Dynamics of magnetization in ferromagnet with spin-transfer torque

We review our recent works on dynamics of magnetization in ferromagnet with spin-transfer torque. Driven by constant spin-polarized current, the spin-transfer torque counteracts both the precession driven by the effective field and the Gilbert damping term different from the common understanding. When the spin current exceeds the critical value, the conjunctive action of Gilbert damping and spin-transfer torque leads naturally the novel screw-pitch effect characterized by the temporal oscillation of domain wall velocity and width. Driven by space- and time-dependent spin-polarized current and magnetic field, we expatiate the formation of domain wall velocity in ferromagnetic nanowire. We discuss the properties of dynamic magnetic soliton in uniaxial anisotropic ferromagnetic nanowire driven by spin-transfer torque, and analyze the modulation instability and dark soliton on the spin wave background, which shows the characteristic breather behavior of the soliton as it propagates along the ferromagnetic nanowire. With stronger breather character, we get the novel magnetic rogue wave and clarify its formation mechanism. The generation of magnetic rogue wave mainly arises from the accumulation of energy and magnons toward to its central part. We also observe that the spin-polarized current can control the exchange rate of magnons between the envelope soliton and the background, and the critical current condition is obtained analytically. At last, we have theoretically investigated the current-excited and frequency-adjusted ferromagnetic resonance in magnetic trilayers. A particular case of the perpendicular analyzer reveals that the ferromagnetic resonance curves, including the resonant location and the resonant linewidth, can be adjusted by changing the pinned magnetization direction and the direct current. Under the control of the current and external magnetic field, several magnetic states, such as quasi-parallel and quasi-antiparallel stable states, out-of-plane precession, and bistable states can be realized. Th     

Phase domain structures in cylindrical magnets under conditions of a first-order magnetic phase transition

The magnetic and resonance properties of cylindrical magnets at first-order phase transition from paramagnetic to ferromagnetic state were theoretically studied. It has been shown that in the external magnetic field directed perpendicularly to the rotation axis, formation of a specific domain structure of paramagnetic and ferromagnetic layers can be energetically favorable. The parameters of cylindrical phase domains as well as their dependences on temperature, magnetic field and material characteristics have been calculated. Peculiarities of the magnetic resonance spectra appearing as a result of the phase domain formation have been considered. Dependence of the resonance field of the system of ferromagnetic domains on magnetization and temperature has been obtained.     

消磁场对纳米铁磁线磁畴壁动力学行为的影响

为了实现基于磁畴壁运动的自旋电子学装置, 掌握磁畴壁动力学行为是重要争论之一.研究了在外磁场驱动下L-型纳米铁磁线磁畴壁的动力学行为. 通过微磁学模拟,在各种外磁场的驱动下考察了纳米铁磁线磁畴壁的动力学特性; 在较强外磁场的驱动下, 在不同厚度纳米线上考察了纳米线表面消磁场对磁畴壁动力学行为的影响. 为了进一步证实消磁场对磁畴壁动力学的影响, 在垂直于纳米线表面的外磁场辅助下分析了磁畴壁的动力学行为变化. 结果表明, 随着纳米线厚度和外驱动磁场强度的增加, 增强了纳米线表面的消磁场的形成, 使得磁畴壁内部自旋结构发生周期性变化, 导致磁畴壁在纳米线上传播时出现Walker崩溃现象. 在垂直于纳米线表面的外磁场辅助下, 发现辅助磁场可以调节消磁场的强度和方向. 这意味着利用辅助磁场可以有效地控制纳米铁磁线磁畴壁的动力学行为.     

Spin orientation and excitation of magnetic nanocluster on metal surface

The spin orientation and excitation of the ferromagnetic nanocluster on the magnetic metal surface are studied numerically. We show that localized magnetic excitation modes are generated by the spin fluctuation of the cluster, when the ferromagnetic interaction J′ between the cluster and the metal surface is small and the spins in the cluster are oriented in the opposite direction with those of the metal surface by the external field. This magnetic structure is similar to the domain wall (DW) structure of a ferromagnetic wire, both sides of which connect with metal surfaces. As the interaction J′ increases, the sign of the thermal average of the spins in the cluster changes, i.e., the spin-flip takes place. In this time, the magnetic fluctuation of the cluster becomes large and the magnetic excitation energies, except for that of one excitation mode, overlap with the excitation spectrum of the spin wave. We also show that, by the overlap, sharp peaks and dips occur in the excitation spectrum of the spin wave.     

