Guiding spin waves with hyperbolic magnets

Conventional magnets that host spin-wave excitations offer a promising platform for charge-free coherent spin transport with low dissipation, but are constrained by limited wavevectors and a weak coupling to other magnetic components. Here, we discuss the realization of hyperbolic magnets which support highly directional spin waves at much higher momentum with an enhanced density of states enabling larger couplings. Based on anisotropic layered Heisenberg magnets, we propose a hyperbolic ferromagnet (HFM) as well as a hyperbolic antiferromagnet (HAFM), which show characteristic hyperbolic dispersions. The corresponding spin configurations are intrinsically unstable but can be dynamically stabilized by an externally applied spin torque and tuned to the operational frequency by magnetic fields. Furthermore, we demonstrate that hyperbolic magnets can naturally act as waveguides that efficiently transmit spin waves across narrow constrictions and facilitate strong focusing effects due to their inherent negative refraction. Hence, hyperbolic magnets provide a promising platform for future quantum magnonic applications.