Magnetic and electron transport properties of the rare earth cobaltates, La_0.7−xLn_xCa_0.3CoO₃ (Ln=Pr, Nd, Gd and Dy): a case of phase separation

Abstract

Magnetic and electrical properties of four series of rare earth cobaltates of the formula La_0.7−xLn_xCa_0.3CoO₃ with Ln=Pr, Nd, Gd and Dy have been investigated. Compositions close to x=0.0 contain large ferromagnetic clusters or domains, and show Brillouin-like behaviour of the field-cooled DC magnetization data with fairly high ferromagnetic T_c values, besides low electrical resistivities with near-zero temperature coefficients. When x>0.0, the zero-field-cooled data generally show a non-monotonic behaviour with a peak at a temperature slightly lower than T_c. The compositions near x=0.0 show a prominent peak corresponding to the T_c in the AC susceptibility data. The ferromagnetic T_c varies linearly with x or the average radius of the A-site cations, <r_A>. With increase in x or decrease in <r_A> , the magnetization value at any given temperature decreases markedly and the AC susceptibility measurements show a prominent transition arising from small magnetic clusters with some characteristics of a spin glass. Electrical resistivity increases with increase in x, showing a significant increase around a critical value of x or <r_A> , at which composition the small clusters also begin to dominate. These properties can be understood in terms of a phase separation scenario wherein large magnetic clusters give way to smaller ones with increase in x, with clusters of both types being present in certain compositions. The changes in magnetic and electrical properties occur in parallel since the large ferromagnetic clusters are hole rich and the small clusters are hole poor. Variable range hopping seems to occur at low temperatures in these cobaltates.