Dynamics of solenoidal magnetic structures in soft magnetic thin-film elements

We report on the dynamics of magnetic domain structure conversions exhibited by soft magnetic thin-film elements of elementary geometrical shape (square, disc, triangle) when exposed to a strong external magnetic field. Starting from flux closure vortex patterns, the magnetic structures evolve towards an in-plane saturated state under the influence of an external field. This irreversible and nucleation-free magnetization process occurs on the time scale of picoseconds. The details of this conversion are investigated by means of a time-resolved micromagnetic finite element modeling. We find a sensitive dependence of the temporal evolution of the magnetic structure on the value of the damping parameter in Gilbert's equation of motion. In the case of high damping, domain wall motion dominates the process, while lower damping leads to the formation of a 360° wall which collapses by emitting magnetization waves. It is shown that the mobility of vortices is generally much lower than that of domain walls. The calculations indicate that at a low damping, a magnetic vortex can act almost as a source for concentric waves in ferromagnetic thin-film elements.     

Formation of phase domain structures in thin films under conditions of a first-order magnetic phase transition

This paper reports on the results of a theoretical investigation into the magnetic and resonance properties of thin films in the range of the transition from a paramagnetic state to a ferromagnetic state in the case where the magnetic transition is a first-order phase transformation. It is demonstrated that, in an external magnetic field directed perpendicular to the film plane, the formation of a specific domain structure consisting of domains of the coexisting paramagnetic and ferromagnetic phases can appear to be energetically favorable. The parameters of the equilibrium system of stripe phase domains and their dependences on the temperature, the magnetic field, and the characteristics of the material are calculated. The specific features of the magnetic resonance spectra under the conditions of formed stripe phase domains are considered. A relationship is derived for the dependence of the resonance field of the system of ferromagnetic domains on the magnetization and temperature. It is shown that the alternating external field can fulfill an orientation function in the formation of stripe phase domains.     

Refraction of surface spin waves in spatially inhomogeneous ferrodielectrics with biaxial magnetic anisotropy

The refractive index of surface spin waves propagating in a ferromagnetic medium with a nonuniform distribution of the parameters of uniaxial and orthorhombic magnetic anisotropies and exchange coupling is determined within the spin-density formalism. The coefficients of reflection and transmission of spin waves at the interface between two homogeneous magnets with different constants of uniaxial and orthorhombic magnetic anisotropies, exchange coupling, and saturation magnetization are calculated. The dependences of the intensity of a reflected wave and the refractive index on the wave frequency and the strength of an external dc homogeneous magnetic field are determined.     

Interaction of a shear wave with a moving domain wall in an iron garnet crystal

The boundary-value problem of the interaction of a plane monochromatic shear wave with a moving Bloch wall in an iron garnet crystal is solved in the framework of the nonexchange magnetostatic approximation on the basis of the method of phase invariants for wave problems with moving boundaries. For a shear wave incident on the domain wall, the possibility of the reflectionless birefringence is demonstrated. Numerical results illustrating the resonance properties of the magnetic subsystem are presented. It is established that, at the upper bound of the reflectionless birefringence range, the interaction of the shear wave with the domain wall manifests itself as a degenerate resonance with the solution in the form of two combined antiphase, collinearly propagating shear waves of infinitely large amplitudes, which form a zero resulting field.     

Spin waves in static non-periodic magnetic structures

We have studied the propagation of spin waves in a number of static non-periodic magnetic structures. We have established that (1) a ferromagnetic spin wave can ride over a domain wall with little reflection if its wavelength is less than twice the thickness of the wall; (2) in a ferromagnet with a set of parallel but irregularly spaced domain walls the spin wave linewidth is determined by the product of the scattering strength of the walls and the degree of randomness of the wall spacings; and (3) spin waves of rather narrow linewidths can exist in continuously varying irregular spin structures.     

Switching of a spin valve with three magnetic layers

A spin valve with two pinned ferromagnetic layers sandwiching a free ferromagnetic layer with a thickness smaller than the spin diffusion length in the same layer and than the domain wall thickness is considered. The instability conditions are determined for various mutual orientations of the magnetization of the layers. The possibility of a considerable decrease in the instability threshold due to joint action of spin-polarized current and an external magnetic field is indicated. It is shown that in addition to collinear states, a nonequilibrium noncollinear state can exist, into which the system is switched after exceeding the instability threshold.     

