Calculation On The Dynamics Of Antiferromagnetic Domain Wall Triggered By The Gradient Of Magnetic Anisotropy Energy

The racetrack memory is a novel non-volatile magnetic memory device.The information unit of the racetrack memory is the magnetization in a magnetic domain of a magnetic nanowire.The stored information is read based on the motion of magnetic domain walls(DWs)induced by a magnetic field or a spin-polarized current.However,the application of the racetrack memory hits a bottleneck owing to the high dissipation from the current.In recent years,the DW motion under the driving force with low dissipation,such as voltage/electric field,spin wave,and surface acoustic wave,has been paid more and more attention.On the other hand,the researches in the past focused on the DW motion in a ferromagnetic nanowire.For collinear antiferromagnets,despite the weak stray field and excellent anti-crosstalk performance,it has not received too much attention for a long time for lacking effective driving methods.In recent decade,the investigation in the field of spintronics and multiferroic materials paves a way to trigger the motion of antiferromagnetic(AFM)DWs.Especially,based on the multiferroic effect,the gradient of magnetic anisotropy energy induced by an electric field can trigger the motion of an AFM DW with low dissipation.However,possible variable velocity due to the inertia of the AFM DW and the special dynamical behaviors in different ranges of velocity for the AFM DW driven by the gradient of magnetic anisotropy energy are still unclear.The research in this thesis may help to reveal the dynamics of the AFM DW induced by voltage,paving a way for developing novel magnetic information devices with ultralow dissipation and high processing speed.Firstly,based on the basic principle of the AFM micromagnetic theory,we have derived the dynamics equation depicting the motion of the AFM DW induced by the magnetic anisotropy gradient.The solution of the equation shows that when the gradient of magnetic anisotropy constant(d K/dx)is in the range between 0 and 300 GJ/m~4,the DW velocity can reach a stable value that is higher than 100 m/s.On the other hand,based on the inertia of the AFM DW motion,the oscillating propulsion of the AFM DW under a pulsed voltage is also investigated,and the result indicates that a low damping coefficient is vital to the continuous AFM DW motion at high velocity.Finally,the inertia of the AFM DW is explained based on the principle of analytical mechanics.In order to verify the results of the theoretical analysis,we also carried out simulation based on numerically solving the atomistic Landau-Lifsthiz-Gilbert equation,and the simulation results are in good agreement with the theoretical solution.Secondly,based on the atomistic simulation,we have also studied the emission of THz spin wave from a moving AFM DW at high velocity under high gradient of magnetic anisotropy energy(d K/dx is 800 GJ/m~4 or higher).The result shows that under high d K/dx,the AFM DW velocity sharply increases with obvious increase in the DW width,which is accompanied by the emission of wake-type spin wave with the frequency between 0.2 THz and 0.4 THz,and the frequency can be tuned by DC voltage.When the DW moves at high velocity,the decrease in DW energy exceeds the increase in kinetic energy of the DW,and the total reducing energy is carried by the emitted spin wave.This work paves a way to develop a novel on-chip THz device with ultralow dissipation since THz spin wave is emitted under DC voltage.