Spin state of Co3+ ions in EuBaCo2–xO5.5–δ in the paramagnetic temperature range

Abstract

The magnetic studies of EuBaCo₂O_5.50, EuBaCo_1.93O_5.40, and EuBaCo_1.90O_5.36 have been performed. Layered cobaltites EuBaCo_2–xO_5.5–δ demonstrated the ferrimagnetic ordering in the temperature range 200 K < T < 650 K. The ferromagnetic Curie temperature T_C almost did not change as the number of Co³⁺–O–Co³⁺ bonds decreased. The contributions of the ferro- and antiferromagnetic interactions were nearly the same and determine the relationship T_C > T_N and T_C < T_N. At T > T_C, the antiferromagnetic order was retained in the EuBaCo1.90O5.36. As temperature increased, EuBaCo_2–xO_5.5–δ conserved the IS state in pyramids, and gradual LS → IS → HS transitions occurred in octahedron sites. The increase in the effective magnetic moment μeff in EuBaCo_1.90O_5.36 at T > 580 K demonstrated the transition of Co³⁺ ions to the HS state at lower temperatures as compared to the EuBaCo₂O_5.50 stoichiometric composition, and the structural distortions and the increase in the unit cell volume favored to this transition.