Temperature dependent magnetization dynamics of magnetic nanoparticles

Recent experimental and theoretical studies show that the switching behavior of magnetic nanoparticles can be well controlled by external time-dependent magnetic fields. In this work, we inspect theoretically the influence of the temperature and the magnetic anisotropy on the spin-dynamics and the switching properties of single domain magnetic nanoparticles (Stoner-particles). Our theoretical tools are the Landau-Lifshitz-Gilbert equation extended as to deal with finite temperatures within a Langevine framework. Physical quantities of interest are the minimum field amplitudes required for switching and the corresponding reversal times of the nanoparticle's magnetic moment. In particular, we contrast the cases of static and time-dependent external fields and analyze the influence of damping for a uniaxial and a cubic anisotropy.Comment: accepted by Journal of Physics: Condensed Matte

Minimal field requirement in precessional magnetization switching

We investigate the minimal field strength in precessional magnetization switching using the Landau-Lifshitz-Gilbert equation in under-critically damped systems. It is shown that precessional switching occurs when localized trajectories in phase space become unlocalized upon application of field pulses. By studying the evolution of the phase space, we obtain the analytical expression of the critical switching field in the limit of small damping for a magnetic object with biaxial anisotropy. We also calculate the switching times for the zero damping situation. We show that applying field along the medium axis is good for both small field and fast switching times.Comment: 6 pages, 7 figure

On the mechanism of irradiation enhanced exchange bias

By means of layer resolved ion irradiation the mechanisms involved in the irradiation driven modifications of the exchange bias effect in NiFe/FeMn bilayers have been investigated. It is shown that not only the locations of the defects but also the magnetic coupling between both layers during the irradiation process is of crucial importance. This requires an extension of current models accounting for defects in exchange bias systems.Comment: 3 pages, 3 figures, revised version, added results from further structural characterization by TEM, submitted to Europhysics Letter

Generation of pulse trains by current-controlled magnetic mirrors

The evolution of a spin-wave packet trapped between two direct current-carrying wires placed on the surface of a ferrite film is observed by Brillouin light scattering. The wires act as semi-transparent mirrors confining the packet. Because the spin-wave energy partially passes through these mirrors, trains of spin-wave packets are generated outside the trap. A numerical model of this process is presented and applied to the case when the current in the wires is dynamically controlled. This dynamical control of the mirror reflectivity provides new functionalities interesting for the field of spin-wave logic like that of a spin-wave memory cell.Comment: 4 pages, 3 figure

Phase sensitive Brillouin scattering measurements with a novel magneto-optic modulator

A recently reported phase sensitive Brillouin light scattering technique is improved by use of a magnetic modulator. This modulator is based on Brillouin light scattering in a thin ferrite film. Using this magnetic modulator in time- and space Brillouin light scattering measurements we have increased phase contrast and excluded influence of optical inhomogeneities in the sample. We also demonstrate that the quality of the resulting interference patterns can be improved by data postprocessing using the simultaneously recorded information about the reference light

Euler equation of the optimal trajectory for the fastest magnetization reversal of nano-magnetic structures

Based on the modified Landau-Lifshitz-Gilbert equation for an arbitrary Stoner particle under an external magnetic field and a spin-polarized electric current, differential equations for the optimal reversal trajectory, along which the magnetization reversal is the fastest one among all possible reversal routes, are obtained. We show that this is a Euler-Lagrange problem with constrains. The Euler equation of the optimal trajectory is useful in designing a magnetic field pulse and/or a polarized electric current pulse in magnetization reversal for two reasons. 1) It is straightforward to obtain the solution of the Euler equation, at least numerically, for a given magnetic nano-structure characterized by its magnetic anisotropy energy. 2) After obtaining the optimal reversal trajectory for a given magnetic nano-structure, finding a proper field/current pulse is an algebraic problem instead of the original nonlinear differential equation

Formation of guided spin-wave bullets in ferrimagnetic film stripes

The formation of quasi-2D nonlinear spin-wave eigenmodes in longitudinally magnetized stripes of a ferrimagnetic film, so-called guided spin-wave bullets, was experimentally observed by using time- and space-resolved Brillouin light scattering spectroscopy and confirmed by numerical simulation. They represent stable spin-wave packets propagating along a waveguide structure, for which both transversal instability and interaction with the side edges of the waveguide are important. The experiments and the numerical simulation of the evolution of the spin-wave excitations show that the shape of the formed packets and their behavior are strongly influenced by the confinement conditions. The discovery of these modes demonstrates the existence of quasi-stable nonlinear solutions in the transition regime between one-dimensional and two-dimensional wave packet propagation.Comment: 4 pages, 3 figure

Revivals, collapses and magnetic-pulse generation in quantum rings

Using a microscopic theory based on the density matrix formalism we investigate quantum revivals and collapses of the charge polarization and charge current dynamics in mesoscopic rings driven by short asymmetric electromagnetic pulses. The collapsed state is utilized for sub-picosecond switching of the current and associated magnetization, enabling thus the generation of pulsed magnetic fields with a tunable time structure and shape asymmetry which provides a new tool to study ultrafast spin-dynamics and ratchet-based effects.Comment: 4 pages, 2 figure