dc magnetic susceptibility $\left(\chi \right)$ and muon-spin-relaxation measurements $\left(\mu \mathrm{SR}\right)$ on the geometrically frustrated systems ${\mathrm{MgCr}}_2{\mathrm{O}}_4$ and ${\mathrm{CdCr}}_2{\mathrm{O}}_4$ 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 $\mu \mathrm{SR}$ 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 $T_N.$

• Received 21 May 2002

DOI:https://doi.org/10.1103/PhysRevB.66.174434