Enhanced orbital magnetic moment in an FeCo-BaF2 granular film revealed by x-ray magnetic circular dichroism

FeCo-insulator fluoride nanogranular films belong to an excellent material group that exhibits a huge Faraday effect, which is several times larger than those of conventional materials, in the optical communication wavelength band (1310 ∼ 1550 nm). In this study, FeCo-BaF ₂ granular films were prepared on glass substrates by a tandem sputtering method. The Faraday effect was measured and a huge Faraday rotation angle of −2.3 deg./μm was observed for the wavelength of 1550 nm. The ratio of orbital magnetic moment to effective spin magnetic moment ( m _orb/ m _spin ^eff) was evaluated by x-ray magnetic circular dichroism (XMCD) measurements, and was found to be 0.43 for Fe and 0.24 for Co. The obtained values are several times larger than those of Fe-Co bulk samples. The elemental selectivity of the XMCD measurements reveals that Fe has a larger m _orb/ m _spin ^eff value than that of Co. Such an increase in m _orb is attributed to the reduction of the symmetry at the interface between FeCo and BaF ₂ in the granular film, and is thought to be the origin of the giant Faraday effect in FeCo-fluoride granular films.