We present an extensive experimental and theoretical study on the low-temperature magnetic properties of the monoclinic anhydrous alum compound BaMo(PO4)2. The magnetic susceptibility reveals strong antiferromagnetic interactions θCW=−167~K and long-range magnetic order at TN=22~K, in agreement with a recent report. Powder neutron diffraction furthermore shows that the order is collinear, with the moments near the ac plane. Neutron spectroscopy reveals a large excitation gap Δ=15~meV in the low-temperature ordered phase, suggesting a much larger easy-axis spin anisotropy than anticipated. However, the large anisotropy justifies the relatively high ordered moment, N\'{e}el temperature, and collinear order observed experimentally, and is furthermore reproduced in a first principles calculations using a new computational scheme. We therefore propose BaMo(PO4)2 to host S=1 antiferromagnetic chains with large easy-axis anisotropy, which has been theoretically predicted to realize novel excitation continua.
@article{arxiv.1912.03969,
title = {Large Easy Axis Anisotropy in the One-Dimensional Magnet BaMo(PO$_4$)$_2$},
author = {Aly H. Abdeldaim and Danis I. Badrtdinov and Alexandra S. Gibbs and Pascal Manuel and Helen C. Walker and Manh Duc Le and Chien Hung Wu and Dariusz Wardecki and Sten-Gunnar Eriksson and Yaroslav O. Kvashnin and Alexander A. Tsirlin and Gøran J. Nilsen},
journal= {arXiv preprint arXiv:1912.03969},
year = {2019}
}