Modelling the interaction between spin waves and domain walls in synthetic antiferromagnets

Abstract

[EN] Efficient modulation, manipulation and propagation of spin wave excitations in ordered materials at the nanoscale are fundamental requisites for the potential utilization of spin waves as information carriers in magnonics applications. The control of spin waves can be achieved through various methods, including electrical fields, strain, or magnetic fields, thereby opening up the possibility of realizing allmagnon spintronic devices. However, the large size of control terminals and high power consumption in these devices pose challenges for practical applications. In response to the modern need to minimize energy consumption for computational and data storage intensifies, alternative strategies to control spin wave dynamics are being actively pursued. Magnetic textures such as domain walls (DW), vortex, and skyrmion is being considered as potential alternatives in magnonics due to their capacity to generate and manipulate magnons at the nanoscale. Among these, domain walls (DW) have recently attracted substantial attention. Research has indicated that DW can be utilized to manipulate the phase and magnitude of coherent spin waves in a nonvolatile manner, underscoring their potential in controlling spin waves. Moreover, it has been demonstrated that spin waves, in turn, can alter the positions of magnetic domain walls through the spin-transfer torque effect generated from magnon spin current. Notwithstanding these advancements, the mutual interaction between domain walls and spin waves remains a subject of fundamental research. This thesis aims to explore novel phenomena based on domain walls in controlling spin wave propagation and generation of spin wave radiation in antiferromagnetic order using synthetic antiferromagnets through simulations and analytical results. First, using micromagnetic simulations we investigate a method to control spin waves propagation in synthetic antiferromagnets. We demonstrate that an external magnetic field can manipulate the interaction between domain walls and linearly polarized propagating spin waves in synthetic antiferromagnets. We find two regimes with a sharp transition between them. At large fields spin waves are strongly reflected by the domain wall and, consequently, the latter is propelled forward. At low fields, however, there is no reflection and yet the domain wall undergoes a small forward displacement, which is attributed to the change in linear momentum of the magnons as they pass through the domain wall and to the imbalance in the population of the two oscillation modes present in the linearly polarized excitation. In particular, we demonstrate that the transition between the two regimes occurs at the field value for which the excitation frequency coincides with that of the righthanded oscillation mode. In the second part, we focus on the response of a magnetic domain wall to an external magnetic field in a perpendicularly magnetized synthetic antiferromagnet using both micromagnetic simulations and a reduced model. We found that the external field induces a sizable displacement between the position of the domain wall in each layer, which can be larger than the domain wall width for a sufficiently strong field. We also study the dynamic evolution of the system when this field is applied or removed. In both cases we find a complex response with two distinct phases that involve both internal domain wall rotation and coupled interlayer domain wall oscillations. As a result of this dynamics spin waves are radiated. The emitted radiation is characterized by a broadband spectrum and can be detected far away from the domain wall.