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.     

MFM probe control of magnetic vortex chirality in elliptical Co nanoparticles

Magnetic force microscopy (MFM) methods were applied to investigate the peculiarities of magnetization distribution in elliptical 400×600×27 nm Co particles. Reversible transitions between the uniform and vortex states under inhomogeneous magnetic field of MFM probe were observed. Possibility to control the chirality of a magnetic vortex in these particles by MFM probe manipulation was shown.     

Micromagnetic simulation of magnetic vortex cores in circular permalloy disks: Switching behavior in external magnetic field

Switching behaviors of magnetic vortex cores under external magnetic field in submicron circular permalloy disks have been systematically studied by using micromagnetic simulations. Simulation results show that the vortex core is stable in out-of-plane field even when it is located at the edge of the disk. The out-of-plane switching field H_sw is strongly dependent on the thickness of the disk. The core polarity and the vortex chirality can be modulated simultaneously on purpose by using a tilted field far smaller than the out-of-plane switching field H_sw. Moreover, it is found that the core polarities in asymmetric disks do not follow the direction of the z projection of the external saturation field.     

Magnetization reversal of microsized Fe elliptic rings

We report on the magnetization reversal of elliptic ring patterns lithographically prepared from Fe films. The elliptic rings in four different arrays are all of the same size , but their geometry is disturbed by introducing an increasing number of slits into the rings. The magnetization reversal is studied by regular longitudinal vector magneto-optical Kerr effect in specular geometry as well as in Bragg geometry, using the diffraction spots from the grating for hysteresis measurements. The measurements are compared with the results of micromagnetic simulation, which allow a detailed interpretation of the experimental data. We find that the remagnetization process in an external magnetic field is characterized by single-step switching or double switching depending on the geometry of the structure.     

电流调控磁涡旋的极性和旋性

提出了一种特殊自旋阀结构, 其极化层(钉扎层)磁矩沿面内方向, 自由层磁矩成磁涡旋结构. 自由层在形状上设计成左右两边厚度不同的阶梯形圆盘. 微磁学模拟研究发现, 通过调控所施加的高斯型脉冲电流的大小、方向和脉冲宽度, 可以实现磁涡旋的不同旋性、不同极性的组态控制. 分析了该结构中电流调控磁涡旋旋性和极性的物理原因和微观机理.     

MFM study of magnetic vortex cores in circular permalloy dots: behavior in external field

In a circular dot of permalloy with an appropriate size, a vortex structure with perpendicular (turned-up) magnetization at the core is realized. The existence of the perpendicular magnetization spot has been confirmed and the direction of the magnetization, up or down, has been determined by magnetic force microscopy (MFM) for permalloy dots with the diameter of 0.1–1 μm. The switching field of turned-up magnetization is determined by applying external fields perpendicularly and in tilted directions to the plane. By comparing the MFM results and the magnetization curves measured by a SQUID magnetometer, the switching process of turned-up magnetization is argued.     

Magnetic states of small cubic particles with uniaxial anisotropy

The lowest energy states in small cubic particles with uniaxial anisotropy are explored as a function of anisotropy strength and particle size. The investigations result in a phase diagram which contains the boundaries between the regions of one, two and three domains (flower, vortex and double vortex states). While the general features of the phase diagram are derived from energy estimates based on domain theory, the details are obtained using numerical micromagnetics. The two-domain and the three-domain phase can be subdivided into subphases. The comparison between different configurations revealed that a twisted vortex configuration with an S-shaped domain wall replaces the symmetric vortex with a straight wall at larger sizes. The three-domain phase contains two subphases which are symmetric with respect to (1 0 0) and (1 1 0) mirror planes, respectively. The transition from two to three domains occurs into the (1 1 0)-three-domain-state (diagonal state). This structure can be described as a configuration with two (quarter-) circular domain walls in two opposing corners. However, this configuration is energetically favored only in a small region within the phase diagram relative to the (1 0 0)-symmetry three-domain state with straight walls (sandwich state).     

Faster vortex core switching with lower current density using three-nanocontact spin-polarized currents in a confined structure

We perform micromagnetic simulations on the switching of magnetic vortex core by using spin-polarized currents through a three-nanocontact geometry. Our simulation results show that the current combination with an appropriate current flow direction destroys the symmetry of the total effective energy of the system so that the vortex core can be easier to excite,resulting in less critical current density and a faster switching process. Besides its fundamental significance, our findings provide an additional route to incorporating magnetic vortex phenomena into data storage devices.     

Calculations of three-dimensional magnetic excitations in permalloy nanostructures with vortex state

Dynamic susceptibility spectra of the vortex state in nanorings and nanodots are studied using three-dimensional micromagnetic simulations. Spatial maps of the susceptibility have enabled identification of various resonance modes. For an exciting field along the x axis, several resonance peaks appear for a thin dot, including a core mode, whereas only one main resonance peak is detected for a ring corresponding to a volume mode with uniform magnetization perpendicular to the exciting field (x direction). A low-frequency resonance peak related to a surface mode and a high-frequency resonance peak viewed as an edge mode are additionally observed for a thick ring. These three resonance modes (surface, volume and edge modes) which correspond to low, intermediate and high-frequency resonance peaks, respectively, are also captured for an exciting field along the y axis. In addition, a mixed edge and volume mode is revealed at a higher frequency.     

