Normal and inverse magnetocaloric effect in colossal magnetoresistive electron-doped manganites R0.15Ca0.85MnO3 (R = Y, Gd and Dy)

Abstract

Abstract Magnetic, magnetocaloric and electrical transport properties of polycrystalline R0.15Ca0.85MnO3 (R = Y, Gd and Dy) manganites (Orthorhombic, Pnma) have been studied. Magnetization measured in 5 kOe field reveals antiferromagnetic transition at 111 K, 119 K and 112 K (TN) respectively in R0.15Ca0.85MnO3 (R = Y, Gd and Dy). Magnetization in the ordered state undergoes field-induced transition and at 2 K, in 140 kOe field, the magnetic moment values are 1.5 μB/f.u., 2.8 μB/f.u. and 2.7 μB/f.u. respectively for R0.15Ca0.85MnO3 (R = Y, Gd and Dy) samples. While the electrical resistivity above TN shows non-adiabatic, small polaron hopping behavior, the resistivity value increases rapidly below TN. Hysteresis is observed around TN between field cooled cooling (FCC) and field cooled warming (FCW) magnetization data in 100 Oe and also between the cooling and warming data of zero-field electrical resistivity. Electrical resistivity exhibits a metal to insulator-like transition at TN in applied magnetic fields of 50 kOe and above. Colossal, negative magnetoresistance of ∼82%, 85% and 76% is observed at 50 K in 50 kOe field in R0.15Ca0.85MnO3 (R = Y, Gd and Dy) manganites. Magnetocaloric effect (MCE) is estimated in terms of isothermal magnetic entropy change (ΔSm) and inverse MCE (+ΔSm) is observed in the antiferromagnetically ordered state. The ΔSm changes sign for field changes greater than the metamagnetic critical magnetic field (Hc). Sign changing MCE signifies the evolution of underlying mixed magnetic interactions in these systems with variations in magnetic field and temperature.