Magnetic and electron transport properties of the rare earth cobaltates,La0.7−xLnxCa0.3CoO3 (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 formulaLa0.7−xLnxCa0.3CoO3 withLn = Pr, Nd, Gd and Dy have been investigated. Compositions close tox = 0.0 contain large ferromagnetic clusters or domains, and show Brillouin-like behaviourof the field-cooled DC magnetization data with fairly high ferromagneticTc values, besides low electrical resistivities with near-zero temperature coefficients. Whenx>0.0, thezero-field-cooled data generally show a non-monotonic behaviour with a peak at a temperature slightly lowerthan Tc. Thecompositions near x = 0.0 show a prominent peak corresponding to theTc in the AC susceptibility data. The ferromagneticTc varieslinearly with x or the average radius of the A-site cations, . With increase in x or decrease in , the magnetization value at any given temperature decreases markedly and the ACsusceptibility measurements show a prominent transition arising from small magnetic clusterswith some characteristics of a spin glass. Electrical resistivity increases with increase inx, showing a significant increase around a critical value ofx or , at which composition the small clusters also begin to dominate. Theseproperties can be understood in terms of a phase separation scenariowherein large magnetic clusters give way to smaller ones with increase inx, with clusters of both types being present in certain compositions. The changes inmagnetic and electrical properties occur in parallel since the large ferromagnetic clustersare hole rich and the small clusters are hole poor. Variable range hopping seems to occur atlow temperatures in these cobaltates.