Nonlinear dynamic structure rearrangement of vortexlike domain walls in magnetic films with in-plane anisotropy

The nonlinear dynamic behavior of vortexlike domain walls in magnetic uniaxial films having an in-plane anisotropy was investigated within a rigorous micromagnetic approach in the framework of a two-dimensional magnetization distribution by numerically solving the Landau–Lifshitz equations (with the Gilbert damping parameter) with allowance for all the main interactions, including the dipole–dipole one. The studies were carried out on magnetic soft films with an anisotropy axis lying in their plane in a dc magnetic field parallel to an easy axis and a pulsed magnetic field normal to it. New possibilities for controlling the nonlinear dynamic rearrangement of the internal structure of domain walls and their velocities in fields both above and below the critical field are established. The wall motion in the field above the critical one is nonstationary.     

Bloch domain walls in type II optical parametric oscillators

Evidence of Bloch domain walls in nonlinear optical systems is given. These walls are found in the transverse fields of optical parametric oscillators when the polarization degree of freedom, the cavity birefringence, and (or) dichroism are taken into account. These domain walls arise spontaneously and exhibit defects where Bloch walls of different chirality join together. Two dynamic regimes are found:In the first one the vector field approaches a final homogeneous state, and in the other the walls are continually generated and annihilated. This dynamic behavior is caused by the fact that walls of opposite chirality move spontaneously with opposite velocity.     

Kinetics of magnetization reversal in a thin La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganite film

The kinetics of magnetization reversal of a thin LSMO film has been studied for the first time. It is shown that the magnetic domain structure critically depends on the conditions of structure formation. In the demagnetized state (after zero-field cooling from T _c ), a maze-like domain microstructure with perpendicular magnetization is formed in the film. However, after field cooling and/or saturating magnetization by a field of arbitrary orientation, the [110] direction of spontaneous magnetization in the film plane is stabilized; this pattern corresponds to macrodomains with in-plane magnetization. Further film magnetization reversal (both quasi-static and pulsed) from this state is implemented via nucleation and motion of 180° “head-to-head” domain walls. Upon pulse magnetization reversal, the walls “jump” at a distance proportional to the applied field strength and then undergo thermally activated drift. All dynamic characterisitcs critically depend on the temperature when the latter varies around the room temperature.     

磁性应力监测中力磁耦合特征及关键影响因素分析

针对磁性应力监测研究中力磁耦合关系有多种变化趋势且一直没有合理理论解释这一问题, 从磁性材料的微观磁畴运动出发, 根据磁致磁化过程, 详细分析了两种不同磁化状态下力致磁化的变化规律, 揭示出力磁耦合关系的变化特征, 进而提出磁化状态是导致力磁耦合关系差异的本质因素; 对工程上实际缆索用镀锌钢丝在不同磁化状态下进行了力磁耦合试验, 结果与理论一致: 力磁耦合关系随磁化状态的不同而不同, 本质上有效地解释了已有研究中力磁耦合关系的矛盾多样性. 关键词： 应力监测 磁化状态 力磁耦合 磁畴运动     

矩形磁性纳米点动力学反磁化过程的微磁学研究

采用微磁学模拟方法研究了初始态为C形磁结构的矩形CoFe纳米点在方波脉冲场作用下的动力学反磁化过程.研究发现,随着脉冲场强的增强,磁体的反磁化模式发生了改变.当场强较弱时反磁化过程通过畴壁移动-单涡旋的形成和移动来完成;当场强较大时反磁化过程模式转变为畴壁移动-双涡旋的形成与移动;在更强的场强下反磁化过程通过畴壁的移动-多涡旋的形成与湮没来实现.由于反磁化模式随场强的变化而改变,反磁化时间随场强的增大出现振荡变化现象. 关键词： 动力学反磁化过程 反磁化时间 微磁学模拟     

Dynamics of domain walls in magnetically multiaxial films in the thickness range corresponding to the Bloch-Néel transformations of the structure

The numerical minimization of the total energy functional and the solution of the nonlinear Landau-Lifshitz equation have been performed exactly taking into account the fundamental (including dipole-dipole) interactions in terms of the two-dimensional magnetization distribution. The equilibrium structure, energy, mobility, and scenario of the dynamic transformation of the domain walls (in their non- steady-state motion) have been determined as a function of the film thickness b and external magnetic field H for two different ((010) and (110)) orientations of the surfaces of magnetically triaxial films. The range of film thicknesses, including the thickness b = b _N, for which the Néel domain walls can be transformed into the Bloch domain walls, has been investigated. The phenomena of anisotropy of the domain-wall energy, the domain-wall mobility, and the period of dynamic transformations of the domain walls have been analyzed as a function of the film thickness b and external magnetic field H. The range of film thicknesses has been determined, in which the non-steady-state motion of the Néel domain walls is accompanied by the creation and annihilation of vortex-like structures despite the one-dimensional character of the magnetization distribution in these walls.     

