Low-energy excitations and magnetic anisotropy of the layered van der Waals antiferromagnet Ni2P2S6

The quasi-two-dimensional antiferromagnet Ni2P2S6 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 Ni2P2S6. The results enable one to reliably determine the positive, ???easy???-plane type of the single ion anisotropy of the Ni2+ ions with the upper limit of its magnitude <0.7 meV. The resonance response reveals strongly anisotropic spin fluctuations setting in shortly above the Neel temperature TN = 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 ???1 meV was found. This magnon mode may explain unusual low temperature relaxation processes observed in the 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 Ni2P2S6.

Automated summary

The quasi-two-dimensional antiferromagnet Ni2P2S6 is an important material for realizing fundamental models of quantum magnetism in low dimensions. Through high-frequency/high-magnetic field electron spin resonance measurements, researchers were able to determine the single ion anisotropy of the Ni2+ ions and observe strongly anisotropic spin fluctuations above the Neel temperature. Additionally, a low-energy magnon excitation was discovered, which may explain unusual low temperature relaxation processes observed in recent nuclear magnetic resonance experiments.