Magnetization switching in single-domain particles and a response to a field pulse

An equation in the Gilbert form that describes the motion of the magnetization vector in intense high-frequency magnetic fields is solved numerically. The solution obtained is used to study switching of the magnetization of a single-domain ferromagnetic particle that has the shape of an ellipsoid of revolution and possesses cubic anisotropy from the position parallel to an easy axis to the position normal to this axis. The ranges of amplitudes and frequencies of the magnetic field where magnetization switching is observed are determined. An expression for the response of an ensemble of variously oriented particles is derived. It is shown that a particle ensemble generated by an rf field may serve as a data carrier on which information is written and read out by means of nonlinear and linear ferromagnetic resonances.     

Spin-wave resonance in magnetic films under conditions of the skin effect

The spin-wave resonance spectrum of a ferromagnetic film magnetized normally to its surface is investigated as a function of the finite depth of penetration of the high-frequency field into the film with due regard for damping in the spin system and different types of surface-spin pinning. The exact numerical solution of the equation of motion for magnetization is obtained by considering the finite thickness of the skin layer. For a substantially inhomogeneous distribution of the high-frequency field over the layer thickness, the change in the resonance shape at frequencies close to the ferromagnetic resonance frequency is observed in addition to the broadening of all the resonance peaks and the decrease in their amplitudes.     

Magnetization stochastic dynamics in exchange-coupled layered structure

The stochastic and self-oscillation regimes established under the action of a longitudinal high-frequency magnetic field in a multilayer magnetic structure with antiferromagnetic exchange coupling are considered. A bifurcation diagram revealing the types of magnetization dynamic states is constructed in various frequency intervals. Poincaré diagrams are constructed for the stochastic regimes.     

Study of surface magnetic properties in Co-rich amorphous microwires

Magneto-optical investigations carried out on a Co-rich glass-covered amorphous microwire is presented. The appearance of circular magnetic bistability and the influence of tensile stress and high-frequency electric current on the surface magnetization reversal have been studied. The change of the mechanism of the magnetization reversal induced by the high-frequency electric current is also discussed.     

Ferromagnetic resonance in films with a plane anisotropy of electrical resistivity

High conductivity of metallic films leads to attenuation of an ultrahigh frequency field at the depth of the skin layer and, consequently, to a nonuniform distribution of the high-frequency magnetization component over the film thickness. This feature leads to a shift in the frequency and to widening of the ferromagnetic resonance line in a metallic film. In the present work, we study the effect of the plane anisotropy of electrical resistivity on the resonance behavior of polycrystalline magnetic films (Ni, Co, permalloy). Appearance of this anisotropy is related to the presence in the film of extended structural defects such as microgaps oriented along the chosen direction of the substrate texture (Lavsan, mica). In the geometry of the perpendicular magnetization and the normal incident angle of the ultrahigh frequency wave with respect to the film there are two resonance frequencies corresponding to the two orthogonal polarizations of the high-frequency field.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 10–13, June, 1985.     

Direct-current induced dynamics in Co90 Fe10/Ni80 Fe20 point contacts

We have directly measured coherent high-frequency magnetization dynamics in ferromagnetic films induced by a spin-polarized dc current. The precession frequency can be tuned over a range of several gigahertz by varying the applied current. The frequencies of excitation also vary with applied field, resulting in a microwave oscillator that can be tuned from below 5 to above 40 GHz. This novel method of inducing high-frequency dynamics yields oscillations having quality factors from 200 to 800. We compare our results with those from single-domain simulations of current-induced dynamics.     

Parametric magnetoelectric effect in an AC magnetic field

Electric polarization is predicted to occur under the conditions of parametric instability of vibrations of magnetization in a longitudinal high-frequency magnetic field (parametric magnetoelectric effect). The requirements on materials are indicated favoring the observation of this effect. An example of such materials is the easy-plane antiferromagnet Cr₂TeO₆.     

