Unusual relaxational dynamics in La_1-xCa_xMnO₃ (x = 0.33 and 0.67)

Muon spin relaxation studies on La_1-xCa_xMnO₃ (x = 0.33 and 0.67) powders show that the muon dynamic relaxation function changes from exponential above the magnetic transition temperature to stretched exponential below the critical temperature (T_C = 274 K for x = 0.33, T_N = 150 K for x = 0.67). This suggests a spatially inhomogeneous distribution of very slow Mn-ion correlation times. An anomalously small applied field destroys the dynamical signature of the ferromagnetic transition for x = 0.33, but not the antiferromagnetic transition for x = 0.67. The x = 0.67 system also shows the evidence of a change in the magnetic phase in zero field near T = 75 K.