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

A detailed study of the structural and magnetic properties of a garnet compound Mn₃Cr₂Ge₃O₁₂ is carried out using x-ray diffraction, magnetization, heat capacity, and neutron diffraction measurements. This compound manifests two successive magnetic transitions at TN1 4.5 K and TN2 2.7 K. Neutron powder diffraction experiments reveal that these two transitions correspond to the orderings of Cr³⁺ and Mn²⁺ sub-lattices, respectively. The ordering at TN1 is found to be collinear antiferromagnetic type while the one at TN2 is of non-collinear antiferromagnetic type. There appears to be several interactions, both ferromagnetic and antiferromagnetic, between two sub-lattices yielding a three-dimensional frustrated pyrochlore geometry. 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. Moreover, it shows a large magnetocaloric effect at low temperatures with a maximum value of isothermal entropy change Δ Sₘ -23 J/kg-K and adiabatic temperature change Δ Tad 9 K for a field change of 7~T. Further, a large value of relative cooling power (RCP 360 J/kg) reflects the usefulness of the compound in magnetic refrigeration purpose.