Selective Doping of Antiferromagnetic Semiconductors

技术：本项目旨在合成一种室温磁性半导体，探索一种新的策略，从反铁磁半导体开始，优先将非磁性施主取代为一个亚晶格。目前合成这些材料的方法主要集中在用过渡金属(TM)如Mn掺杂半导体。磁性离子负责提供局域磁矩和空穴(例如在GaMnAs中)。在空穴的作用下，Mn的定域d电子之间的交换作用导致了铁磁有序。在这种方法中，只能得到一种类型的掺杂，例如，掺锰的III-V的掺杂是p型的，并且达到居里温度300K以上所需的浓度高于锰的溶解度。在这个项目中，方法是通过选择性地掺杂反铁磁(AFM)半导体和非磁性掺杂剂来模拟铁磁半导体--优先将非磁性掺杂剂，即铜，加入到AFM半导体的一个磁性亚晶格中，导致局部磁矩减少，并产生整体净亚铁磁矩。这种掺杂原子力显微镜材料的总磁化强度由掺杂剂的数量和掺杂剂的选择来控制，因此可以同时引入自旋极化的电子和空穴。这种方法可以避免溶解度极限的问题，并允许对磁性半导体中不同类型的载流子进行独立控制。在这个项目中，早期的研究范围被扩大到一种新的材料：TMGeV2，通过改变V族元素(As，P和Sb)和Tm(Cr，Fe，Co和Ni)，通过DFT计算，初步研究了TMGeV2的性质。在理论预测的指导下，实验工作将集中在当掺杂时产生转变温度超过300K并且具有独立可控的自旋极化电子和空穴的亚铁磁性半导体。采用分子束外延技术生长外延合金薄膜。它们的结构将用高分辨率的电子显微镜和相关的衍射技术进行表征。磁性将通过温度和磁场相关的磁化强度测量、霍尔效应和磁阻来研究。电子和磁性性质将由阿贡国家实验室(ANL)先进光子源的X射线吸收光谱(XAS)和X射线磁性圆二色谱(XMCD)确定。橡树岭国家实验室(ORNL)纳米尺度材料研究中心将利用带有极化分析的扫描电子显微镜(SEMPA)获得磁畴结构及其演化。非技术性：该项目涉及具有高度技术相关性的电子/光子材料科学专题领域的基础研究问题。它被认为是一个高风险/高潜在回报的项目。如果成功，将实现转变温度超过300K，载流子独立可控的磁性半导体。该项目涉及使用PI实验室以及ANL和ORNL的设施对研究生和本科生进行磁性半导体合成和表征方面的培训。研究的跨学科性质和理论/实验相结合的方法为研究生和本科生提供了更多的机会来扩大他们的教育经验。RET计划将继续与当地高中合作，让高中生接触自旋电子学，并激发他们对科学的兴趣。

Technical: This project aims for synthesis of a room temperature magnetic semiconductor, exploring a new strategy by starting with an antiferromagnetic semiconductor, and preferentially substituting nonmagnetic donors into one sublattice. Current approaches to synthesize these materials focus on doping semiconductors with a transition metal (TM) such as Mn. The magnetic ion is responsible both for providing a localized magnetic moment and a hole (e.g. in GaMnAs). The exchange interaction amongst the localized d electrons of Mn, mediated by the holes, leads to ferromagnetic ordering. In this approach only one type of doping can be obtained, e.g., p-type for Mn doped III-V's, and the concentration required to achieve Curie temperatures above 300K is higher than the solubility of Mn. In this project the approach is to emulate ferromagnetic semiconductors by selective doping of an antiferromagnetic (AFM) semiconductor with nonmagnetic dopants--to preferentially incorporate a nonmagnetic dopant, i.e., Cu, into one of the magnetic sublattices of the AFM semiconductor, resulting in local reduction of the magnetic moment, and in an overall net ferrimagnetic moment. The total magnetization of such doped AFM material is controlled by the number of dopants, and the choice of dopants, so both spin-polarized electrons and holes can be introduced. This approach may evade the problem of solubility limit and permit the independent control of different carrier types in magnetic semiconductors. The scope of earlier research is expanded in this project to investigate the properties of a new class of material: TMGeV2, by varying the group V element (As, P, and Sb) and TM (Cr, Fe, Co, and Ni), through DFT calculations, initially. Guided by the theoretical predictions, experimental efforts will focus on compounds that, when doped, yield ferrimagnetic semiconductors with transition temperature in excess of 300 K and have independently controllable spin-polarized electrons and holes. Epitaxial thin alloy films will be grown by MBE. Their structures will be characterized using high resolution transmission electron microscopy and related diffraction techniques. Magnetic properties will be investigated by temperature and field dependent magnetization measurements, Hall effect and magnetoresistance. Electronic and magnetic properties will be determined by x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at the Advanced Photon Source, Argonne National Laboratory (ANL). Magnetic domain structures and their evolution will be obtained by scanning electron microscopy with polarization analysis (SEMPA) at the Center for Nanoscale Materials Research, Oak Ridge National Laboratory (ORNL). Non-technical: The project addresses basic research issues in a topical area of electronic/photonic materials science with high technological relevance. It is considered a high risk/high potential pay-off project. If successful, magnetic semiconductors with transition temperature in excess of 300 K and independently controllable carriers will be realized. The project involves training of graduate and undergraduate students in the synthesis and characterization of magnetic semiconductors using facilities at the PI's labs, as well as at ANL and ORNL. The interdisciplinary nature of the research and the combined theoretical/experimental approach provide additional opportunities for graduate and undergraduate students to broaden their educational experience. An RET program will be continued with local high schools to expose high school students to spintronics, and to inspire their interests in science in general.