Abstract

Under certain conditions, spin waves can be channeled into a broad angular spectrum of wave vectors, where the direction of the group velocity becomes independent of those wave vectors. Such highly focused waves are called caustic waves, whose properties can be manipulated by anisotropies or chiral interactions, like the Dzyaloshinskii-Moriya interaction. In this paper, we theoretically study the focusing features of the spin waves induced by the dipole-dipole interaction in synthetic antiferromagnets. For stacked systems, the dipolar interaction causes a noticeable frequency nonreciprocity when the magnetizations in both films are antiparallelly aligned, and then the focusing properties of the spin waves are enhanced. The role of thicknesses and magnetic graduation along the film's normal are systematically analyzed. We found that the degree of focalization of the spin waves can be manipulated by increasing the layers' thickness. Also, we show that the low- and high-frequency modes exhibit different focalization properties; the low-frequency mode manifests a similar behavior to the heavy-metal/ferromagnet systems with interfacial Dzyaloshinskii-Moriya interaction, while the high-frequency one tends the generate almost reciprocal interference patterns along one axis. In the case of magnetization-graded synthetic antiferromagnets, we demonstrate that the graduation slightly influences the low-frequency mode, while the focusing and nonreciprocal dynamic properties of the high-frequency ones are notoriously altered. The theoretical calculations are compared with micromagnetic simulations, where a good agreement is found between both methods. Our results demonstrate that a synthetic antiferromagnetic system allows for controlling the propagation of spin waves, assisting in the transfer of angular momentum and energy.