Spin state of Co3+ and magnetic transitions in RBaCo2O5.50 (R=Pr,Gd):: Dependence on rare-earth size

We have studied the magnetic and structural properties of PrBaCo2O5.50 and GdBaCo2O5.50 using neutron and synchrotron techniques between 10 and 425 K. A unique structural transition has been detected in this temperature range. The changes observed in Co-O bond lengths across the metal-insulator transition confirm the spin state change of octahedral Co ions with R=Pr, the member of the RBaCo2O5.50 family with the largest R-ion (and R/Ba order). Magnetic structures have been thoroughly investigated. Different magnetic models previously reported for smaller lanthanides were checked against our data. We have found an evolution of the magnetic order, across the successive magnetic transitions, which can be interpreted on the basis of a hierarchical set of exchange interactions.