NIRT: Formation and Properties of Spin-Polarized Quantum Dots in Magnetic Semiconductors by Controlled Variation of Magnetic Fields on the Nanoscale

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This proposal was received in response to the Nanoscale Science and Engineering solicitation, NSF 01-157, category NIRT. The goal of this project is to explore the issues of confining spin-polarized charges on the nano-scale, in zero-dimensional (quantum-dot-like) semiconductor geometries. The approach is to do this in III-V- and II-VI-based magnetic semiconductors (such as GaMnAs or ZnMnSe) subjected to a magnetic field that is modulated on the nano-scale. The giant Zeeman splitting characteristic of these materials is expected to exhibit nano-scale modulation resulting in tight localization of spin-polarized states. The idea is to produce such nano-scale field modulation in a magnetic semiconductor (MS) in three ways: (1) by lithographically-patterned ferromagnetic (FM) layers deposited on the MS surface (e.g., sub-micron Co/Pt multilayer discs); by depositing and patterning superconducting (SC) overlayers on the MSs, where the magnetic field modulation is achieved as the magnetic flux by-passes the nano-scale SC "obstacles" due to the Meissner effect; and by using unpatterned SC films, where the field modulation at the MS/SC interface is achieved via the Abrikosov vortex lattice, whose "lattice constant"-and thus the Zeeman-induced quantum dots separation-can be controlled by an external magnetic field. Prior to patterning, the SC films will either be deposited epitaxially, in the same molecular beam epitaxy (MBE) chamber as the MS layers; or ex-situ, by sputtering. Key aspects of the project are: theory, materials growth, lithography, characterization, and development of spintronic concepts. The project is highly collaborative involving researchers at Notre Dame, Purdue, and U. IL at Chicago. Additionally, the Superconductivity and Magnetism Group at Argonne National Laboratory, led by Dr. George Crabtree, will collaborate by providing magneto-transport, near-field magneto-optical mapping, as well as STM and (eventually) spin-polarized STS mapping of the SC-MS hybrid structures. %%% The project addresses basic research issues in a topical area of materials science with high technological relevance. 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. In addition to training graduate students in nano-science, undergraduates will also participate in research through senior projects and the NSF REU program. It is also expected that a regular for-credit course on nano-science at the senior and first-year-graduate level will be developed. The project combines experiment and theory, and brings together researchers from physics, electrical engineering, and material science. Activities are designed to develop strong technical, communication, and organizational/management skills in undergraduate and graduate students and postdoctorals through unique educational experiences made possible by a forefront research environment. Cross-disciplinary research and site visits between organizations will enhance the education and training process. The project is co-supported by the DMR/CMP, DMR/EM, and ECS/EPDT Divisions/Programs.***