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We present magneto-optical evidence for the formation of a magnetic-field-induced spin superlattice in modulated ZnSe/Zni-vMnvSe structures. In the samples studied, the offsets in both the conduction band and the valence band are very small at zero magnetic field. When a magnetic field is applied, the large Zeeman splitting of the Zni -vMnvSe band edges overcomes the zero-field offsets and results in the formation of a spin superlattice in which spin states of both electrons and holes are spatially and periodically separated.PACS numbers: 73.20.Dx, 68.55.Bd, 75.30.Et, 78.20.Ls The experimental and theoretical study of quantum confinement of carriers in spatially modulated semiconductor structures has been an area of intense activity over the past decade. Magnetic semiconductor quantum wells and superlattices-which are relatively new in this context-have extended this field to include novel spindependent phenomena [1-3]. A unique property of magnetic semiconductor quantum wells is that the band alignment can be tuned by the application of a magnetic field. An early theoretical study pointed out that-under favorable circumstances-a magnetic field could be used to induce a spin-dependent potential in a magnetic semiconductor superlattice so as to form a "spin superlattice" consisting of spatially separated spin states [4].The essential idea is explained in Fig. I. Consider a superlattice formed by alternating layers of a magnetic and a nonmagnetic semiconductor such that the band offsets at zero magnetic field are small. On applying a magnetic field, the band edges in the magnetic semiconductor undergo a huge spin splitting due to the sp-d exchange between carriers and localized magnetic ions [5], while the splitting in the nonmagnetic layers is much smaller. When the large Zeeman shift in the magnetic layers overcomes the band offsets in both conduction and valence bands, the magnetic layers act as barriers for electrons and holes in the spin-up state, and as quantum wiells for the spin-down state. Hence, spin-up holes and electrons are localized in the nonmagnetic regions of the superlattice, while those of opposite spin are localized in the magnetic layers. The spin states are then spatially modulated with the same period as the structural superlattice, thus forming a spin superlattice. Apart from their novelty, such structures will be crucially important to the understanding of fundamental spin-dependent phenomena of contemporary interest, such as carrier-spin scattering mechanisms in low-dimensional systems [1,2].Although magnetic-field-induced spatial spin separation for hole states has been demonstrated in ZnSe/ Zno.9Feo.1Se [3] and ZnSe/Zno.9Mno.iSe [6] single quantum wells, the conduction-band offsets were in those cases too large to form a spin superlattice as described above. Further, since single quantum wells were used, only one spin component could be spatially localized. Our approach to the problem is to use ZnSe/Zni-^Mn^Se superlattices in which the magnetic alloy composition is judiciously chosen...

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