Muon-spin-relaxation investigation of the spin dynamics of geometrically frustrated chromium spinels

dc magnetic susceptibility (X) and muon-spin-relaxation measurements (μSR) on the geometrically frustrated systems MgCr₂O₄ and CdCr₂O₄ indicate a transition from a paramagnetic state to a quasistatic spin state at 12.5 K and 7.8 K, respectively. Comparison of low-temperature field strength measured by μSR to that predicted by a dipole model suggests that only about 15% of the spin is static on the muon time scale. We observe slow spin fluctuations persisting to millikelvin temperatures, indicating that extremely low-energy magnetic excitations are present in these materials to the lowest temperatures. Our results demonstrate that substantial magnetic frustration remains, in spite of the tetragonal distortion occurring at T_N.