We show experimentally and by model calculations that in finite, nonellipsoidal, micrometer size ... more We show experimentally and by model calculations that in finite, nonellipsoidal, micrometer size magnetic thin film elements the dynamic magnetic eigenexcitations (spin waves) may exhibit strong spatial localization. This localization is due to the formation of a potential well for spin waves in the highly inhomogeneous internal magnetic field within the element.
We present spectral measurements of spin-wave excitations driven by direct spin-polarized current... more We present spectral measurements of spin-wave excitations driven by direct spin-polarized current in a free layer of nanoscale Ir 20 Mn 80 /Ni 80 Fe 20 /Cu/Ni 80 Fe 20 spin valves. The measurements reveal that large-amplitude coherent spin-wave modes are excited over a wide range of bias current. The frequency of these excitations exhibits a series of jumps as a function of current due to transitions between different localized nonlinear spin-wave modes of the Ni 80 Fe 20 nanomagnet. We find that micromagnetic simulations employing the Landau-Lifshitz-Gilbert equation of motion augmented by the Slonczewski spin-torque term ͑LLGS͒ accurately describe the frequency of the current-driven excitations including the mode transition behavior. However, LLGS simulations give qualitatively incorrect predictions for the amplitude of excited spin waves as a function of current.
We consider a partial-sum process generated by a sequence of nonidentically distributed independe... more We consider a partial-sum process generated by a sequence of nonidentically distributed independent random variables. Assuming that this process is available for observation along an arbitrary time sequence, we fill the gaps by linear interpolation and prove the functional law of the iterated logarithm (FLIL) for sample paths obtained in this way. Assuming that the V. A. Egorov condition holds, we show that FLIL is valid, while under other conditions sufficient for the usual law of the iterated logarithm FLIL may fail. Bibliography: 16 titles.
In this contribution, we present the comparative analysis of the simple Ewald, lattice-based Ewal... more In this contribution, we present the comparative analysis of the simple Ewald, lattice-based Ewald and Fast Multipole (FMM) methods for the evaluation of the long-range dipolar field in disordered systems. We show that the FMM method with the optimally chosen hierarchical structure and the improved lattice version of the Ewald algorithm are superior with respect to original Ewald method starting from the particle number N$10 4 . FMM and lattice Ewald versions demonstrate comparable performance for systems up to N$4 Â 10 4 particles.
For numerical studies of a ferrofluid dynamics we have developed a model which includes internal ... more For numerical studies of a ferrofluid dynamics we have developed a model which includes internal magnetic degrees of freedom of ferrofluid particles. Contrary to standard models, we take into account that the magnetocrystalline anisotropy of a ferrofluid particle material is finite, so that the particle moment is allowed to rotate with respect to the particle itself. Simulating magnetization relaxation of a ferrofluid after switching off the external field and comparing results with those obtained for rigid dipoles model, we demonstrate that for anisotropy typical for commonly used ferrofluid materials inclusion of 'magnetic' degrees of freedom is essential for a correct description of ferrofluid dynamics.
We have developed a new specialized version of the fast multipole method (FMM) for dipolar system... more We have developed a new specialized version of the fast multipole method (FMM) for dipolar systems. For this purpose we have derived general expressions of the multipole expansion coefficients (in spherical coordinates) for a system of point dipoles with the potential j dip B1=r 2 : Our version is especially useful for simulations of fine magnetic particle systems (magnetic nanocomposites, ferrofluids), molecular dipolar fluids or electric dipolar glasses.
Using Langevin dynamics simulations, we have calculated the AC-susceptibility " #i of disordered ... more Using Langevin dynamics simulations, we have calculated the AC-susceptibility " #i of disordered "ne magnetic particle systems with the dipolar interparticle interaction. We have shown that the shift of the (¹)-peak with increasing particle concentration depends qualitatively on the single-particle anisotropy and quantitatively on the precession damping.
We present a powerful method for simulations of fast remagnetization processes in ferrofluids whi... more We present a powerful method for simulations of fast remagnetization processes in ferrofluids which combines the stochastic (Langevin) dynamics and Monte-Carlo method. Our Langevin equations for the description of ferrofluid dynamics include both the mechanical (translational and rotational particle motion) and magnetic (rotation of the magnetic moment with respect to the particle) degrees of freedom. As an application example we present new physical results concerning the dependence of the magnetization relaxation in ferrofluids after switching off the external field.
