Large Easy Axis Anisotropy in the One-Dimensional Magnet BaMo(PO${}_4$)${}_2$

Abstract

We present an extensive experimental and theoretical study on the low-temperature magnetic properties of the monoclinic anhydrous alum compound BaMo(PO${}_4$)${}_2$. The magnetic susceptibility reveals strong antiferromagnetic interactions $\hat{I}{¸}_{CW}=-167$~K and long-range magnetic order at $T_N=22$~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 $\hat{I}”=15$~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, Néel temperature, and collinear order observed experimentally, and is furthermore reproduced in a first principles calculations using a new computational scheme. We therefore propose BaMo(PO${}_4$)${}_2$ to host $S=1$ antiferromagnetic chains with large easy-axis anisotropy, which has been theoretically predicted to realize novel excitation continua.

11 pages, 7 figures