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 ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ (CCPS) presents an interesting class of vdW solids where the coupling of $\mathrm{C}{\mathrm{r}}^{3+}$ moments and ordering of $\mathrm{C}{\mathrm{u}}^{1+}$ ions give rise to a multiferroic ground state. Here we investigate the spin dynamics of $\mathrm{C}{\mathrm{r}}^{3+}$ ions in CCPS through magnetization and broadband as well as single sub-THz magnetic resonance measurements. The orbital moment of $\mathrm{C}{\mathrm{r}}^{3+}$ 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 ($T_{\mathrm{N}}\approx 32\mathrm{K}$) points to a short-range magnetic order in CCPS and motivate 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.

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• Antiferroelectricity
• Ferroelectricity
• Magnetic interactions
• Magnetic order
• Magnetic phase transitions
• Magnetic susceptibility
• Magnetization dynamics
• Spin dynamics