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

University Of California-Santa Barbara, Santa Barbara CA

Investigators

Abstract

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.