Effect of a pulsed magnetic field on the nonlinear dynamics of vortexlike domain walls in magnetic films

The Landau-Lifshitz equation is numerically solved to study the nonlinear dynamic behavior of domain walls with the 2D vortexlike magnetization distribution in magnetically uniaxial films that have in-plane anisotropy and are exposed to a pulsed magnetic field. It is shown that a pulsed magnetic field H _p may induce transitions between various steady wall motions that differ in magnetization distribution. Solitary rectangular pulses, as well as a regular train of rectangular pulses, may be used to control the period of nonlinear dynamic transformations of the wall internal structure and the related period of variation of the wall velocity.     

Magnetization dynamics under nonlinear ferromagnetic resonance conditions in a (111) film

The equation of motion of magnetization of a garnet ferrite (111) film is solved numerically. It is shown that different regimes of the resonance precession of magnetization are realized depending on the bias and microwave fields; the regimes differ in amplitude and in the shape of their magnetization precession trajectories. In addition, bifurcations occur which lead to an abrupt change in amplitude, to bistability, and to precession with a period that is a multiple of the microwave-field period.     

Magnetization switching modes in nanopillar spin valve under the external field

The current-induced magnetic switching is studied in Co/Cu/Co nanopillar with an in-plane magnetization traversed under the perpendicular-to-plane external field.Magnetization switching is found to take place when the current density exceeds a threshold.By analyzing precessional trajectories,evolutions of domain walls and magnetization switching times under the perpendicular magnetic field,there are two different magnetization switching modes:nucleation and domain wall motion reversal;uniform magnetization ...     

Regular and chaotic precession of magnetization in magnetic films with a stripe domain structure

Based on a numerical solution of the equations of motion found over a wide range of frequencies of an alternating magnetic field, the nonlinear precession dynamics of magnetization are studied in thin-film structures of the (100) type with a stripe domain structure in a perpendicular bias field. The conditions are determined under which high-amplitude regular and chaotic dynamic regimes occur. Bifurcational variations in the precession of coupled magnetic moments and dynamic-bistability states are detected. The specific features of the spectrum of Lyapunov exponents and of time analogs of Poincaré cross sections of trajectories in chaotic regimes are considered.     

Effect of the electron energy on the entry blockage interval during electron scattering by a magnetic film with a strip domain structure

The trajectory of electrons in the stray field of a uniaxial magnetic film containing a strip domain structure (SDS) with a magnetization perpendicular to the sample surface are determined by numerical solution of the equations of motion. The interval of angles between the electron velocity projection onto the film surface and domain walls is determined, in which entry blockage takes place (electron scattering occurs without collisions with the sample surface). It is found that an increase in the electron energy leads to a decrease in this angular interval. The results are analyzed for multiple interactions of an electron moving in the gradient field of an SDS with attracting and repelling domain walls.     

Dynamics of domain walls in ultrathin magnetic films

The domain walls in ultrathin ferromagnetic films with uniaxial magnetic anisotropy are investigated theoretically. It is shown that taking account of the magnetodipole and magnetoelastic interactions leads to the appearance of an effective anisotropy with respect to the direction of the normal to the plane of the wall. The existence of a new type of domain walls—“corner” walls, at which the magnetization vector is rotated in the plane making a certain angle, which depends on the film parameters, with the plane of the domain wall and the static and dynamic properties of these walls are investigated. The dependence of the limiting velocity of the domain walls on the film thickness is found. Zh. éksp. Teor. Fiz. 112, 1476–1489 (October 1997)     

T=0 phase diagram and nature of domains in ultrathin ferromagnetic films with perpendicular anisotropy

We present the complete zero temperature phase diagram of a model for ultrathin films with perpendicular anisotropy. The whole parameter space of relevant coupling constants is studied in first order anisotropy approximation. Because the ground state is known to be formed by perpendicular stripes separated by Bloch walls, a standard variational approach is used, complemented with specially designed Monte Carlo simulations. We can distinguish four regimes according to the different nature of striped domains: a high anisotropy Ising regime with sharp domain walls, a saturated stripe regime with thicker walls inside which an in-plane component of the magnetization develops, a narrow canted-like regime, characterized by a sinusoidal variation of both the in-plane and the out of plane magnetization components, which upon further decrease of the anisotropy leads to an in-plane ferromagnetic state via a spin reorientation transition (SRT). The nature of domains and walls are described in some detail together with the variation of domain width with anisotropy, for any value of exchange and dipolar interactions. Our results, although strictly valid at T=0, can be valuable for interpreting data on the evolution of domain width at finite temperature, a still largely open problem.     

