Low-frequency spin dynamics of the quasi-two-dimensional S = 12 antiferromagnet BaCdVO(PO4)2

Abstract

A quasi-two-dimensional quantum magnet with competing ferro- and antiferromagnetic exchange interactions, $\mathrm{BaCdVO}{({\mathrm{PO}}_{4})}_{2}$, is studied using the magnetic resonance technique in a wide frequency range. The magnetic resonance spectra of the ordered phase confirm the collinear antiferromagnetic spin structure oriented along the $a$ axis. The frequency-field diagram of antiferromagnetic resonance demonstrates two low-frequency excitation branches with different energy gaps, indicating a biaxial nature of anisotropy. We observe a critical softening of the longitudinal resonance mode in a field of 3.8 T, which is the lower boundary of a previously proposed presaturation phase positioned below the saturation field of 6.5 T. An interpretation of the presaturation phase as a state with variable magnetization of spin-vacancy defects is proposed. It may be considered an alternative to the spin-nematic phase initially supposed.