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

TL;DR

This study reveals that BaMo(PO$_4$)$_2$ exhibits unexpectedly large easy-axis anisotropy in its magnetic properties, leading to collinear antiferromagnetic order and a significant excitation gap, supported by experimental and first-principles calculations.

Contribution

The paper demonstrates the presence of large easy-axis anisotropy in BaMo(PO$_4$)$_2$, combining experimental measurements with a new computational scheme to explain its magnetic behavior.

Findings

Strong antiferromagnetic interactions with $ heta_{CW} = -167$~K

Long-range magnetic order at $T_N=22$~K with collinear moments

Large excitation gap $oxed{ ext{15 meV}}$ indicating significant anisotropy

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 $\theta_{CW} = -167$~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 $\Delta = 15$~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(PO$_4$)$_2$ to host $S=1$ antiferromagnetic chains with large easy-axis anisotropy, which has been theoretically predicted to realize novel excitation continua.