Dynamics in quantum Ising chain driven by inhomogeneous transverse magnetization

We study the dynamics caused by transport of transverse magnetization in one dimensional transverse Ising chain at zero temperature. We observe that a class of initial states having product structure in fermionic momentum-space and satisfying certain criteria, produce spatial variation in transverse magnetization. Starting from such a state, we obtain the transverse magnetization analytically and then observe its dynamics in presence of a homogeneous constant field Γ. In contradiction with general expectation, whatever be the strength of the field, the magnetization of the system does not become homogeneous even after infinite time. At each site, the dynamics is associated with oscillations having two different timescales. The envelope of the larger timescale oscillation decays algebraically with an exponent which is invariant for all such special initial states. The frequency of this oscillation varies differently with external field in ordered and disordered phases. The local magnetization after infinite time also characterizes the quantum phase transition.     

Scaling and front dynamics in Ising quantum chains

We study the relaxation behaviour of the quantum Ising chain, focusing our attention onto the non-equilibrium dynamics of the transverse magnetization. The initial states, from which the magnetization relaxes, are product states such as those generated by setting in contact several systems, each initially equilibrated at a given temperature. Due to the free fermionic structure of the chain, the dynamics of the transverse magnetization is easily expressed in a compact form. In the completely factorized initial state, corresponding to a situation where all the spins are thermalized independently, we obtain in the scaling limit the Green function associated to the transverse magnetization. The relaxation behaviour is also considered in the system-bath case, where part of the chain called the system is thermalized at a temperature T_s and the remaining part is at a temperature T_b. The magnetization profiles show a scaling behaviour. Moreover, in the extreme case T_b=∞ and T_s=0, it is shown that the magnetization relaxes in quantized steps in the strong transverse field region.     

Dynamic orientational phase transition in a two-layer magnetically coupled structure

The magnetization dynamics was studied for a system of two magnetic layers separated by a nonmagnetic spacer providing their antiferromagnetic coupling. A new effect of orientational phase transition occurring upon a change in the amplitude (frequency) of a microwave field was observed near the edge of the orientational hysteresis loop.     

Reversing the training effect in exchange biased CoO/Co bilayers

We performed a detailed study of the training effect in exchange biased CoO/Co bilayers. High-resolution measurements of the anisotropic magnetoresistance (AMR) display an asymmetry in the first magnetization reversal process and training in the subsequent reversal processes. Surprisingly, the AMR measurements as well as magnetization measurements reveal that it is possible to partially reinduce the untrained state by performing a hysteresis measurement with an in-plane external field perpendicular to the cooling field. Indeed, the next hysteresis loop obtained in a field parallel to the cooling field resembles the initial asymmetric hysteresis loop, but with a reduced amount of spin rotation occurring at the first coercive field. This implies that the antiferromagnetic domains, which are created during the first reversal after cooling, can be partially erased.     

Rosenzweig instability in a thin layer of a magnetic fluid

A simple mathematical model of the initial stage of nonlinear evolution of the Rosenzweig instability in a thin layer of a nonlinearly magnetized viscous ferrofluid coating a horizontal nonmagnetizable plate is constructed on the basis of the system of equations and boundary conditions of ferrofluid dynamics. A dispersion relation is derived and analyzed using the linearized equations of this model. The critical magnetization of the initial layer with a flat free surface, the threshold wavenumber, and the characteristic time of evolution of the most rapidly growing mode are determined. The equation for the neutral stability curve, which is applicable for any physically admissible law of magnetization of a ferrofluid, is derived analytically.     

Relaxing-precessional magnetization switching

A new way of magnetization switching employing both the spin-transfer torque and the torque by a magnetic field is proposed. The solution of the Landau–Lifshitz–Gilbert equation shows that the dynamics of the magnetization in the initial stage of the switching is similar to that in the precessional switching, while that in the final stage is rather similar to the relaxing switching. We call the present method the relaxing-precessional switching. It offers a faster and lower-power-consuming way of switching than the relaxing switching and a more controllable way than the precessional switching.     

The study of ultrafast magnetization dynamics induced by femtosecond laser pulses in GdFeCo amorphous film

The time-resolved magneto-optical Kerr spectroscopy technique is used to study the ultrafast magnetization dynamics induced by femtosecond laser pulses in GdFeCo amorphous film. We study concretely the influence of the different pump fluence and the different external magnetic field on magnetization dynamics of ultrafast demagnetization, magnetization reversal and magnetization recovery. The pump fluence dependence magnetization dynamics shows that the degree of demagnetization, the degree of magnetization reversal and the time of magnetization recovery increase with pump fluence, which can be interpreted by the “three-temperature” model. The external magnetic field dependence magnetization dynamics shows that the rate of magnetization reversal increases with the external field, which accord with the magnetization reversal mechanism based on the reversed magnetic domain nucleation and domain-wall motion.     

A study of the Barkhausen effect in U3P4 single crystal

The present work is the study of the amplitude distribution and the course of the power spectra of the Barkhausen impulses during the magnetization reversal of U₃P₄ single-crystal. On the basis of the obtained results the dynamics of the magnetization processes is discussed. In agreement with some preceding experimental papers it is shown that U₃P₄ has a large anisotropy of initial susceptibility.     

