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Dynamic interaction of acoustic and magnetic systems is of strong current interest, triggered by the promises of almost lossless new concepts of magnet-based information technology. In such concepts, a significant role is often given to domain walls (DW). Therefore, here we investigate how launching an acoustic shear wave, we can control the DW motion. Surprisingly, at sufficiently large amplitudes of the shear displacement, the speed of the forced DW motion can reach sizeable fraction of the speed of sound. This was shown to happen due to certain resonance conditions depending on the wave frequency, its angle of incidence, and shear displacement amplitudes, leading to a total reflection of the wave and maximizing the impact. Most interesting, strong nonlinearity appears in the interaction of the elastic and magnetic subsystems, expressed by the negative slope of the resonant reflection peak and the s-shaped dependence of the domain wall velocity on the shear displacement amplitude, typical for nonlinear systems.

The problem of the stress state in an orthotropic piezoelectric body with a triaxial ellipsoidal inclusion under homogeneous force and electric loads is considered. The problem is solved by the Eshelby method of equivalent inclusion generalized to the case of a piezoelectric orthotropic space. The approach is validated against the example of a spheroidal cavity in a transversely isotropic material (the axis of revolution coincides with the symmetry axis) for which the exact solution is known. The stress distribution over the surface of the ellipsoidal cavity subject to tension is analyzed numerically.

Although photonics presents the fastest and most energy-efficient method of data transfer, magnetism still offers the cheapest and most natural way to store data. The ultrafast and energy-efficient optical control of magnetism is presently a missing technological link that prevents us from reaching the next evolution in information processing. The discovery of all-optical magnetization reversal in GdFeCo with the help of 100 fs laser pulses has further aroused intense interest in this compelling problem. Although the applicability of this approach to high-speed data processing depends vitally on the maximum repetition rate of the switching, the latter remains virtually unknown. Here we experimentally unveil the ultimate frequency of repetitive all-optical magnetization reversal through time-resolved studies of the dual-shot magnetization dynamics in Gd27Fe63.87Co9.13. Varying the intensities of the shots and the shot-to-shot separation, we reveal the conditions for ultrafast writing and the fastest possible restoration of magnetic bits. It is shown that although magnetic writing launched by the first shot is completed after 100 ps, a reliable rewriting of the bit by the second shot requires separating the shots by at least 300 ps. Using two shots partially overlapping in space and minimally separated by 300 ps, we demonstrate an approach for GHz magnetic writing that can be scaled down to sizes below the diffraction limit., Optical magnetization: extreme limits for magnetic data recording with light The potential for combining the speed of photonics with the advantages of magnetic data storage, using pulsed laser light to control the magnetization of alloys of gadolinium, iron and cobalt (GdFeCo), has been explored and quantified. Achieving ultrafast and energy-efficient optical control of magnetism could revolutionize information-processing technology. Researchers led by Xiangping Li at Jinan University in China and Alexey V. Kimel at Radboud University in The Netherlands have experimentally determined the fastest possible rate of the optical reversal of magnetization achieved by this promising new technology. They suggest data switching repetition rates of up to three billion cycles per second (3 GHz) are feasible. They also propose a method to achieve spatial resolution of data recording at scales below light’s ‘diffraction limit’, which is generally believed to restrict the attainable resolution.

Ferrimagnetic materials represent unique systems where the ease of manipulating the spins with applied magnetic fields is combined with exchange-driven acceleration of the internal spin dynamics. Of particular interest is the temperature range around the magnetic and spin compensation points, finely balancing both magnetic moment and angular momentum of the system, and leading to a very particular character of magnetic switching by the domain wall motion. Here we present our studies of the temperature-dependent domain wall dynamics in the temperature range covering both angular momentum and magnetization compensation points in garnet film, and reaching up to the Curie temperature. Drastic difference in the domain wall mobility and maximum achievable velocity in the vicinities of these two compensation points is demonstrated. Also a remarkably high mobility in weak applied magnetic fields is indicated.

