Cr3+ spin dynamics under the octahedral crystal field in van der Waals antiferromagnets

Abstract: The magnetic moment in van der Waals (vdW) materials containing 3d transition metals originates from unpaired d-electron spins and their interaction with surrounding ligands. The interplay between exchange interactions and magnetic anisotropy stabilizes long-range ordering of such moments. The compound CuCrP2S6 (CCPS) presents an interesting class of vdW solids where the coupling of Cr3+ moments and ordering of Cu1+ ions give rise to a multiferroic ground state. Here we investigate the spin dynamics of Cr3+ ions in CCPS through magnetization and broadband as well as single sub-THz magnetic resonance measurements. The orbital moment of Cr3+ is quenched under the octahedral crystal field of surrounding chalcogen ions, resulting in negligible magnetic anisotropy-a feature common to Cr-based vdW antiferromagnets (AFM). Resonance spectra over a wide frequency-field-temperature range reveal quasi-2D AFM dynamics governed mainly by isotropic Cr-Cr exchange interactions, which determine the magnetic order, spin reorientation, and damping. Sub-THz resonance spectra also uncover a field-induced ferromagnetic polarization, highlighting the universal role of Cr-Cr exchange in layered Cr compounds. Moreover, persistent magnetic correlations far above the Néel temperature (TN ~ 32 K) points to short-range magnetic order in CCPS and motivates future studies of a possible interplay between AFM and antiferroelectric orders. These results establish CCPS as an exemplary system for exploring 2D magnetism and electric-field-tunable spintronic functionalities in layered multiferroics.