Evidence of spin transition and charge order in cobalt substituted La0.7Ca0.3MnO3

The transport and magnetic studies of a series of compounds having the general formula La0.7Ca0.3Mn1-xCoxO3 (0.1 # < x < 0.9) have shown that Co3+ can exist both in high spin and low spin states (diamagnetic state). This is found to affect greatly the transport and magnetic properties of the system at low temperature by varying the strengths of both double-exchange (DE) and superexchange (SE) interactions. The introduction of cobalt changes the mobile carrier concentration, so even with 10% of Co concentration the strength of DE is found to decrease considerably. For x # > 0.25 there is a clear spin transition at low temperature from the high to the low spin state of trivalent cobalt and this leads to change in ferromagnetic (FM) and antiferromagnetic (AFM) phases. For x # > 0.25 there are two transitions for each value of x: the upper one gives the FM and AFM spin arrangement depending upon whether the DE or the SE dominates; the lower one is obtained due to the transition from the high to the low spin state of the trivalent cobalt ion.