On purpose to increase efficiency of an orbital-frequency resource the usage of polarization-division multiplexing (PDM) is extended in satellite communication systems. For this aim the axis ratio (AR) of polarization ellipses on satellite radio lines with circular (rotating) polarization must be not less 0,94. Nowadays widespread techniques of experimental defining AR, are based on measurements of orthogonal components intensity of electromagnetic fields created by aerials. But, at high, more than 0,85 axis ratios, the information about a difference in components intensity loses on errors of the measurements realizing these techniques. The method of defining AR, close to extremely achievable value (nearly equal to unit) is presented in the article. The basic idea of this method consists in inclusion to the measurement scheme of two identical devices (DUT), working one towards to another. Also the scheme of counter inclusion of two identical DUT, similar used for measurements, may be applied for designing antenna elements with high AR. Good coincidence of results of modeling, laboratory measurements and field tests have confirmed suitability of the offered method for the designing and testing of aerials with high axis ratio.

We resolve a significant controversy about how to understand and engineer single-shot all-optical switching of magnetization in ferrimagnets using femto- or picosecond-long heat pulses. By realistically modelling a generic ferrimagnet as two coupled macrospins, we comprehensively show that the net magnetization can be reversed via different pathways, using a heat pulse with duration spanning all relevant timescales within the non-adiabatic limit. This conceptual understanding is fully validated by experiments studying the material and optical limits at which the switching process in GdFeCo alloys loses its reliability. Our interpretation and results constitute a blueprint for understanding how deterministic all-optical switching can be achieved in alternative ferrimagnets using short thermal pulses.

Identifying an efficient pathway to change the order parameter via a subtle excitation of the coupled high-frequency mode is the ultimate goal of the field of ultrafast phase transitions. This is an especially interesting research direction in magnetism, where the coupling between spin and lattice excitations is required for magnetization reversal. Despite several attempts however, the switching between magnetic states via resonant pumping of phonon modes has not yet been demonstrated. Here we show how an ultrafast resonant excitation of the longitudinal optical phonon modes in magnetic garnet films switches magnetization into a peculiar quadrupolar magnetic domain pattern, unambiguously revealing the magneto-elastic mechanism of the switching. In contrast, the excitation of strongly absorbing transverse phonon modes results in thermal demagnetization effect only., Comment: 16 pages, 12 figures

On the base of a rigorous mathematical model, the problem of the contact interaction of a heated flat punch of an elliptical section with a transversely isotropic elastic half-space is investigated. It is assumed that the half-space surface is the isotropy plane of a transversely isotropic material, and also that there is a smooth (without friction) contact. Expressions of contact stresses and displacements of a heated flat elliptical punch are found explicitly. In the form of a simple inequality, a condition for separating the elastic material from the surface of a flat elliptical punch is obtained. Numerical calculations are carried out. Contact interaction of a heated flat punch is studied taking into account the separation of material from the punch.

In this letter, a method to realize all-optical helicity-dependent magnetic switching (AO-HDS) using a first-order azimuthally polarized vortex (FAPV) beam is demonstrated. Numerical calculations of the focal fields of FAPV beams reveal that left-handed and right-handed circular polarizations are generated due to the interaction between the polarization singularity and the helical wave front. Its feasibility for AO-HDS is experimentally demonstrated in Gd27Fe63.87Co9.13 under low numerical aperture (NA) conditions and within a narrow fluence window. It is numerically predicted that under high NA conditions, the lateral size of magnetic bits recorded by FAPV beams can be nearly 30% smaller than that obtained by circularly polarized beams, which opens a promising route to realize ultrafast and ultrahigh-density magnetic recording.

Here we study both static and time-resolved dynamic magnetic properties of the compensated ferrimagnet from room temperature down to 10K, thus crossing the magnetic compensation temperature $T_{M}$. The behaviour is analysed with a model of a simple collinear ferrimagnet with uniaxial anisotropy and site-specific gyromagnetic ratios. We find a maximum zero-applied-field resonance frequency of $\sim$160GHz and a low intrinsic Gilbert damping $\alpha$$\sim$0.02, making it a very attractive candidate for various spintronic applications., Comment: 6 pages, 6 figures, accepted paper in PRB (https://journals.aps.org/prb/accepted/df07aOb2T7f1e845c21429304f825e825c3a5f190)

In the last decade it was demonstrated that the fastest way to write information employs ultrashort laser pulses. Naturally such experiments raise questions about the ultimate limit of repetition rate at which light can switch a medium between stable bit states. Here the authors demonstrate that with femtosecond pulses it is possible to write and rewrite magnetic bits in iron garnet with a frequency of up to 20 GHz, with the maximum repetition rate being defined by the frequency of ferromagnetic resonance in the field of photoinduced magnetic anisotropy. This finding reveals the principles to be employed in achieving magnetic recording at frequencies far beyond today's state of the art.