Numerical simulations of fast remagnetization processes using the stochastic dynamics are widely ... more Numerical simulations of fast remagnetization processes using the stochastic dynamics are widely used to study various magnetic systems. In this paper we first address several crucial methodological problems of such simulations: (i) the influence of the finite-element discretization on the simulated dynamics, (ii) choice between Ito and Stratonovich stochastic calculi by the solution of micromagnetic stochastic equations of motion and (iii) non-trivial correlation properties of the random (thermal) field. Next we discuss several examples to demonstrate the great potential of the Langevin dynamics for studying fast remagnetization processes in technically relevant applications: we present numerical analysis of equilibrium magnon spectra in patterned structures, study thermal noise effects on the magnetization dynamics of nanoelements in pulsed fields and show some results for a remagnetization dynamics induced by a spin-polarized current.
for a Wiener process W and a class of" twice weakly differentiable functions h E C[0, 1], thus so... more for a Wiener process W and a class of" twice weakly differentiable functions h E C[0, 1], thus solving the problem of the convergence rate in Chung's functional law for the so-called "slowest points". Our description is closely related to an interesting functional emerging from a large deviation problem for the Wiener process in a strip.
A Brillouin light scattering study and theoretical interpretation of spin-wave modes in arrays of... more A Brillouin light scattering study and theoretical interpretation of spin-wave modes in arrays of in-plane magnetized micron-size rectangular Ni 80 Fe 20 elements are reported. It is shown that two-dimensional spinwave eigenmodes of these elements can be approximately described as products of one-dimensional spin-wave eigenmodes of longitudinally and transversely magnetized long finite-width permalloy stripes. The lowest eigenmodes of rectangular elements are of dipole-exchange nature and are localized near the element edges, while the higher eigenmodes are of a mostly dipolar nature and are weakly localized near the element center. The frequency spectra and spatial profiles of these eigenmodes are calculated both analytically and numerically, and are compared with the results of the Brillouin light scattering experiment.
We show experimentally and by model calculations that in finite, nonellipsoidal, micrometer size ... more We show experimentally and by model calculations that in finite, nonellipsoidal, micrometer size magnetic thin film elements the dynamic magnetic eigenexcitations (spin waves) may exhibit strong spatial localization. This localization is due to the formation of a potential well for spin waves in the highly inhomogeneous internal magnetic field within the element.
We present spectral measurements of spin-wave excitations driven by direct spin-polarized current... more We present spectral measurements of spin-wave excitations driven by direct spin-polarized current in a free layer of nanoscale Ir 20 Mn 80 /Ni 80 Fe 20 /Cu/Ni 80 Fe 20 spin valves. The measurements reveal that large-amplitude coherent spin-wave modes are excited over a wide range of bias current. The frequency of these excitations exhibits a series of jumps as a function of current due to transitions between different localized nonlinear spin-wave modes of the Ni 80 Fe 20 nanomagnet. We find that micromagnetic simulations employing the Landau-Lifshitz-Gilbert equation of motion augmented by the Slonczewski spin-torque term ͑LLGS͒ accurately describe the frequency of the current-driven excitations including the mode transition behavior. However, LLGS simulations give qualitatively incorrect predictions for the amplitude of excited spin waves as a function of current.
We consider a partial-sum process generated by a sequence of nonidentically distributed independe... more We consider a partial-sum process generated by a sequence of nonidentically distributed independent random variables. Assuming that this process is available for observation along an arbitrary time sequence, we fill the gaps by linear interpolation and prove the functional law of the iterated logarithm (FLIL) for sample paths obtained in this way. Assuming that the V. A. Egorov condition holds, we show that FLIL is valid, while under other conditions sufficient for the usual law of the iterated logarithm FLIL may fail. Bibliography: 16 titles.
In this contribution, we present the comparative analysis of the simple Ewald, lattice-based Ewal... more In this contribution, we present the comparative analysis of the simple Ewald, lattice-based Ewald and Fast Multipole (FMM) methods for the evaluation of the long-range dipolar field in disordered systems. We show that the FMM method with the optimally chosen hierarchical structure and the improved lattice version of the Ewald algorithm are superior with respect to original Ewald method starting from the particle number N$10 4 . FMM and lattice Ewald versions demonstrate comparable performance for systems up to N$4 Â 10 4 particles.
