Brillouin spectroscopy of spin waves in metallic ferromagnetic crystals

在本研究项目中，我们利用布里渊光散射（BLS）研究了各种金属铁磁晶体中的热激发自旋波。在项目的第一年，我们研究了典型的铁磁半导体EuS单晶在7.5 K到300 K的宽温度范围内的自旋波，其中铁磁转变温度为16.5 K。即使在远高于转变温度的室温下，我们也能观察到磁散射。然而，在转变温度附近，我们既没有观察到自旋波频率的软化，也没有观察到峰宽的展宽。我们发现铁磁相的自旋波频率远低于由EuS的磁化强度和外加磁场计算得到的值。这一观察结果有力地表明，激光照射表面附近的退磁系数远小于薄膜的预期值4p。为了弄清退磁系数效应，我们计划对同样经历非常相似的铁磁相变的溅射EuO薄膜进行新的BLS研究。第二年主要研究了MBE制备的Fe (n ML)/Au (n ML) （n=1 ~ 5）细层超晶格的层间交换耦合现象，该超晶格可以看作是D_<4h>四方体系的规则有序Fe-Au合金单晶。我们研究了整数和非整数原子层超晶格的自旋波。我们发现跨Au层之间的层间交换耦合对于我们的超晶格来说总是铁磁性的，并且在3ml和5ml左右变得非常弱，这是在超晶格结构中首次观察到这种偶奇效应的层间交换耦合现象。

In this research project we have investigated thermally excited spin waves in various metallic ferromagnetic crystals by means of Brillouin light scattering (BLS).In the first year of our project we have examined spin waves of a typical ferromagnetic semiconductor EuS single crystal in a wide temperature range between 7.5 K and 300 K including the ferromagnetic transition temperature at 16.5 K.We could observe magnetic scattering even at room temperature which is far above the transition temperature. However we could observe neither softeing of the spin wave frequency nor broadening of peak width around the transition temperature. We found that the spin wave frequencies in the ferromagnetic phase are much lower than the values evaluated from the magnetization of EuS and the applied magnetic field. This observation strongly indicates that the demagnetization coefficient near the surface illuminated by laser beam is much less than the value of 4p which is expected for a thin film. In order to clarify the demagnetization coefficient effect, we are planning a new BLS study of sputtered EuO thin films which also undergo a very similar ferromagnetic phase transition.In the second year we have mainly investigated the interlayer exchange coupling phenomena of MBE prepared fine-layered Fe (n ML)/Au (n ML) (n=1 to 5) superlattices which can be regarded as regularly ordered Fe-Au alloy single crystals with D_<4h> tetragonal sysmetry. We have studied spin waves for both of integer and non-integer atomic layer superlattices. We found that the interlayer exchange coupling between the Fe layers across the Au layers is always ferromagnetic for our superlattices and becomes very weak around at 3 ML and 5 ML.This is the first observation of such even-odd effect of the interlayer exchange coupling phenomena in superlattice structures.