Interplay of structural and magnetic nanoscale phase separation in layered cobaltites red cobaltites

We report on the structural, electronic, and magnetic phases of a previously unexplored region in the phase diagram of GdBaCo2O5+? (?=0.57 and 0.63). Despite a homogenous average structure displayed by both the samples, the orthorhombic highly oxygenated GdBaCo2O5.63 shows clear signatures of structural nanoscale phase separation. By combining a pair distribution function with photoluminescence and electron spin resonance techniques, we found that the nanoscale phase separation is induced by an inhomogeneous distribution of ferromagnetic Co3+-Co4+ clusters embedded in an antiferromagnetic Co3+-rich matrix. In addition, we uncovered a phase evolution involving the collapse of the orthorhombic strain below room temperature. The origin of this noncanonical transition seems to be associated with the interplay of the observed nanoscale phase separation and a new magnetic phase transition occurring below T~180K.