Neutron Scattering Studies of Magnetic Semiconductor Nanostructures

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This research is focused on an understanding of the magnetic structure and exchange interactions present in hybrid magnetic semiconductor materials. Semiconductor superlattices consisting of conventional II-VI materials in which alternate layers are antiferromagnetic and non-magnetic exhibit interlayer spin correlations of a surprisingly long range comparable to that in metallic systems. Neutron scattering is used as a probe of the magnetic structure, order parameter, and correlation range in these systems as a function of semiconductor doping. The results indicate that a magnetic interaction other than conventional superexchange is dominant. One of the present research objectives is to investigate in detail this exchange coupling and its dependence on electronic structure, strain effects, and types of doping. In addition to the II-VI materials, the research will focus on a detailed investigation of new ferromagnetic III-V systems, including the unusual dependence of the ferromagnetic transition temperature on composition and on the effect of thermal annealing. Neutrons will also provide direct information on the formation of ferromagnetic clusters or other nanoscale inhomogeneities that may affect the onset of infinite-range ferromagnetism. Finally, neutron reflectivity will be used to investigate the spatial dependence of the order parameter, and to investigate proximity effects between a hybrid III-V ferromagnetic semiconductor and an adjacent II-VI antiferromagnetic layer.Semiconductor and magnetic materials both play essential, but distinct, roles in many common electronic devices. Examples are computer memories made of semiconductors and hard disk drives utilizing magnetic materials. Recently the potential has arisen for a marriage of semiconducting and magnetic properties in a single material utilizing the concept of "spintronics." This research program is designed to use the powerful technique of neutron scattering to probe the magnetic state of materials that are both semiconducting and magnetic. The neutrons can be scattered by interacting with nuclei inside these systems, and in addition, since neutrons possess a magnetic moment, they can interact with magnetic fields inside materials. The result of such scattering provides direct information about the magnetic structure inside a material and over what distances the individual magnetic moments are coupled together. This type of information on the magnetism can be correlated with changes in the semiconducting properties of the materials as the system is "doped" with various chemical elements. The result provides necessary information to simultaneously optimize both the magnetic and the semiconducting aspects of these new compounds for future hybrid spintronic devices.