Conductivity, spin-state and magnetostructural transitions in cobalt perovskites investigated by x-ray spectroscopies and neutron diffraction

Abstract

The spin-state of trivalent cobalt is being examined in cobaltites with CoO6 octahedral units because of its proved ability to condition their transport, magnetic and electronic properties. We have investigated metal-insulator and conductivity transitions in a variety of cobalt compounds with perovskite and related structures such as LnCoO3, Ln1-xAxCoO3, LnBaCo2O5.50 or Ba2Co9O14 (BCO) presenting spin-state instabilities. Neutron and synchrotron diffraction, x-ray absorption and x-ray emission spectroscopies, and x-ray magnetic circular dichroism techniques have all been employed to probe the electronic and spin state of Co ions in different regimes and across the magnetostructural transitions of these compounds. In particular cases films could also be grown by pulsed laser deposition. In half-doped Pr0.50A0.50CoO3 metallic perovskites, the spin-lattice coupling brings about distinct magnetostructural transitions for A=Ca and A=Sr close to ~100 K. However, the ground magnetic properties of Pr0.50Sr0.50CoO3 (PSCO) strongly differ from Pr0.50Ca0.50CoO3 (PCCO) ones, where a partial Pr3+ to Pr4+ valence shift and Co spin transition makes the system insulating. Instead of spin-state changes, the magnetostructural transition and the magnetic anomalies in the PSCO perovskite can be ascribed to a magnetic symmetry transition from Im'm'a to Fm'm'm, which encompasses a sudden reorientation of the magnetization axis by 45º within the a-b plane and the appearance of conjugated magnetic domains. Finally, Ba2Co9O14 presents five different Co sites and its low temperature magnetic symmetry is PS-1. Two out of the three low-spin CoO6 octahedral sites adopt a higher spin state at the conductivity transition around 567 K. The origin of the strong moment reduction found at the nominally “divalent high spin” Co5 octahedral positions was carefully investigated.