Spin transition and thermal expansion in the layered cobaltite GdBaCo2O5.5

Abstract

The electrical conductivity and thermal expansion coefficient of GdBaCo2O5.5 samples have been measured in order to reveal the mechanism of the metal-insulator transition in cation-ordered cobaltites RBaCo2O5.5 (where R is a rare-earth element) and its relation to the change in the spin state of cobalt ions. It has been established that the unit cell volume considerably increases upon the transition to the metallic state at TMI ≈ 360–365 K and that the thermal expansion exhibits anomalies (which are two orders of magnitude weaker) due to the ferromagnetic and antiferromagnetic orderings. The data obtained confirm that the spin transition in Co3+ ions actually proceeds simultaneously with the metal-insulator transition and excludes the possibility of stepwise spin transitions occurring at lower temperatures.