Dao-wei Wang

Strain induced topological phase transitions in monolayer honeycomb structures of group-V binary compounds

We present first-principles calculations of electronic structures of a class of two-dimensional (2D) honeycomb structures of group-V binary compounds. Our results show these new 2D materials are stable semiconductors with direct or indirect band gaps. The band gap can be tuned by applying lattice strain. During their stretchable regime, they all exhibit metal-indirect gap semiconductor-direct gap semiconductor-topological insulator (TI) transitions with increasing strain from negative (compressive) to positive (tensile) values. The topological phase transition results from the band inversion at the Γ point which is due to the evolution of bonding and anti-bonding states under lattice strain.

Spin-wave propagation in domain wall magnonic crystal

We present a new type of magnonic crystal consisting of a series of periodically distributed magnetic domain walls in a uniform strip. When spin waves propagate in such a structure, allowed and forbidden bands are formed due to translation symmetry and scattering of the spin waves at the domain wall boundaries caused by the dynamic stray field in the domain wall region. The control of the bandgap position in frequency and its width by the period of magnonic crystal and the domain wall width is investigated. It is found that the bandgap position decreases monotonously with the increase of the period or domain wall width, while the bandgap width displays an oscillated behavior. The origin of the oscillation of the bandgap width is discussed. This work may provide a new way of designing reconfigurable magnonic devices.

Steady-state domain wall motion driven by adiabatic spin-transfer torque with assistance of microwave field

We have studied the current-induced displacement of a 180° Bloch wall by means of micromagnetic simulation and analytical approach. It is found that the adiabatic spin-transfer torque can sustain a steady-state domain wall (DW) motion in the direction opposite to that of the electron flow without Walker Breakdown when a transverse microwave field is applied. This kind of motion is very sensitive to the microwave frequency and can be resonantly enhanced by exciting the domain wall thickness oscillation mode. A one-dimensional analytical model was established to account for the microwave-assisted wall motion. These findings may be helpful for reducing the critical spin-polarized current density and designing DW-based spintronic devices.

Spin waves in the soft layer of exchange-coupled soft/hard bilayers

The magnetic dynamical properties of the soft layer in exchange-coupled soft/hard bilayers have been investigated numerically using a one-dimensional atomic chain model. The frequencies and spatial profiles of spin wave eigenmodes are calculated during the magnetization reversal process of the soft layer. The spin wave modes exhibit a spatially modulated amplitude, which is especially evident for high-order modes. A dynamic pinning effect of surface magnetic moment is observed. The spin wave eigenfrequency decreases linearly with the increase of the magnetic field in the uniformly magnetized state and increases nonlinearly with field when spiral magnetization configuration is formed in the soft layer.

Engineering irreversibility of exchange springs in antiferromagnetic DyFe2/YFe2 superlattices

Irreversible exchange springs are experimentally observed in an antiferromagnetic (DyFe2 40 A/YFe2 80 A/DyFe2 8 A/YFe2 80 A) × 20 superlattice. Two irreversible exchange springs can be observed at 80 K with field along the [00] crystal direction. OOMMF micromagnetic simulation is employed to understand the underlying physics. It is confirmed that the induced anisotropy in the soft phase, by the 8 A thick DyFe2 layer, is mainly responsible for the observed irreversibility, which corresponds to switching between energy minima of the thin hard inclusion in the soft phase. The qualitative agreement between experiment and simulation is satisfactory. This demonstrates the feasibility to control the irreversibility of exchange spring winding and unwinding processes in antiferromagnetically coupled superlattices. This additional degree of freedom in the demagnetization process of the soft layers of exchange coupled superlattices can be utilized to reduce the switching field of a hybrid magnet through spin-wave assisted magnetization reversal.

Domain wall motion driven by adiabatic spin transfer torque through excitation of nonlinear dynamics.

Domain wall dynamics under the joint action of a linearly polarized microwave magnetic field and spin transfer torque was analysed in terms of the domain wall collective coordinates. It was found that a microwave-assisted steady domain wall motion driven by adiabatic spin transfer torque can be adequately described by three domain wall collective coordinates. Analytical expression for the domain wall velocity showed that there are two contributions to the steady domain wall motion. One is derived from the nonlinear oscillation of domain wall width excited by the microwave field, and the other is from the heterodyne process between the width oscillation and the microwave field. The former always propels a domain wall to move in the positive direction, which is defined as the direction of the applied current. The latter contribution to the domain wall velocity can be positive or negative, depending on the polarization of the microwave field. The final domain wall velocity is determined by the competition between those two contributions, which indicates that by simply changing the polarization of the microwave field, the direction of the domain wall motion can be reversed. Our analysis demonstrated that the characteristics of domain wall motion can be tuned by selective excitation of nonlinear domain wall dynamics.

Determination of the spin polarization of RFe2 (R = Dy, Er, Y) by point contact Andreev reflection

Epitaxially grown intermetallic RFe2 (R = Dy, Er, Y) thin films have been studied by point contact Andreev reflection. Spin polarization values were extracted by fitting normalized conductance curves for mechanical Nb/RFe2 point contacts, using a modified Blonder-Tinkham-Klapwijk model. Good agreement is found between this model and the experimentally obtained data. Extracted values of spin polarization, which are close to the spin polarization of Fe, reveal no variation with the rare earth component for the measured intermetallic compounds. This suggests that using this technique we probe the Fe sub-lattice, and that this lattice drives spintronic effects in these compounds.