AC-current-induced magnetization switching in amorphous microwires

We studied the influence of AC current flowing through microwires, on magnetization dynamics. We used a previously developed Sixtus-Tonks modified setup to evaluate the domain wall (DW) velocity within the microwire. However, instead of a magnetizing solenoid, we used a current flowing through the microwire. We observed that the AC current flowing through the annealed Co-rich microwire leads to remagnetization by fast domain wall propagation. The estimated DW velocity was approximately 4.5 km/s, which is similar to and even higher than that reported for the magnetic-field-driven domain wall propagation in Fe- and Co-rich microwires. We measured the DW velocity under tensile stress, and found that the DW velocity decreases under applied stress. An observed DW propagation induced by the current flowing through the microwire is explained considering the influence of an Oersted magnetic field on the outer domain shell. This field has a circular easy magnetization direction and magnetostatic interaction between the outer circumferentially magnetized shell and the inner axially magnetized core.     

Domain wall motion in perpendicular anisotropy nanowires with edge roughness

We study field-driven domain wall (DW) motion in nanowires with perpendicular magnetic anisotropy using finite element micromagnetic simulations. Edge roughness is introduced by deforming the finite element mesh, and we vary the correlation length and magnitude of the roughness deformation separately. We observe the Walker breakdown both with and without roughness, with steady DW motion for applied fields below the critical Walker field H(c), and oscillatory motion for larger fields. The value of H(c) is not altered in the presence of roughness. The edge roughness introduces a depinning field. During the transient process of depinning, from the initial configuration to steady DW motion, the DW velocity is significantly reduced in comparison to that for a wire without roughness. The asymptotic DW velocity, on the other hand, is virtually unaffected by the roughness, even though the magnetization reacts to the edge distortions during the entire course of motion, both above and below the Walker breakdown. A moving DW can become pinned again at some later point ('dynamic pinning'). Dynamic pinning is a stochastic process and is observed both for small fields below H(c) and for fields of any strength above H(c). In the latter case, where the DW shows oscillatory motion and the magnetization in the DW rotates in the film plane, pinning can only occur at positions where the DW reverses direction and the instantaneous velocity is zero, i.e., at the beginning or in the middle of a positional oscillation cycle. In our simulations pinning was only observed at the beginnings of cycles, where the magnetization is pointing along the wire. The depinning field depends linearly on the magnitude of the edge roughness. The strongest pinning fields are observed for roughness correlation lengths that match the domain wall width.     

Role of anisotropy on the domain wall properties of ferromagnetic nanotubes

In this paper we investigate the role of magneto-crystalline anisotropy on the domain wall (DW) properties of tubular magnetic nanostructures. Based on a theoretical model and micromagnetic simulations, we show that either cubic or uniaxial magneto-crystalline anisotropies have some influence on the domain wall properties (wall size, propagation velocity and energy barrier) and then on the overall magnetization reversal mechanism. Besides the characterization of the transverse and vortex domain wall sizes for different anisotropies, we predict an anisotropy dependent transition between the occurrence of transverse and vortex domain walls in tubular nanowires. We also discuss the dynamics of the vortex DW propagation gradually increasing the uniaxial anisotropy constant and we found that the average velocity is considerably reduced. Our results show that different anisotropies can be considered in real samples in order to manipulate the domain wall behavior and the magnetization reversal process.     

Current induced domain wall motion in GaMnAs close to the Curie temperature

Domain wall dynamics produced by spin transfer torques is investigated in (Ga, Mn)As ferromagnetic semiconducting tracks with perpendicular anisotropy, close to the Curie temperature. The domain wall velocities are found to follow a linear flow regime which only slightly varies with temperature. Using the D?ring inequality, boundaries of the spin polarization of the current are deduced. A comparison with the predictions of the mean field k·p theory leads to an estimation of the carrier density whose value is compatible with results published in the literature. The spin polarization of the current and the magnetization of the magnetic atoms present similar temperature variations. This leads to a weak temperature dependence of the spin drift velocity and thus of the domain wall velocity. A combined study of field- and current-driven motion and deformation of magnetic domains reveals a motion of domain walls in the steady state regime without transition to the precessional regime. The ratio between the non-adiabatic torque β and the Gilbert damping factor α is shown to remain close to unity.     

铁磁/非磁金属异质结中的拓扑霍尔效应

在铁磁/非磁金属异质结中,界面处的Dzyaloshinskii-Moriya相互作用会诱导诸如磁性斯格明子等手性磁畴壁结构的形成.当巡游电子通过手性磁畴壁结构时,会获得一个贝里相位,而相应的贝里曲率则等效于一个外磁场,它将诱导额外的霍尔效应,即拓扑霍尔效应.拓扑霍尔效应是当前磁性斯格明子和自旋电子学研究领域的热点之一.本文由实空间贝里相位出发,简要介绍了拓扑霍尔效应的物理机制;然后着重讨论了铁磁/非磁金属异质结中的拓扑霍尔效应,包括磁性多层膜中和MnGa/重金属双层膜中的拓扑霍尔效应.这两种结构都可以通过改变材料的厚度、种类、生长方式等调控界面Dzyaloshinskii-Moriya相互作用,从而有效地调控磁性斯格明子和拓扑霍尔效应.     

