Disorder Effects on Magnetism in Dilute Magnetic Semiconductors

技术描述：III-V族化合物稀磁半导体（DMS）提供了操纵自旋和电荷的潜力，是一类具有磁性、磁光和半导体特性的材料。然而，由于它们的低磁跃迁（居里）温度，它们的利用一直受到阻碍。结果表明，金属有机气相外延法制备的InMnSb合金的居里温度超过400k。采用横断面扫描隧道显微镜（STM）对Mn掺杂物原子进行了成像。未观察到Mn的聚集，但观察到位置紊乱。磁性离子在原子尺度上的不均匀分布在磁性行为中起主导作用。本课题的研究目的是确定DMS在稀浓度极限下的磁相图。感兴趣的合金体系是窄间隙DMS InMnSb。研究了描述无序DMS磁性的理论模型的适用性。需要回答的主要问题是：(a)为什么这些合金具有高居里温度和(b) Mn掺杂剂的无序性对转变温度的作用。本课题采用金属有机气相外延法沉积外延合金薄膜。合成了Mn浓度小于1%、载流子浓度小于10^19 cm-3的合金。研究了无序在稳定铁磁相中的作用。提出在载流子浓度为2 × 10^17 cm-3时，铁磁相稳定，该载流子浓度高于金属-绝缘体过渡载流子浓度。使用的实验表征技术包括：透射电子显微镜，温度相关磁化测量，霍尔效应，电阻率和磁阻测量。埃因霍温技术大学的低温横截面STM和自旋隧道STM用于确定InSb中Mn取代的性质及其在原子水平上的磁性。非技术描述：对于更广泛的影响，金属有机气相外延制备的合金薄膜提供给美国和国外大学和Argonne国家实验室的同事，用于表征其结构，磁光和磁性能。西北大学和埃因霍温工业大学在稀磁半导体中单掺杂原子成像方面建立了扩展合作。该项目涉及培养铁磁半导体合成和性能测量方面的博士后、研究生和本科生。研究生通过伊利诺伊州埃文斯顿港学校的HANDS项目参与STEM外展。磁性半导体材料对自旋电子器件如自旋阀、双极磁结晶体管、自旋二极管逻辑电路和量子计算器件具有潜在的技术重要性。

Technical Description: Group III-V compound dilute magnetic semiconductors (DMS) offer the potential for manipulating both spin and charge and a class of materials that exhibit magnetic, magneto-optical and semiconducting properties. Their utilization, however, has been impeded due to their low magnetic transition (Curie) temperatures. It was demonstrated that InMnSb alloys prepared by metalorganic vapor phase epitaxy exhibit a Curie temperature in excess of 400 K. Mn dopant atoms were imaged using cross-sectional scanning tunneling microscopy (STM). No clustering of Mn was noted but positional disorder was observed. The inhomogeneous distribution of magnetic ions on the atomic scale is proposed to play a dominant role in the magnetic behavior.The objective of this research project centers on determining the magnetic phase diagram of DMS in the dilute concentration limit. The alloy system of interest is the narrow-gap DMS InMnSb. The applicability of a theoretical model for describing the magnetic properties of disordered DMS is investigated. The major questions to be answered are (a) why these alloys have high Curie temperatures and (b) what the role of disorder of Mn dopants is on the transition temperature. In the project, epitaxial thin alloy films are deposited by metalorganic vapor phase epitaxy. Alloys with Mn concentrations of less than 1% and carrier concentrations of lower than 10^19 cm-3 are synthesized. The role of disorder in stabilizing the ferromagnetic phase is investigated. It is proposed that the ferromagnetic phase is stabilized at carrier concentrations 2 x 10^17 cm-3, which is above the metal-insulator transition carrier concentration. Experimental characterization techniques used include: transmission electron microscopy, temperature dependent magnetization measurements, Hall effect, resistivity and magnetoresistance measurements. Low-temperature cross-sectional STM and spin-tunneling STM at the Technical University at Eindhoven are used to determine the nature of Mn substitution in InSb and its magnetic properties at the atomic level.Non-technical Description: As to broader impact, alloy films prepared by metalorganic vapor phase epitaxy are provided to colleagues at universities in the US and abroad and Argonne National Laboratories for characterizing their structural, magneto-optical and magnetic properties. An extended collaboration is established between Northwestern University and Technical University of Eindhoven on the imaging of single dopant atom in dilute magnetic semiconductors. The project involves the training of post-doctoral, graduate and undergraduate students in the synthesis and property measurements of ferromagnetic semiconductors. Graduate students are involved in STEM outreach through the HANDS program at the Haven School in Evanston, Illinois. The magnetic semiconductor materials have potential technological importance for spintronic devices such as spin valves, bipolar magnetic junction transistors, spin diode logic circuits and quantum computation devices.