Double magnetic transitions, complex field-induced phases, and large magnetocaloric effect in the frustrated garnet compound Mn3Cr2Ge3O12

A detailed study of the magnetic and magnetocaloric properties of the garnet compound ${\mathrm{Mn}}_{3}{\mathrm{Cr}}_{2}{\mathrm{Ge}}_{3}{\mathrm{O}}_{12}$ is carried out using x-ray diffraction, magnetization, heat capacity, and neutron diffraction measurements as well as ab initio band-structure calculations. This compound manifests two successive magnetic transitions at ${T}_{\mathrm{N}1}\ensuremath{\simeq}4.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{N}2}\ensuremath{\simeq}2.7\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Neutron powder diffraction experiments reveal that these two transitions correspond to the collinear and noncollinear antiferromagnetic ordering of the nonfrustrated ${\mathrm{Cr}}^{3+}$ and frustrated ${\mathrm{Mn}}^{2+}$ sublattices, respectively. The interactions within each of the Cr and Mn sublattices are antiferromagnetic, while the intersublattice interactions are ferromagnetic. The H-T phase diagram is quite complex and displays multiple phases under magnetic field, which can be attributed to the frustrated nature of the spin lattice. ${\mathrm{Mn}}_{3}{\mathrm{Cr}}_{2}{\mathrm{Ge}}_{3}{\mathrm{O}}_{12}$ shows a large magnetocaloric effect with a maximum value of isothermal entropy change $\mathrm{\ensuremath{\Delta}}{S}_{\mathrm{m}}\ensuremath{\simeq}\ensuremath{-}23$ J/kg K and adiabatic temperature change $\mathrm{\ensuremath{\Delta}}{T}_{\mathrm{ad}}\ensuremath{\simeq}9\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for a field change of 7 T. Further, a large value of the relative cooling power ($RCP\ensuremath{\simeq}360$ J/kg) demonstrates the promise of using this compound in magnetic refrigeration.