Dynamics of sandwich domain structure in Co-based amorphous ribbons with helical magnetic anisotropy: Part I

The distribution of axes of easy magnetization close to a homogeneous distribution is revealed in each half-thickness of a ribbon after annealing it in a helical magnetic field. The transition from magnetic reversal of a ribbon by the displacement of two domain walls formed near a middle plane of a ribbon to magnetic reversal of a ribbon by displacement of two domain walls formed near to the main surfaces of a ribbon is found out during each half-period of a magnetic reversal.     

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.     

Detection of domain wall propagation in a mesoscopic wire

Extraordinary Hall effect was used to detect the propagation of a domain wall in magnetic devices patterned in sputter grown Pt/Co(1 nm)/Pt sandwiches with perpendicular easy magnetization axis. In such films, domain walls propagate as coherent 1D nano-object in a 2D medium with weak fluctuation energy density. In a patterned device, the competition between global wall energy and Zeeman energy strongly influences the wall propagation.     

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.     

Domain wall dynamics near the limit velocity in a cubic ferromagnet with induced magnetic anisotropy

A research has been conducted into the dynamics of the 180° domain wall in a cubic ferromagnet with induced magnetic anisotropy, this domain wall moving at a velocity close to the limit one. The Landau–Lifshits equation has been reduced to a modified double Sine–Gordon equation with the highest dispersion. A solution has been found which corresponds to the moving 180° domain wall. This paper has determined the dependencies of the velocity of the domain wall's stationary movement on the quality factor and on the ratio of the induced and the cubic magnetic anisotropy constants in slabs with the developed (0 0 1) and (0 1 1) surfaces.     

Manipulating the magnetoimpedance by dc bias current in amorphous microwire

We have studied the effect of the internal circumferential magnetic field H_B created by the dc bias current I_B on longitudinal and off-diagonal magnetoimpedance (MI) in amorphous microwire with helical anisotropy and experimentally demonstrated that by changing the dc current I_B it is possible to considerably change the MI dependencies. The bias current applied to such microwire transforms the symmetric and hysteretic MI curve to asymmetrical and anhysteretic. The minimum of longitudinal MI curve shifts from the zero-field point. Reversing the bias current causes reversal of the bias field direction and results in a mirroring of the MI curves. It is proposed to apply a cross-checking of two MI curves with I_B of different polarity for magnetic field sensing. In particular, this method allows to overcome the drawbacks usually associated with longitudinal MI—namely the impossibility to determine the direction of an external axial magnetic field H_E and the low sensitivity near the zero-field point. Moreover, the operating range of the longitudinal MI sensor, in contrast to the off-diagonal one, can be much extended as it exhibits a quite high sensitivity in the field range up to one order of magnitude higher than the anisotropy field.     

Magnetic domain wall dynamics in an inhomogeneous magnetic field

A new experimental method for the study of single magnetic domain wall dynamics in bistable microwires is presented. It raises new possibilities for experimenting with a single magnetic domain wall moving in an inhomogeneous magnetic field. Models for a wall with fixed length were confronted with experimental data obtained on amorphous glass-coated ferromagnetic Fe_77.5B₁₅Si_7.5${\mathrm{Fe}}_{77.5}{\mathrm{B}}_{15}{\mathrm{Si}}_{7.5}$ microwire. These models qualitatively describe the observed behavior. The accord between models and experiment increases as the field disturbance decreases due to its inhomogeneity. A better match between experimental and model curves can probably be obtained if the changes in the wall dimensions and wall mass are taken into account.     

Single domain wall dynamics in ferromagnetic lamination with variable conductivity

The influence of variable conductivity and thickness of two outer non-ferromagnetic layers on magnetization reversal of one central ferromagnetic layer is theoretically investigated. The model of a thin rigid 180°$180°$ domain wall moving transversely through the axially magnetized ferromagnetic layer is used to calculate induced eddy currents in lamination from which the domain wall mobility is determined. The effect of asymmetric distribution of eddy currents around moving domain wall results in acceleration of the wall near the edge of the lamination. The known domain wall mobility in ferromagnetic lamination can then be used to determine either the conductivity or the thickness of deposited outer non-ferromagnetic layers as proposed in discussion.     