末端形状对NiFe纳米薄膜磁反转动力学特性的影响

基于微磁学模拟方法研究末端形状对NiFe纳米薄膜的磁反转和自旋波本征动力学特性的调制及磁反转与自旋波模式软化间的内在联系.纳米薄膜微磁结构的相变总是伴随着某种自旋波模式的软化,软化自旋波模式空间分布预示微磁结构相变的路径.存在一临界裁剪度（h₀）.当裁剪度h<h₀时,磁振荡局域于末端边缘的EM自旋波软化诱导磁反转从磁体末端边缘磁矩失稳开始,边缘失稳区域向中央扩展形成反转畴,最后反转畴逐渐移出膜面外而实现反转.当h≥h₀时,形状各向异性导致边缘局域化模式自旋波被抑制,反转场附近一致模式自旋波的软化诱导磁体一致反转.     

Self-consistent calculation of spin transport and magnetization dynamics

A spin-polarized current transfers its spin-angular momentum to a local magnetization, exciting various types of current-induced magnetization dynamics. So far, most studies in this field have focused on the direct effect of spin transport on magnetization dynamics, but ignored the feedback from the magnetization dynamics to the spin transport and back to the magnetization dynamics. Although the feedback is usually weak, there are situations when it can play an important role in the dynamics. In such situations, simultaneous, self-consistent calculations of the magnetization dynamics and the spin transport can accurately describe the feedback. This review describes in detail the feedback mechanisms, and presents recent progress in self-consistent calculations of the coupled dynamics. We pay special attention to three representative examples, where the feedback generates non-local effective interactions for the magnetization after the spin accumulation has been integrated out. Possibly the most dramatic feedback example is the dynamic instability in magnetic nanopillars with a single magnetic layer. This instability does not occur without non-local feedback. We demonstrate that full self-consistent calculations generate simulation results in much better agreement with experiments than previous calculations that addressed the feedback effect approximately. The next example is for more typical spin valve nanopillars. Although the effect of feedback is less dramatic because even without feedback the current can make stationary states unstable and induce magnetization oscillation, the feedback can still have important consequences. For instance, we show that the feedback can reduce the linewidth of oscillations, in agreement with experimental observations. A key aspect of this reduction is the suppression of the excitation of short wavelength spin waves by the non-local feedback. Finally, we consider nonadiabatic electron transport in narrow domain walls. The non-local feedback in these systems leads to a significant renormalization of the effective nonadiabatic spin transfer torque. These examples show that the self-consistent treatment of spin transport and magnetization dynamics is important for understanding the physics of the coupled dynamics and for providing a bridge between the ongoing research fields of current-induced magnetization dynamics and the newly emerging fields of magnetization-dynamics-induced generation of charge and spin currents.     

基于时空变换恒定磁化的起始磁化曲线推算方法

起始磁化曲线作为铁磁材料磁学特性的重要表征, 研究其获取方法具有重要意义. 现有方法采用随时间变化磁场作为激励磁场, 通过改变激励磁场大小, 逐步改变试件内的磁场及磁感应强度并进行测量以得到起始磁化曲线, 效率较低, 准备繁琐. 为此, 本文从基本的磁学物理定理出发, 提出一种基于时空变换的起始磁化曲线推算方法. 该方法以细长棒状试件或环形试件作为被测试件; 采用恒定磁化在被测试件内产生空间变化磁场作为激励磁场; 通过测量试件表面的磁场值来推算试件内磁场值, 从而获取铁磁材料起始磁化曲线. 直流线圈恒定磁化环形和棒状试件仿真实验验证了该方法的理论正确性. 进一步地, 考虑实际应用限制因素的推算结果表明了该方法在实际应用中是可行的, 可为探索新的起始磁化曲线测量方法提供理论指导.     

Three-dimensional simulation of irregular dynamics of topological solitons in moving magnetic domain walls

A three-dimensional computer simulation of dynamic processes occurring in a domain wall moving in a soft-magnetic uniaxial film with in-plane anisotropy has been performed based on the micromagnetic approach. It has been shown that the domain wall motion is accompanied by topological transformations of the magnetization distribution, or, more specifically, by “fast” processes associated with the creation and annihilation of vortices, antivortices, and singular (Bloch) points. The method used for visualizing the topological structure of magnetization distributions is based on the numerical determination of topological charges of two types by means of the integration over the contours and surfaces with variable geometry. The obtained data indicate that the choice of the initial configuration predetermines the dynamic scenario of topological transformations.     

Complete magnetizing field relating with magnetization reversal dynamics

We report the experimental finding that a complete magnetizing field H_M exists in magnetization reversal dynamics of ferromagnetic thin films, which is much larger than the apparent magnetic saturation field measured from the major hysteresis loop. Magnetization reversal dynamics contrastingly changes from nucleation dominated to wall-motion dominated according to an initial magnetization state magnetized by a field below H_M, whereas it is basically unchanged when the field is larger than H_M. The complete magnetizing field is found to be 1.5–2.0 times larger than the apparent magnetic saturation field and 6–10 times smaller than the anisotropy field in Co/Pd multilayer thin films.     

