Effect of Illumination on Magnetic Properties of Materials Composed of a Semiconductor and a Magnetic Component

with Mark Filipkowski

Because of their inherent read/write capability and nonvolatility, magnetic storage media form the basis for essentially all mass storage of electronic data. The use of magnetooptical techniques to read and write data has added to the versatility of these media by increasing the density of storage. Our interest is in exploring another potential advantage of magnetooptical read/write methods, i.e., speed, through possible novel approaches to magnetization reversal in thin magnetic films.

Magnetooptic writing of data is presently accomplished by local optical heating of a magnetic thin film in the presence of a magnetic field: A local increase in temperature causes a decrease in the coercive field of the thin film to a point where the magnetic field is sufficient to reverse the film magnetization. The rate at which this process occurs is limited by thermal transport effects to a minimum switching time on the order of ns. As such, the thermal process does not appear to take full advantage of the available switching times of thin films, which have been shown to be as short as a few ps.

Optically driven electronic processes in solids, such as photogeneration of carriers in semiconductors, occur on extremely short time scales. When such processes are coupled to a magnetic system within the same material, for example via carrier spin scattering, changes in the state of the magnetic system can be induced on the same short time scale. An example of such an instance is dynamic polarization of Mn ions in Zn1-xMnxSe.

Recently it was shown that optical excitation of carriers in a semiconductor can influence an adjacent magnetic system. In these experiments, the coercivity of an interfacial ferromagnetic layer is modified by photogeneration of carriers in a substrate GaAs epilayer. Although modification of the interfacial acceptor population depends on diffusion of photogenerated carriers from the bulk, a slow process, these experiments demonstrate the possibility of modifying the behavior of a ferromagnetic film via optoelectronic excitation.

The proposed REU experiments will involve measurements of the (steady state) magnetic properties, under illumination, of ferromagnetic/semiconductor bilayer systems and of bulk semiconductors containing magnetic dopants (e.g. InP:Fe). Materials are available via in house synthesis and via collaboration. The student will investigate quantitative changes in the magnetic properties (susceptibility, coercivity, etc.) of these two systems as a function of the wavelength and intensity of illumination, in support of future experiments investigating dynamical effects. The experiments will be conducted utilizing an existing SQUID-based magnetometer fitted with a probe containing fiber optics for illumination of the sample.


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