Magneto-optical spectroscopy in fields up to 30 Tesla reveals anomalies in the equilibrium and ultrafast magnetic properties of the ferrimagnetic rare-earth-transition metal alloy TbFeCo. In particular, in the vicinity of the magnetization compensation temperature, each of the magnetizations of the antiferromagnetically coupled Tb and FeCo sublattices show triple hysteresis loops. Contrary to state-of-the-art theory, which explains such loops by sample inhomogeneities, here we show that they are an intrinsic property of the rare-earth ferrimagnets. Assuming that the rare-earth ions are paramagnetic and have a non-zero orbital momentum in the ground state and, therefore, a large magnetic anisotropy, we are able to reproduce the experimentally observed behavior in equilibrium. The same theory is also able to describe the experimentally observed critical slowdown of the spin dynamics in the vicinity of the magnetization compensation temperature, emphasizing the role played by the orbital momentum in static and ultrafast magnetism of ferrimagnets.

Monte-Carlo methods to asses a statistical validity of the relationship between coefficients of time series regression model were proposed. In economics such a relationship is present in the case when constant return to scale in production functions is assumed. The techniques being discussed here are virtually free from assumptions about underlying probability distributions and may be used in the case, when target variable or regressors are time series with random walk. This is achieved by comparing the regression model built on truly multivariate time series with those built on simulated time series with random walk. It has been shown that for the production functions of most Russian regions, the returns to scale significantly differs from a constant value at p

The nonlinear magneto-plasmonics aims to utilize plasmonic excitations to control the mechanisms and taylor the efficiencies of the non-linear light frequency conversion at the nanoscale. We investigate the mechanisms of magnetic second harmonic generation in hybrid gold-cobalt-silver multilayer structures, which support propagating surface plasmon polaritons at both fundamental and second harmonic frequencies. Using magneto-optical spectroscopy in Kretschmann geometry, we show that the huge magneto-optical modulation of the second harmonic intensity is dominated by the excitation of surface plasmon polaritons at the second harmonic frequency, as shown by tuning the optical wavelength over the spectral region of strong plasmonic dispersion. Our proof-of-principle experiment highlights bright prospects of nonlinear magneto-plasmonics and contributes to the general understanding of the nonlinear optics of magnetic surfaces and interfaces., Main Manuscript: 5 pages, 3 figures. Supplementary Information: 10 pages, 7 figures

Using a novel heterodyne magneto-optical Kerr-effect technique, we have studied the spectral characteristics of a room-temperature magnon Bose–Einstein condensate (BEC) in yttrium-iron-garnet (YIG) films. Thanks to the ultimate frequency resolution of the technique of better than 100 Hz, we were able to directly determine the spectral linewidth of the spontaneously formed condensate of microwave-frequency magons. The value of 1.7 MHz provides direct evidence of high temporal coherence of the magnon condensate, and shows that the condensation phenomenon can be used for implementation of tunable microwave oscillators with a quality factor ${{Q}} = {{f}}/\Delta {{f}}= {3}{\times}{10}^{3}$ . The measured linewidth is close to, but larger than, the lower limit estimated from previous indirect experiments based on the measurements of the condensate lifetime. Additionally, the technique enables a determination of the wavevector of the condensate ${{k}}_{\text{BEC}} = {5}{\times}{10}^{4}\ {\text{cm}}^{-1}$ , which is in agreement with previous studies.

We demonstrate an approach to parallel all-optical writing of magnetic domains using spatial and temporal interference of two ultrashort light pulses. We explore how the fluence and grating periodicity of the optical transient grating influence the size and uniformity of the written bits. Using a total incident optical energy of 3.5 μJ, we demonstrate the capability of simultaneously writing 102 spatially separated bits, each featuring a relevant lateral width of ∼1 μm. We discuss viable routes to extend this technique to write individually addressable, sub-diffraction-limited magnetic domains in a wide range of materials.We demonstrate an approach to parallel all-optical writing of magnetic domains using spatial and temporal interference of two ultrashort light pulses. We explore how the fluence and grating periodicity of the optical transient grating influence the size and uniformity of the written bits. Using a total incident optical energy of 3.5 μJ, we demonstrate the capability of simultaneously writing 102 spatially separated bits, each featuring a relevant lateral width of ∼1 μm. We discuss viable routes to extend this technique to write individually addressable, sub-diffraction-limited magnetic domains in a wide range of materials.

