SPIN ELECTRONICS: Band-Offset and Time-Resolved Nonlinear-Optical Studies of Magnetic Heterostructure Interfaces

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This proposal was received in response to the Spin Electronics of the 21st Century Initiative, Program Solicitation NSF 02-036. The proposal focuses on spin-dependent transport and magnetic phenomena at interfaces of magnetic semiconductor quantum well structures and magnetic tunneling diodes which have potent ml applications in spin electronics. As interfaces in these heterostructures appear to play an essential role for the device characteristics, a detailed study of the electronic and magnetic interface properties as well as split transport across the heterojunctions is needed. The objective of our experimental program is to resolve fundamental questions regarding spin scattering, the magnetic state and the hand alignment at the buried heterointerfaces. The effects of compound formation (intermixing), roughness, magnetic state and defects will be studied and correlated with growth conditions, structural properties and magneto-transport measurements. Spin lifetimes and decoherence times will he measured by femtosecondresolved pump-probe linear and nonlinear magneto-optical experiments to elucidate spin scattering and spin relaxation processes at the buried heterointerfaces. Both electrical (via a photoconductive switch) and optical injection (with linear and circular-polarized ultrafast pulses) will be used to excite a spin population. Internal photoemission experiments (with circular polarization) will be employed to map the alignment of the spin-hands at the heterojunctions. The magnetization dynamics at the buried interface will be measured using the surface-sensitive magnetization-induced second-harmonic generation (MSGH) technique. Spectroscopic MSGH studies will he performed to reveal interface states, which may act as spin scattering centers at the heterojunctions. These measurements will he performed as a function of: temperature, applied magnetic field, applied electrical bias, composition, well thickness and/or harrier height, built-in strain and growth conditions. The information on the spin-hand alignment obtained from these measurements can be used to tune into resonant states thereby enhancing the efficiency of the spin-injection process. A major focus of our program will be the education of graduate and undergraduate students in the nascent field of spin electronics. The students will benefit from the proposed research program, providing them with excellent training in an interdisciplinary field including optics and electronic materials and devices.