磁畴壁手性和磁斯格明子的拓扑性表征及其调控

磁性斯格明子由于拓扑的保护性,具有很高的稳定性和较小的临界驱动电流,有望应用于未来的赛道存储器件中.而在中心对称体系,由于偶极作用的各向同性,磁泡的拓扑性和螺旋度都呈现出多样性的特征.其中非平庸的磁泡即等同于磁性斯格明子.我们通过近期实验结果,结合微磁学模拟的方法,发现在中心对称体系中磁斯格明子的拓扑性会受到体系垂直各向异性的调控.另外在加磁场的演变过程中,会很大程度上依赖于基态畴的畴壁特性.磁场的倾斜或者一定的面内各向异性也会改变磁斯格明子的形态.通过对材料的基态磁结构及磁各向异性的调节,辅助以面内分量的控制,可以对基态磁畴、进而对磁斯格明子的拓扑性实现调控.这对磁斯格明子在电流驱动存储器件中的应用具有重要意义.     

Magnetization processes in the narrow domain structures of amorphous ferromagnetic alloys

The magnetization processes within the narrow domain laminae of amorphous ferromagnetic alloys have been investigated by means of the magneto-optical Kerr effect. Changes of the domain width of the closure domains by a magnetic field applied perpendicular to the laminae have been determined for the alloys Fe₈₀B₂₀ and Fe₄₀Ni₄₀P₁₄B₆. These results are compared with theoretical calculations, assuming that wall displacements within the closure domains and rotations of the magnetization in the bulk domains take place simultaneously and a stray field free domain structure is developed. It turned out, that the closure domain structure on the surface of the sample vanishes at the same magnetic field where magnetic saturation is approached.     

Observation and theory of strong stripe domains in amorphous sputtered FeB films

Domain structures in thin sputtered amorphous FeB films are studied by means of the longitudinal Kerr effect. In addition to the irregular domain structure characteristic of soft magnetic materials, we observe in certain regions a fine equilibrium domain structure with periodicity of a few micrometers. The Kerr contrast indicates that the magnetization at the surface of the film lies partially along the stripe direction. These characteristics and the behavior in applied fields suggests that the domains are similar to type II “strong stripe domains” observed earlier in permalloy films. Extending an earlier theory by Hara, we use a stray-field-free model with tilted orthorhombic anisotropy to show that there are at least two qualitatively different strong stripe structures: type IIa with surface magnetization perpendicular to the stripes and type IIb with surface magnetization at least partially parallel to the stripes. Type IIb is favored when K_p/K₀<cos 2θ ₀ where K₀ is the anisotropy component with axis tilted by θ₀ out of the film plane, and K_p is an in-plane anisotropy perpendicular to K₀. Strong stripes in amorphous FeB appear to be type IIb while those in permalloy are usually type IIa.     

On magnetic remanence

The magnetic retentivity of many materials is about half of the magnetization at saturation, a fact accounted for by simple domain theory. In some materials, however, the retentivity is only a small fraction of saturation, sometimes less than 10 per cent. The explanation of this fact is discussed. It is suggested that in materials with almost zero magnetic anisotropy the Bloch walls between domains increase in thickness until they envelope the whole specimen and the domain structure disappears.     

Magnetization reversal in magnetic nanostripes via Bloch wall formation

We present results of micromagnetic simulations of the magnetization reversal in permalloy nanostripes with 5-10 nm thickness and 200-500 nm width under a longitudinal field of 0.4-16 kA m(-1). The data show four distinct field regions: the well-known regions of uniform and oscillating domain wall movement as well as a process with multiple vortices, and finally a new process including Bloch walls and the generation of vortex-antivortex pairs in the inner part of the stripe rather than at the edges. We investigate this process in detail and derive a criterion for the formation of Bloch walls.     

New method to determine the domain wall energy in thin uniaxial films with perpendicular easy axis and large saturation magnetization

The surface energy density, γ, of domain walls in thin uniaxial films with perpendicular easy axis is usually found either by measurements of the domain pattern period of the stripe domain pattern or by determination of the bubble collapse field and diameter. However, when the saturation magnetization is large, the widths of stripe domains and bubbles become smaller than the minimum optical resolution, and optical observation becomes impossible. The method proposed in this paper is based on measurements of the susceptibility S = d (M/M_s)/d(H_a/4πM_s) of the stripe pattern at M = 0. Optical observations are avoided. The only additional important quantities are the saturation magnetization M_s and the film thickness. The method has been successfully applied to MnBi films (M_s = 625 G). The determination of γ in this material yields γ = (15 ± 1) erg/cm².     

