Magnetization reversal dynamics in Co nanowires with competing magnetic anisotropies

We present the magnetization reversal dynamics of Co nanowires with competing magnetic anisotropies. The aspect ratio (R) of the nanowires is varied between 2.5 and 60, and we observe a cross-over of the directions of the magnetic easy and hard axes at R=6.8. Micromagnetic simulations qualitatively reproduce the observed cross-over and give detailed insight into the reversal mechanisms associated with the cross-over. The reversal mechanism for a field applied along the long axis of the nanowire exhibits a quasi-coherent rotation mode and a corkscrew-like mode, respectively, above and below the cross-over, with the formation of a Bloch domain near the cross-over region. For a field applied along the short axis, the reversal occurs by nucleation and propagation of reversed domains from the two ends of the nanowires for very high values of the aspect ratio down to the cross-over region, but it transforms into quasi-coherent rotation mode for smaller aspect ratios (below the cross-over region).     

Thermal activation of magnetization in Pr_2Fe_(14)B ribbons

The aftereffect field of thermal activation,which corresponds to the fluctuation field of a domain wall,is investigated via specific measurements of the magnetization behavior in Pr2Fe14B nanocrystalline magnets.The thermal activation is a magnetization reversal arising from thermal fluctuation over an energy barrier to an equilibrate state.According to the magnetic viscosity and the field sweep rate dependence of the coercivity,the calculated values of the fluctuation field are lower than the aftereffect field and in a range between those of domain walls and individual grains.Based on these results,we propose that the magnetization reversal occurs in multiple ways involving grain activation and domain wall activation in thermal activation,and the thermal activation decreases the coercivity by~0.2 kOe in the Pr2Fe14B ribbons.     

Thermally activated magnetization reversal in magnetic tunnel junctions

In this paper, the magnetization reversal of the ferromagnetic layers in the IrMn/CoFe/AlO_x/CoFe magnetic tunnel junction has been investigated using bulk magnetometry. The films exhibit very complex magnetization processes and reversal mechanism. Thermal activation phenomena such as the training effect, the asymmetry of reversal, the loop broadening and the decrease of exchange field while holding the film at negative saturation have been observed on the hysteresis loops of the pinned ferromagnetic layer while not on those of the free ferromagnetic layer. The thermal activation phenomena observed can be explained by the model of two energy barrier distributions with different time constants.     

Magnetization behavior of nanomagnets for patterned media application

Bit patterned media (BPM) which utilize each magnetic nanostructured dot as one recorded bit has attracted much interest as a promising candidate for future high-density magnetic recording. In this study, the magnetization reversal behaviors of nanostructured L1₀-FePt, Co/Pt multilayer (ML), and CoPt/Ru dots are investigated. For Co/Pt and CoPt/Ru nanodots, the bi-stable state is maintained in a very wide size range up to several hundred nm, and the magnetization reversal is dominated by the nucleation of a small reversed nucleus with the dimension of domain wall width. On the other hand, the critical size for the bi-stability of L1₀-FePt is about 60 nm, and its magnetization reversal proceeds via domain wall displacement even for such a small dot size. These reversal behaviors, depending on the magnetic materials, might be attributed to the difference in structural inhomogeneity, such as defects. In addition to the magnetic properties, the structural uniformity of the material could be crucial for the BPM application.     

Magnetization reversal measurements in mesoscopic amorphous magnets by magneto-optical Kerr effect

A magneto-optical setup based on the transverse Kerr effect has been designed to study the magnetization reversal processes by vector magnetometry in arrays of magnetic nanostructures with a reduced total volume. This system allows the measurement of both the parallel and perpendicular to the field components of the magnetization. It has been used to analyze the behavior of amorphous Co _x Si_1-x lines fabricated by electron beam lithography that present a very well defined shape induced uniaxial anisotropy. When the field is applied near to the hard direction, coherent rotation processes are found to occur with a collapse of this reversal mode at fields very close to the hard axis that allows to estimate the very low anisotropy dispersion of these samples. The analysis of the vector hysteresis loops reveals that the magnetization switches via an incoherent process that starts prior to the Stoner-Wohlfarth instability and that can be described in terms of a localized curling-like reversal mode.Received: 16 June 2004, Published online: 24 September 2004PACS: 75.75. + a Magnetic properties of nanostructures - 75.60.Jk Magnetization reversal mechanisms - 75.50.Kj Amorphous and quasicrystalline magnetic materials     

Inter-relation between reversible and irreversible magnetization components in nanocrystalline SmCo films

A study of the experimental behavior of the reversible (M_rev) and the irreversible (M_irr) DCD magnetization components in enhanced-remanence SmCo films was carried out. The evolution of the DCD M_rev and M_irr components were determined at room temperature by measuring sets of first-order reversal curves from different points on the initial magnetization curve and the demagnetization curve. From these data, M_rev(M_irr)_Hi curves were built, H_i being the internal field of the sample. Recent studies show that the behavior of the M_rev(M_irr) curves at a constant internal field can be used as an indicator of the mechanism of reversal magnetization. The non-monotonic behavior showed by these curves at low fields in our material, seems to be associated with an incoherent mechanism of magnetization reversal in the nanograins of SmCo.     

