Nonlinear Magnetization Oscillations Research Articles

Light-Induced Ultrafast Dynamics of Spin Crossovers under High Pressure

Within the multielectron model of magnetic insulator with two different spin terms at each cation and spin crossover under high pressure we have studied dynamics of a sudden excited non equilibrium spin state. We obtain the different relaxation of the magnetization, high spin/low spin occupation numbers, and the metal-oxygen bond length for different values of the external pressure. For each pressure-temperature values stationary state agrees to the mean field phase diagrams. We found the long living oscillations of magnetization for the high spin ground state at small pressure. Close to crossover pressure the smooth relaxation is accompanied with a set of sharp strongly non linear oscillations of magnetization and HS/LS occupation numbers that are accompanied by the Franck–Condon resonances.

Nonlinear Oscillations of Magnetization in a Tangentially Magnetized Ferromagnetic Film Resonator

Introduction. Resonators based on epitaxially grown single-crystal films of yttrium iron garnet are used in various applications of microwave electronics. It is known that with increasing of microwave power incident on a resonator, various nonlinear effects begin to manifest themselves. There are: bistability effect, nonlinear frequency shift, nonlinear damping, etc. By now, the listed nonlinear effects have been quite good studied experimentally. Previously, when describing oscillations of various dynamical systems, the nonlinear damping and the nonlinear frequency shift were usually considered separately. At the same time, it was known that, when studying nonlinear magnetization oscillations in ferromagnetic film resonators with an increase in oscillation amplitude, these effects could occur simultaneously. Aim. Development of a model of magnetization oscillations taking into account the nonlinear frequency shift and nonlinear damping, as well as its experimental justification for a ferromagnetic film resonator. Materials and methods. The development of the model was carried out by the method of slowly varying amplitudes. An experimental study was carried out with a ferromagnetic film resonator. For the measurements, we used Rohde & Schwarz ZVA 40 vector network analyzer. We measured the frequency dependence of the reflection coefficient of the microwave signal from the resonator. Results. A model of nonlinear magnetization oscillations was developed taking into account both a nonlinear frequency shift and a nonlinear attenuation. The resonance curves were experimentally measured at various levels of the microwave power incident on the resonator. It was shown that nonlinear damping limits the nonlinear frequency shift of the magnetization oscillations in a tangentially magnetized ferromagnetic film resonator. Conclusion. The developed model adequately describes behavior of the resonance curves of ferromagnetic film resonators at high microwave power levels. The nonlinear damping leads to broadening of the resonance curves, thereby increasing losses. This effect also increases the reflection coefficient of the microwave signal from the resonator.

Studying the effect of biaxial anisotropy on nonlinear magnetization oscillations accompanying the 90° pulsed magnetization process in ferrite–garnet films with easy-plane anisotropy

Time dependences of the azimuthal component of the torque T φ(t) acting on magnetization are calculated to understand the nature of the delayed magnetization acceleration effect observed during the 90° pulsed magnetization of real ferrite–garnet films, in which biaxial anisotropy exists alongside with in-plane anisotropy. A calculation technique based on analyzing an operating point trajectory is used. Calculations show that if the effective anisotropy field H K2 is comparable to the magnetizing pulse amplitude H ma, abruptly ascending regions at characteristic times t* in curves T φ(t) arise, in the limit of which nonlinear magnetization oscillations formed. The shape of these regions depends weakly on the magnetizing pulse front duration τf. This explains the reason of the weak dependence of the nonlinear magnetization oscillations on duration of the magnetizing pulse front. Calculations also show that the main features of the delayed acceleration effect are less clear upon an increase of the pulse amplitude: the behavior of curves T φ(t) becomes smoother near times t*, and an increase in the pulse front duration is accompanied by a stronger drop in the intensity of magnetization oscillations.

Torque acting on the magnetization vector during 90° pulsed magnetization of real garnet ferrite films with in-plane anisotropy

The torque acting on the magnetization vector in the course of 90° pulsed magnetization of real garnet ferrite films with in-plane and biaxial anisotropy is calculated by a method in which the operating point trajectory is analyzed. The position of the operating point is described by azimuthal angle φ and torque component Tm produced by pulsed magnetizing field Hm. The time dependence of resultant torque Tφ has a sharply ascending portion, within which the nonlinear magnetization oscillations are excited. Additionally, the shape of the curve Tφ(t) within this portion depends on pulse rise time τf only slightly. These results explain the weak dependence of the magnetization oscillation strength on τf, which was experimentally found previously. It is shown analytically that when τf decreases to 2.5–3.0 ns within the initial portion of the curve Tφ(t) at φ ≤ 10°, there arises an extra maximum of torque Tφ. Simultaneously, an additional voltage peak appears in the initial part of the longitudinal magnetization signal. The appearance of the additional voltage peak is confirmed experimentally.

Nonlinear low-frequency oscillations of magnetization in ferromagnetic materials near phase transition

The paper deals with the study of nonlinear oscillations of magnetization, which evolve in ferromagnetic materials at low frequencies in the vicinity of the spin-orientation transition. It is shown that these effects occur when easy-axis anisotropy is realized, as occurs in the case of the magnetic impedance in wires covered with a magnetic coating and when anisotropy is caused by the specimen shape alone.

Effect of easy-plane anisotropy on the transient process time in magnetic films and plates

The effect of easy-plane anisotropy on the damping of the nonlinear magnetization oscillations that accompany 90° pulsed magnetization of magnetic films and plates is analyzed numerically. It is shown that the magnetization time can be decreased to ∼0.5 ns at an effective anisotropy field H K p ≥ 6 kOe and that magnetization oscillations are fully damped at H K p ≥ 20–40 kOe. The magnetization time can be ∼0.15–0.20 ns at a magnetizing pulse amplitude H m ∼ 20–40 Oe.