Remanence due to wall magnetization and counterintuitive magnetometry data in 200-nm films of Ni

200-nm-thick Ni films in an epitaxial Cu/Ni/Cu/Si(001) structure are expected to have an in-plane effective magnetic anisotropy. However, the in-plane remanence is only 42%, and magnetic force microscopy domain images suggest perpendicular magnetization. Quantitative magnetic force microscopy analysis can resolve the inconsistencies and show that (i) the films have perpendicular domains capped by closure domains with magnetization canted at 51 degrees from the film normal, (ii) the magnetization in the Bloch domain walls between the perpendicular domains accounts for the low in-plane remanence, and (iii) the perpendicular magnetization process requires a short-range domain wall motion prior to wall-magnetization rotation and is nonhysteretic, whereas the in-plane magnetization requires long-range motion before domain-magnetization rotation and is hysteretic.     

Asymmetry in the dynamics of domain walls in thin exchange-coupled ferromagnetic films

The kinetics of remagnetization after magnetic field switching was studied by the magneto-optic visualization technique in the bilayer hybrid structure composed of exchange-coupled FeNi and FeMn films. It was observed that not only the static but the dynamic characteristics of remagnetization as well depend on the polarity of a field applied along the direction of easy magnetization. The rate of the process was found to be exponentially dependent on the field strength in both directions, but the rate varied by factors of 10 upon inversion of the field. It was shown that this difference is the consequence of variation of both the time of domains nucleation and the velocity of domain walls motion.     

Nonlinear dynamic magnetization regimes in (100) ferrite-garnet films

To study the nonlinear dynamics of a uniformly precessing magnetization in perpendicularly magnetized (100) ferrite-garnet films, equations of motion are numerically solved over a wide ac field frequency range. Bifurcation changes in the magnetization precession and the states of dynamic bistability have been detected. The conditions of high-amplitude regular and stochastic dynamic regimes are revealed, and the possibilities of controlling these precession regimes by applied magnetic fields are shown.     

Magnetization distribution in submicrometer iron-nickel antidot arrays patterned by a focused ion beam

Antidots of size 0.5 μm are prepared by patterning iron-nickel films with a focused ion beam. The magnetization distribution in antidot arrays is examined with Lorentz transmission electron microscopy. It is shown that one side of the array makes an angle of about 20° with the easy magnetic axis of the film. Magnetization reversal in the direction close to the easy magnetic axis starts with domain nucleation at the antidot edges that are perpendicular to the applied field and adjacent to the unpatterned region of the film, and propagates as the domain walls move. Magnetization reversal in the direction close to the hard magnetic axis starts with magnetization rotation outside the patterned region at the antidot edges and propagates as the domain walls execute a complicated motion. It is demonstrated that some areas between the edges of adjacent antidots can carry information bits. Results obtained are explained in terms of competition between the demagnetizing energy, energy of internal anisotropy, and misorientation effect. The feasibility of such structures as high-density storage elements is discussed.     

Mechanism of the motion of domain walls with spin wave radiation in uniaxial magnetic films

It is shown, using the example of epitaxial ferrite—garnet films with (111) orientation whose dimensionless damping parameter was varied over a wide range, that for uniaxial magnetic films the mechanism of the motion of the domain walls is universal and includes local rotation of the magnetization ahead of the moving domain wall. The threshold field for the transition to this mechanism of motion of the domain walls is proportional to the uniaxial magnetic anisotropy field. On the curve of the dependence of the domain wall velocity on the acting magnetic field, this mechanism corresponds to the section with increased differential mobility of the domain walls. Magnitooptoelktronika Joint Self-Financing Laboratory, N. R. Ogarev Institute of General Physics, Russian Academy of Sciences, Mordovian State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 9–14, July, 1997.     

Competition between domain walls and the reverse magnetization in the magnetic relaxation of a Pt/Co/Ir/Co/Pt spin switcher

A multilayer Pt/Co/Ir/Co/Pt/GaAs heterostructures demonstrates a long term (to several hours) magnetic relaxation between two stable states of the magnetization of the system. The magnetization reversal of the heterostructure layers occurs both due to the formation of nuclei of the reverse magnetization domains and as a result of their further growth by means of motion of domain walls. The competition between two these processes provides a nonexponential character of the magnetic relaxation. At 300 K, the contributions of these processes to the relaxation are commensurable, while, at temperatures lower than 200 K, the contribution of the nucleation is suppressed and the magnetic relaxation occurs as a result of motion of the domain walls.     

Nonlinear precession dynamics of magnetization in (111) garnet ferrite films

The nonlinear dynamics of magnetization precession in perpendicular-magnetized (111) garnet ferrite films is studied by numerically solving the equations of motion of magnetization. Bifurcational changes in the magnetization precession and dynamic-bistability states are detected. The conditions are found under which both regular and stochastic large-amplitude dynamic regimes arise.