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Thomas J Silva - One of the best experts on this subject based on the ideXlab platform.
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resolving the controversy of a possible relationship between perpendicular magnetic anisotropy and the magnetic Damping Parameter
Applied Physics Letters, 2014Co-Authors: Justin M Shaw, Hans T Nembach, Thomas J SilvaAbstract:We use broadband ferromagnetic resonance spectroscopy to systematically measure the Landau-Lifshitz Damping Parameter, perpendicular anisotropy, and the orbital moment asymmetry in Co90Fe10/Ni multilayers. No relationship is found between perpendicular magnetic anisotropy and the Damping Parameter in this material. However, inadequate accounting for inhomogeneous linewidth broadening, spin-pumping, and two-magnon scattering could give rise to an apparent relationship between anisotropy and Damping. In contrast, the orbital-moment asymmetry and the perpendicular anisotropy are linearly proportional to each other. These results demonstrate a fundamental mechanism by which perpendicular anisotropy can be varied independently of the Damping Parameter.
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roughness induced magnetic inhomogeneity in co ni multilayers ferromagnetic resonance and switching properties in nanostructures
Journal of Applied Physics, 2010Co-Authors: Justin M Shaw, Hans T Nembach, Thomas J SilvaAbstract:We report on the effect roughness has on the magnetic properties of Co/Ni multilayers with perpendicular anisotropy. We can systematically vary the surface roughness and grain size by the variation in the Cu seed layer thickness. The roughness has a significant effect on the lateral anisotropy variation across the material, which was studied through the size dependence of the switching field and switching field distribution in nanostructures. Ferromagnetic resonance measurements show a large dependence of the linewidth on the topography. However, only the inhomogeneous linewidth broadening varied significantly with roughness with little to no effect on the intrinsic Damping Parameter of α=0.015–0.018.
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Advances in Magnetics Theory of Thermally Induced Phase Noise in Spin Torque Oscillators
2010Co-Authors: Thomas J Silva, Mark W KellerAbstract:We derive equations for the phase noise spectrum of a spin torque oscillator in the macrospin approximation for the highly symmetric geometry where the equilibrium magnetization, applied field, anisotropy, and spin accumulation are all collinear. This particular problem is one that can be solved by analytical methods, but nevertheless illustrates several important general principles for phase noise in spin torque oscillators. In the limit, where the restoring torque is linearly proportional to the deviation of the precession amplitude from steady-state, the problem reduces to a sum of the Wiener-Lévy (W-L) and Ornstein-Uhlenbeck (O-U) processes familiar from the physics of random walks and Brownian motion. For typical device Parameters, the O-U process dominates the phase noise and results in a phase noise spectrum that is nontrivial, with dependence at low Fourier frequencies, and dependence at high Fourier frequencies. The contribution to oscillator linewidth due to the O-U process in the low temperature limit is independent of magnetic anisotropy field and scales inversely with the Damping Parameter, whereas in the high temperature limit the oscillator linewidth is independent of the Damping Parameter and scales as. Numerical integration of the fully nonlinear stochastic differential equations is used to determine the temperature and precession amplitude ranges over which our equations for phase noise and linewidth are valid. We then expand the theory to include effects of spin torque asymmetry. Given the lack of experimental data for nanopillars in the geometry considered here, we make a rough extrapolation to the case of nanocontacts, with reasonable agreement with published data. The theory does not yield any obvious means to reduce phase noise to levels required for practical applications in the geometry considered here. Index Terms—Langevin equations, macrospin, spin torque, spin torque oscillator, Ornstein-Uhlenbeck, Wiener-Lévy, phase noise. I
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theory of thermally induced phase noise in spin torque oscillators for a high symmetry case
IEEE Transactions on Magnetics, 2010Co-Authors: Thomas J Silva, Mark W KellerAbstract:We derive equations for the phase noise spectrum of a spin torque oscillator in the macrospin approximation for the highly symmetric geometry where the equilibrium magnetization, applied field, anisotropy, and spin accumulation are all collinear. This particular problem is one that can be solved by analytical methods, but nevertheless illustrates several important general principles for phase noise in spin torque oscillators. In the limit, where the restoring torque is linearly proportional to the deviation of the precession amplitude from steady-state, the problem reduces to a sum of the Wiener-Levy (W-L) and Ornstein-Uhlenbeck (O-U) processes familiar from the physics of random walks and Brownian motion. For typical device Parameters, the O-U process dominates the phase noise and results in a phase noise spectrum that is nontrivial, with 1/ω2 dependence at low Fourier frequencies, and 1/ω4 dependence at high Fourier frequencies. The contribution to oscillator linewidth due to the O-U process in the low temperature limit is independent of magnetic anisotropy field Hk and scales inversely with the Damping Parameter, whereas in the high temperature limit the oscillator linewidth is independent of the Damping Parameter and scales as √(|Hk|) . Numerical integration of the fully nonlinear stochastic differential equations is used to determine the temperature and precession amplitude ranges over which our equations for phase noise and linewidth are valid. We then expand the theory to include effects of spin torque asymmetry. Given the lack of experimental data for nanopillars in the geometry considered here, we make a rough extrapolation to the case of nanocontacts, with reasonable agreement with published data. The theory does not yield any obvious means to reduce phase noise to levels required for practical applications in the geometry considered here.