Magnetization reversal in weakly coupled magnetic patterns

Ion irradiation is an original process to pattern the structural and as a consequence the magnetic properties of ultra-thin films, down to the nanometer scale. Patterns of dots and tracks have been fabricated by focused Ga⁺ ion beam scanned onto a Co layer with perpendicular magnetic anisotropy. Depending on the dose, the magnetic behaviour of the nanometric irradiated lines can be tuned from the ferromagnetic with reduced coercivity to paramagnetic. The larger the fluence, the smaller is the exchange between dots or tracks. These systems enabled investigations of the competition between exchange and dipolar interactions. For arrays designed with high irradiation doses and only coupled by dipolar interactions, the magnetic relaxation proceeds by the magnetization reversal of individual dots and follows a power-law time decay. Monte Carlo simulations reproduce this time dependence.     

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).     

Nonreciprocity engineering in magnetostatic spin waves

Magnetostatic surface spin waves (MSSW) excited from a coplanar waveguide antenna travel in different directions with different amplitudes. This effect, called nonreciprocity of MSSW, has been investigated by micromagnetic simulations. The ratio of amplitude of two counter propagating spin waves, the nonreciprocity parameter κ, is obtained for different ferromagnetic materials, such as NiFe (Py), CoFeAl, yttrium iron garnet (YIG), and GaMnAs. A device schematic has been proposed in which κ can be tuned to a large value by varying simple geometrical parameters of the device.     

Deforming and moving a vortex by the tip of a magnetic force microscope

Recent experiments [O.M. Auslaender et al., Nat. Phys. 5 (2009) 35] use a magnetic force microscope not only to image but also to move and deform an individual vortex line in a bulk YBCO type-II superconductor. The theory of this experiment is presented accounting for pinning and curving of the vortex and for the full three-dimensional anisotropy of pinning and of vortex line tension in this material.     

Site-specific chirality in magnetic transitions

We report site-specific energy loss magnetic dichroism measurements of the technologically interesting Heusler alloy Ni₂MnSn. In addition, we confirm the theoretical prediction that under certain conditions, two different atoms on inequivalent lattice sites give dichroic signals with opposite signs. With this, it is possible to distinguish the magnetic moments of atomic columns that are merely 1.5 Å apart using a conventional transmission electron microscope without the need for aberration corrections.     

Nonlinear behavior in magnetic transients

We examine the motion of a spin subject to a static external field, dynamic demagnetizing fields, and a relatively weak oscillatory driving field. More specifically, we study the initial transient behavior just after all these fields are turned on and present analytic calculations based on an approximation scheme for the motion of the spin. We find that two modes - a mode at the driving frequency and a mode at the natural resonant frequency - play an important role in the initial transient. Because of the nonlinear nature of the spin equations of motion, one also finds, only in the initial transient, modes with doubled frequencies of the initial two modes and modes involving the sum and difference frequencies of the initial two modes. We also examine large amplitude transients in specific cases through numerical calculations.     

Magnetization and magnetic susceptibility of InAs nano-rings

In this work, we investigate theoretically the magnetic susceptibility of the three-dimensional cut-torus-shaped quantum rings. The calculations are carried out for the realistic three-dimensional model of InAs/GaAs nano-rings with the hard-wall confinement potential, three-dimensional effective one-electronic-band Hamiltonian with position and energy-dependent effective mass, and Ben Daniel–Duke boundary conditions. This allows us to describe the penetration of the magnetic field into the torus region. At zero temperature, the ring's differential susceptibility demonstrates delta-like paramagnetic peaks, which are generated by aperiodic oscillations of magnetization. With increasing temperature, the peaks gradually transform into Lorenz-shaped peaks and then disappear. As opposed to meso-scopic quantum rings we found a dependence of the peak's amplitude on the dimensions of the rings.     

Field-controllable injection of virtual magnetic domain wall in discrete magnetic nanodot chains

Periodic injection behaviors of virtual magnetic domain wall (VDW) have been systematically investigated in asymmetrically shaped nanodot chains by means of micromagnetic simulations. Systematic investigation on a controllable VDW injection has been carried out. We demonstrate that precise control of VDW injection is achievable by using different nanodot shapes as well as by changing alternating magnetic field (AC field) profiles. The VDW position can be tuned by adjusting AC field frequency and amplitude. Field-controllable periodic VDW injection phenomenon is found to be sustainable over wide ranges of phase diagram spanned by AC field frequency and amplitude.     

磁涡旋极性翻转的局域能量

针对坡莫合金纳米圆盘中的单个磁涡旋结构,采用微磁学模拟研究了磁涡旋极性翻转过程中的局域能量密度.磁涡旋的极性翻转通过与初始涡旋极性相反的涡旋与反涡旋对的生成,以及随后发生的反涡旋与初始涡旋的湮没来实现.模拟结果显示当纳米圆盘样品中局域能量密度的最大值达到一临界值时,磁涡旋将会实现极性翻转,其中交换能起主导作用.基于涡旋极性翻转过程中出现的三涡旋态结构,应用刚性磁涡旋模型对局域交换能量密度进行了理论分析.通过刚性磁涡旋模型得到的磁涡旋极性翻转所需的局域交换能量密度的临界值与模拟结果符合得较好.     

Impact of magnetic surface anisotropy on the precessional switching of magnetization in Pt-alloy nanofilms

Precessional switching of magnetization in CoPt and FePt nanofilms is investigated by solving the Landau–Lifshitz–Gilbert (LLG) equation analytically and numerically. Switching in these films occurs only above a critical value of the magnetic field, and it further depends on the magnetocrystalline anisotropy and saturation magnetization of the film. The presence of magnetic surface anisotropy in these films reduces the switching time significantly. Also, the switching time in the case of Pt-alloys of Co and Fe is low compared to that in the case of pure Co and Fe films.