For numerical studies of a ferrofluid dynamics we have developed a model which includes internal ... more For numerical studies of a ferrofluid dynamics we have developed a model which includes internal magnetic degrees of freedom of ferrofluid particles. Contrary to standard models, we take into account that the magnetocrystalline anisotropy of a ferrofluid particle material is finite, so that the particle moment is allowed to rotate with respect to the particle itself. Simulating magnetization relaxation of a ferrofluid after switching off the external field and comparing results with those obtained for rigid dipoles model, we demonstrate that for anisotropy typical for commonly used ferrofluid materials inclusion of 'magnetic' degrees of freedom is essential for a correct description of ferrofluid dynamics.
We have developed a new specialized version of the fast multipole method (FMM) for dipolar system... more We have developed a new specialized version of the fast multipole method (FMM) for dipolar systems. For this purpose we have derived general expressions of the multipole expansion coefficients (in spherical coordinates) for a system of point dipoles with the potential j dip B1=r 2 : Our version is especially useful for simulations of fine magnetic particle systems (magnetic nanocomposites, ferrofluids), molecular dipolar fluids or electric dipolar glasses.
Using Langevin dynamics simulations, we have calculated the AC-susceptibility " #i of disordered ... more Using Langevin dynamics simulations, we have calculated the AC-susceptibility " #i of disordered "ne magnetic particle systems with the dipolar interparticle interaction. We have shown that the shift of the (¹)-peak with increasing particle concentration depends qualitatively on the single-particle anisotropy and quantitatively on the precession damping.
We present a powerful method for simulations of fast remagnetization processes in ferrofluids whi... more We present a powerful method for simulations of fast remagnetization processes in ferrofluids which combines the stochastic (Langevin) dynamics and Monte-Carlo method. Our Langevin equations for the description of ferrofluid dynamics include both the mechanical (translational and rotational particle motion) and magnetic (rotation of the magnetic moment with respect to the particle) degrees of freedom. As an application example we present new physical results concerning the dependence of the magnetization relaxation in ferrofluids after switching off the external field.
Numerical simulations of fast remagnetization processes using the stochastic dynamics are widely ... more Numerical simulations of fast remagnetization processes using the stochastic dynamics are widely used to study various magnetic systems. In this paper we first address several crucial methodological problems of such simulations: (i) the influence of the finite-element discretization on the simulated dynamics, (ii) choice between Ito and Stratonovich stochastic calculi by the solution of micromagnetic stochastic equations of motion and (iii) non-trivial correlation properties of the random (thermal) field. Next we discuss several examples to demonstrate the great potential of the Langevin dynamics for studying fast remagnetization processes in technically relevant applications: we present numerical analysis of equilibrium magnon spectra in patterned structures, study thermal noise effects on the magnetization dynamics of nanoelements in pulsed fields and show some results for a remagnetization dynamics induced by a spin-polarized current.
for a Wiener process W and a class of" twice weakly differentiable functions h E C[0, 1], thus so... more for a Wiener process W and a class of" twice weakly differentiable functions h E C[0, 1], thus solving the problem of the convergence rate in Chung's functional law for the so-called "slowest points". Our description is closely related to an interesting functional emerging from a large deviation problem for the Wiener process in a strip.
A Brillouin light scattering study and theoretical interpretation of spin-wave modes in arrays of... more A Brillouin light scattering study and theoretical interpretation of spin-wave modes in arrays of in-plane magnetized micron-size rectangular Ni 80 Fe 20 elements are reported. It is shown that two-dimensional spinwave eigenmodes of these elements can be approximately described as products of one-dimensional spin-wave eigenmodes of longitudinally and transversely magnetized long finite-width permalloy stripes. The lowest eigenmodes of rectangular elements are of dipole-exchange nature and are localized near the element edges, while the higher eigenmodes are of a mostly dipolar nature and are weakly localized near the element center. The frequency spectra and spatial profiles of these eigenmodes are calculated both analytically and numerically, and are compared with the results of the Brillouin light scattering experiment.
Uploads
Papers by Natalia Gorn