Magnetic Properties and Magnetocaloric Effect in Pb-Doped Manganites

We have studied the magnetic properties and magnetocaloric effect of rhombohedral (La_0.9Dy_0.1)_1-xPb_xMnO₃ (x = 0.1, 0.2, and 0.3) fabricated by solid-state reaction. Thermomagnetization curve reveals that the increase of Pb doping in (La_0.9Dy_0.1)_1-xPb_xMnO₃ shifts the ferromagnetic (FM)-paramagnetic (PM) phase-transition temperature (T_C) toward room temperature; the TCvalues determined for x = 0.1, 0.2, and 0.3 are about 172, 249, and 322 K, respectively. Based on magnetic-field dependencies of magnetization, M(H), the magnetic entropy changes (ΔS_m) of the samples were calculated. Under an applied magnetic field of H = 10 kOe, the |ΔS_max| value slightly increases from 0.74 J/kg · K for x = 0.1 to about 1.1 J/kg· K for x = 0.2 and 0.3. These results correspond to a relative cooling power of ~50 J/kg, which is comparable with some perovskite manganites, indicating the applicability of (La_0.9Dy_0.1)_1-xPb_xMnO₃ in magnetic refrigeration devices working around room temperature. Particularly, we find that the variation of |ΔS_max| versus the magnetic field H can be described by a power law |ΔS_max| α Hn, where the magnetic-ordering parameter (n) decreases from 0.861 for x = 0.1-0.834 for x = 0.3. A deviation of the n values estimated from the mean-field theory (n = 2/3) demonstrates an existence of short-range FM order in the samples.