Correlation of Atomic Level Growth, Characterization and Electronic Properties of Epitaxial Ferromagnetic Alloys on Compound Semiconductors

技术：理论研究表明，磁性材料的自旋极化对成分变化和表面和界面缺陷非常敏感。这种对成分和缺陷的敏感性被认为是限制基于自旋输运的器件性能的原因，本提案的目的是通过实验确定这些相关性。单晶铁磁合金将通过分子束外延（MBE）生长在原子清洁、表征良好的MBE生长的半导体表面上。原位沉积和退火将用于改变表面成分和结构。原位扫描隧道显微镜（STM）、反射高能电子衍射、低能电子衍射、x射线光电子能谱、俄歇电子能谱和磁光克尔效应（MOKE）测量将结合非原位卢瑟福后向散射、高分辨率x射线衍射、透射电子显微镜（TEM）、振动样品（VSM）和超导量子干涉器件（SQUID）磁强计，以获得对原子水平结构，化学和磁性质的详细了解。自旋极化将通过原位制备超导铁磁金属隧道结、原位制备铁磁金属隧道结和原位点接触安德烈夫反射（PCAR）的测量来确定。原位PCAR有利于确定表面组成和结构的影响，因为它不需要在铁磁层顶部沉积隧道阻挡材料，这可能会改变表面性质。它还具有可以连续测量同一样品表面改性后的自旋极化的优点。通过对比三种自旋极化测量，实验确定了表面、界面和隧道势垒对隧道结中自旋输运的影响。非技术：该项目涉及材料科学主题领域的基础研究问题，具有很高的技术相关性。这项研究将为潜在的下一代电子/自旋电子器件的新理解和能力提供基础材料科学知识。该计划的一个重要特点是通过培养学生在一个基础和技术上重要的领域的研究和教育的整合。该项目的跨学科性质包括磁学、材料科学、分子束外延、表面科学、低温物理、超导、超高真空技术和电子显微镜，为未来的本科和研究生水平的材料科学家提供了一个独特的培训基地。学生将有机会在这些跨学科领域的各个方面接受培训和教育。对表面、界面和大部分材料的原子结构如何影响薄膜的磁性和自旋特性有基本的了解，将是他们教育的关键组成部分。PI致力于促进多样性，培养学生和博士后与其他学生和科学家合作。合作项目提供的机会使这种培训对未来的材料科学家来说是独特和重要的。目前，PI的团队由4名研究生和2名博士后组成，他们来自不同的民族背景，包括美国、德国、台湾、印度和韩国。一半的学生是女性。拓宽学生和博士后所知的领域范围有助于他们未来的职业选择。学生将有机会接触到最先进的原子水平表征技术，并了解通过各种技术可能获得或可能无法获得的信息。PI已经成功地让四年制大学的本科生男女比例平衡（50%）参加暑期研究和培训项目，并将继续努力。重点是通过与互补性专业知识小组的合作和研究互动来发展互动和协作技能。

Technical: Theoretical studies have suggested that the spin polarization of magnetic materials is very sensitive to compositional changes and defects at surfaces and interfaces. This sensitivity to composition and defects is believed to be responsible for limiting the performance of spin transport based devices and the aim of this proposal is to experimentally determine these correlations. Single crystal ferromagnetic alloys will be grown by molecular beam epitaxy (MBE) on atomically clean, well characterized, MBE-grown semiconductor surfaces. In-situ deposition and annealing will be used to modify the surface composition and structure. In-situ scanning tunneling microscopy (STM), reflection high energy electron diffraction, low energy electron diffraction, x-ray photoelectron spectroscopy, Auger electron spectroscopy and magneto optic Kerr effect (MOKE) measurements will be combined with ex-situ Rutherford backscattering, high resolution x-ray diffraction, transmission electron microscopy (TEM), vibration sample (VSM) and superconducting quantum interference device (SQUID) magnetometry to obtain a detailed understanding of the atomic level structural, chemical and magnetic properties. The spin polarization will be determined by measurements of in-situ fabricated superconductor-ferromagnetic metal tunnel junctions, in-situ fabricated ferromagnetic metal tunnel junctions, and in-situ point contact Andreev reflections (PCAR). In-situ PCAR is advantageous for determining the effect of surface composition and structure, as it does not require deposition of a tunnel barrier material on top of the ferromagnetic layer, which is likely to change the surface properties. It also has the advantage that the spin polarization of the same sample can be measured sequentially after surface modification. By comparing the three spin polarization measurements, the influence the surfaces, interfaces and tunnel barrier on the spin transport in tunnel junctions will be determined experimentally. Non-Technical: The project addresses basic research issues in a topical area of materials science having high technological relevance. The research will contribute basic materials science knowledge at a fundamental level to new understanding and capabilities for potential next generation electronic/spintronic devices. An important feature of the program is the integration of research and education through the training of students in a fundamentally and technologically significant area. The interdisciplinary nature of the project including magnetism, materials science, molecular beam epitaxy, surface science, low-temperature physics, superconductivity, ultra high vacuum technology, and electron microscopy provides a unique training ground for future materials scientists at the undergraduate and graduate levels. Students will have opportunities and be trained and educated in all aspects of these interdisciplinary fields. The ability to gain a fundamental understanding of how atomic structure at surfaces, interfaces and in the bulk of the materials affects magnetic and spin properties of thin films will be a critical component of their education. The PI has a strong commitment to assisting diversity and to training students and postdoctoral associates to collaborate with other students and scientists. Opportunities provided by collaborative projects make this training unique and important to future materials scientists. Currently, the PI's group consists of 4 graduate students and two postdoctoral associates from different ethnic backgrounds, including US, Germany, Taiwan, India, and Korea. Half of the students are women. Broadening the range of areas in which students and postdoctoral associates have knowledge enhances their future career choices. Students will gain invaluable exposure to state of the art atomic level characterization techniques and learn what information may or may not be gained through various techniques. The PI has been successful in involving a balance of male (50%) and female (50%) undergraduate students from four-year colleges to participate in summer research and training programs and will continue these efforts. Emphasis is made on developing interactive and collaborative skills through collaborations and research interactions with groups of complementary expertise.