Magnetic properties of polycrystalline LnNi0.3Co0.7O3 (Ln=La, Pr) compounds

Abstract

Polycrystalline samples of LnNi0.3Co0.7O3 (Ln=La, Pr) were prepared through sol-gel precursors. The samples were subjected to several heat treatments at temperatures ranging from 650 to 1000 °C and under oxygen pressure as high as 70 bar. Characterizations of x-ray powder diffraction XRD magnetization M(T), and ac magnetic susceptibility χac(T) were performed on these compounds. The results of XRD showed that samples of LaNi0.3Co0.7O3 crystallize in a rhombohedral distorted perovskite structure (R3̄c) and that PrNi0.3Co0.7O3 compounds displayed an orthorhombically distorted structure (Pbnm). The temperature dependence of M(T) revealed the presence of a sharp cusp occurring at temperatures TP∼60 and ∼ 30 K for Ln=La and Pr, respectively. Below TP, the M(T) curves taken after different thermal cycles, i.e., field cooled and zero-field cooled processes, are histories dependent on the magnitude of M(T) decreasing with increasing applied magnetic field. We have also observed that TP decreases with increasing applied magnetic field. Measurements of χac(T) at zero applied magnetic field in all samples studied revealed one peak anomaly occurring at temperatures Tf=58 and 30 K for La and Pr, respectively. In fact, increasing frequency over four orders of magnitude results in a shift of the peak anomaly to higher temperatures, as expected for a spin-glass system. Also, a broadening of the peak anomaly is observed with the application of an external magnetic field and it evolves in two peaks at H as high as ∼ 500 Oe. Thus the χac(T) curves reveal two well-defined peaks: one, which is essentially frequency independent and another that is shifted to higher temperatures with increasing frequency. The origin of these magnetic behaviors is discussed.