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Damping as a function of pulsed field amplitude and bias field in thin film permalloy
Applied Physics Letters, 2003Co-Authors: J P Nibarger, Radek Lopusnik, Thomas J SilvaAbstract:We have measured the step response in thin film Permalloy as a function of both a hard-axis pulsed field amplitude and an easy-axis longitudinal magnetic bias field using a pulsed inductive microwave magnetometer. The bias field ranged from 0 to 8000 A/m (0 to 100 Oe) and the pulsed field varied from 0.32 to 320 A/m (0.004 to 4 Oe). The rotation angle of the equilibrium magnetization direction varied from 0.002° to 40° for this range of field values. Data were analyzed to extract the Gilbert Damping Parameter, α. The Damping Parameter decreased monotonically with an increase in longitudinal bias field. However, there is no observed dependence of α on the pulse amplitude, indicating that the Damping is independent of the angle of rotation. We conclude that there is no significant nonlinear generation of spin waves that affects the Damping in the case of free induction decay for the range of field pulses employed.
H Ebert - One of the best experts on this subject based on the ideXlab platform.
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calculating linear response functions for finite temperatures on the basis of the alloy analogy model
Physical Review B, 2015Co-Authors: H Ebert, Sergiy Mankovsky, Kristina Chadova, S Polesya, J Minar, Diemo KoedderitzschAbstract:A scheme is presented that is based on the alloy analogy model and allows one to account for thermal lattice vibrations as well as spin fluctuations when calculating response quantities in solids. Various models to deal with spin fluctuations are discussed concerning their impact on the resulting temperature-dependent magnetic moment, longitudinal conductivity, and Gilbert Damping Parameter. It is demonstrated that, by using the Monte Carlo (MC) spin configuration as input, the alloy analogy model is capable of reproducing the results of MC simulations on the average magnetic moment within all spin fluctuation models under discussion. On the other hand, the response quantities are much more sensitive to the spin fluctuation model. Separate calculations accounting for the thermal effect due to either lattice vibrations or spin fluctuations show that they give comparable contributions to the electrical conductivity and Gilbert Damping. However, comparison to results accounting for both thermal effects demonstrates violation of Matthiessen's rule, showing the nonadditive effect of lattice vibrations and spin fluctuations. The results obtained for bcc Fe and fcc Ni are compared with the experimental data, showing rather good agreement for the temperature-dependent electrical conductivity and the Gilbert Damping Parameter.
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first principles calculation of the gilbert Damping Parameter via the linear response formalism with application to magnetic transition metals and alloys
Physical Review B, 2013Co-Authors: Sergiy Mankovsky, Diemo Ködderitzsch, Georg Woltersdorf, H EbertAbstract:A method for the calculations of the Gilbert Damping Parameter $\ensuremath{\alpha}$ is presented, which, based on the linear response formalism, has been implemented within the fully relativistic Korringa-Kohn-Rostoker band structure method in combination with the coherent potential approximation alloy theory. To account for thermal displacements of atoms as a scattering mechanism, an alloy-analogy model is introduced. This allows the determination of $\ensuremath{\alpha}$ for various types of materials, such as elemental magnetic systems and ordered magnetic compounds at finite temperature, as well as for disordered magnetic alloys at $T=0$ K and above. The effects of spin-orbit coupling, chemical- and temperature-induced structural disorder, are analyzed. Calculations have been performed for the 3$d$ transition metals bcc Fe, hcp Co, and fcc Ni; their binary alloys bcc Fe${}_{1\ensuremath{-}x}$Co${}_{x}$, fcc Ni${}_{1\ensuremath{-}x}$Fe${}_{x}$, fcc Ni${}_{1\ensuremath{-}x}$Co${}_{x}$ and bcc Fe${}_{1\ensuremath{-}x}$V${}_{x}$; and for $5d$ impurities in transition-metal alloys. All results are in satisfying agreement with experiment.
