Ching-Ray Chang

MICROMAGNETIC STUDIES OF COHERENT ROTATION WITH QUARTIC CRYSTALLINE ANISOTROPY

The nucleation fields under coherent rotation for uniaxial ferromagnets are rigorously determined including the second anisotropy constant K2. Theoretical studies cover the whole range of the relationships between K1 and K2. We show that the reversal processes can be classified into eight types depending on the parameters characterizing the magnetic materials. The results show that the value of K2 affects both the hysteresis loop and nucleation field significantly. The nucleation field along the c‐axis is not always equal to 2K1/Ms. For certain values of second anisotropy, the easy axes may be oblique to the c‐axis. When the ratio of K2 and K1 is large, the easy cone or hard cone will form at a certain oblique angle. The effects of K2 on the saturation field of hard axis loops have also been investigated. It is suggested that the values of K1 and K2 can be determined independently from the slope and saturation field of hard axis loops.

Nonuniform magnetization structures in thin soft type ferromagnetic elements of elliptical shape

Stable micromagnetic configurations are investigated numerically in thin soft type ferromagnetic elements of elliptical shape with typical thickness of Lz=30 nm and with aspect ratios 1⩽Lx/Ly⩽3.5. In this range of aspect ratios, a complete description of stable magnetization patterns is obtained including the critical sizes for stability of one-vortex and quasiuniform micromagnetic configurations. Due to the absence of sharp corners, the behavior of the quasiuniform state in an external magnetic field resembles that of a single-domain ellipsoidal particle of similar shape. Nevertheless, the calculated hysteresis loops of elliptical elements show a considerable reduction of the switching fields in comparison with the Stoner–Wohlfarth model due to the development of a new mode of instability. The magnetization curves for elliptical elements, initially in one-, two- or three- vortex states, show significant dependence on the direction of external magnetic field with respect to the longest particle’s axis.

MAGNETIC FORCE MICROSCOPY STUDY OF DOMAIN WALLS ON A THIN COBALT FILM

Two types of 180° domain walls on a 115 nm cobalt film are observed by magnetic force microscopy. The surface Neel wall on top of an asymmetric Bloch wall is found inhomogeneous with the width estimated to be around 200 nm. The width of the rhomb-structured cross-tie wall, on the other hand, is approximately 400 nm. The coexistence of these two types of domain walls indicates that their wall energies are comparable at the film thickness.

Control of microstructure in (001)-orientated FePt–SiO2 granular films

Highly ordered L10 FePt–SiO2 granular films with a (001)-preferred orientation were obtained by depositing atomic-scale [Fe∕Pt∕SiO2(tox)] multilayers on glass substrates and subsequently annealing multilayers at the temperature of as low as 350°C. The tox value plays an important role on both microstructure and magnetic properties. The average grain sizes of samples A, B, and C (tox=0.11,0.28,0.56) are 12.12, 15.46, and 5.14nm, respectively, and the corresponding perpendicular coercivities are 6800, 5550, and 7700Oe. Due to different microstructures, samples show distinct coercivity dependence on annealing time.

Persistent spin helix in Rashba-Dresselhaus two-dimensional electron systems

A persistent spin helix (PSH) in spin-orbit-coupled two-dimensional electron systems was recently predicted to exist in two cases: [001] quantum wells (QWs) with equal coupling strengths of the Rashba and the Dresselhaus interactions (RD), and Dresselhaus-only [110] QWs. Here we present supporting results and further investigations, using our previous results [Phys. Rev. B 72, 153305 (2005)]. Refined PSH patterns for both RD [001] and Dresselhaus [110] QWs are shown, such that the feature of the helix is clearly seen. We also discuss the time dependence of spin to reexamine the origin of the predicted persistence of the PSH. For the RD [001] case, we further take into account the random Rashba effect, which is much more realistic than the constant Rashba model. The distorted PSH pattern thus obtained suggests that such a PSH may be more observable in the Dresselhaus [110] QWs, if the dopants cannot be regularly enough distributed.A quantum world-line Monte Carlo method for high-symmetrical quantum models is proposed. Firstly, based on a representation of a partition function using the Matsubara formula, the principle of quantum world-line Monte Carlo methods is briefly outlined and a new algorithm using non-binary loops is given for quantum models with high symmetry as SU(N). The algorithm is called non-binary loop algorithm because of non-binary loop updatings. Secondary, one example of our numerical studies using the non-binary loop updating is shown. It is the problem of the ground state of two-dimensional SU(N) anti-ferromagnets. Our numerical study confirms that the ground state in the small N<= 4 case is a magnetic ordered Neel state, but the one in the large N>= 5 case has no magnetic order, and it becomes a dimer state.

