Jyh-Shinn Yang

Detection of anisotropy in the reversible transverse susceptibility

A model of reversible transverse susceptibility is proposed. It is based on the Stoner–Wohlfarth coherent rotation model, but with a macroscopic mean field interaction included to represent mutual interaction between particles. Unlike previous predictions, our calculated results show that the determination of the magnetic anisotropy field strength by means of the location of the peak in the transverse susceptibility curve works only in a weakly interacting particle system. For a strongly correlated system, nucleation does not start at the anisotropy field; therefore, it can be concluded that the peak detection method of anisotropy in transverse susceptibility should fail in this system.

Hysteresis of binary clusters

A pair of parallel uniaxial particles with dipole‐dipole coupling has, for bond angles β=0 and π/2, up to four locally stable configurations: ↑↑, ↑↓, ↓↑, and ↓↓. Assuming thermal relaxation via coherent rotation and a periodic driving field we solve a master equation for the occupation probabilities of these states and find the coercivity and switching field distribution of the ensemble of coupled particles. For either value of the bond angle we compare numerical solutions of the master equation with approximate expressions based on extremal analysis and find good agreement between the two.

The influence of interfacial exchange on the coercivity of acicular coated particle

The acicular coated magnetic particles are considered as magnetostatic interacting dual‐component particles with interfacial exchange coupling. The calculations show that the coercivity varies with shell thickness and the relative magnetic properties between core and shell. When increasing the shell thickness, it is found that the initial slope of the increasing coercivity depends on the anisotropy energy of the shell, while the saturation level of the coercivity depends on the interfacial interaction. The calculated results show that a thin layer of shell is responsible for the increase of the coercivity. The best fitting curve for the available experimental data gives interfacial exchange coupling, A=5.87×10−8 erg/cm, and shell anisotropy constant, K=0.52×106 erg/cm3.

Micromagnetic Simulation for Spin-Transfer Switching With a Tilted Spin Polarizer

This paper reports the micromagnetic simulations of spin-wave modes excited by spin-transfer torque for an elliptical nanomagnet sized 178 × 133 × 2.1 nm3. Through power spectral analysis we discovered that, for the collinear case, the excited spin-wave mode exhibits the same spatial symmetry as the initial ground state. The additional action of the Oersted field not only changes the mode pattern but also shifts the eigenfrequency from 8.24 GHz to 2.99 GHz due to the degradation of symmetry. For the non-collinear case, without taking into account the Oersted field, a tilted polarizer leads to the excitation of the quasi-uniform mode and reduces the switching time substantially. However, considering the Oersted field, the activated mode is the center mode. The simulated results indicate that the spatial symmetry of the Oersted field and the spin-torque term play crucial roles in determining the activated spin-wave eigenmodes.

Investigation on the Magnetization Reversal of Nanostructured Magnetic Tunnel Junction Rings

Nanostructured magnetic tunnel junction rings (MTJ) based on CoFeB free layer were fabricated to investigate the corresponding magnetization reversal processes. The dimensions of the MTJ rings were with outer diameter/linewidth of 1 mum/100 nm and 0.7 mu m/100 nm , respectively, in which the fabrication was carried out by a top-down technique in combination with electron beam lithography, ion-milling and etch-back processes. Double transition processes in magnetoresistance (MR) responses are observed in these nanostructured MTJ rings. With considering the surface roughness, an OOMMF simulation was employed to explore the magnetization evolution. The simulated hysteresis loops are consistent with the MR behaviors, unfolding that the magnetic switching process initiates from the forward to reversed onion states through an incomplete vortex state. This two-step switching behavior is reasonably assumed to be due to a pinning site resulting from the surface roughness in a region of the ring. In addition, a series of in situ magnetic force microscopy images extracted from nanostructured MTJ rings reveals the similar magnetization evolution with the simulated results.

Calculation of exact head and image fields of recording heads by conformal mapping

An exact conformal mapping calculation is carried out, which maps the entire upper edge of an idealized ring head and a symmetric thin-film head in the complex z plane to the real axis of the complex t plane, and uses the slotted-plane approximation to obtain the inverse mapping function t(z). The latter is used to calculate not only the head fields of a given geometry, but also the kernal function to evaluate the total demagnetization field of the interaction head-media system including the media self-demagnetization and the image field induced on the head poles. The total demagnetization field contours for a single-bit arctangent transition and a dibit biquadratic transition corresponding to various head configurations have been obtained and their physical meaning elaborated on. >

Magnetization configurations and magnetization reversal of thin submicron permalloy ellipses with structural defects.

Quaternary ammonium salts based on monofunctionalized Perfluoropolyether (PFPE) oligomers were synthesized and used for the cation exchange process of sodium Montmorillonite nanoclays. The new fluoromodified nanoclays were characterized through X-rays diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), termogravimetric analysis (TGA), differential scanning calorimetry (DSC), electronic microscopy (SEM-EDS), and contact angle measurements (CA). In particular XRD showed rather complex patterns (presence of higher order reflections) which allowed the calculation of basal spacings, regularly increasing with the molecular weight of the fluorinated macrocation. Both IR and SEM confirmed the presence of fluorinated segments at clays interface, while TGA showed a limited thermal stability with an onset of degradation temperature which seems not dependent on the molecular weight of the macrocation. CA measurements showed a peculiar behaviour, with evident dynamic hysteresis phenomena and surface tension components quite different from those of commercially available, organomodified clays.

Effects of laminated soft layer on magnetization reversal of exchange coupled composite media

The laminated soft layer (LSL) that comprises the granular [CoPtCr–SiO2/Pt]N multilayers with perpendicular magnetization is designed to reduce the switching field of exchange coupled composite (ECC) media. The magnetic simulation shows that the reduction in the switching field can be optimized by changing the coupling strength between the adjacent CoPtCr–SiO2 layers in LSL. The reversal mechanism of ECC media with LSL depends on the bilayers number N of [CoPtCr–SiO2/Pt]N. Both simulation and experiments reveal that the domain-wall assisting reversal strongly depends on the thickness of LSL. By properly adjusting the coupling strength inside the LSL, the switching and saturation fields can be significantly reduced at a limited thickness of the soft layer due to decreased domain-wall length by the Pt lamination.

Exact field calculations for asymmetric ring heads

An exact solution for the fields of an unconventional ring head with an inclined gap is derived. The Schwartz-Christoffel transformation is used to determine the distribution on the fringe field components in the gap vicinity, and an analytical field equation is obtained. The transcendental equation of the mapping function is solved using Newtons method. >

Mean field interaction and transverse susceptibility

To analyze the behavior of the peaks of the reversible transverse susceptibility, magnetic interparticle interactions are included with a mean field approximation, and the orientation of media is described with a texture function that had independent in-plane and out-of-plane distributions. In contrast to the noninteracting models, it is found that the peaks at /spl plusmn/H/sub k/ can be suppressed at a strong interaction strength and the peak at H/sub c/ critically depends on both the orientation of the media and the coupling strength. Calculated results indicate that the determination of anisotropy constants from the peak positions of transverse susceptibility is only valid in weakly interacting particulate media. >