Using spin-wave tomography (SWaT), we have investigated the excitation and the propagation dynamics of optically-excited magnetoelastic waves, i.e. hybridized modes of spin waves and elastic waves, in a garnet film. By using time-resolved SWaT, we reveal the excitation dynamics of magnetoelastic waves through coherent-energy transfer between optically-excited pure-elastic waves and spin waves via magnetoelastic coupling. This process realizes frequency and wavenumber selective excitation of spin waves at the crossing of the dispersion relations of spin waves and elastic waves. Finally, we demonstrate that the excitation mechanism of the optically-excited pure-elastic waves, which are the source of the observed magnetoelastic waves, is dissipative in nature., 5 pages, 4 figures

We recorded IR spectra for neutral cobalt clusters via two-color IR-UV ionization, using the Free Electron Laser for intracavity experiments (FELICE). Well-resolved IR spectra are presented for [Formula: see text] (n = 4-10, 13) and analyzed with the help of Density Functional Theory calculations using two different correlation exchange functionals: the revisited Tao-Perdew-Staroverov-Scuseria (revTPSS) and the frequently used Perdew-Burke-Ernzerhof (PBE) approaches. Although we have not performed an extensive structure search, we tentatively assign the spectra for all cluster sizes except for n = 7, and n = 10. We find that neither of the two functionals chosen clearly outperforms the other in predicting IR spectra, and that relatively low scaling factors of 0.82 (PBE) and 0.8 (revTPSS) are required. In contrast to the magnetic moments, the calculated electric dipole moments fluctuate strongly as a function of cluster size and could therefore be used as an indirect probe to the cluster structure.

Contains fulltext : 181970.pdf (Publisher’s version ) (Open Access) Light propagation effects can strongly influence the excitation and the detection of laser-induced magnetization dynamics. We investigated experimentally and analytically the effects of crystallographic linear birefringence on the excitation and detection of ultrafast magnetization dynamics in the rare-earth orthoferrites (Sm 0.5 Pr 0.5 )FeO 3 and (Sm 0.55 Tb 0.45 )FeO 3 , which possess weak and strong linear birefringence, respectively. Our finding is that the effect of linear birefringence on the result of a magneto-optical pump-probe experiment strongly depends on the mechanism of excitation. When magnetization dynamics, probed by means of the Faraday effect, is excited via a rapid, heat-induced phase transition, the measured rotation of the probe pulse polarization is strongly suppressed due to the birefringence. This contrasts with the situation for magnetization dynamics induced by the ultrafast inverse Faraday effect, where the corresponding probe polarization rotation values were larger in the orthoferrite with strong linear birefringence. We show that this striking difference results from an interplay between the polarization transformations experienced by pump and probe pulses in the birefringent medium. 10 p.

Discovering ways to control the magnetic state of media with the lowest possible production of heat and at the fastest possible speeds is important in the study of fundamental magnetism, with clear practical potential. In metals, it is possible to switch the magnetization between two stable states (and thus to record magnetic bits) using femtosecond circularly polarized laser pulses. However, the switching mechanisms in these materials are directly related to laser-induced heating close to the Curie temperature. Although several possible routes for achieving all-optical switching in magnetic dielectrics have been discussed, no recording has hitherto been demonstrated. Here we describe ultrafast all-optical photo-magnetic recording in transparent films of the dielectric cobalt-substituted garnet. A single linearly polarized femtosecond laser pulse resonantly pumps specific d-d transitions in the cobalt ions, breaking the degeneracy between metastable magnetic states. By changing the polarization of the laser pulse, we deterministically steer the net magnetization in the garnet, thus writing '0' and '1' magnetic bits at will. This mechanism outperforms existing alternatives in terms of the speed of the write-read magnetic recording event (less than 20 picoseconds) and the unprecedentedly low heat load (less than 6 joules per cubic centimetre).

