| Antiferromagnetic materials generally exhibit ultrafast spin dynamics with resonance frequencies in the terahertz range,which have broad applications in future high-frequency information processing and communication technologies.However,exciting antiferromagnetic terahertz signals require overcoming strong exchange fields,which need significant excitation power.Therefore,research on achieving low-power excitation of antiferromagnetic signals is of great academic significance and practical value.Antiferromagnetic domain walls are typical magnetic structures in antiferromagnetic media that can be effectively driven by their strong exchange energy arising from the non-parallel alignment of internal magnetic moments at low power consumption.To date,experimental and theoretical studies have revealed various methods for effectively driving antiferromagnetic domain wall motion,such as spin waves,electrical manipulation,temperature gradients,and microwave fields.Furthermore,antiferromagnetic domain walls exhibit dynamic behaviors analogous to inertial mass objects.When domain wall motion approaches the maximum group velocity,the domain wall width undergoes Lorentz contraction.However,current research is focused on the unidirectional motion of antiferromagnetic domain walls,and the spin oscillations excited by dynamics behavior of inertial mass has not yet to be explored.The possible spin oscillations excited by the motion of antiferromagnetic domain walls under various driving forces have been theoretically investigated.The research includes low-frequency（megahertz）oscillations and gigahertz chaotic precession related to the inertial motion of antiferromagnetic domain walls,THz magnon emission with tunable frequency caused by variable speed motion and inertia mass loss of domain walls.The research in this thesis covers the following four aspects:（1）Inertial motion induced megahertz oscillations of antiferromagnetic domain walls.Based on the inertial properties,we predict that antiferromagnetic domain walls driven by spatially sinusoidal stress can be excited to oscillate at frequencies up to hundreds of megahertz when the spatial period of stress is much larger than the domain wall width.Moreover,based on the damping oscillations of antiferromagnetic domain walls,we have developed a spatial stress phase switching circuit,which enables the continuous unidirectional motion of domain walls.The device exhibits a low power dissipation of approximately 3.5×10-15 W.Our research has discovered a new approach to extend the working frequency and decrease the power consumption of antiferromagnetic devices.（2）GHz chaotic spin precession of antiferromagnetic domain walls.The inertia of antiferromagnetic domain walls has been widely studied,but the rotational inertia of domain walls has received little attention due to the lack of effective excitation methods.In an antiferromagnetic spin valve containing domain walls,we predict that the precession of antiferromagnetic domain walls can be excited by spin-polarized currents along the anisotropy easy axis direction.Moreover,by introducing the interface Dzyaloshinskii-Moriya interaction into the antiferromagnetic layer,the azimuthal angle of the Néel vector at the center of the antiferromagnetic domain wall satisfies an equation similar to that of a simple pendulum motion,with the DMI acting as a restoring force.With low damping and moderate DMI,chaotic behavior appears in the domain wall precession.By controlling the driving current,interesting intermittent chaos can be generated.This work provides valuable references for the design of chaos-based signal devices based on antiferromagnetic domain walls.（3）The variable-speed motion of antiferromagnetic domain walls excites THz spin waves emission and magnetic frequency comb.Based on the spin-orbit torque effect,we numerically studied the THz spin waves excited by domain wall variable-speed motion during rising and falling stages of current pulses in a heavy metal/antiferromagnetic composite system.We found that during domain wall acceleration（deceleration）,forward and backward spin wave oscillations accompany it,with a frequency one order of magnitude lower than the exchange spin wave emitted by the domain wall due to Lorentz contraction under high-speed motion.The variation of spin wave frequency with current density can be explained by a two-magnon emission model.In addition,using the periodic back-and-forth variable-speed motion of the domain wall driven by pulsed current,a magnetic magnon frequency comb with a frequency spacing of GHz can be generated.（4）Loss of domain wall mass induces THz magnon.Under the driving force of magnetic anisotropy gradient,domain walls move towards low anisotropy regions,and their mass decreases as the anisotropy constant decreases.We demonstrated that the decrease in energy due to domain wall mass loss and the increase in magnon energy satisfy the Einstein mass-energy equivalence.The magnon spectrum has a series of characteristic peaks,and frequency tuning can be achieved by changing the anisotropy gradient via voltage control.This work promotes the study of relativistic effects in antiferromagnetic domain walls and provides new ideas for researching antiferromagnetic THz devices with tunable frequency.In summary,based on the inertia dynamics of antiferromagnetic domain walls,we theoretically reveal that the self-motion and magnon excitation associated with the antiferromagnetic domain walls can serve as potential sources of signals.The frequency of characteristic signals ranges from MHz to THz,providing a theoretical basis for nanoscale and high-frequency spintronic devices. |
