Spin injection into magnetic alloys using optical spin excitation technique

本研究的目的是研究在铁磁性金属/砷化镓界面处圆偏振光照射下，自旋极化电子在砷化镓中激发的自旋滤波效应。采用磁控溅射和反应MBE技术制备了Fe/GaAs、Fe/MgO/GaAs、Fe_3O_4/GaAs等样品。用波长为800-850 nm的左右圆偏振光从样品法向穿过磁层照射样品，在垂直于膜平面的磁场中检测到螺旋度相关的光电流。与螺旋度相关的光电流表现出明显的场依赖性，与磁层的磁化过程相对应。在足以饱和磁化的磁场中，还测量了螺旋度相关光电流的偏置依赖性。磁圆二色性在Fe/GaAs样品的反向偏置中占主导地位，而在正向偏置中观察到显著的特征。我们将这一特性归因于Fe/GaAs界面上的自旋滤波效应。然而，自旋过滤的效率被限制在几个百分点的范围内。此外，在界面处插入MgO隧道势垒并没有显著提高效率，这可能是由于Mg0层的粗糙度和针孔的存在。对于Fe_3O_4/GaAs样品，我们观察到零偏压下螺旋度相关光电流的偏压依赖性具有类似的特征。综合这些结果，我们得出结论，磁性金属层可以作为自旋过滤器和自旋探测器，尽管效率很小。原子平面层是提高自旋检测效率的必要条件。

The purpose of this study is to examine the spin filtering effect of spin polarized electrons excited in GaAs with illumination of circularly polarized light at the ferromagnetic metal/GaAs interfaces.Several types of samples, e.g., Fe/GaAs, Fe/MgO/GaAs, Fe_3O_4/GaAs, were grown using magnetron sputtering and reactive MBE techniques. These samples were illuminated with right or left circularly polarized light with a wavelength of 800-850 nm from sample normal direction through the magnetic layer, thereby helicity dependent photocurrent was detected in a magnetic field normal to the film plane. The helicity dependent photocurrent showed clear field dependence, corresponding to the magnetization process of the magnetic layer. The bias dependence of the helicity dependent photocurrent was also measured in a magnetic field sufficient to saturate the magnetization. Magnetic circular dichroism was predominant for reverse bias for Fe/GaAs samples, while a significant feature was observed at a forward bias. We attribute this characteristic feature in the bias dependence of helicity dependent photocurrent to spin filtering effect at the Fe/GaAs interface. The efficiency of the spin filtering, however, is limited to a value to a few %. Also, any significant enhancement in the efficiency was not obtained by inserting an MgO tunneling barrier at the interface, presumably due to the roughness of the Mg0 layer and the presence of pin holes. For Fe_3O_4/GaAs samples, we observed a similar feature in the bias dependence of the helicity dependent photocurret at zero bias.From these combined results, we conclude that magnetic metal layer can act as a spin filter and work as a spin detector, although the efficiency is small. Atomically flat layers are necessary to improve the spin detection efficiency.