Effect of Ru doping on magnetoresistance and magnetocaloric effect in Bi0.4Ca0.6Mn1−xRuxO3 (≤x≤0.2)

Abstract

We show that Ru doping in Bi0.4Ca0.6Mn1−xRuxO3, unlike other magnetic ions, destabilizes charge ordering and transforms the charge-ordered antiferromagnetic insulator (x=0) into a ferromagnetic metal for x≥0.1. The ferromagnetic transition temperature (TC) increases from TC=130 K for x=0.1 to 272 K for x=0.2 and the resistivity of the later compound at 10 K is eight orders of magnitude lower than that of x=0. The magnetoresistance at H=7 T has a maximum value (−98%) for x=0.05 but it decreases to −20% for x=0.2. While magnetic entropy (ΔSm) for x=0.05 is positive just above the charge ordering temperature, it changes sign with lowering temperature, and is negative over a wide temperature range for x=0.1 and 0.2. The x=0.2 sample shows the highest value of ΔSm=−1.83 J/kg K for ΔH=5 T in the series. Our results suggest that while Ru doping induces ferromagnetic clusters locally in the charge-ordered matrix for x=0.05, charge ordering is completely absent in x=0.2. The greater ability of Ru to induce long range ferromagnetism and insulator-metal transition in robust charge-ordered materials such as Bi0.4Ca0.6MnO3 is quite interesting from the point of view of fundamental physics and applications.