Magnetization reversal processes in FeSm thin films

We have investigated the in-plane magnetization reversal in FeSm thin films and discovered that it can be controlled through an induced anisotropy. For films with an induced easy direction, reversal is ultra fast and can be characterized approximately using the Fatuzzo model. In films with no pronounced induced easy axis, the reversal is much slower and can be described using a logarithmic model. We have also investigated the short time (1–50 s) dependence of the remanent coercivity and fitted to logarithmic equations. For films with no pronounced easy axis, the time dependence of the coercivity correlates with the film thickness, indicating that the switching volume scales with thickness. For films with an induced easy direction, the time dependence of the coercivity is essentially constant, independent of film thickness, indicating no scalable switching volume.     

Reversal modes in particles for magnetic recording modified with ionic substitutions

The particles used in magnetic recording are often subjected to ionic substitutions in the crystal lattice in order to make them suitable in advanced recording technology. In this paper it is experimentally investigated the influence of such doping on the magnetization reversal mode of the particles. The result is that the ionic modifications seem to favour the incoherent reversal of the magnetization, as an effect of the evolution of the crystalline and magnetic order induced by the doping process.     

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.     

斯托纳粒子的磁矩翻转

文章根据朗道-利夫席茨-吉尔伯特（Landau-Lifshitz-Gilbert）理论,介绍了斯托纳（Stoner）粒子（单个磁畴的磁性颗粒）磁矩翻转的相关理论.其中指出了有关磁矩翻转的斯托纳-沃尔法特（Wohlfarth）极限（SW极限）只有在阻尼系数无穷大时才是真正准确的.在此极限下,磁矩是沿着能量下降最快的路径翻转.最小的翻转磁场出现在当系统能量曲面中只有一个稳定的不动点的情形.文中还指出了对于一个给定的各向异性的磁体,阻尼系数存在一个临界值,超过它时,最小翻转磁场与SW极限是相同的.低于此临界值,最小翻转磁场可以小于SW极限.对于在有阻尼情况下的弹道式磁矩翻转,文中指出,施加的磁场方向应该处在一特定的方向内.这个方向窗口的宽度与阻尼系数和磁内能有关.对于一给定的磁内能,窗口的上下边界随着阻尼系数的增加而增加,窗口的宽度则随着阻尼系数的增加而呈振荡的变化.在没有阻尼和阻尼无穷大的极限下,窗口宽度变为零。     

Modeling of irreversible switching and viscosity phenomena in perpendicular thin films

We have developed a simple numerical model for simulating domains as well as remanence and viscosity curves in the slow dynamics regime, for thin films characterized by perpendicular magnetization and irregular domain configurations due to strong disorder. The physical system is represented as constituted of identical switching units, described by proper switching field distributions and energy barrier laws for pinning and nucleation processes. The model also includes an effective field which accounts for magnetic forces proportional to magnetization, on average. Simulations of DCD curves show that when the reversal of magnetization is governed by pinning, the coercive field depends on the physical size of the film area on which the external field is applied. In the case of viscosity phenomena described by a linear energy barrier law associated with a single predominant reversal process (pinning or nucleation), universal viscosity curves can be generated by properly transforming the DCD curve of the system. We also demonstrate that a reduction of the maximum viscosity coefficient can coexist with a reduction of the energy barrier heights.     

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.     

垂直自由层倾斜极化层自旋阀结构中的磁矩翻转和进动

在理论上研究了垂直自由层和倾斜极化层自旋阀结构中自旋转移矩驱动的磁矩翻转和进动.通过线性展开包括自旋转移矩项的Landau-Lifshitz-Gilbert方程并使用稳定性分析方法,得到了包括准平行稳定态、准反平行稳定态、伸出膜面进动态以及双稳态的磁性状态相图.发现通过调节电流密度和外磁场的大小可以实现磁矩从稳定态到进动态之间的转化以及在两个稳定态之间的翻转.翻转电流随外磁场的增加而增加,并且受自旋极化方向的影响.当自旋极化方向和自由层易磁化轴方向平行时,翻转电流最小;当自旋极化方向和自由层易磁化轴方向垂直时,翻转电流最大.通过数值求解微分方程,给出了不同磁性状态磁矩随时间的演化轨迹并验证了相图的正确性.     

