Magnetic structure of the magnetoelectric material Ba2MnGe2O7

A detailed investigation of ${\mathrm{Ba}}_{2}{\mathrm{MnGe}}_{2}{\mathrm{O}}_{7}$ 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 ${T}_{\text{N}}\ensuremath{\approx}4\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ were determined using unpolarized neutron diffraction. These data imply no structural phase transition from 10 K down to 2.5 K and are well described within the tetragonal space group $P\overline{4}{2}_{1}m$. We found that in zero magnetic field the magnetic space group is either ${C}_{c}mc{2}_{1}$ or ${P}_{c}{2}_{1}{2}_{1}{2}_{1}$ with antiferromagnetic order along the [110] or [100] direction, respectively, while neighboring spins along the [001] axis are ordered antiferromagnetically. A noncollinear 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) ${\ensuremath{\mu}}_{\text{B}}/{\mathrm{Mn}}^{2+}$ 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 ${\mathrm{Ba}}_{2}{\mathrm{MnGe}}_{2}{\mathrm{O}}_{7}$ as compared to those of ${\mathrm{Ba}}_{2}{\mathrm{CoGe}}_{2}{\mathrm{O}}_{7}$ and ${\mathrm{Ca}}_{2}{\mathrm{CoSi}}_{2}{\mathrm{O}}_{7}$ are discussed.