The Analysis of Large Barkhausen Effect in the FeSiB Amorphous Wire

Amorphous FeSiB wires with positive magnetostriction are very perspective soft magnetic materials for many applications, e.g. torque, field or current sensors, pulse generators and highly sensitive magnetometers. The appearance of the Large Barkhausen Effect (LBE) during slow magnetization of FeSiB wires is described by means of the core-shell model assuming a residual radial tensile stresses in the as-cast state. In this work, the LBE during magnetization reversal of Fe_77.5Si_7.5B₁₅ amorphous wire in the as-cast state was analysed. We have studied the kinetics of the reverse domain in the core region of the wire by means of Sixtus-Tonks method of two small pick-up coils placed in an asymmetric way with respect to the ends of the wire. We estimated the velocity of the reverse domain wall and the core region volume of the wire. It was found that the residual radial tensile stress distribution of the shell region strongly influences the magnetization reversal in the FeSiB wire.     

Experimental characterization of domain walls dynamics in a photorefractive oscillator

We report on the experimental characterization of domain wall dynamics in a photorefractive resonator in a degenerate four-wave mixing configuration. We show how the non-flat profile of the emitted field affects the velocity of domain walls as well as the variations of intensity and phase gradient during their motion. We find a clear correlation between these two last quantities that allows the experimental determination of the chirality that governs the domain walls’ dynamics. PACS 42.65.Sf; 47.54.+r; 42.65.Hw     

Dynamic vortex-antivortex interaction in a single cross-tie wall

A fascinating property of micromagnetism comes from the possibility to control the domain and vortex configuration through the sample shape and size. For instance, in a rectangular platelet a configuration containing a stable combination of vortices and an antivortex can be created. Such a single cross-tie wall can be understood as being a coupled micromagnetic system with three static solitons. Here we report on its magnetization dynamics including the vortex-antivortex interactions. The spectrum of eigenmodes is investigated as well as the effect of different vortex core orientations. We show that the vortex dynamics can be used to identify the core configuration, which is not directly accessible to x-ray microscopy because of its limited spatial resolution.     

Dynamics of vortexlike domain walls in triaxial magnetic films with the Goss orientation of the surface

The stationary dynamics of vortexlike domain walls in films with three magnetic axes and Goss orientation of the surface is studied for the first time with a micromagnetic method that exactly takes into account all basic types of interaction (including dipole-dipole interaction). Consideration is carried out using a 2D model of magnetization distribution by numerically solving Landau and Lifshitz’s nonlinear equations with attenuation in the Gilbert form. Dynamic configurations of domain walls are established, and the dependences of the domain wall velocity on an applied magnetic field, damping parameter, and magnetic film thickness are found.     

微磁动力学的新进展

本文介绍微磁动力学领域的一个最新进展,我们的研究发现在磁场驱动下且保持畴结构不变地沿着纳米磁线运动的磁畴壁,其运动源于能量耗散,磁畴壁运动速度正比于能量耗散率。与此同时,我们根据能量守恒原则,给出了磁畴壁速度的一个合理定义,该定义适用于任意的磁畴壁结构。在此定义下,即使磁畴壁没有做刚性运动,我们也能得到磁畴壁运动的瞬时速度和平均速度。我们的结果不仅能重复低磁场下的沃克（Walker）解,还能反映出当磁场高于沃克极限（Walker limit）时速度{磁场的依赖关系,该结果跟数值模拟和实验数据都符合得很好。我们根据微磁动力学研究的这一新进展,最终澄清了一个事实即“磁畴壁质量”这个概念是错误的。     

Thermally assisted current-driven domain-wall motion

Starting from the stochastic Landau-Lifschitz-Gilbert equation, we derive Langevin equations that describe the nonzero-temperature dynamics of a rigid domain wall. We derive an expression for the average drift velocity of the domain wall r(dw) as a function of the applied current, and find qualitative agreement with recent magnetic semiconductor experiments. Our model implies that at any nonzero-temperature r(dw) initially varies linearly with current, even in the absence of nonadiabatic spin torques.     

Phenomenology of current-induced dynamics in antiferromagnets

We derive a phenomenological theory of current-induced staggered magnetization dynamics in antiferromagnets. The theory captures the reactive and dissipative current-induced torques and the conventional effects of magnetic fields and damping. A Walker ansatz describes the dc current-induced domain-wall motion when there is no dissipation. If magnetic damping and dissipative torques are included, the Walker ansatz remains robust when the domain wall moves slowly. As in ferromagnets, the domain-wall velocity is proportional to the ratio between the dissipative torque and the magnetization damping. In addition, a current-driven antiferromagnetic domain wall acquires a net magnetic moment.     