The Bose-Einstein condensate of magnons (mBEC) that is formed at room temperature in parametrically pumped magnetic films is doubly degenerate: it is formed simultaneously in two spectral minima corresponding to the lowest-energy magnons propagating in opposite directions along the in-plane bias magnetic field. In this work the interactions of magnons in the mBEC are studied both theoretically and experimentally. It is shown by direct calculation that the magnons residing in each of the degenerate spectral minima of mBEC form a practically ideal magnon gas, as the attractive self-interaction between these magnons is very weak. At the same time, the interaction between the magnons residing in different spectral minima, corresponding to opposite directions of the magnon wave vector, is relatively strong and repulsive, leading to a repulsive total intermagnon interaction. By measuring the spectral characteristics of the mBEC it is shown that with increased magnon density the energy per magnon in the mBECs increases, thus confirming experimentally that the net intermagnon interaction in a doubly degenerate mBEC is repulsive.

Excitation of antiferromagnetic HoFeO3 with a single 80 fs laser pulse triggers a first-order spin-reorientation phase transition. In the ultrafast kinetics of the transition one can distinguish the processes of impulsive excitation of spin precession, nucleation of the new domain and growth of the nuclei. The orientation of the spins in the nuclei is defined by the phase of the laser-induced coherent spin precession. The growth of the nuclei is further promoted by heating induced by the laser excitation. Hereby we demonstrate that in HoFeO3 coherent control of the spin precession allows an effective control of the route of the heat-induced first-order magnetic phase transition. The theoretical description of the excitation of the spin precession by linearly-polarized ultrashort laser pulses is developed with the sigma model. The analysis showed high sensitivity of the excited dynamics to the initial spin orientations with respect to the crystallographic axes of the material.

In this paper, we propose an approximate nonlinear theory of a spintronic terahertz-frequency emitter based on canted antiferromagnet-platinum bilayers. We present a model accounting for the excitation of nonlinear oscillations of the Neel vector in an antiferromagnet using terahertz pulses of an electromagnetic field. We determine that, with increasing amplitude of the pumping pulse, the spin system’s response increases nonlinearly in the fundamental quasiantiferromagnetic mode. We demonstrate control of the Neel vector trajectory by changing the terahertz pulse peak amplitude and frequency and determine the bands of nonlinear excitation using Fourier spectra. Finally, we develop an averaging method which gives the envelope function of an oscillating output electromagnetic field. The nonlinear dynamics of the antiferromagnet-based emitters discussed here is of importance in terahertz-frequency spintronic technologies.

Recent experiments on isolated Co clusters have shown huge orbital magnetic moments in comparison with their bulk and surface counterparts. These clusters hence provide the unique possibility to study the evolution of the orbital magnetic moment with respect to the cluster size and how competing interactions contribute to the quenching of orbital magnetism. We investigate here different theoretical methods to calculate the spin and orbital moments of Co clusters, and assess the performances of the methods in comparison with experiments. It is shown that density-functional theory in conventional local density or generalized gradient approximations, or even with a hybrid functional, severely underestimates the orbital moment. As natural extensions/corrections, we considered the orbital polarization correction, the LDA+$U$ approximation as well as the LDA+DMFT method. Our theory shows that of the considered methods, only the LDA+DMFT method provides orbital moments in agreement with experiment, thus emphasizing the importance of dynamic correlations effects for determining fundamental magnetic properties of magnets in the nanosize regime.

Contains fulltext : 166215.pdf (Publisher’s version ) (Open Access)

We present helicity and field dependent magnetization dynamics of ferromagnetic Co/Pt multilayers, suitable for all-optical helicity-dependent switching. Employing single-shot time-resolved magneto-optical Kerr effect imaging, our study demonstrates an ultra-fast quenching of the magnetization after a single 60 fs laser pulse excitation followed by a recovery. Full demagnetization occurs within 1 ps after laser excitation. The magnetization dynamics reveals a small helicity dependence caused by magnetic circular dichroism. When an external magnetic field is applied, a heat-assisted magnetization reversal occurs on a nanosecond time scale.