Calculations of three-dimensional magnetic excitations in permalloy nanostructures with vortex state

Dynamic susceptibility spectra of the vortex state in nanorings and nanodots are studied using three-dimensional micromagnetic simulations. Spatial maps of the susceptibility have enabled identification of various resonance modes. For an exciting field along the x axis, several resonance peaks appear for a thin dot, including a core mode, whereas only one main resonance peak is detected for a ring corresponding to a volume mode with uniform magnetization perpendicular to the exciting field (x direction). A low-frequency resonance peak related to a surface mode and a high-frequency resonance peak viewed as an edge mode are additionally observed for a thick ring. These three resonance modes (surface, volume and edge modes) which correspond to low, intermediate and high-frequency resonance peaks, respectively, are also captured for an exciting field along the y axis. In addition, a mixed edge and volume mode is revealed at a higher frequency.     

Micromagnetic calculation of magnetostatic oscillation modes of an orthogonally magnetized disk of yttrium iron garnet

The absorption spectrum of a normally magnetized disk of yttrium iron garnet, which is associated with resonances of magnetostatic oscillation modes excited by a homogeneous high-frequency magnetic field, has been investigated using a numerical analysis of the micromagnetic model developed for ferromagnetic objects. The distribution of magnetization oscillation amplitudes over the disk surface area has been obtained for the first four modes. A good agreement between the results of the micromagnetic simulation and the data of analytical calculations for special cases has proved the reliability and efficiency of the proposed approach in numerical experiments on the study of the magnetization dynamics in objects with different geometries and shapes, including multilayer magnetic film structures.     

Magnetic particle detection by frequency mixing for immunoassay applications

For magnetic immunoassay applications, a novel magnetic reader for quantification of magnetic particle concentrations has been developed. Upon magnetic excitation at two distinct frequencies incident on the sample, the response signal generated at a sum frequency is detected. The low-frequency field component periodically drives the magnetic particles into saturation, which is probed by the high-frequency field. The appearance of frequency mixing lines is highly specific to the nonlinearity of the superparamagnetic particles’ magnetization. The optimization of field coils, differential pickup coil, preamplifier, demodulators and filters are discussed. The output signal is linear for four orders of magnitude in iron concentration.     

Absorption of electromagnetic radiation by systems with a noncollinear magnetization distribution

The high-frequency absorption of electromagnetic radiation in systems with a noncollinear spatial magnetization distribution has been calculated. A medium with a helicoidal magnetic structure and a superlattice whose period contains two layers with noncollinear magnetizations are considered. An additional absorption peak related to the electron transitions from one spin subband to another under the action of a variable linearly polarized electric field is shown to appear in such systems at frequencies near the spin splitting of the conduction band.     

Photogalvanic effect in ferromagnets with a noncoplanar magnetization distribution

The photogalvanic effect in ferromagnetic materials is predicted. This is the appearance of the direct electric current in a ferromagnet sample due to a high-frequency alternating electric field. The phenomenological theory of this photogalvanic effect in ferromagnets is developed and the simplest microscopic model explaining the occurrence of the photogalvanic effect in a medium with the helical magnetic structure is proposed. The photogalvanic effect arises owing to the breaking of the symmetry with respect to the reversal of the electron motion in the medium with a noncoplanar magnetization distribution.     

Singularity in high-frequency susceptibility of thin magnetic films with uniaxial anisotropy

A sharp peak of magnetic susceptibility has been observed in the ferromagnetic resonance spectra of uniaxial magnetic films placed in a planar field directed orthogonal to the easy magnetization axis, along which a pumping high-frequency magnetic field has been oriented. The peak width is considerably narrower than the line width of the uniform ferromagnetic resonance, and its position in a field equal to the film anisotropy field does not depend on the pumping frequency. The nature of the peak is associated with a drastic increase in the static transverse susceptibility of the film in the vicinity of the anisotropy field. It is shown phenomenologically that the peak can be observed only for quality samples with small angular and amplitude dispersion of the uniaxial anisotropy.     

