High-Temperature Ferrimagnetism with Large Coercivity and Exchange Bias in the Partially Ordered 3d/5d Hexagonal Perovskite Ba₂Fe_1.12Os_0.88O₆

Double perovskite oxides A 2 BB ′O 6 combining 3 d and 4 d or 5 d transition metal ions at the B and B ′ sites feature a variety of magnetic and magneto-electric properties. Targeting Ba 2 FeOsO 6, we synthesized powder samples of nonstoichiometric Ba 2 Fe 1.12 Os 0.88 O 6 by solid-state reaction from the oxides. The crystal structure was investigated by using synchrotron powder X-ray and powder neutron diffraction. In contrast to Ca 2 FeOsO 6 and Sr 2 FeOsO 6, the compound adopts the hexagonal 6L perovskite structure (space group P 3̅ m 1) with partial Fe–Os order at both the face-sharing B 2 O 9 dimer and the corner sharing B O 6 transition metal sites. Magnetization, neutron diffraction, and 57 Fe Mössbauer spectroscopy results show that Ba 2 Fe 1.12 Os 0.88 O 6 develops ferrimagnetic order well above room temperature at T C ≈ 370 K. The nonsaturated magnetization curve at 2 K features a magnetic moment of 0.4 μ B per formula unit at 7 T and a pronounced hysteresis with a coercive field of about 2 T. Large exchange bias effects are observed when the magnetization curves are measured after field cooling. The peculiar magnetic properties of Ba 2 Fe 1.12 Os 0.88 O 6 are attributed to an inhomogeneous magnetic state formed as a consequence of the atomic disorder. Our results indicate that hexagonal double-perovskite-related oxides are a promising class of compounds for finding new materials with potential applications as hard magnets or in the area of spintronics.