Transport and Magnetic Properties of R1-xAxCoO3 (R=La, Pr and Nd; A=Ba, Sr and Ca)

TL;DR

This study investigates how doping with different alkaline earth metals affects the transport and magnetic properties of R1-xAxCoO3 perovskites, revealing phase transitions and spin state changes related to ionic radius and doping level.

Contribution

It provides new insights into the relationship between ionic radius, doping, and spin states, and reports a pressure-induced phase transition in Pr-based cobaltates.

Findings

Doping with Sr or Ba induces metallic ferromagnetism.

Ca-doped systems approach but do not reach metallic state.

Pressure reveals a phase transition in Pr1-xCaxCoO3.

Abstract

Transport and magnetic measurements have been carried out on perovskite Co-oxides R1-xAxCoO3 (R=La, Pr, and Nd; A=Ba, Sr and Ca; 0<x<0.5: All sets of the R and A species except Nd1-xBaxCoO3 have been studied.). With increasing the Sr- or Ba-concentration x, the system becomes metallic ferromagnet with rather large magnetic moments. For R=Pr and Nd and A=Ca, the system approaches the metal- insulator phase boundary but does not become metallic. The magnetic moments of the Ca-doped systems measured with the magnetic field H=0.1 T are much smaller than those of the Ba- and Sr-doped systems. The thermoelectric powers of the Ba- and Sr-doped systems decrease from large positive values of lightly doped samples to negative ones with increasing doping level, while those of Ca-doped systems remain positive. These results can be understood by considering the relationship between the average ionic radius of R1-xAx and the energy difference between the low spin and intermediate spin states. We have found the resistivity-anomaly in the measurements of Pr1-xCaxCoO3 under pressure in the wide region of x, which indicates the existence of a phase transition different from the one reported in the very restricted region of x~0.5 at ambient pressure [Tsubouchi et al. Phys. Rev. B 66 (2002) 052418.]. No indication of this kind of transition has been observed in other species of R.