Chao-Hsien Huang

Study of One-Side-Flat Edge on Vortex in Submicro-Scaled Permalloy Disks

By introducing a one-side-flat edge to a submicroscaled Permalloy disk, the isotropy of the disk is broken to achieve the control on vortex nucleation and annihilation. Series one-side-flat magnetic disk arrays with different cutting percentages of disk are fabricated to observe the influence from asymmetry on the vortex configuration in submicroscaled magnetic disks. In the excised angles were varied from 15° to 90° in 500-nm-diameter and 40-nm-thick NiFe disk arrays, it is observed the dependences of vortex nucleation and annihilation fields on the asymmetry. Linear relations of vortex nucleation and annihilation fields to aspect ratio were found that would be useful for controlling nucleation and annihilation fields on purpose. Micromagnetism simulations for Permalloy disk array with a diameter of 500 nm and different excise dangles were executed to compare with experimental data. The relations of vortex existence field to the aspect ratio were found that could be taken into account for designing single vortex element applications. In addition to magnetic field parallel to excised flat edge, the constant perpendicular field (Hy) applied to sample could change vortex nucleation and annihilation field of Py disk array. Finally, It was found that a higher asymmetry effect could depress the unpleasantly interactions and lead to a narrow range of switching field distribution, which was an important index for designing high-density magnetic devices.

Influence of asymmetric disks on control over the existence of vortex in submicrometer-scaled Permalloy disks

The influence of the degree of asymmetry of a Py disk in a vortex configuration by changing an excised angle is studied in this paper. Samples of 40 nm thick NiFe disk arrays, with excised angles varying from 15° to 90° and diameters of 800, 500, and 300 nm, were investigated to observe the dependence of vortex nucleation and annihilation fields on the asymmetry and dimensions. Linear relations of vortex nucleation and annihilation fields to aspect ratio were found, and are useful mainly for controlling the nucleation and annihilation fields. Relationships between the vortex existence field and the aspect ratio, a thickness and diameter were found, and these should be taken into account when designing single vortex element applications.

Vortex annihilation in magnetic disks with different degrees of asymmetry

We investigate the influence of one-side-flat asymmetric degrees on vortex annihilation behavior in different chirality, clockwise or counterclockwise. The vortex annihilation fields are found to depend not only on the vortex chirality but also strongly on the degrees of asymmetry. The sequence of vortex annihilation from the flat to the round edges is observed in low asymmetric disks, and interestingly, the sequence is reversed in high asymmetric disks. Fast and non-contact vortex chirality detection can be realized in high asymmetric disks by analyzing hysteresis loop of focused magneto-optic Kerr effect on vortex annihilation. The experimental results agree well with the micromagnetic simulations.

Adjustment of Demagnetizing Field in Permalloy Nanowires to Control Domain Wall Motion

The domain wall motion in permalloy nanowires has been investigated widely due to its potential for a new type of memory. In this paper. we use the LLG Simulator based on the Landau-Lifshitz-Gilbert (LLG) equation to investigate the field-driven domain-wall motion in a long, straight ferromagnetic strip. An injection field of 60 Oe is applied to inject a domain wall from an extended disk into the nanowire. We found a dependence of nanowire dimensions with the velocity of domain wall. By increasing the width of the nanowire, the velocity of the domain wall motion also increases, while the Walker breakdown field ( HWB) decreases. On the other hand, increasing the thicknesses of the nanowire, the domain wall velocity, HWB, and demagnetizing field all decrease. By applying a vertical field from 0 to 1000 Oe in order to enhance the demagnetizing field, it is found the HWB is increased from 16 to 20 Oe.

Effect of Asymmetric Shape on Vortex Control in Permalloy Rings

A method to control the vortex state (flux-closure state) nucleation and annihilation field in asymmetric permalloy (Py) rings is presented. By introducing an inner circle shifted from the center of outer circle by a shift length, samples of asymmetric permalloy rings with different asymmetric ratios were fabricated by using E-beam lithography, sputtering, lift-off techniques, and measured by focused magneto-optic Kerr effect magnetometry. The vortex nucleation (Hn) and vortex annihilation (Han) fields, analyzed from the simulated and experimental hysteresis loops of 500 nm and 1000 nm outer diameters, have linear relations to the asymmetric ratio. When the lateral size reduces down to nanosize, two switching behaviors for the simulated hysteresis loops were observed. The single switching loop indicates that the switching process is single-domain switching and the all the spins rotate coherently. The double switching loop corresponds to the switching process via the formation of a vortex state. Both analyzed switching fields from the experiment and simulation present a linear relation with the aspect ratio in the small range of variation. This relation indicates that the Hn and Han of asymmetric ring arrays are mainly controlled by the shape anisotropy. It is observed that the switching behavior is in agreement with the experimental results.

Influence of asymmetry of Kerr signal on vortex configuration in submicro-scaled permalloy disc array

A self-alignment method for local coating fabrication in submicro-scaled discs is described. The adjustable area of the capping layer on the magnetic disc can create asymmetric Kerr signals for vortex configuration and provide different asymmetries of the Kerr signal. Nondegenerate Kerr signals of vortex chirality in 300?nm diameter and 40?nm thick Ni80Fe20 discs locally capped with 25?nm thickness Al were investigated. Various capping area ratios ranging from 0 to 0.5 were fabricated to observe the influence of Kerr signal asymmetry. This modifiable asymmetric Kerr signal advances the time-resolved measurements of vortex chirality dynamics.

Exchange Bias in NiFe/IrMn/CoFeB Antidot Arrays

A study was made of the exchange bias effect in a structure of SiO2/Ta (5 nm)/NiFe (7 nm)/IrMn (10 nm)/CoFeB (7 nm)/Ta (5 nm) with nanoscale antidot arrays. The nanostructure comprised rhomboid lattice antidot arrays with antidot diameters from 150 to 300 nm and a rhomboid lattice period of 500 nm. Enhancements of exchange bias HEX and coercivity HC were observed in the nanostructure antidot arrays compared to continuous film. These effects were mainly ascribed to the physical limitations on ferromagnetic and antiferromagnetic layers due to the presence of antidots. In antidot arrays, due to the presence of nonmagnetic holes, the ferromagnetic-ferromagnetic interactions in FM layer are reduced, leading to smaller ferromagnetic domains and larger exchange bias with the random-field model. In this paper, the relation between the antidot diameter size and exchange bias was studied.

Brief rapid thermal treatment effect on patterned CoFeB-based magnetic tunneling junctions

The brief thermal treatment effects on the magnetoresistance of microstructured Co60Fe20B20-based magnetic tunneling junctions have been studied. The elliptical shape of devices with long/short axis of 4∕2μm was patterned out of film stack of seed layer (20)∕PtMn(15)∕Co60Fe20B20(3)∕Al(0.7)oxide∕C60Fe20B20(20)∕capping layer (48) (thickness unit in nanometers) combining conventional lithography and inductively coupled plasma reactive ion beam etching technologies. The thermal annealing was carried out with device loading into a furnace with preset temperatures ranging from 100to400°C for only 5min in the absence of any external magnetic field. The magnetoresistance was found to increase with increasing annealing temperatures up to 250°C and then decrease at higher annealing temperatures. In addition, the magnetoresistance ratio of around 35%, similar to that of as-fabricated devices, sustains up to annealing temperature of 350°C. This survival of magnetoresistance at higher annealing temperature is due to b...