Unusual relaxational dynamics in La1 − xCaxMnO3 (x = 0.33 and 0.67)

Muon spin relaxation studies on La1 − xCaxMnO3 (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 (TC = 274 K for x = 0.33, TN = 150 K for x = 0.67). This suggests a spatially inhomogeneous distribution of very slow Mn-ion correlation times. An anomalously small applied field destrroys the dynamical signature of the ferromagnetic transition for x = 0.3, 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.