Xi-guang Wang

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.

An analytical approach to the interaction of a propagating spin wave and a Bloch wall

The spin wave propagation and the spin-wave induced domain wall motion in a nanostrip with a Bloch domain wall are studied. The spin-wave dispersion relation and the transmission coefficients across the wall are derived analytically. A one-dimensional model for the domain wall motion is constructed. It is found that the spin wave can drive the wall to move either in the same direction or in the opposite direction to that of spin-wave propagation depending on the transmission coefficient. The transmitted magnons drag the wall moving backward without inertia by the adiabatic and nonadiabatic spin-transfer torques, while the reflected magnons push the wall moving forward by the linear momentum transfer torque.

Thermally induced magnonic spin current, thermomagnonic torques, and domain-wall dynamics in the presence of Dzyaloshinskii-Moriya interaction

Thermally activated domain-wall (DW) motion in magnetic insulators has been considered theoretically, with a particular focus on the role of Dzyaloshinskii-Moriya interaction (DMI) and thermomagnonic torques. The thermally assisted DW motion is a consequence of the magnonic spin current due to the applied thermal bias. In addition to the exchange magnonic spin current and the exchange adiabatic and the entropic spin transfer torques, we also consider the DMI-induced magnonic spin current, thermomagnonic DMI fieldlike torque, and the DMI entropic torque. Analytical estimations are supported by numerical calculations. We found that the DMI has a substantial influence on the size and the geometry of DWs, and that the DWs become oriented parallel to the long axis of the nanostrip. Increasing the temperature smoothes the DWs. Moreover, the thermally induced magnonic current generates a torque on the DWs, which is responsible for their motion. From our analysis it follows that for a large enough DMI the influence of DMI-induced fieldlike torque is much stronger than that of the DMI and the exchange entropic torques. By manipulating the strength of the DMI constant, one can control the speed of the DW motion, and the direction of the DW motion can be switched, as well. We also found that DMI not only contributes to the total magnonic current, but also it modifies the exchange magnonic spin current, and this modification depends on the orientation of the steady-state magnetization. The observed phenomenon can be utilized in spin caloritronics devices, for example in the DMI based thermal diodes. By switching the magnetization direction, one can rectify the total magnonic spin current.

Spin-wave resonance reflection and spin-wave induced domain wall displacement

Spin-wave propagation and spin-wave induced domain wall motion in nanostrips with a Neel wall are studied by micromagnetic simulations. It is found that the reflection of spin waves by the wall can be resonantly excited due to the interaction between spin waves and domain-wall normal modes. With the decrease of the saturation magnetization Ms (and the consequent increase of the wall width), the reflection is diminished and complete transmission can occur. The domain wall motion induced by spin waves is closely related to the spin-wave reflectivity of the wall, and may exhibit different types of behavior. The reflected spin waves (or magnons) give rise to a magnonic linear momentum-transfer torque, which drives the wall to move along the spin wave propagation direction. The maximal velocity of the domain wall motion corresponds to the resonance reflection of the spin waves. The transmitted spin waves (or magnons) lead to a magnonic spin-transfer torque, which drags the wall to move backwardly. The complica...

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.

Crossover from reversible to irreversible magnetic exchange-spring processes in antiferromagnetically exchange-coupled soft/hard bilayer structures

The demagnetization processes of antiferromagnetically exchange-coupled soft/hard bilayer structures have been studied using a one-dimensional atomic chain model, taking into account the anisotropies of both soft and hard layers. It is found that for bilayer structures with strong interfacial exchange coupling, the demagnetization process exhibits typical reversible magnetic exchange-spring behavior. However, as the strength of the interfacial exchange coupling is decreased, there is a crossover point Ashc, after which the process becomes irreversible. The phase diagram of reversible and irreversible exchange-spring processes is mapped in Ash and Ns plane, where Ash and Ns are the interfacial exchange coupling and soft layer thickness, respectively. The thickness dependence of the bending field, which characterizes the onset of the exchange spring in the soft layer, is numerically examined and compared with analytical models.

Electric field controlled spin waveguide phase shifter in YIG

We propose a new type of a spin waveguide in yttrium iron garnet solely controlled by external electric fields. Spin waves are generated by microwave electric fields while the shift of the phase between spin waves is achieved by means of static electric fields. The phase shifter operation is based on the magneto-electric coupling and effective Dzyaloshinskii Moriya interaction. The special geometry of the waveguide imposes certain asymmetry in the dispersion relationships of the spin waves. Depending on the propagation direction, the phases of the spin waves are shifted differently by the external electric field. The phase difference is entirely controlled by the driving electric fields. The proposed phase shifter can be easily incorporated into electronic circuits and in spin wave logical operations.We propose a new type of a spin waveguide in yttrium iron garnet solely controlled by external electric fields. Spin waves are generated by microwave electric fields while the shift of the phase between spin waves is achieved by means of static electric fields. The phase shifter operation is based on the magneto-electric coupling and effective Dzyaloshinskii Moriya interaction. The special geometry of the waveguide imposes certain asymmetry in the dispersion relationships of the spin waves. Depending on the propagation direction, the phases of the spin waves are shifted differently by the external electric field. The phase difference is entirely controlled by the driving electric fields. The proposed phase shifter can be easily incorporated into electronic circuits and in spin wave logical operations.

Conversion of electronic to magnonic spin current at a heavy-metal magnetic-insulator interface

Electronic spin current is convertible to magnonic spin current via the creation or annihilation of thermal magnons at the interface of a magnetic insulator and a metal with a strong spin-orbital coupling. So far this phenomenon was evidenced in the linear regime. Based on analytical and fulledged numerical results for the non-linear regime we demonstrate that the generated thermal magnons or magnonic spin current in the insulator is asymmetric with respect to the charge current direction in the metal and exhibits a nonlinear dependence on the charge current density, which is explained by the tuning effect of the spin Hall torque and the magnetization damping. The results are also discussed in light of and are in line with recent experiments pointing to a new way of non-linear manipulation of spin with electrical means.

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.

Magnonic momentum transfer force on domain walls confined in space

Momentum transfer from incoming magnons to a Bloch domain wall is calculated using one-dimensional continuum micromagnetic analysis. Due to the confinement of the wall in space, the dispersion relation of magnons is different from that of a single domain. This mismatch of dispersion relations can result in reflection of magnons upon incidence on the domain wall, whose direct consequence is a transfer of momentum between magnons and the domain wall. The corresponding counteraction force exerted on the wall can be used for the control of domain wall motion through magnonic linear-momentum transfer, in analogy with the spin transfer torque induced by magnonic angular-momentum transfer.