Magnetism of transition metal (TM) oxides is usually described in terms of the Heisenberg model, with orientation-independent interactions between the spins. However, the applicability of such a model is not fully justified for TM oxides because spin polarization of oxygen is usually ignored. In the conventional model based on the Anderson principle, oxygen effects are considered as a property of the TM ion and only TM interactions are relevant. Here, we perform a systematic comparison between two approaches for spin polarization on oxygen in typical TM oxides. To this end, we calculate the exchange interactions in NiO, MnO, and hematite (Fe2O3) for different magnetic configurations using the magnetic force theorem. We consider the full spin Hamiltonian including oxygen sites, and also derive an effective model where the spin polarization on oxygen renormalizes the exchange interactions between TM sites. Surprisingly, the exchange interactions in NiO depend on the magnetic state if spin polarization on oxygen is neglected, resulting in non-Heisenberg behavior. In contrast, the inclusion of spin polarization in NiO makes the Heisenberg model more applicable. Just the opposite, MnO behaves as a Heisenberg magnet when oxygen spin polarization is neglected, but shows strong non-Heisenberg effects when spin polarization on oxygen is included. In hematite, both models result in non-Heisenberg behavior. General applicability of the magnetic force theorem as well as the Heisenberg model to TM oxides is discussed., Comment: 19 pages, 2 figures

Here we follow, both experimentally and theoretically, the development of magnetism in Tb clusters from the atomic limit, adding one atom at a time. The exchange interaction is, surprisingly, observed to drastically increase compared to that of bulk and to exhibit irregular oscillations as a function of the interatomic distance. From electronic structure theory we find that the theoretical magnetic moments oscillate with cluster size in exact agreement with experimental data. Unlike the bulk, the oscillation is not caused by the RKKY mechanism. Instead, the inter-atomic exchange is shown to be driven by a competition between wave-function overlap of the 5d shell and the on-site exchange interaction, which leads to a competition between ferromagnetic double-exchange and antiferromagnetic super-exchange. This understanding opens up new ways to tune the magnetic properties of rare-earth based magnets with nano-sized building blocks.

The contact interaction of two compressed electroelastic transversely isotropic half-spaces with different properties is studied. One of the half-spaces has an axisymmetric notch of special shape. The contact plane coincides with the plane of isotropy of the half-spaces. Explicit formulas for the contact pressure, displacements, and the size of the gap between piezoelectric half-spaces are derived. These contact characteristics for transversely isotropic and isotropic elastic half-spaces are obtained as special cases

The paper discusses problems associated with the use of data mining tools to study discrepancies between countries with different types of institutional matrices by variety of potential explanatory variables: climate, economic or infrastructure indicators. An approach is presented which is based on the search of statistically valid regularities describing the dependence of the institutional type on a single variable or a pair of variables. Examples of regularities are given.

Contains fulltext : 83735.pdf (Publisher’s version ) (Open Access) 54 p.

The static equilibrium of a transversely isotropic magnetoelectroelastic body with a plane crack of arbitrary shape in the isotropy plane under antisymmetric mechanical loading is studied. The relationships between the stress intensity factors (SIFs) for an infinite magnetoelectroelastic body and the SIFs for a purely elastic body with the same crack and under the same antisymmetric loading are established. This enables the SIFs for a magnetoelectroelastic body to be found directly from the analogous problem of elasticity. As an example of using this result, the SIFs for penny-shaped, elliptic, and parabolic cracks in a magnetoelectroelastic body under antisymmetric mechanical loading are found

The elastic stress state in a piezoelectric body with an arbitrarily oriented elliptic crack under mechanical and electric loads is analyzed. The solution is obtained using triple Fourier transform and the Fourier-transformed Green’s function for an unbounded piezoelastic body. Solving the problem for the case of a crack lying in the isotropy plane, for which there is an exact solution, demonstrates that the approach is highly efficient. The distribution of the stress intensity factors along the front of a crack in a piezoelectric body under uniform mechanical loading is analyzed numerically for different orientations of the crack

This review summarizes the recent progress in the study of ultrafast nonthermal effects of light on magnetic materials. It is demonstrated that due to opto-magnetic phe- nomena an intense 100 fs circularly polarized laser pulse acts on the spins similar to an equivalently short effective mag- netic field pulse up to 1 T. The review shows that using such opto-magnetic phenomena one may selectively excite differ- ent modes of magnetic resonance, realize quantum control of magnons, trigger magnetic phase transitions and switch spins on a subpicosecond time-scale. All these findings open new in- sights into the understanding of ultrafast magnetic excitation and, considering recent progress in the development of compact ultrafast lasers, may provide new prospects for applications of ultrafast opto-magnetic phenomena in magnetic storage and in- formation processing technology.