Ultrastrong Magnon-Magnon Coupling and Chirality Switching in Antiferromagnet CrPS₄

As new information carriers, antiferromagnetic magnons have great potential in the fields of spintronics and quantum information. However, the strong exchange interaction between sublattice spins in conventional antiferromagnets results in their frequencies up to the terahertz (THz) range, hindering further exploration of related applications and physics. Recently, emerging van der Waals A-type antiferromagnets with the weak exchange interaction may bring about a change. In this study, it demonstrates two distinct tunable ultrastrong magnon-magnon couplings in the gigahertz (GHz) band using this type of antiferromagnet, CrPS4, with a maximum normalized coupling strength (& eta;) of 0.31. It establishes orthorhombic and monoclinic models for theoretical analyses, unambiguously showing that the ultrastrong coupling strength is caused by unique magnetocrystalline anisotropy rather than exchange enhancement. Furthermore, for the first time, it observes a continuous switching process of sublattice magnon chirality arising from the orthorhombic nature of anisotropy. These findings not only deepen the understanding of antiferromagnetic spin dynamics but also offer a powerful platform for building magnonic quantum systems and chirality-based spintronics.