Ordered magnetic phases of the frustrated spin-dimer compound Ba3Mn2O8

TL;DR

This study investigates the high-field magnetic phases of the frustrated spin-dimer compound Ba3Mn2O8, revealing distinct ordered states influenced by anisotropy and frustration through thermodynamic measurements up to 32 T.

Contribution

First thermodynamic characterization of Ba3Mn2O8's high-field ordered phases, identifying different magnetic structures depending on field orientation and analyzing their origins.

Findings

Single magnetic phase for H parallel to c above 8.7 T.

Two distinct phases for H perpendicular to c.

Analysis suggests spiral and modulated magnetic structures.

Abstract

Ba3Mn2O8 is a spin-dimer compound based on pairs of S=1, 3d^2, Mn^{5+} ions arranged on a triangular lattice. Antiferromagnetic intradimer exchange leads to a singlet ground state in zero-field. Here we present the first results of thermodynamic measurements for single crystals probing the high-field ordered states of this material associated with closing the spin gap to the excited triplet states. Specific heat, magnetocaloric effect, and torque magnetometry measurements were performed in magnetic fields up to 32 T and temperatures down to 20 mK. For fields above H_{c1} ~ 8.7 T, these measurements reveal a single magnetic phase for H parallel to c, but two distinct phases (approximately symmetric about the center of the phase diagram) for H perpendicular to c. Analysis of the simplest possible spin Hamiltonian describing this system yields candidates for these ordered states corresponding to a simple spiral structure for H parallel to c, and to two distinct modulated phases for H perpendicular to c. Both single-ion anisotropy and geometric frustration play crucial roles in defining the phase diagram.