Spectroscopic characterization of magnetic frustration in Mn₂Ga₂S₅

Geometrical spin frustration is of great interest in creating novel spin disordered states such as spin glasses or spin liquids. Mn₂Ga₂S₅ exhibits a large frustration index ~ 20, the ratio of Weiss temperature to the spin freezing temperature, and there is no long-range order above 2 K, which suggests the possibility of disordered spin states in this material. We observe a broad and strongly shifted peak in the ferromagnetic resonance (FMR) spectra of Mn₂Ga₂S₅ below 50 K. As the temperature is lowered, this peak shifts to fields lower than the free ion (g=2) spin resonance field. The peak shift (from the g=2 value) and linewidth increase almost linearly with temperature. Furthermore, while this shifted peak is observed at 240 GHz and 336 GHz, it is not observed at 120 GHz. These unusual spectral characteristics are unlike from those due to typical magnetocrystalline anisotropies and suggest an effective field of spins influenced by strong exchange interactions with surrounding frustrated spins.