Orbital Moment Unquenching and Large Enhancement of Local Magnetic Anisotropy in the M-type Sr-Hexaferrites by La and Co Substitutions

The hard magnetic property M-type hexaferrites are originated basically from the uniaxial crystal structure of magnetoplumbite and there have been a lot of works to enhance the property by modifying the magentic crystalline anisotropy energy or local spin magnetic moments through substituting varieties of cations for Sr and Fe sites. For example, smaller size of La³⁺ ions induce a structural transition at low temperature and the excess charges by them can increase the ratio of Fe²⁺ ions with smaller spin magnetic moment in the antiparallel sites. The substituted Co ions for Fe sites have smaller local magnetic moments than Fe ions and their uneven distribution in differerent local sites is expected to change the magnetization too. In this work, we have measured X-ray absorption spectroscopy and X-ray magnetic circular dichroism on a series of M type Sr hexaferrites which have different substitution ratios of La and Co. They directly show that the Fe²⁺ are formed by the La and Co substitution and they are localized mostly at O_h sites. Also, we will show that the local symmetry distortion at O_h sites taken by Fe²⁺ induces a large amount of orbital magnetic moment at the Fe²⁺ sites.