Perspectives on spin hydrodynamics in ferromagnetic materials

The field of spin hydrodynamics aims to describe magnetization dynamics from a fluid perspective. For ferromagnetic materials, there is an exact mapping between the Landau-Lifshitz equation and a set of dispersive hydrodynamic equations. This analogy provides ample opportunities to explore novel magnetization dynamics and magnetization states that can lead to potential applications that rely entirely on magnetic materials, for example, long-distance transport of information. This article provides an overview of the theoretical foundations of spin hydrodynamics and their physical interpretation in the context of spin transport. We discuss other proposed applications for spin hydrodynamics as well as our view on challenges and future research directions.     

Dynamics of paramagnetic agents by off-resonance rotating frame technique

Off-resonance rotating frame technique offers a novel tool to explore the dynamics of paramagnetic agents at high magnetic fields (B0 > 3T). Based on the effect of paramagnetic relaxation enhancement in the off-resonance rotating frame, a new method is described here for determining the dynamics of paramagnetic ion chelates from the residual z-magnetizations of water protons. In this method, the dynamics of the chelates are identified by the difference magnetization profiles, which are the subtraction of the residual z-magnetization as a function of frequency offset obtained at two sets of RF amplitude omega(1) and pulse duration tau. The choices of omega(1) and tau are guided by a 2-D magnetization map that is created numerically by plotting the residual z-magnetization as a function of effective field angle theta and off-resonance pulse duration tau. From the region of magnetization map that is the most sensitive to the alteration of the paramagnetic relaxation enhancement efficiency R(1rho)/R1, the ratio of the off-resonance rotating frame relaxation rate constant R(1rho) verse the laboratory frame relaxation rate constant R(1), three types of difference magnetization profiles can be generated. The magnetization map and the difference magnetization profiles are correlated with the rotational correlation time tauR of Gd-DTPA through numerical simulations, and further validated by the experimental data for a series of macromolecule conjugated Gd-DTPA in aqueous solutions. Effects of hydration water number q, diffusion coefficient D, magnetic field strength B0 and multiple rotational correlation times are explored with the simulations of the magnetization map. This method not only provides a simple and reliable approach to determine the dynamics of paramagnetic labeling of molecular/cellular events at high magnetic fields, but also a new strategy for spectral editing in NMR/MRI based on the dynamics of paramagnetic labeling in vivo.     

Magnetization filters: applications to NMR imaging of elastomers.

The general Fourier scheme for parameter selective imaging is subdivided into three periods: A preparation period for application of magnetization filters, an evolution period for space encoding, and a detection period for acquisition of the spectroscopic dimension. Depending on sample requirements and available time, the first and the last period can be omitted. Preparation of the initial magnetization for space encoding by appropriate filters is a powerful and time-saving way to introduce parameter-selective image contrast. This is illustrated by filters of molecular dynamics, which are sensitive to segmental motion in different time windows for contrast enhancement in composite and aged elastomers. Further contrast amplification is achieved by computing the difference of images acquired with different filter settings.     

Symmetry ensemble theory of spin wave emitting effectdriven by current in nanoscale magnetic multilayer

<正>This paper proposes a symmetry ensemble model for the magnetic dynamics caused by spin transfer torque in nanoscale pseudo-spin-valves,in which individual spin moments in the free layer are considered as subsystems to form a spinor ensemble.The magnetization dynamics equation of the ensemble was developed.By analytically investigating the equation,many magnetization dynamics properties excited by polarized current reported in experiments,such as double spin wave modes and the abrupt frequency jump,can be successfully explained.It is pointed out that an external field is not necessary for spin wave emitting(SWE) and a novel perpendicular configuration structure can provide much higher SWE efficiency in zero magnetic field.     

Antivortex gyrotropic motion in a nanocontact

A vortex-antivortex pair can form in the free layer of a nanocontact device as a result of the Oersted field produced by the current. In a large-scale free layer having uniform magnetization boundary conditions an in-plane external magnetic field will tend to confine the vortex-antivortex pair, which undergoes gyrotropic motion about the nanocontact after an initial displacement from the static equilibrium position. With the vortex pinned to a defect at the nanocontact the antivortex dynamics can be isolated and gyrotropic precession of the antivortex will be the dominant mode. The frequency of antivortex precession increases as the external magnetic field increases, and the frequency decreases as the nanocontact current increases.     

Current-driven magnetization dynamics in magnetic trilayers with a tilted spin polarizer

The magnetization dynamics is studied theoretically in the tilted-polarizer magnetic trilayers. Utilizing stability analysis, we obtain the phase diagrams in the plane defined by the current density and the direction of pinned-layer magnetization. With the pinned-layer magnetization oriented in a certain range, one can realize different magnetic states, such as quasi-parallel and quasi-antiparallel stable states, in-plane and out-of-plane precessions, and out-of-plane stable states by varying the current. We find that the free-layer magnetization prefers reversal for small deviation of the fixed-layer magnetization from the film plane, while precession for big deviation.