X-ray magneto-optic Kerr effect studies of spring magnet heterostructures

The complex 3-dimensional magnetization reversal behavior of Sm–Co/Fe exchange spring films is used to test the sensitivity of different resonant soft X-ray magneto-optical Kerr effect (MOKE) measurements to changes in longitudinal and transverse moments within the soft Fe layer and to changes in these moments in depth within the Fe layer. As in the visible MOKE, changes in longitudinal and net transverse moments are resolved by measuring both Kerr rotation and intensity loops, respectively, near the Fe 2p core resonance. These X-ray MOKE signals measured using linear incident polarization are more directly interpreted in terms of longitudinal and transverse moments than are the same signals measured using elliptical polarization. Varying photon energy near the Fe L3 line is shown to be an effective means of resolving distinctly different reversal behavior at the top and bottom of the 20 nm thick Fe layer resulting from the strong exchange coupling at the Sm–Co/Fe interface. Measured X-ray MOKE spectra and signals are in qualitative agreement with those calculated using standard magneto-optical formalisms incorporating interference between different layers and measured helicity-dependent magneto-optical constants for Fe.