Nanostructure of exchange spring magnet multilayers examined by resonant X-ray magnetic scattering

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Spring magnets comprising of magnetically soft materials such as NiFe films on top of hard materials such as SmCo, exhibits unique magnetization characteristics. The present research aims to examine the twisted magnetic structure appearing in the magnetically soft layers by means of resonant X-ray magnetic scattering measurements.The sample used in the present experiments was grown on Si substrate as SmCo_5(16.5)/Ni_<80>Fe_<20>(60)/SmCo_516.5 nm) trilayers by Molecular Beam Epitaxy(MBE). In-plane magnetic field was applied during the film growth. The direction of applied field was rotated by 90 degree for the top hard layer (SmCo). The magnetization curve of the sample exhibited a two-step magnetization, characteristic of spring magnet structure. Resonant X-ray reflectivity measurements were carried out at beam-line 39XU of Spring 8. The charge contribution of the reflectivity gives a clear Kiessig fringes. The difference reflectivity obtained by the difference in the reflectivity between right and left circularly polarized X-ray near the critical angle, where the preliminary simulation suggest best signal, was well above the statistical variation, but still remain noisy for quantitative analysis. Considering quite small cross section of the magnetic scattering at K edge, much better statistics is required. The present study has shown that the limiting factor in the RMXS measurement is the maximum counting rate of the detector.