This proposal was received in response to the Nanoscale Science and Engineering Initiative, Program Solicitation NSF 01-157, in the NER category. The proposal focuses on the development and characterization of one unique class of metal-semiconductor heterostructures, which potentially will result in production of a spin injector. Specifically, problems exist in characterizing the nature of spin transport and scattering through the interfaces of nanomagnetic and spintronic materials. As one considers various potential hybrid structures, the need to be able to probe the spin-dependent transmission properties of the interfaces between semiconductors, ferromagnets, non-magnetic metals, and insulators becomes critical.
Our objective is to fabricate and characterize a structure that will allow one to interrogate the spin-dependent transmission properties by providing an easily-controlled source of spin-polarized electrons. The device uses circularly-polarized light to excite spin-dependent carriers within GaAs, and incorporates a Ag thin-film quantum-well spin-filter to energy select the hot electrons. This source of energy-selectable spin-polarized electrons then forms the substrate for the subsequent growth of magnetic structures. We will characterize the degree of spin-polarization by using the empty minority states of a Co film grown on this spin injector as an analyzer. While changing the doping of the GaAs provides a coarse energy selection, judicious choice of the Ag thickness provides a fine-tuning of the energy of these electrons via the energetics of the relevant Ag quantum-well state.
Our plan to demonstrate, refine, and calibrate this device involves the growth of quantum-stabilized Ag films on cleaved GaAs(110) and on GaAs(100) wafers. We have already shown that these Ag films exhibit quantum-well states and they will be used to further energy-filter the spin-polarized electrons. To analyze the spin-polarization of these electrons, we will epitaxially grow Co on the Ag/GaAs structure and calibrate the spin-dependent transport into the empty minority-state bands, both as a function of the polarization of the electrons from GaAs as well as the magnetization of the Co. Future extensions include the incorporation of an Al2O3 insulating barrier to test the operation in practical spin-tunneling devices. Throughout the research plan, emphasis will be placed on correlating atomic/morphological and electronic properties of nanophase heterostructures with ensuing spin-dependent conductance.
This project impacts the field of magnetics in several ways. This structure provides an inexpensive tool to interrogate spin transport properties and provides an alternative to spin-polarized photoemission and inverse photoemission. Once calibrated, it can be used to quickly evaluate the spin-dependent transport properties of advanced materials such as half-metallic thin-films. Furthermore, it can be used in a direct manner to provide valuable data on the spin-dependent scattering processes that occur at the interfaces of spintronic structures.
