Thermal and magnetic instability near the percolation threshold of Nd0.5„Ca, Sr...0.5MnO3

We report the observation of an instability phenomena in Nd0.5~Ca,Sr!0.5MnO3 for cyclic runs between 300 and 10 K through resistivity, thermopower, and magnetization measurements. The instability is especially dramatic when the concentration of Sr is near the metal/insulator percolation threshold, and it manifests itself in the delicate balance between the external magnetic field and the intrinsic instability. We also demonstrate an effective way of turning the metallic or insulating phases by switching off the external field at particular temperatures. Further studies on time-relaxation effect reveal that the change of resistivity is logarithmic-time dependent and it occurs only within a small temperature range, suggesting that the transition is not only thermal assisted but also magnetically correlated.