Magnetic properties and magnetocaloric effect of field-induced ferro magnet BaFeO3

T alkaline-earth iron perovskites AFeO3 (A=Ca, Sr, Ba), containing iron in a high valence state of Fe4+ (3d4), have been prepared using high-pressure apparatases to generate strongly oxidizing atmosphere. Recently, however, Hayashi et al. obtained BaFeO3 by heating easily prepared BaFeO2.5 at 200oC in flowing oxygen containing ~10 % of ozone at ambient pressure. This oxide shows ferromagnetism at Tc=111K with a large moment of 3.5 μB/Fe in the presence of an external field of 3 kOe; the magnetic structure at lower fields is a helical structure of the A-type. Our recent study proposed that the material is an interesting candidate for refrigeration based on the magnetocaloric effect (MCE). The ferromagnetism leads to a change of magnetic entropy of 5.8 J kg-1 K-1 and a refrigerant capacity of 170 J kg-1 under an external field of 50 kOe, both being comparable to those of perovskite manganites. The reversibility in both the thermal and magnetic cycles is beneficial to efficient refrigeration. This property arises from the absence of an orbital moment in Fe4+, which is actually in a Fe3+L state (L: ligand hole). In this presentation, the magnetism and MCE of BaFeO3 will be discussed. Also, the author will show Fe-sitesubstitution effects, carried out for the purpose of chemical control of MCE.