, which governs the spin wave characteristics.

• 12.
KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar. KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Univ Gothenburg, Sweden. Faculty of Physics, Amirkabir University of Technology. KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Laboratory for Nanoelectronics and Spintronics, RIEC, Tohoku University. Center for Spintronics Integrated Systems, Tohoku University. Laboratory for Nanoelectronics and Spintronics, RIEC, Tohoku University. Laboratory for Nanoelectronics and Spintronics, RIEC, Tohoku University. KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Univ Gothenburg, Sweden; NanOsc AB, Sweden.
Ultra-high frequency tunability in low-current and low-field spin-torque oscillators based on perpendicular magnetic tunnel junctionsManuskript (preprint) (Annet (populærvitenskap, debatt, mm))

We demonstrate ultra-high frequency tunability of up to 4.4 GHz/mA, and low threshold currents of about -21 $\mu$A, in spin-torque oscillators based on CoFeB/MgO/CoFeB magnetic tunnel junctions, in which both free and fixed layers have perpendicular magnetic anisotropy (PMA). By using different thicknesses of the two CoFeB layers, their individual PMA strengths can be tailored to achieve significant relative misalignment of their respective magnetizations in moderate in-plane fields. We observe a broad maximum in both the device resistance and the generated microwave power around maximum misalignment. Maximum frequency tunability is observed at low-to-moderate fields and decrease rapidly after maximum misalignment.

• 13.
KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
University of Gothenburg. KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik. KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik. University of Gothenburg. University of Gothenburg. University of Gothenburg. KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik. University of Gothenburg.
Auto-oscillating spin-wave modes of constriction-based spin Hall nano-oscillators in weak in-plane fieldsManuskript (preprint) (Annet vitenskapelig)

We experimentally study the auto-oscillating spin-wave modes in Ni80Fe20/β-W constriction-based spin Hall nano-oscillators as a function of bias current, in-plane applied field strength, and azimuthal field angle, in the low-field range of 40-80 mT. We observe two different spin-wave modes: i) a linear-like mode confined to the minima of the internal field near the edges of the nanoconstriction, with weak frequency dependencies on the bias current and the applied field angle, and ii) a second, lower frequency mode that has significantly higher threshold current and stronger frequency dependencies on both bias current and the external eld angle. Our micromagnetic modeling qualitatively reproduces the experimental data and reveals that the second mode is a spin-wave bullet and that the SHNO mode hops between the two modes, resulting in a substantial increase in linewidths. In contrast to the linear-like mode, the bullet is localized in the middle of the constriction and shrinks with increasing bias current. Utilizing intrinsic frequency doubling at zero eld angle we can reach frequencies above 9 GHz in fields as low as 40 mT, which is important for the development of low-eld spintronic oscillators with applications in microwave signal generation and neuromorphic computing.

• 14. Mohseni, Seyed Morteza
KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101), Materialfysik, MF (Stängd 20120101). KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
Magnetic droplet soliton nucleation in oblique fields2018Inngår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 97, nr 184402Artikkel i tidsskrift (Fagfellevurdert)

We study the auto-oscillating magnetodynamics in orthogonal spin-torque nano-oscillators (STNOs) as a function of the out-of-plane (OOP) magnetic-field angle. In perpendicular fields and at OOP field angles down to approximately 50°, we observe the nucleation of a droplet. However, for field angles below 50°, experiments indicate that the droplet gives way to propagating spin waves, in agreement with our micromagnetic simulations. Theoretical calculations show that the physical mechanism behind these observations is the sign changing of spin-wave nonlinearity (SWN) by angle. In addition, we show that the presence of a strong perpendicular magnetic anisotropy free layer in the system reverses the angular dependence of the SWN and dynamics in STNOs with respect to the known behavior determined for the in-plane magnetic anisotropy free layer. Our results are of fundamental interest in understanding the rich dynamics of nanoscale solitons and spin-wave dynamics in STNOs.

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