Autoresonance Excitation of Nonlinear Oscillations of Magnetization and Domain Walls in Ferromagnets

We investigate the conditions of capturing into resonance and exciting nonlinear ferromagnetic resonance in a ferromagnetic film with the anisotropic easy plane, as well as autoresonance excitation of nonlinear oscillations of the domain wall in uniaxial ferromagnets. The investigations demonstrate that in easy-plane ferromagnets with a narrow resonance line nonlinear oscillations of magnetization in the autoresonance mode can be generated. This autoresonance takes place if the resonance field grows slowly and pumping frequency is the constant which is equal to the frequency of linear resonance. It has been established that effectively exciting nonlinear oscillations of the domain wall in uniaxial ferromagnets and controlling the wall dynamics by low-amplitude alternating fields with the slow variation of the planar field in the autophasing mode are possible in the case of weak dissipation.

Analysis of 90° pulsed magnetization signals of ferrite-garnet films with easy plane-type anisotropy

The results from a numerical analysis of 90° pulsed magnetization signals of ferrite-garnet films with easy plane-type anisotropy are discussed. The possibility of nonlinear magnetization oscillations occurring at a magnetizing pulse front several times greater than the oscillation period is shown. The results from the calculations describe the actual shape of the longitudinal and transverse signals quite well. The calculated values of the oscillation frequency (∼500 MHz) are also in good agreement with the experimental data.

Formation of nonlinear magnetization oscillations by spin waves transmission through the boundary of two uniaxial ferromagnets

In this paper, the possibility of nonlinear spin wave formation at transmission of a bulk spin wave through the boundary of two ferromagnets with uniaxial magnetic anisotropy in the external magnetic field was shown at the certain conditions. The bulk spin waves with different values of precession angles of magnetizations can propagate in the second ferromagnet so both linear and nonlinear states can be excited. Also, expressions for coefficients of reflection and transmission of spin waves were obtained depending on value of a precession angle of magnetization in the second ferromagnet.

Nonlinear oscillations of magnetization for ferromagnetic particles in the vortex state and their ordered arrays

The dynamics of magnetization oscillations with a considerable amplitude and a radial symmetry in small ferromagnetic particles in the form of a thin disk with a magnetic vortex has been investigated. The collective variables that describe radially symmetric oscillations of the magnetization dynamics for particles in the vortex state are introduced, and the dependence of the particle energy is studied as a function of these variables. The analytical expressions describing the frequency of magnetization oscillations with a radial symmetry, including nonlinear oscillations, are derived using the collective variables. It is shown that the presence of a magnetic field oriented perpendicular to the particle plane reduces the oscillation frequency and can lead to hybridization of this mode with other modes of spin oscillations, including the mode of translational oscillations of the vortex core. The soliton solutions describing the propagation of collective oscillations along the chain of magnetic particles are found.

Nonlinear oscillations of magnetization accompanying pulsed 90°-magnetization of garnet ferrite films with easy-plane anisotropy

Abstract Nonlinear oscillations of magnetization (with a fundamental harmonic frequency of ≈0.5 GHz) emerging during 90°-pulsed magnetization of garnet ferrite films with easy-plane anisotropy are studied. Analysis of longitudinal and transverse magnetization signals and of the magnetization vector hodograph plotted on their basis shows that the weak dependence of the oscillation intensity on duration tf of the magnetizing magnetic field pulse (upon its variation from 0.4 to 6–8 ns), which is typical of these films, can be explained by the presence of biaxial anisotropy in the plane of actual films.

How to fix a broken symmetry: quantum dynamics of symmetry restoration in a ferromagnetic Bose–Einstein condensate

We discuss the dynamics of a quantum phase transition in a spin-1 Bose–Einstein condensate when it is driven from the magnetized broken-symmetry phase to the unmagnetized ‘symmetric’ polar phase. We determine where the condensate goes out of equilibrium as it approaches the critical point, and compute the condensate magnetization at the critical point. This is done within a quantum Kibble–Zurek scheme traditionally employed in the context of symmetry-breaking quantum phase transitions. Then we study the influence of the non-equilibrium dynamics near a critical point on the condensate magnetization. In particular, when the quench stops at the critical point, nonlinear oscillations of magnetization occur. They are characterized by a period and an amplitude that are inversely proportional. If we keep driving the condensate far away from the critical point through the unmagnetized ‘symmetric’ polar phase, the amplitude of magnetization oscillations slowly decreases reaching a nonzero asymptotic value. That process is described by an equation that can be mapped onto the classical mechanical problem of a particle moving under the influence of harmonic and `anti-friction' forces whose interplay leads to surprisingly simple fixed-amplitude oscillations. We obtain several scaling results relating the condensate magnetization to the quench rate, and verify numerically all analytical predictions.

On the Quasistochastic Behavior of Nonlinear Magnetization Oscillations in Orthorhombic Ferromagnets Induced by a Magnetic Pulse Field

AbstractIt is shown that large oscillations of magnetization (induced by fast changes of external magnetic field) in ferromagnets, of orthorhombic symmetry can show quasistochastic features: a slight deviation in initial state essentially changes the magnetization behavior for t → ∞. In contrast to the simplest uniaxial anisotropy case (where the quasistochastic behavior should be expected only after switching the in‐plane field on), the presence of orthorhombic anisotropy creates the new possibility of quasistochastic excitation after switching the external field off. This anisotropy component may also considerably change the ranges of quasistochastic behavior.