Prediction of a domain-drag effect in uniaxial,non-compensated,ferromagnetic metals

An electric current is predicted to exert a dragging force on ferromagnetic domain walls. The effect arises from the non-uniform current distribution, recently predicted and observed at 4·2 K in the neighborhood of a wall in metals (Co, Ni) with large Hall angle. When the average current density j? exceeds a certain “coercive” value j?_c, the domains are set into motion in the same direction as the charge carriers. When j?j?_c, the wall speed approaches the drift speed of the carriers. Ohm's law fails when the walls move. The limits of very small and of very large Hall angle are both considered, using the single-wall model of Williams, Shockley and Kittel, and also the stratified-medium model of Herring. We assume B = M_s inside each domain; this is appropriate if the sample thickness t in the direction of the saturation magnetization M_s, and the wall spacing a, obey t > a. Materials with large anisotropy field H_A (h.c.p. Co, Gd) are necessary if the field H of the current is not to remove all walls normal to the current. For similar reasons, the sample should be in the shape of a flat ribbon (or rod) normal to M_s, of thickness (or diameter) t obeying t < dH_A/H_c, where H_c is the coercive field and d the wall length in the direction normal to the current and to M_s.     

Stripe domain statics in garnet films

Magnetic garnet films grown epitaxially on nonmagnetic garnet substrates exhibit a growth or stress-induced uniaxial anisotropy in addition to the cubic magnetocrystalline anisotropy associated with their crystal symmetry. When the uniaxial anisotropy is dominant over the cubic, such films exhibit stripe or bubble domain structures; even a small cubic anisotropy component can have a decisive effect on the behavior of the domains in applied fields. We report an experimental study of the quadistatic behavior of domains in fields applied to a (111) film in the film plane along (11$\text{2}$) and ($\text{1}$10). The experimental results are interpreted by a new theory that gives good agreement with the observed behavior, and yields an accurate measurement of the cubic and uniaxial anisotropy constants.The main qualitative features of the results are: In a ($\text{1}$10) field, the walls are Neél walls perpendicular to the field. In a (11$\text{2}$) field the walls are Bloch walls parallel to the field, the domain magnetization in adjacent stripes is not symmetrical about the film plane, and adjacent stripes are not of equal width; the domain period first shrinks and then expands with increasing field; and even though the applied field has no component perpendicular to the film plane, the film develops a net perpendicular magnetic moment.     

Domain and wall structures in films with helical magnetization profile

We study soft magnetic bilayers having orthogonal, in-plane easy axes. The layers are thicker than the Bloch wall width linked to the anisotropy, so that a helical magnetization with a large angle exists across the sample thickness. The magnetic domains structure has been investigated at both sample surfaces, using magneto-optical microscopy. The domain structure is found to be similar to that of double films with biquadratic coupling. Two kinds of domain walls are identified, namely with a 90° and 180° rotation of the average magnetization. The detailed structure and energy of these walls are studied by micromagnetic calculations.     

Numerical simulation of wall dynamics in (111)-oriented garnet films in the presence of an in-plane magnetic field

The domain wall motion in the presence of an in-plane magnetic field H_y perpendicular to the wall is simulated using a fall implicit numerical scheme. Calculations are performed for the drive fields 0 Oe<H_z<15 Oe and in-plane fields -210 Oe?H_y?210 Oe. The relation between the average wall velocity $\text{v}$ and the drive field H_z is discussed considering the wall structure. It was found that an in-plane field increases the peak velocity of the wall and extends the range of the drive fields, where the linear mobility relation is valid. A dynamical Bloch line stacking was found for sufficiently large drives. The influence of an in-plane field on the angular span of horizontal Bloch lines is discussed also. In particular the occurrence of 2π-horizontal Bloch lines is described. Numerical results obtained with a full implicit method are compared with the experimental observations of bubble motion and good agreement is found for |H_y|≤100 Oe.     

Current induced domain wall motion in nanostripes with perpendicular magnetic anisotropy

We report micromagnetic modeling results of current induced domain wall (DW) motion in magnetic devices with perpendicular magnetic anisotropy by solving the Landau-Lifschitz-Gilbert equation including adiabatic and non-adiabatic terms. A nanostripe model system with dimensions of 500 nm (L)×25 nm (W)×5 nm (H) was selected for calculating the DW motion and its width, as a function of various parameters such as non-adiabatic contribution, anisotropy constant (K_u), saturation magnetization (M_s), and temperature (T). The DW velocity was found to increase when the values of K_u and T were increased and the M_s value decreased. In addition, a reduction of the domain wall width could be achieved by increasing K_u and lowering M_s values regardless of the non-adiabatic constant value.     

