Time dependent effects and transport evidence for phase separation in La_0.5Ca_0.5MnO₃

The ground state of La_1-xCa_xMnO₃ changes from a ferromagnetic metallic to an antiferromagnetic charge-ordered state as a function of Ca concentration at x∼0.50. We present evidence from transport measurements on a sample with x=0.50 that the two phases can coexist, in agreement with other observations of phase separation in these materials. We also observe that, by applying and then removing a magnetic field to the mainly charge-ordered state at some temperatures, we can "magnetically anneal" the charge order, resulting in a higher zero-field resistivity. We also observe logarithmic time dependence in both resistivity and magnetization after a field sweep at low temperatures.