How a certain ground state of complex physical systems emerges, especially in two-dimensional materials, is a fundamental question in condensed-matter physics. A particularly interesting case is systems belonging to the class of XY Hamiltonian where the magnetic order parameter of conventional nature is unstable in two-dimensional materials leading to a Berezinskii-Kosterlitz-Thouless transition. Here, we report how the XXZ-type antiferromagnetic order of a magnetic van der Waals material, NiPS3, behaves upon reducing the thickness and ultimately becomes unstable in the monolayer limit. Our experimental data are consistent with the findings based on renormalization group theory that at low temperatures a two-dimensional XXZ system behaves like a two-dimensional XY one, which cannot have a long-range order at finite temperatures. This work provides experimental examination of the XY magnetism in the atomically thin limit and opens new opportunities of exploiting these fundamental theorems of magnetism using magnetic van der Waals materials.
@article{arxiv.1901.10890,
title = {Suppression of magnetic ordering in XXZ-type antiferromagnetic monolayer NiPS3},
author = {Kangwon Kim and Soo Yeon Lim and Jae-Ung Lee and Sungmin Lee and Tae Yun Kim and Kisoo Park and Gun Sang Jeon and Cheol-Hwan Park and Je-Geun Park and Hyeonsik Cheong},
journal= {arXiv preprint arXiv:1901.10890},
year = {2019}
}