末端形状对NiFe纳米薄膜磁反转动力学特性的影响

基于微磁学模拟方法研究末端形状对NiFe纳米薄膜的磁反转和自旋波本征动力学特性的调制及磁反转与自旋波模式软化间的内在联系.纳米薄膜微磁结构的相变总是伴随着某种自旋波模式的软化,软化自旋波模式空间分布预示微磁结构相变的路径.存在一临界裁剪度（h₀）.当裁剪度h<h₀时,磁振荡局域于末端边缘的EM自旋波软化诱导磁反转从磁体末端边缘磁矩失稳开始,边缘失稳区域向中央扩展形成反转畴,最后反转畴逐渐移出膜面外而实现反转.当h≥h₀时,形状各向异性导致边缘局域化模式自旋波被抑制,反转场附近一致模式自旋波的软化诱导磁体一致反转.     

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.     

Simulation of magnetization reversal processes in finite ferromagnets with defects

The magnetization and magnetization reversal processes that occur through the mechanism of incoherent rotation of magnetic moments in cubic ferromagnets with limited sizes are investigated theoretically. It is established that the appropriate model representation of magnetic inhomogeneities arising in the region of defects is provided by 0° domain walls. The influence of the external magnetic field on the structure and the stability region of the 0° domain walls is determined. This makes it possible to reveal the characteristic features of the magnetization reversal of real crystals as a function of the material and defect parameters, in particular, in the vicinity of the spin-reorientation phase transition.     

In-plane current-induced magnetization reversal of Pd/CoZr/MgO magnetic multilayers

High critical current density ($> 10^{6}$ A/cm$^{2})$ is one of major obstacles to realize practical applications of the current-driven magnetization reversal devices. In this work, we successfully prepared Pd/CoZr(3.5 nm)/MgO thin films with large perpendicular magnetic anisotropy and demonstrated a way of reducing the critical current density with a low out-of-plane magnetic field in the Pd/CoZr/MgO stack. Under the assistance of an out-of-plane magnetic field, the magnetization can be fully reversed with a current density of about 10$^{4}$ A/cm$^{2}$. The magnetization reversal is attributed to the combined effect of the out-of-plane magnetic field and the current-induced spin-orbital torque. It is found that the current-driven magnetization reversal is highly relevant to the temperature owing to the varied spin-orbital torque, and the current-driven magnetization reversal will be more efficient in low-temperature range, while the magnetic field is helpful for the magnetization reversal in high-temperature range.     

Vortex chirality control in magnetic submicron dots with asymmetrical magnetic properties in lateral direction

The authors use micromagnetic simulation to investigate the magnetization reversal process of a new ferromagnetic submicron dot structure composed of a lateral gradient magnetization. The reversal process in this new structure begins at the both edges along and is perpendicular to the applied magnetic field due to introducing a demagnetizing field from the interface of the magnetization gradient. This leads to a two-stage nucleation process. Based on the analytical results, a novel submicron structure with a quarter of lateral gradient magnetization is proposed to control the chirality of a vortex, which is important for applications that use the vortex's chirality.     

3D calculation of hysteresis loops,magnetic orientations and reversal processes for exchange-spring bilayers with perpendicular anisotropy

Hysteresis loops and magnetic reversal processes have been determined by a three dimensional (3D) micromagnetic model for exchange-coupled Nd₂Fe₁₄B/α-Fe bilayers and carefully compared with a popular one-dimensional (1D) micromagnetic model. It is found that the calculated hysteresis loops, the critical fields and the magnetic phase diagrams agree well with the results given by the 1D model. However, the calculated nucleation mode is a quasi-curling one where the magnetic moment exhibits a curling in the film plane and varies in the thickness direction, in contrast with the reported quasi-coherent mode. The calculated spatial distribution of the magnetization orientation in the thickness direction at various applied fields signifies a three-step magnetic reversal process, which includes nucleation, growth and displacement of the domain wall as well as the rotation and the reversal of magnetization in the hard phase. The magnetic reversal of the hard phase is much slower than that given by the 1D model, leading to a more slant hysteresis loop near the coercivity point.     

Magnetization reversal of individual Co nanoislands

We investigate the magnetization reversal of individual Co islands on Cu(111) in the size range of N=700 to 18,000 atoms by spin-polarized scanning tunneling microscopy at 8 K. The switching field H(sw) changes with island size in a nonmonotonic manner: it increases with island size and reaches a maximum value of 2.4 T at N=5500 atoms, and it decreases for larger islands. We extract the energy barrier for magnetization reversal as a function of island size. The maximum H(sw) corresponds to an energy barrier of 1 eV. Our results elucidate a crossover of the magnetization reversal from an exchange-spring behavior to domain wall formation with increasing size at around 7500 atoms.