Magnetic structure of the magnetoelectric material Ba$_2$MnGe$_2$O$_7$

TL;DR

This study elucidates the magnetic structure of Ba$_2$MnGe$_2$O$_7$ in its low-temperature magnetoelectric state using neutron diffraction and magnetization measurements, revealing antiferromagnetic order, possible non-collinear spin arrangements, and a detailed phase diagram.

Contribution

The paper provides the first detailed determination of the magnetic structure of Ba$_2$MnGe$_2$O$_7$ in its magnetoelectric state, including symmetry analysis and comparison with related compounds.

Findings

Magnetic structure determined below 4 K with antiferromagnetic order along [110] or [100]

Ordered magnetic moment of 3.24(3) μ_B/Mn at 2.5 K

Temperature-field phase diagram mapped via magnetization measurements

Abstract

A detailed investigation of Ba$_2$MnGe$_2$O$_7$ was performed in its low-temperature magnetoelectric state combining neutron diffraction with magnetization measurements on single crystals. In the paramagnetic state at 10 K, polarized neutron diffraction was applied to reveal the components of the susceptibility tensor. The crystal and magnetic structures below the antiferromagnetic transition temperature of $T_N \approx 4$ K were determined using unpolarized neutron diffraction. This data implies no structural phase transition from 10 K down to 2.5 K and is well described within the tetragonal space group $P\bar{4}2_1m$. We found that in zero magnetic field the magnetic space group is either $C_cmc2_1$ or $P_c2_12_12_1$ with antiferromagnetic order along the [110] or [100] directions, respectively, while neighboring spins along the [001] axis are ordered antiferromagnetically. A non-collinear spin arrangement due to small canting within the $ab$ plane is allowed by symmetry and observed experimentally. The ordered moment is found to be 3.24(3) $\mu_B$/Mn$^{2+}$ at 2.5 K and the temperature-field dependent magnetic phase diagram is mapped out by macroscopic magnetization. Distinct differences between the magnetic structure of Ba$_2$MnGe$_2$O$_7$ as compared to those of Ba$_2$CoGe$_2$O$_7$ and Ca$_2$CoSi$_2$O$_7$ are discussed.