  Antiferromagnetic (AFM) magnons and the manipulation of magnetisation via
spin currents in van der Waals (vdW) materials offer substantial potential for
applications in magnonics and spintronics. In this study, we demonstrate
ultrastrong magnon-magnon coupling in the GHz regime within a vdW AFM,
achieving a remarkable maximum normalised coupling strength of 0.47. Our
investigation unveils the tunability of coupling strength through
temperature-dependent magnetic anisotropies. Additionally, we show the prospect
for further enhancement to the deep-strong magnon-magnon coupling regime by
manipulating the anisotropy. Furthermore, we report spin Hall magnetoresistance
in a vdW AFM-based device and study the field and temperature dependence. These
findings highlight the transformative potential of vdW AFMs in advancing the
field of spin-based technologies.