Quantum interference and spin polarization on Rashba double quantum dots embedded in a ring

We report on the quantum interference and spin accumulation on double quantum dots with Rashba spin-orbit coupling and electron-electron interaction based on the Keldysh nonequilibrium Greens function formalism. It is shown that Rashba spin-orbit interaction can strongly affect the conductance spectrum. By gradually increasing the Rashba parameter from zero, Fano resonances in strong overlap regime continuously evolve to resolve antiresonances. This transition is ascribed to the phase shift of couplings between molecular states and the lead due to spin precession. We also show that both bias and Rashba effects strengthen the induced spin polarization in this device. For particular energy position, up- and down-spin electron occupations can intersect to form a crossing point. Spin polarization on different sides of this point has opposite sign in consequence. The magnitude and direction of spin polarization are therefore controllable by tuning the dot levels and the Rashba parameter through gates.

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.

Microwave-driven ferromagnet–topological-insulator heterostructures: The prospect for giant spin battery effect and quantized charge pump devices

We study heterostructures where a two-dimensional topological insulator (TI) is attached to two normal metal (NM) electrodes while an island of a ferromagnetic insulator (FI) with precessing magnetization covers a portion of its lateral edges to induce time-dependent exchange field underneath via the magnetic proximity effect. When the FI island covers both lateral edges, such device pumps pure spin current in the absence of any bias voltage, thereby acting as an efficient spin battery with giant output current even at very small microwave power input driving the precession. When only one lateral edge is covered by the FI island, both charge and spin current are pumped into the NM electrodes. We delineate conditions for the corresponding conductances (current-to-microwave-frequency ratio) to be quantized in a wide interval of precession cone angles, which is robust with respect to weak disorder and can be further extended by changes in device geometry.

Evolution of vortex states under external magnetic field

Abstract The behavior of one-, two-, and three-vortex states of submicron-sized elliptical elements under the influence of external magnetic field is investigated here by means of micromagnetic simulations. The total magnetization curves show abrupt changes at the nucleation or annihilation of vortices. The magnetization curves and critical field show significant dependence on the field direction with respect to the particle’s long axis.

Magnetoresistance effect in Ag–Fe3O4 and Al–Fe3O4 composite films

The Agx–(Fe3O4)1−x and Agx–(Fe3O4)1−x composite films were prepared by dc sputtering on Si(100) substrates. The x-ray diffraction results show that the films contain essentially only the cubic inverse spinal phase from Fe3O4 and face-centered cubic phase from Ag or Al. The transmission electron microscopy images indicate that the metal granules are randomly distributed with Fe3O4 grains. The resistivity determined from the four-probe method decreases rapidly with increasing metal content. At x≒0.5, a percolation occurs. The conducting path is formed from metal granules in series with Fe3O4 grains. The magnetoresistance (MR) is defined to be {R(H=0.8 T)−R(H=0)}/R(H=0). It has been found that MR is isotropic and the appearance of Ag granules has significant impact on the MR effect. Furthermore, a positive MR region appears with 0.011<x<0.1 in Agx–(Fe3O4)1−x. On the contrary, the incorporation of Al granules does not have the same effect on MR as in Agx–(Fe3O4)1−x. A slow increase of MR with Al content might...