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

弹簧磁体由磁性软材料（如NiFe薄膜）和硬材料（如SmCo）组成，具有独特的磁化特性。本研究的目的是通过共振x射线磁散射测量来检测磁性软层中出现的扭曲磁结构。采用分子束外延法（MBE）在Si衬底上生长SmCo_5(16.5)/Ni_<80>Fe_<20>(60)/SmCo_516.5 nm三层薄膜。在薄膜生长过程中施加面内磁场。对于顶部硬层（SmCo），施加磁场的方向旋转90度。样品的磁化曲线表现为两步磁化，具有弹簧磁铁结构的特点。在Spring 8的光束线39XU处进行了共振x射线反射率测量。反射率的电荷贡献产生了清晰的基斯格条纹。左右圆偏振x射线在临界角附近的反射率差，即初步模拟的最佳信号，其反射率差远高于统计变化，但仍然存在噪声，无法进行定量分析。考虑到K边的磁散射截面很小，需要更好的统计。本研究表明，RMXS测量中的限制因素是探测器的最大计数率。

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.