Fast domain wall dynamics in amorphous and nanocrystalline magnetic microwires

We have studied the effect of thermal treatment on the domain wall dynamics of FeSiB and FeCoMoB microwires. It was shown that annealing in transversal magnetic field increases the domain wall mobility as well as the domain wall velocity. Annealing under the tensile stress hinders the appearance of the monodomain structure but application of tensile stress leads to the magnetic bistability having the domain wall mobility twice higher that in as-cast state. Further increase of the tensile stress reduces the domain wall mobility but the domain wall velocity increases as a result of the decrease of critical propagation field. Annealing of the FeCoMoB microwire by Joule heating leads to introduction of the circular anisotropy that favors the vortex domain wall. Such treatment increases the domain wall mobility as well as the maximum domain wall velocity.     

A model for torsion-stress effect on nonlinear magnetoimpedance in amorphous wires with negative magnetostriction

A model to describe the influence of torsional stress on nonlinear magnetoimpedance in amorphous wires with negative magnetostriction is proposed. The nonlinear voltage response is found in the framework of the low-frequency approximation taking into account the spatial distribution of the circular magnetic field and the magnetoelastic anisotropy induced by the torsional stress. It is demonstrated that the application of torsional stress results in an increase of the second harmonic amplitude in voltage due to a reinforcement of helical anisotropy in the wire. The second harmonic amplitude is analyzed as a function of external field, torsional stress and current amplitude. The ranges of torsional stress and current amplitude to achieve maximal field sensitivity of the second harmonic are found.     

Micromagnetic study of magnetic domain structure and magnetization reversal in amorphous wires with circular anisotropy

In this work we present a detailed numerical investigation on the magnetic domain formation and magnetization reversal mechanism in sub-millimeter amorphous wires with negative magnetostriction by means of micromagnetic calculations. The formation of circular magnetic domains surrounding a multidomain axially oriented central nucleus was observed for the micromagnetic model representing the amorphous wire. The magnetization reversal explained by micromagnetic computations for the M-H curve is described in terms of a combined nucleation-propagation−rotational mechanism after the saturated state. Results are interpreted in terms of the effective magnetic anisotropy.     

Domain walls confined in magnetic nanotubes with uniaxial anisotropy

We present calculations of the different domain wall structures confined in magnetic nanotubes, such as transverse wall, asymmetric vortex wall, branch fashion wall, and horse-saddle wall. The wall structures were calculated by micromagnetic simulations. The tube radii R=50 nm and 100 nm, and aspect ratios length/radius L/R≤15 were considered. The magnetic phase diagrams of the stability of different kinds of the domain walls were plotted as function of the tube aspect ratio L/R and the tube thickness (difference of the outer and inner tube radii).     

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.     

CoFeB/Pt多层膜的垂直磁各向异性研究

具有垂直磁各向异性的磁性纳米结构是自旋转移力矩器件的重要研究内容, 本文采用反常霍尔效应系统地研究了磁控溅射法制备的[CoFeB/Pt]_n多层膜的垂直磁各向异性. 当CoFeB的厚度小于0.6 nm时, 可以在[CoFeB/Pt]_n多层膜中观察到清晰的垂直磁各向异性, 其垂直磁各向异性强烈依赖于CoFeB和Pt层厚度及多层膜周期数. 当多层膜周期数n ≥ 5时, 出现零剩磁现象. 另外, [CoFeB/Pt]_n多层膜的矫顽力均小于2 kA·m^-1, 有望作为垂直自由层的重要侯选材料应用于垂直磁纳米结构中.     

Co/Ni多层膜垂直磁各向异性的研究

采用直流磁控溅射法在玻璃基片上制备了Pt底层的Co/Ni多层膜样品, 对影响样品垂直磁各向异性的各因素进行了调制, 通过样品的反常霍尔效应系统的研究了Co/Ni多层膜的垂直磁各向异性. 结果表明, 多层膜中各层的厚度及周期数对样品的反常霍尔效应和磁性有重要的影响. 通过对多层膜各个参数的调制优化, 最终获得了具有良好的垂直磁各向异性的Co/Ni多层膜最佳样品Pt(2.0)/Co(0.2)/Ni(0.4)/Co(0.2)/Pt(2.0), 经计算, 该样品的各向异性常数K_eff 达到了3.6×10⁵ J/m³, 说明样品具备良好的垂直磁各向异性. 最佳样品磁性层厚度仅为0.8 nm, 样品总厚度在5 nm以内, 可更为深入的研究其与元件的集成性.     

