Anisotropic phase diagram of the frustrated spin dimer compound Ba3Mn2O8

TL;DR

This study maps the anisotropic phase diagram of Ba3Mn2O8 using heat capacity and magnetic torque measurements, revealing complex field-dependent phase behavior and non-monotonic evolution with field rotation.

Contribution

It provides the first detailed experimental phase diagram of Ba3Mn2O8 under various field orientations and tests a minimal spin Hamiltonian against observed phenomena.

Findings

Single magnetic phase along c axis

Two distinct phases along a axis

Non-monotonic phase evolution with field rotation

Abstract

Heat capacity and magnetic torque measurements are used to probe the anisotropic temperature-field phase diagram of the frustrated spin dimer compound Ba3Mn2O8 in the field range from 0T to 18T. For fields oriented along the c axis a single magnetically ordered phase is found in this field range, whereas for fields oriented along the a axis two distinct phases are observed. The present measurements reveal a surprising non-monotonic evolution of the phase diagram as the magnetic field is rotated in the [001]-[100] plane. The angle dependence of the critical field (Hc1) that marks the closing of the spin gap can be quantitatively accounted for using a minimal spin Hamiltonian comprising superexchange between nearest and next nearest Mn ions, the Zeeman energy and single ion anisotropy. This Hamiltonian also predicts a non-monotonic evolution of the transition between the two ordered states as the field is rotated in the a-c plane. However, the observed effect is found to be significantly larger in magnitude, implying that either this minimal spin Hamiltonian is incomplete or that the magnetically ordered states have a slightly different structure than previously proposed.