两端线型双量子点分子Aharonov-Bohm干涉仪电输运

设计一个两端线型双量子点分子Aharonov-Bohm (A-B)干涉仪. 采用非平衡格林函数技术, 理论研究无含时外场作用下的体系电导和引入含时外场作用下的体系平均电流. 在不考虑含时外场时, 调节点间耦合强度或磁通可以诱导电导共振峰劈裂. 控制穿过A-B干涉仪磁通的有无, 实现了共振峰电导数值在0与1之间的数字转换, 为制造量子开关提供了一个新的物理方案. 同时借助磁通和Rashba自旋轨道相互作用, 获得了自旋过滤. 当体系引入含时外场时, 平均电流曲线展示了旁带效应. 改变含时外场的振幅, 实现了体系平均电流的大小与位置的有效控制, 而调节含时外场的频率, 则可以实现平均电流峰与谷之间的可逆转换. 通过调节磁通与Rashba自旋轨道相互作用, 与自旋相关的平均电流亦得到有效控制. 研究结果为开发利用耦合多量子点链嵌入A-B 干涉仪体系电输运性质提供了新的认知. 上述结果可望对未来的量子器件设计与量子计算发挥重要的指导作用.     

垂直场下磁性薄膜中的铁磁共振现象

将铁磁共振频率看成外磁场的函数, 讨论了垂直场下磁性膜中的铁磁共振现象. 结果显示: 当外磁场平行于膜面, 并考虑磁膜具有垂直磁晶各向异性情形时, 其磁共振频率随外磁场的变化分为高频支和低频支两种情况, 具体的依赖关系取决于磁膜内磁晶的各向异性; 当外磁场垂直于膜面, 其磁共振频率随外磁场的关系仅存在一支, 一般地, 磁共振频率随外磁场的增加单调地非线性减小, 但当立方磁晶各向异性场H_k1 与单轴磁晶各向异性场H_a之比值介于2/3 < H_k1/H_a <1时, 其磁共振频率随外磁场的增加单调增加, 这与相关的实验结果一致. 研究结果表明: 磁薄膜中有无垂直于膜面的磁各向异性可以通过其磁共振谱的测量进行辨析.     

Untersuchung der Neutronendepolarisation von Dy in der Umgebung von magnetischen Umwandlungspunkten

It is possible to determine the dimensions of the ferromagnetic correlation range by depolarisation measurements of polarized thermal neutrons near magnetic transition points, where small magnetic domains (< 10^?4 cm) are present. The dimensions of the magnetic domains of Dy were determined in the temperature range from 4,2 °K to room temperature and in an external magnetic field from 0 to 2,4 kOe. The size of the domains increases with decreasing temperature and increasing external field. For low temperatures a sort of internal coercive force for the wall mobility was observed, which strongly hinders the formation of greater ferromagnetic domains. At the Curie-point the ferromagnetic correlation range shows a continuous transition and goes only slowly to zero when the temperature increases.     

Domain wall dynamics and Hall effect in eddy current loop in amorphous ferromagnetic wire with small helical anisotropy

Small helical anisotropy was induced in amorphous ferromagnetic Co_68.2Fe_4.3Si_12.5B₁₅ wire by current annealing and simultaneous application of tensile stress and torsion. Presence of helical anisotropy was confirmed by measurement and analysis of the circular magnetic flux versus axial magnetic field hysteresis loops. These measurements also showed that a single domain wall between circular domains can be created by placing the wire in a sufficiently high inhomogeneous magnetic field generated by Helmholtz coils with opposite currents. The domain wall velocity versus axial driving field was measured. The results show that the basic dynamic properties (magnitude of the wall mobility, field interval in which linear dependencies between velocity and field are observed, accelerated increase of the velocity for higher fields) are very similar to those obtained for the domain wall between circular domains driven by a constant circular field. The Hall effect was detected in the eddy current loop generated by the moving domain wall.     

