A.E. Carrillo

Magnetism and vacancy ordering in PrBaCo2O5+δ (δ⩾0.50)

This work presents an investigation of the central role of oxygen on the physical properties of highly ordered layered cobaltites. It includes the study of the structural and magnetic properties of PrBaCo2O5+δ compounds with high oxygen content (0.5⩽δ⩽0.90), with special emphasis on PrBaCo2O5.75. For δ=0.75 the ordering of oxygen vacancies gives rise to a tetragonal (P4∕mmm) structure with a≈c≈2ap. On cooling, a ferromagnetic spin alignment precedes the apparition of antiferromagnetic order (canted G-type). The compounds are ferromagnetic for δ⩾0.80. Measurements on PrBaCo2O5.75 support that, above the magnetic ordering temperature, Co3+ ions in octahedral environment are in HS state.

Spin state of Co{sup 3+} and magnetic transitions in RBaCo{sub 2}O{sub 5.50} (R=Pr,Gd): Dependence on rare-earth size

We have studied the magnetic and structural properties of PrBaCo{sub 2}O{sub 5.50} and GdBaCo{sub 2}O{sub 5.50} 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 RBaCo{sub 2}O{sub 5.50} 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.