Domain dynamics and magneto-electronics in magnetic thin films and nanowires

Recent theoretical results are highlighted that illustrate some of the interesting phenomena associated with magnetic domain boundary walls. Two problems will be discussed: dynamics associated with domain wall propagation, and effects related to spin transport through domain walls. For the first problem, an example of wall interaction and motion through a random potential will be discussed with reference to the general problem of roughening transitions. Images of domain dynamics in thin films of ion irradiated Co reveal a de-roughening transition associated with long range magnetostatic interactions between pairs of domain walls. A scaling theory of this transition is described in which a curious type of dynamic hysteresis can occur. For the second problem, results from calculations of ballistic charge and spin transport through domain boundary walls are discussed in terms of an effective circuit model.     

Domain wall nucleation in epitaxial exchange-biased Fe/IrMn bilayers with highly misaligned anisotropies

The angular dependence of the magnetization reversal in epitaxial Fe/IrMn bilayers with collinear and non-collinear cubic and unidirectional anisotropies is investigated. Multistep loops with different magnetization reversal processes are observed for either positive or negative angles with respect to the Fe easy axis. The angular dependence of the switching fields displays the broken symmetry of the induced non-collinearity. The experimental results are reproduced with a generalized domain wall nucleation model that includes the induced anisotropy configuration and the peculiar asymmetric magnetic switching behavior. These results highlight the importance of the relative angle between anisotropies in epitaxial exchange bias systems with incoherent rotation reversal mechanism, opening a new pathway for tailoring the magnetic properties of such systems.     

Anomalous Hall effect and perpendicular magnetic anisotropy in Sm28Fe72 and Sm32Fe68 thin films

Sm₂₈Fe₇₂ and Sm₃₂Fe₆₈ films of 100 nm thickness were grown using DC magnetron sputter deposition and their structure, magnetization, electrical and Hall resistance characteristics were investigated. An increase in electrical resistivity from 4.75×10⁻⁶ to 5.62×10⁻⁶ Ω m and from 2.26×10⁻⁶ to 2.84×10⁻⁶ Ω m for Sm₂₈Fe₇₂ and Sm₃₂Fe₆₈ films, respectively, with decrease in temperature from 300 to 40 K is attributed to the strain induced anisotropy that dominates at lower temperatures. The positive extraordinary Hall coefficients (R_S) are observed for both films at 300 and 80 K. The existence of hysteresis indicates that Sm₂₈Fe₇₂ and Sm₃₂Fe₆₈ films possess perpendicular anisotropy at 300 K. Hysteresis loop becomes narrow at 80 K for both Sm₂₈Fe₇₂ and Sm₃₂Fe₆₈ films. Magnetization measurements at 300 K exhibiting small coercive field values of 31 and 49 Oe for Sm₂₈Fe₇₂ and Sm₃₂Fe₆₈ films, respectively, confirm the existence of perpendicular anisotropy at 300 K.     

Field-driven creep motion of a composite domain wall in a Pt/Co/Pt/Co/Pt multilayer wire

We have studied the field-driven motion of a pair of coupled Bloch domain walls in a perpendicular magnetic anisotropy Pt/Co/Pt/Co/Pt multilayer Hall bar. The nucleation of an isolated but coincident pair of walls in the two Co layers, observed by Kerr microscopy, took place at an artificial nucleation site created by Ga⁺ ion irradiation. The average velocity v of the wall motion was calculated from time-resolved magnetotransport measurements at fixed driving field H, where the influence of the extraordinary Hall effect leads to the observation of voltages at the longitudinal resistance probes. We observed a good fit to the scaling relation lnv∝H^−1/4, consistent the motion of a single 1-dimensional wall moving in a 2-dimensional disordered medium in the creep regime: the two walls are coupled together into a 1-dimensional composite object.