Magnetic anisotropy in Ce/Fe and Ce/FeCoV multilayers

Multilayer samples of Ce/Fe and Ce/FeCoV, fabricated by sputtering and characterized by means of x-ray diffraction and reflectivity, were studied by ⁵⁷Fe Mössbauer spectroscopy and polarized neutron reflectivity. In the Ce/Fe system a paramagnetic Fe phase is observed at room temperature for Fe thicknesses below 20 Å. This phase is ordered at 4.2 K, and is attributed to amorphous Fe at the interface with Ce. In contrast, the Ce/FeCoV system is ferromagnetic at room temperature for all layer thicknesses studied. From the relative intensity of the Mössbauer lines we show that both systems exhibit a perpendicular magnetic anisotropy for magnetic layers of less than 17 Å thickness, indicating a positive interface anisotropy constant, K_S. For a selection of Ce/Fe samples, the reorientation of the magnetic moments in magnetic fields applied normal to the layers enabled the magnetic anisotropy energy K_eff of each sample to be evaluated. The trend of the values of K_eff gave volume and interface anisotropy constants K_V = 1.1 (±1.2) × 10⁵ J m^-3 and K_S = 0.98 (±0.15) × 10^-3 J m^-2 for the Ce/Fe system, where the positive values indicate a preference for an out-of-plane orientation of the magnetization.