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ab initio calculation of the gilbert Damping Parameter via the linear response formalism
Physical Review Letters, 2011Co-Authors: H Ebert, Diemo Ködderitzsch, Sergiy Mankovsky, Paul J KellyAbstract:A Kubo-Greenwood-like equation for the Gilbert Damping Parameter α is presented that is based on the linear response formalism. Its implementation using the fully relativistic Korringa-Kohn-Rostoker band structure method in combination with coherent potential approximation alloy theory allows it to be applied to a wide range of situations. This is demonstrated with results obtained for the bcc alloy system Fe1−xCox as well as for a series of alloys of Permalloy with 5d transition metals. To account for the thermal displacements of atoms as a scattering mechanism, an alloy-analogy model is introduced. The corresponding calculations for Ni correctly describe the rapid change of α when small amounts of substitutional Cu are introduced.
Yu K Guslienko - One of the best experts on this subject based on the ideXlab platform.
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low frequency vortex dynamic susceptibility and relaxation in mesoscopic ferromagnetic dots
Applied Physics Letters, 2006Co-Authors: Yu K GuslienkoAbstract:Vortex dynamics in a restricted geometry is considered for a magnetic system consisting of ferromagnetic cylindrical dots. To describe the vortex dynamic susceptibility and relaxation the equation of motion for the vortex center position is applied. The dependencies of the vortex dynamic susceptibility and resonance linewidth on geometrical Parameters are calculated. A method of extracting Damping Parameter from the vortex low-frequency resonance peaks is proposed and applied for interpretation of resonance data on FeNi circular dots.
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low frequency vortex dynamic susceptibility and relaxation in mesoscopic ferromagnetic dots
arXiv: Mesoscale and Nanoscale Physics, 2006Co-Authors: Yu K GuslienkoAbstract:Vortex dynamics in a restricted geometry is considered for a magnetic system consisting of ferromagnetic cylindrical dots. To describe the vortex dynamic susceptibility and relaxation the equation of motion for the vortex center position is applied. The dependencies of the vortex dynamic susceptibility and resonance linewidth on geometrical Parameters are calculated. A new method of extracting Damping Parameter from the vortex low-frequency resonance peaks is proposed and applied for interpretation of resonance data on FeNi circular dots.
Sergiy Mankovsky - One of the best experts on this subject based on the ideXlab platform.
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calculating linear response functions for finite temperatures on the basis of the alloy analogy model
Physical Review B, 2015Co-Authors: H Ebert, Sergiy Mankovsky, Kristina Chadova, S Polesya, J Minar, Diemo KoedderitzschAbstract:A scheme is presented that is based on the alloy analogy model and allows one to account for thermal lattice vibrations as well as spin fluctuations when calculating response quantities in solids. Various models to deal with spin fluctuations are discussed concerning their impact on the resulting temperature-dependent magnetic moment, longitudinal conductivity, and Gilbert Damping Parameter. It is demonstrated that, by using the Monte Carlo (MC) spin configuration as input, the alloy analogy model is capable of reproducing the results of MC simulations on the average magnetic moment within all spin fluctuation models under discussion. On the other hand, the response quantities are much more sensitive to the spin fluctuation model. Separate calculations accounting for the thermal effect due to either lattice vibrations or spin fluctuations show that they give comparable contributions to the electrical conductivity and Gilbert Damping. However, comparison to results accounting for both thermal effects demonstrates violation of Matthiessen's rule, showing the nonadditive effect of lattice vibrations and spin fluctuations. The results obtained for bcc Fe and fcc Ni are compared with the experimental data, showing rather good agreement for the temperature-dependent electrical conductivity and the Gilbert Damping Parameter.
