Cluster-glass behaviour and large magnetocaloric effect in frustrated hyperkagome ferromagnet Li₂MgMn₃O₈

A detailed study of the structural and magnetic properties of the spin-$3/2$ hyperkagome lattice compound Li₂MgMn₃O₈ is reported. This material shows ferromagnetic response below TC 20.6 K, the temperature almost three times lower than the Curie-Weiss temperature θCW 56.6 K. Density-functional band-structure calculations suggest that this reduction in TC may be caused by long-range antiferromagnetic couplings that frustrate nearest-neighbor ferromagnetic couplings on the hyperkagome lattice. Large magnetocaloric effect is observed around the TC with a maximum value of isothermal entropy change Δ Sₘ 20 J/kg-K and a maximum relative cooling power of RCP 840 J/kg for the 7 T magnetic field change. Critical analysis of the magnetization data and scaling analysis of the magnetocaloric effect suggest the 3D Heisenberg/XY universality class of the transition. The DC and AC magnetization measurements further reveal glassy nature of the ferromagnetic transition. A detailed study of the non-equilibrium dynamics via magnetic relaxation and memory effect measurements demonstrates that the system evolves through a large number of intermediate metastable states and manifests significant memory effect in the cluster-glass state.