Competing spin exchange interactions in ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se/${\mathrm{Zn}}_{1\mathrm{-}\mathit{y}}$${\mathrm{Fe}}_{\mathit{y}}$Se multiple-quantum-well structures

We report the study of a mixed-spin multiple-quantum-well system consisting of alternating ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se and ${\mathrm{Zn}}_{1\mathrm{-}\mathit{y}}$${\mathrm{Fe}}_{\mathit{y}}$Se layers, grown by molecular-beam epitaxy. The two spin components of the heavy-hole exciton are each dominated by different exchange interactions as revealed by their temperature and magnetic-field dependence. We find that the energy of the spin-down ${\mathrm{\sigma }}_{+}$ component (-3/2,-1/2) is well described by exchange interactions of the electrons and holes with ${\mathrm{Mn}}^{2+}$ ions (Brillouin paramagnet), while the energy of the spin-up ${\mathrm{\sigma }}_{\mathrm{-}}$ component (+3/2,+1/2) is determined by the interactions of carriers with ${\mathrm{Fe}}^{2+}$ ions (Van Vleck paramagnet). Thus, these tailored heterostructures permit simultaneous study of Brillouin and Van Vleck paramagnetism as well as associated carrier dynamics in a single structure.