Magnetic skyrmion dynamics in thin cylindrical dots

We calculate high‐frequency spin excitations of the skyrmion ground state cylindrical magnetic dots. The skyrmion is assumed to be stabilized at room temperature due to interplay of the isotropic and Dzyaloshinskii–Moriya exchange interactions, perpendicular magnetic anisotropy and magnetostatic interaction. The Skyrmion ground state is represented as combination of two radially symmetric bubble domains. To consider the Bloch‐ and Néel‐type magnetic skyrmion dynamics we apply an approximation of ultrathin domain wall between the circular domains and assume that the magnetic dot is thin enough (magnetization does not depend on the thickness coordinate). The eigenfunctions/eigenfrequencies of spin wave excitations over the skyrmion background are calculated as a function of the skyrmion radius. The developed approach allows predicting spin wave eigenfrequencies in the skyrmion ground state magnetic dots. Recent experiments on magnetic skyrmion dynamics are discussed. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Nonlinear reconfiguration of vortexlike domain walls in magnetically uniaxial films under the action of an external field normal to the easy magnetic axis

The effect of external magnetic field H normal to the anisotropy axis on the energy and configuration of vortexlike asymmetric magnetic walls in a magnetically uniaxial film with an easy magnetic axis parallel to its surface is studied. The investigation is based on minimizing the energy functional of the film with due regard to exchange energy, magnetic anisotropy energy, magnetostatic energy, and Zeeman energy. The range of H below the anisotropy field is found where the asymmetric Néel wall is stable, unlike the case H = 0, when the asymmetric Bloch wall is stable. It is shown that an asymmetric Bloch wall becomes absolutely unstable and reconfigures into an asymmetric Néel wall at some critical values of H = H _⊥. The dependences of critical field H _⊥ on the film thickness and saturation induction at different values of the anisotropy field are determined: field H _⊥ depends on the thickness nonlinearly and on the saturation induction nonmonotonically.     

3-D FEM analysis of thermal degradation in writing and reading characteristics of a perpendicular magnetic head

In order to achieve high-density recording, the detailed behavior of thermal degradation should be investigated. In this paper, the degradation of magnetization of high-density recording medium is examined using the 3-D finite element method (FEM) combined with the modeling of Stoner–Wohlfarth (SW) particles and Neel–Arrhenius switching probability. It is shown that the anisotropy field H_k suppressed the thermal degradation and the saturation magnetization M_s enhances it. The thermal degradation is also changed by the amplitude of magnetization.     

Influence of three-dimensional inhomogeneities of the magnetic parameters on the dynamics of vortex-like domain walls

The interaction of a vortex-like domain wall moving in an external magnetic field with a three-dimensional periodic chain of cubic volumes with high values of the saturation magnetization and magnetic anisotropy constant has been investigated theoretically. It has been found that the result of the interaction depends on the initial distance between the wall and the region of inhomogeneity of magnetic parameters at the moment of turning on the external magnetic field. The pinning of domain walls near the regions with high values of the saturation magnetization and magnetic anisotropy constant has been investigated, and the anisotropy of the corresponding depinning fields has been revealed. The method of investigation is the numerical micromagnetic simulation.     

Anomalous magnetization processes and non-symmetrical domain wall displacements in L10 FePt particulate films

Anomalous magnetization processes and non-symmetrical domain wall displacements in the minor loop of L1₀ FePt particulate films were investigated by magnetization measurements and in situ magnetic force microscopy. Magnetization (M) decreases dramatically on increasing the magnetic field to ∼3 kOe after which M becomes small and constant in the range of 5–20 kOe as observed in the successive measurement of minor loops. The domain wall displacement is non-symmetrical with respect to the field direction. The anomalous magnetization behavior was attributed to the non-symmetrical domain wall displacement and large magnetic field required for domain wall nucleation. Energy calculations from modeling suggest that non-symmetrical domain wall displacement is caused by the existence of metastable domains in which the domain edges are stuck to the particle boundaries.     

Spontaneous transformations of the magnetic structure of a film nanocontact

The magnetization distributions in a symmetric magnetic film nanocontact for oppositely magnetized ferromagnetic electrodes are analyzed based on numerically solving the Landau-Lifshitz and magnetostatic equations as a function of magnetic and geometrical factors. It is found that a symmetric magnetic configuration is unstable when the head-to-head domain wall dividing the regions with opposite orientations of magnetization is located at the center of the nanocontact. The instability arises when the uniaxial magnetic anisotropy constant reaches a certain critical value K _c below which it spontaneously leaves the center of the nanocontact. The transition from the symmetric state (wall at the center) to an asymmetric one can be continuous (second order) or discrete (first order), depending on the geometrical and physical parameters of the nanocontact (length to width ratio, anisotropy constant, and saturation magnetization). The phase diagram is constructed in terms of the variable’s nanocontact length vs. anisotropy constant. This diagram divides the symmetric and asymmetric magnetic configurations of the system. The occurrence of a tricritical point in the phase diagram is its characteristic feature.     

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.     

面内场和静态偏磁场作用下条畴和泡畴的稳定性

实验研究面内场H_in和静态偏磁场H_b作用下,(111)面磁泡膜内条畴的消失过程。保持H_b恒定,增加H_in,测量条畴消失场H_s^*和泡畴消失场H_k^*与面内场方向β的变化关系。计及立方磁晶各向异性的影响,建立H_in和H_b共同存在时的条畴稳定性理论。定性解释了实验的主要特点。导出黑、白条畴同时消失时的角度 β_n=1/3(2nπ±arc cos│3/(2^1/2)(M_sH_b)/K₁│)(n=0,±1,±2,…)与实验基本符合。 关键词：