Eur. Phys. J. B 70, 327-334 (2009)
https://doi.org/10.1140/epjb/e2009-00228-x

Magnetic ground state and the spin-state transitions in YBaCo₂O_5.5

¹ Frank Laboratory of Neutron Physics, JINR, 141980 Dubna Moscow Reg., Russia
² Institute of Physics, Cukrovarnická 10, 162 53 Prague, Czech Republic

Corresponding author: ^a denk@nf.jinr.ru

Received: 19 November 2008
Revised: 27 May 2009
Published online: 3 July 2009

Abstract

The crystal and magnetic structure of the perovskite-like, oxygen deficient cobalt oxide YBaCo₂O_5.5 has been studied by means of neutron and X-ray diffraction in the 10–300 K temperature range. The magnetic ground state is characterized by a coexistence of two distinct antiferromagnetic phases. In the first one, the ionic moments of high-spin Co³⁺ ions in the pyramidal sites are ordered in a spiral arrangement, while octahedral sites are non-magnetic due to presence of low-spin Co³⁺ ions. The arrangement in the second phase is collinear of the G-type, with non-zero moments both in pyramidal (high-spin Co³⁺ ions) and octahedral sites (presumably a mixture of the low- and high-spin states). With increasing temperature, at 260–300 K, the system develops a gradual structural transformation, which is associated with appearance of spontaneous magnetic moment. This process is related to a thermally induced reversion of low- and high-spin states at the octahedral sites to the intermediate-spin Co³⁺ states, resulting in an insulator-metal transition at T_C ≈ T_IM ≈ 295 K.