Skin effect under the conditions of ferromagnetic and spin-wave resonance

An expression for the electromagnetic field penetration depth in a metallic magnetic layer is derived taking into account the complexity of the high-frequency permeability under the conditions of ferromagnetic and spin-wave resonance. The frequency dependences of the skin depth and its dependences on the magnitude and type of spin pinning are determined from a numerical analysis of the solutions of the motion equation for magnetization. Some features of the dependences associated with the nature of the spin-wave spectrum are revealed.     

Influence of an electric field on the ferromagnetic resonance in a plane-layered magnetic system

The influence of an electric field on the ferromagnetic resonance (FMR) in a multilayer magnetic system consisting of two magnetic layers separated by a thin nonmagnetic interlayer has been investigated. It has been shown that, upon the excitation of magnetization oscillations by a microwave magnetic field, the eigenfrequencies of the ferromagnetic resonance depend on the stationary electric field applied in the plane of the layers. It has also been demonstrated that, in this system, high-frequency magnetization oscillations can be excited by an electric microwave field. The results of the investigation of the polarization properties of the excitation mechanism indicate that this effect can be observed experimentally.     

Nonlinear theory of the excitation of spin waves in ferrites by a longitudinal ultrahigh-frequency magnetic field

The problem of parametric excitation of spin waves in ferrites due to the action of a perturbing super high-frequency (SHF) magnetic field polarized along the direction of the constant magnetic field H₀ is considered in a nonlinear approximation. Such an examination enables one to determine the conditions for parametric excitation of spin waves and to find the amplitude and phase of the oscillations of the system's magnetization in the steady state. The frequency interval, within whose limits parametric excitation of spin waves is possible, is determined. Stability of the steady state under the conditions of parametric resonance is investigated. The results obtained are compared with existing experimental data.     

Requirements for soft magnetic underlayer (SUL)—Micromagnetic simulations of single-pole-type write heads and SUL systems

Landau-Lifshitz-Gilbert micromagnetic models were used to analyze the head field distributions and high-frequency responses for various soft magnetic underlayer thicknesses (t-SUL) and saturation magnetization flux densities (B_s) of single-pole-type (SPT) head-SUL systems. It was found that B_s of 10-12 kG and t-SUL of 30-45 nm would be sufficient for the examined head and perhaps for most next generation head-SUL systems. Antiferromagnetic coupling in the SUL affects the head and SUL magnetization and, eventually, the head field. With regard to the head magnetization response, it was found that the magnetization under the coil responded first, while the response at the main pole tip was quite slow, even compared with the head field. The main pole tip of the head was fully saturated and the main pole yoke was almost saturated, while the SUL was not saturated in the examined head-SUL system.     

Exchange-conductivity shift of ferromagnetic antiresonance (FMAR) and the behaviour of surface impedance in the neighbourhood of FMAR for perpendicular configuration

We have occupied ourselves with studying Larmor and antilarmor ferromagnetic antiresonance (AR) in bulk isotropic and saturated ferromagnetic metals in the region of the normal skin effect, on the basis of a macroscopic theory, for the case of the static external magnetic field,H ₀, perpendicular to the surface of a sample. We have solved Maxwell's equations for the propagation of high-frequency electromagnetic waves and, at the same time, the equation of motion for the magnetization in a Landau-Lifshitz form. On including the spatial dispersion of the permeability, we have found an exchange-conductivity shift of the antiresonance field compared to the AR field derived in the work of Kittel (J. de phys. et rad.12 (1951) 291). In the immediate vicinity of AR we have also derived the surface impedance of the metal and the dispersion relations for all waves produced in it by a linearly polarized high-frequency electromagnetic wave perpendicularly falling on the surface.The author is grateful to Dr. D. Fraitová for useful suggestions and discussions.