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first principles calculation of the gilbert Damping Parameter via the linear response formalism with application to magnetic transition metals and alloys
Physical Review B, 2013Co-Authors: Sergiy Mankovsky, Diemo Ködderitzsch, Georg Woltersdorf, H EbertAbstract:A method for the calculations of the Gilbert Damping Parameter $\ensuremath{\alpha}$ is presented, which, based on the linear response formalism, has been implemented within the fully relativistic Korringa-Kohn-Rostoker band structure method in combination with the coherent potential approximation alloy theory. To account for thermal displacements of atoms as a scattering mechanism, an alloy-analogy model is introduced. This allows the determination of $\ensuremath{\alpha}$ for various types of materials, such as elemental magnetic systems and ordered magnetic compounds at finite temperature, as well as for disordered magnetic alloys at $T=0$ K and above. The effects of spin-orbit coupling, chemical- and temperature-induced structural disorder, are analyzed. Calculations have been performed for the 3$d$ transition metals bcc Fe, hcp Co, and fcc Ni; their binary alloys bcc Fe${}_{1\ensuremath{-}x}$Co${}_{x}$, fcc Ni${}_{1\ensuremath{-}x}$Fe${}_{x}$, fcc Ni${}_{1\ensuremath{-}x}$Co${}_{x}$ and bcc Fe${}_{1\ensuremath{-}x}$V${}_{x}$; and for $5d$ impurities in transition-metal alloys. All results are in satisfying agreement with experiment.
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Ab Initio Calculation of the Gilbert Damping Parameter via Linear Response Formalism
Magnetics, IEEE Transactions on, 2013Co-Authors: Diemo Ködderitzsch, Sergiy Mankovsky, Hubert EbertAbstract:A Kubo-Greenwood-like equation for the Gilbert Damping Parameter α is presented that is based on the linear response formalism. Its implementation on the basis of the fully relativistic Korringa-Kohn-Rostoker (KKR) band structure method together with the Coherent Potential Approximation (CPA) alloy theory allows application to a wide range of situations. This is demonstrated by results obtained for the alloy system bcc FexCo1-x as well as for a series of alloys of permalloy with 5d transition metals. To account for the thermal displacements of atoms as a scattering mechanism, an alloy-analogy model is introduced. In line with experiment, calculations for Ni correctly describe the rapid change of α when small amounts of Cu are introduced substitutionally.
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ab initio calculation of the gilbert Damping Parameter via the linear response formalism
Physical Review Letters, 2011Co-Authors: H Ebert, Diemo Ködderitzsch, Sergiy Mankovsky, Paul J KellyAbstract:A Kubo-Greenwood-like equation for the Gilbert Damping Parameter α is presented that is based on the linear response formalism. Its implementation using the fully relativistic Korringa-Kohn-Rostoker band structure method in combination with coherent potential approximation alloy theory allows it to be applied to a wide range of situations. This is demonstrated with results obtained for the bcc alloy system Fe1−xCox as well as for a series of alloys of Permalloy with 5d transition metals. To account for the thermal displacements of atoms as a scattering mechanism, an alloy-analogy model is introduced. The corresponding calculations for Ni correctly describe the rapid change of α when small amounts of substitutional Cu are introduced.
Paul J Kelly - One of the best experts on this subject based on the ideXlab platform.
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ab initio calculation of the gilbert Damping Parameter via the linear response formalism
Physical Review Letters, 2011Co-Authors: H Ebert, Diemo Ködderitzsch, Sergiy Mankovsky, Paul J KellyAbstract:A Kubo-Greenwood-like equation for the Gilbert Damping Parameter α is presented that is based on the linear response formalism. Its implementation using the fully relativistic Korringa-Kohn-Rostoker band structure method in combination with coherent potential approximation alloy theory allows it to be applied to a wide range of situations. This is demonstrated with results obtained for the bcc alloy system Fe1−xCox as well as for a series of alloys of Permalloy with 5d transition metals. To account for the thermal displacements of atoms as a scattering mechanism, an alloy-analogy model is introduced. The corresponding calculations for Ni correctly describe the rapid change of α when small amounts of substitutional Cu are introduced.
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unified first principles study of gilbert Damping spin flip diffusion and resistivity in transition metal alloys
Physical Review Letters, 2010Co-Authors: A A Starikov, Yaroslav Tserkovnyak, Paul J Kelly, Arne Brataas, G BauerAbstract:Using a formulation of first-principles scattering theory that includes disorder and spin-orbit coupling on an equal footing, we calculate the resistivity ?, spin-flip diffusion length lsf, and Gilbert Damping Parameter ? for Ni1-xFex substitutional alloys as a function of x. For the technologically important Ni80Fe20 alloy, Permalloy, we calculate values of ?=3.5±0.15?????cm, lsf=5.5±0.3??nm, and ?=0.0046±0.0001 compared to experimental low-temperature values in the range 4.2–4.8?????cm for ?, 5.0–6.0 nm for lsf, and 0.004–0.013 for ?, indicating that the theoretical formalism captures the most important contributions to these Parameters.