Minimization of Ohmic losses for domain wall motion in a ferromagnetic nanowire

We study current-induced domain-wall motion in a narrow ferromagnetic wire. We propose a way to move domain walls with a resonant time-dependent current which dramatically decreases the Ohmic losses in the wire and allows driving of the domain wall with higher speed without burning the wire. For any domain-wall velocity we find the time dependence of the current needed to minimize the Ohmic losses. Below a critical domain-wall velocity specified by the parameters of the wire the minimal Ohmic losses are achieved by dc current. Furthermore, we identify the wire parameters for which the losses reduction from its dc value is the most dramatic.     

The interaction of transverse domain walls

The interaction between transverse domain walls is calculated analytically using a multipole expansion up to third order. Starting from an analytical expression for the magnetization in the wall, the monopole, dipole, and quadrupole moments are derived and their impact on the interaction is investigated using the surface and volume charges. The surface charges are important for the dipole moment while the volume charges constitute the monopole and quadrupole moments. For domain walls that are situated in different wires it is found that there is a strong deviation from the interaction of two monopoles. This deviation is caused by the interaction of the monopole of the wall in the first wire with the dipole of the wall in the second wire and vice versa. The dipole-dipole and the quadrupole-monopole interactions are found to be also of considerable size and non-negligible. A comparison with micromagnetic simulations shows a good agreement.     

Fast domain wall dynamics in amorphous glass-coated microwires

Here we report on the domain wall dynamics in amorphous glass-coated FeCuNbSiB microwires measured in the temperature range from 77 up to 400 K. At low temperatures below 200 K, the domain wall velocity is proportional to the applied magnetic field. At temperatures above 200 K, two regions have been found: one with low domain wall mobility at low fields and another one with high domain wall mobility at high fields. The different regions of the domain wall dynamics are treated in terms of the change of the domain wall configuration from transversal to vortex one. Moreover, non-linear regime is shown at low fields at the temperature 373 K as a result of the domain wall interaction with the local defects.     

Resonance interaction of an edge-dislocation wall with a traveling sound wave

The dynamics of an infinite edge-dislocation wall and fragments of this wall in their interaction with a monochromatic sound wave with a nonzero wave vector has been investigated taking into account the mass of dislocations. It has been shown that the drift velocities of the wall fragments significantly increase when the sound wave frequency approaches the natural frequencies of small-amplitude vibrations, including zero frequency. For the infinite wall, a similar increase in the drift velocity is observed for low frequencies and for frequencies close to half the maximum frequency of small-amplitude vibrations of the edge-dislocation wall. The resonance increase in the drift velocity at low frequencies of the sound wave can be important for practical applications.     

Giant current-driven domain wall mobility in (Ga,Mn)As

We study theoretically hole current-driven domain wall dynamics in (Ga,Mn)As. We show that the spin-orbit coupling causes significant hole reflection at the domain wall, even in the adiabatic limit when the wall is much thicker than the Fermi wavelength, resulting in spin accumulation and mistracking between current-carrying spins and the domain wall magnetization. This increases the out-of-plane nonadiabatic spin-transfer torque and consequently the current-driven domain wall mobility by 3 to 4 orders of magnitude. Trends and magnitude of the calculated domain wall current mobilities agree with experimental findings.     

Critical Dzyaloshinskii–Moriya interaction energy density for the skyrmion states formation in ultrathin ferromagnetic layer

We investigate the condition for the skyrmion state formation in ultrathin ferromagnetic layers with the Dzyaloshinskii–Moriya interaction (DMI). By using micromagnetic simulations with DMI term, we found skyrmion state is a ground state instead of a collinear single domain when DMI is larger than a critical value. When the skyrmion arrays are formed, a specific length scale of skyrmion is governed the spin configurations. We found that the critical DMI energy density for the skyrmion state is related with not only crystalline anisotropy energy, but also the dipole–dipole interaction.     

Compressed exponential form for disordered domain wall motion in ultra-thin Au/Co/Au ferromagnetic films

Magnetization reversal in ultra-thin Au/Co/Au films deposited on single crystal silicon (1 0 0) was investigated using Kerr microscopy. In the considered ultra-thin Co films, with a thickness between 0.7 and 1 nm, the coercivity and magnetic anisotropy decrease with decrease in cobalt layer thickness and the magnetization reversal dynamics is dominated by disordered domain wall motion. An analysis of the observed magnetization reversal dynamics is proposed, starting from the Fatuzzo-Labrune model. We show that the relaxation curves of these samples are well described by a function obtained by a technical transformation of Fatuzzo-Labrune model in the regime dominated by domain wall motion.     

Nonlinear domain-wall dynamics in a system of two magnetic layers

The nonlinear dynamics of the magnetization in a spin-valve structure is investigated. Equations describing the dynamics of the magnetization in such a structure are obtained. The stability of the solution corresponding to a motionless flat domain wall is investigated. The nonlinear domain-wall dynamics are investigated in the approximation of a strong exchange interaction between the magnetic layers and in the approximation of a large magnetostatic energy. In the former case the nonlinear dynamical equations are shown to be similar to the equations describing the dynamics of the magnetization in a weak ferromagnet, and in the latter case they are similar to the equations of motion of a magnetic vortex (i.e., a vertical Bloch line) in a domain wall. Zh. éksp. Teor. Fiz. 116, 1365–1374 (October 1999)