Low-energy excitations and magnetic anisotropy of the layered van der Waals antiferromagnet Ni$_2$P$_2$S$_6$

TL;DR

This study investigates the magnetic excitations and anisotropy in Ni$_2$P$_2$S$_6$, revealing a positive easy-plane anisotropy, anisotropic spin fluctuations near the Néel temperature, and a low-energy magnon mode, aiding in modeling its magnetic behavior.

Contribution

First high-frequency electron spin resonance measurements on Ni$_2$P$_2$S$_6$, determining its single ion anisotropy and revealing detailed magnetic excitation spectra.

Findings

Positive easy-plane anisotropy of Ni$^{2+}$ ions with an upper limit of ~0.7 meV

Strong anisotropic spin fluctuations near $T_N=158$ K

Low-energy magnon excitation ~1 meV gap

Abstract

The quasi-two-dimensional antiferromagnet Ni$_2$P$_2$S$_6$ belongs to the family of magnetic van der Waals compounds that provides a rich material base for the realization of fundamental models of quantum magnetism in low dimensions. Here, we report high-frequency/high-magnetic field electron spin resonance measurements on single crystals of Ni$_2$P$_2$S$_6$. The results enable one to reliably determine the positive, "easy"-plane type of the single ion anisotropy of the Ni$^{2+}$ ions with the upper limit of its magnitude $\lesssim 0.7$ meV. The resonance response reveals strongly anisotropic spin fluctuations setting in shortly above the N\'eel temperature $T_{\rm N} = 158$ K and extending in the antiferromagnetically ordered state down to low temperatures. There, a low-energy magnon excitation gapped from the ground state by only $\sim 1$ meV was found. This magnon mode may explain unusual low temperature relaxation processes observed in a latest nuclear magnetic resonance experiments and together with the estimate of the single ion anisotropy they should enable setting up a realistic spin model for an accurate description of magnetic properties of Ni$_2$P$_2$S$_6$.