Transverse acoustoelectric effect in ferromagnetic superconductors

A longitudinal ultrasonic wave in a type-II superconductor with a ferromagnetic subsystem and negligible Hall effect carries the vortex structure in its propagation direction and generates a constant transverse electric (acoustoelectric) field. This field has a maximum in temperature and external magnetic field. The magnitudes and positions of these maxima depend on the magnitude and direction of the internal ferromagnetic moment of the superconductor. It is shown that experimental investigation of the temperature dependence of the acoustoelectric field in a fixed external magnetic field or at a fixed temperature on the external magnetic field strength makes it possible to measure the magnetic moment and magnetic susceptibility of the superconductor ferromagnetic subsystem and the viscosity coefficient of the vortex structure.     

Mobility of domain walls in low-loss garnet films

The dependence of the domain wall mobility on the strength of a static magnetic field applied in the plane of the sample is investigated in single-crystal garnet films of the system YBiFeGa with perpendicular magnetic anisotropy and a narrow ferromagnetic resonance line. It is shown that, as in the case of YIG single crystals with cubic magnetic anisotropy, wall motion gives rise to an additional energy loss contribution far greater than the relativistic contribution also present in the case of homogeneous magnetization. It is established that a mechanism recently proposed in theory does not give a correct explanation for this additional contribution, because qualitative as well as quantitative discrepancies exist between the theoretical conclusions and measurement data. Fiz. Tverd. Tela (St. Petersburg) 39, 1253–1256 (July 1997)     

Shear Surface Wave on a Moving Bloch Wall

Using Galileo's transformation for moving to the rest frame of the Bloch wall in the exchange-free magnetostatic approximation, we obtain the dispersion relation for a shear surface wave guided by a moving 180-degree domain boundary of a ferromagnetic crystal. It is found that the motion of the domain boundary has the orienting action on the wave normal of the shear surface wave and significantly changes the spectrum of forward-propagating waves in the frequency band below the scattered-field ferromagnetic resonance.     

Determining the magnetic ground state of TbNi5 single crystal using polarized neutron scattering technique

The TbNi₅ compound shows an interesting magnetic phase transition with an incommensurate structure below 23 K, whose true nature remains unresolved. In order to solve this question, we have carried out polarized neutron diffraction experiments by measuring temperature and field dependence of the intensities of satellites and Bragg reflections. From the temperature dependence of both satellite peaks and Bragg reflection, we demonstrated that it has only one magnetic structure at a given temperature. Furthermore, unlike previous reports, we found that both ferromagnetic and modulated components of the Tb ion magnetic moments should be collinear to each other. Our data also show strong depolarisation effects that are most likely to arise from domain structure of ferromagnetic component. A critical field, which destroyers a modulated magnetic structure is found to be lower than a field value to saturate the ferromagnetic component, in which the intensity of Bragg ferromagnetic reflections reaches saturation.     

Electric-field and magnetic-field effects on electronic tunneling through magnetic-barrier nanostructures

We have theoretically investigated electric-field and magnetic-field effects on electronic transport properties in nanostructures consisting of realistic magnetic barriers created by lithographic patterning of ferromagnetic or superconducting films. The results indicate that the characteristics of transmission resonance are determined not only by the magnetic configuration and the incident wave vector but also strongly by the applied electric and magnetic fields. It is shown that transmission resonance shifts towards the low-energy region by applying the electric field, and that with increasing the electric field transmission resonance is suppressed for the entire incident wave vector in the magnetic nanostructures with antisymmetric magnetic profile, while for the magnetic nanostructures with symmetric magnetic profile transmission resonance is enhanced for certain incident wave vector. It is also shown that both transmission and conductance shift towards high-energy direction and are greatly suppressed with the increase of the external magnetic field.Received: 20 May 2003, Published online: 11 August 2003PACS: 73.40.Gk Tunneling - 73.23.-b Electronic transport in mesoscopic system - 75.70.Cn Interfacial magnetic properties (multilayers, superlattices)     

Pinning of domain walls in two-layer ferromagnetic nanowire with scattering fields of nanoparticles

The results of a micromagnetic simulation of the pinning-depinning processes of a domain wall (DW) in a rectangular ferromagnetic nanowire (NW) consisting of two magnetic layers with scattering fields of two rectangular two-layer nanoparticles (NPs) located on NW opposite sides and oriented perpendicular to its axis are presented. The features of magnetization reversal of this system in the external magnetic field are studied depending on direction of the magnetic moments of the nanoparticle layers. The value of the depinning field in such a system depends essentially on mutual orientation of NP magnetic moments and NW magnetization. The possibility to realize a magnetic logic cell performing the “conjunction” operation of ternary logic is discussed.