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Mat. Res. Soc. Symp. Proc. Vol. 517 © 1998 Materials Research Society

in significant field-induced modifications of the band structure, that can be harnessed to produce striking magnetooptical phenomena -- especially so in multilayers. THE sp-d EXCHANGE AND BAND-OFFSET TUNING As has been remarked, one of the most interesting mechanisms characteristic of DMSs is the sp-d exchange interaction of band electrons with localized magnetic moments (e.g., those associated with Mn÷÷ ions). As a consequence of this sp-d exchange the spin-up and spin-down states of electronic levels (e.g., band edge energies, shallow impurity states, etc.) experience enormous Zeeman splittings when a magnetic field is applied, that can achieve values of as much as 100 meV at low temperatures in relatively modest magnetic fields. We will not discuss the underlying physics of this sp-d exchange, since it has been extensively presented in the literature [1], but rather will focus on its effect on thin film structures involving DMSs. It is important to appreciate, however, that the above interaction is inherently a spin phenomenon. The electronic spin -- which in the context of semiconductor physics has traditionally been only of academic interest -- is now playing a central role. One of the most interesting magnetooptic manifestations of the above exchange phenomena arises in thin film systems comprised of alternating DMS and non-DMS layers. Since the band edges of the DMS material experience a large Zeeman shift, while the corresponding splitting in the non-magnetic layers is negligible by comparison, we can use this splitting to "tune" the valence and conduction band offsets simply by applying an external magnetic field -- thus tuning the properties of the multilayer as a whole. Furthermore, since such tuning is spin-selective (electrons with one spin orientation in the DMS layers experience a large upward shift of the band edge, while those with the opposite spin experience a comparable downward shift), this band-offset tuning also results in spin segregation: electrons with opposite spin orientation can be separated into different layers of the multilayer structure. This phenomenon has been used to form "spin superlattices" [3,4]; to convert type-II band alignment into type-I, and vice versa [5]; to control the coupling between double quantum wells separated by a DMS barrier [6]; etc. Although these effects are extremely interesting, their impact has been primarily on basic studies, because the Zeeman splitting which underlies the above effects is large only at low temperatures (typically below 10K). Even with this limitation, however, one aspect of band-offset tuning has already had a major -- albeit indirect -- practical application, in the form of "mapping" of electron probability distributions in various semiconductor heterostructures [7]. For example, this DMS property has been used to determine that states at energies above the barriers are -- perhaps counter-intuitively -- localized in the barrierlayers [8]. It has also been use