Magnetic anisotropy and low-energy spin dynamics in the van der Waals compounds Mn2P2S6 and MnNiP

The multifrequency electron spin resonance study of the van der Waals compounds Mn${}_{2}$P${}_{2}$S${}_{6}$ and MnNiP${}_{2}$S${}_{6}$ shows that they are strongly anisotropic, each having a unique ground state and type of magnetic order. The authors find that increasing the Ni content yields a larger magnon gap in the ordered state and a larger $g$ factor value and its anisotropy in the paramagnetic state. Moreover, the character of the excitations in Mn${}_{2}$P${}_{2}$S${}_{6}$ undergoes a field-induced crossover from a 3D-like toward a 2D XY regime.