Fabrication of Tunneling Magnetoresistance Devices using Novel Room-temperature Ferromagnetic Semiconductor MnGeP_2

本报告描述了日本科学促进会2005-2006年度科学研究资助项目（B类）(2)（17360009）“利用新型室温铁磁半导体MnGeP2制备隧道磁阻器件”的研究成果。本课题的目的是对2001-2003年度MEXT-JSPS科学研究财政资助项目中研究黄铜矿型室温铁磁半导体时发现的MnGeP2薄膜的最佳制备条件进行研究，并利用这些薄膜制备隧道磁阻（TMR）器件。为了实现TMR器件，必须制备平面度更高的磁膜。然而，在GaAs和InP衬底上制备的MnGeP2薄膜表面形貌差，粗糙度大，不适合用于tMR器件。为了提高表面的平整度，我们引入了一种更优的磨粉层，并取得了良好的效果。然而，TEM和EDX观察发现MnP晶粒混合在MnGeP2织构中。为了抑制相分离，我们根据热力学第一原理计算提高了衬底温度。然而，所得到的薄膜在室温下没有自发磁化，尽管薄膜处于单相状态。因此，我们改变了策略，并开始探索最佳条件，以获得具有足够平坦度的MnP薄膜，因为MnP的磁性已经众所周知。以MnP和CoFe为电极制备了具有磁阻效应的磁隧道结。因此，我们建立了一种获得由MnGeP2组成的MTJ的技术。此外，在提高衬底温度制备高质量MnGeP2薄膜的过程中，通过仔细的扫描电镜观察，我们发现衬底表面有纳米尺寸的晶须自组装生长。通过TEM观察和成分分析，晶须可以是锗基或MnP基晶体。由于以前的报道大多使用Au作为催化剂，纳米晶须的无催化剂生长受到了人们的关注。因此，尽管这一发现不是本研究计划的主要主题，但结果也包括在本报告中。少

This report describes achievements of the research project "Fabrication of Tunneling Magneto-resistance Devices using Novel Room-temperature Ferromagnetic Semiconductor MnGeP2" conducted under the support of the Grant-in-Aid for Scientific Research from Japan Society for Promotion of Science for 2005-2006 FY, Category (B) (2), No.17360009.The purpose of this project is to elucidate the optimum fabrication condition of MnGeP2 films which was discovered during the research for chalcopyrite-type room-temperature ferromagnetic semiconductors conducted under the 2001-2003 FY Grant-in-Aid for Scientific Research of MEXT-JSPS, and fabricate tunneling magneto-resistance (TMR) devices using these films.For realization of TMR devices it is essential to prepare magnetic films with improved flatness. However, MnGeP2 films prepared on GaAs and InP substrates suffer poor surface morphology with roughness and were found to be unsuitable for tMR devices. To improve surface flatness we introduced Ge bu … More ffer layer, which proved to be effective. However, it is found by TEM and EDX observation that MnP grains are mixed among MnGeP2 texture.In order to suppress phase separation we raised the substrate temperature following the first principle thermodynamic calculation. The resulting films, however, showed no spontaneous magnetization at room temperature, despite that the films are in single phase state. We therefore changed the strategy and started to exploit optimum conditions to obtain MnP thin films with sufficient flatness, since the magnetic properties of MnP has been well known.Using MnP and CoFe as electrodes we prepared magnetic tunnel junctions which showed magneto-resistance effect. Thus we have established a technique to obtain MTJ consisting of MnGeP2.In addition, during preparation of high quality MnGeP2 films at elevated substrate temperatures we have found self-assembled growth of nanometer-sized whiskers on the surface of the substrates by careful SEM observations. By TEM observations and compositional analyses whiskers are found to be either Ge-based or MnP based crystals. Catalyst-free growth of nano-whiskers is of interest since most of previous report used Au as catalyst. Therefore the results are included in this report although this finding is not a main subject of this research program. Less