Study of Mn₂Ga₂S₅ spin system and its magnetic frustration via electron spin resonance

Geometrical spin frustration, which can give rise to exotic disordered spin phases such as spin glasses, spin liquids and spin ices, is of great interest. Mn₂Ga₂S₅ exhibits a large frustration index of about 20, the ratio of Weiss temperature to the spin freezing temperature, and exhibits classical spin glass behavior, where the AC susceptibility cusp shifts to higher temperatures with increasing frequency. We observe a broad electron spin resonance (ESR) line separated from the free ion (g=2) line of Mn₂Ga₂S₅ below 50 K. This line shifts toward lower magnetic field as the temperature decreases, and its shift and width vary almost linearly with temperature, both reaching nearly a couple of Tesla at 4 K. When a magnetic field is applied in the direction along the crystal plane, the ESR signal spectral range spreads over almost the entire measurement spectral range of about 10 T. These unusual ESR line characteristics suggest magnetic frustration in combination with strong spin-spin interactions. We discuss the dependence of the ESR line shift on the orientation of the applied magnetic field with respect to the crystal plane using a model spin Hamiltonian incorporating a two-sublattice antiferromagnet with single-ion anisotropy. We further discuss a more general spin Hamiltonian model to account for possible magnetic frustration in the spin system.