Effect of Cu doping at the Mn site on the transport aaand magnetic behaviors of La _2/3 Ca _1/3 MnO₃

Abstract:

We show that the replacement of Mn with Cu, without any structure change, strongly affects the transport and magnetic behaviors in La _2/3 Ca _1/3 Mn_1-xCu_xO _{3 - δ} (x ?0.3). Doping largely shifts the insulator-metal transition towards the low-temperature regime and causes substantial enhancements in both resistance and magnetoresistance near the transition for doping range of x ? 15%, and further doping to x ≥ 20% shows insulating behavior. For 15% doping, the sample shows large magnetoresistances (∼10⁴%) over the whole low temperature range. Magnetization measurement at helium temperature reveals that samples for x ? 10% and ≥ 20% basically show ferromagnetic (FM) and antiferromagnetic (AF) behaviors, respectively, while the 15% Cu doped sample undergoes a metamagnetic transition from AF state to FM state on increasing field. We argue that doping introduces the AF insulating phase into the system, causing unusual transport and magnetic behaviors.