Hybrid structures synthesized from di erent materials have attracted considerable attention because they may allow not only combination of the functionalities of the individual constituents but also mutual control of their properties. To obtain such a control an interaction between the components needs to be established. For coupling the magnetic properties, an exchange interaction has to be implemented which typically depends on wavefunction overlap and is therefore short-ranged, so that it may be compromised across the hybrid interface. Here we study a hybrid structure consisting of a ferromagnetic Co layer and a semiconducting CdTe quantum well, separated by a thin (Cd,Mg)Te barrier. In contrast to the expected p-d exchange that decreases exponentially with the wavefunction overlap of quantum well holes and magnetic atoms, we find a long-ranged, robust coupling that does not vary with barrier width up to more than 30 nm. We suggest that the resulting spin polarization of acceptor-bound holes is induced by an e ective p-d exchange that is mediated by elliptically polarized phonons. E xchange interactions are the origin for correlated magnetism in condensed matter with multi-faceted behaviour such as ferro-, antiferro-or ferrimagnetism. In magnetic semiconductors (SCs), the exchange occurs between free charge carriers and localized magnetic atoms 1-4 and is determined by their wavefunction overlap. To control this overlap, hybrid structures consisting of a ferromagnetic (FM) layer and a semiconductor quantum well (QW) are appealing objects because they allow wavefunction engineering. Furthermore, the mobility of QW carriers will not be reduced by inclusion of magnetic ions in the same spatial region in these systems.More specifically, for a two-dimensional hole gas (2DHG, the p-system) in a QW the overlap of the hole wavefunction with the magnetic atoms in a nearby ferromagnetic layer (the d-system) is believed to result in p-d exchange interaction 5-8 . This exchange interaction may cause strong coupling between the SC and FM spin systems 9 , through which the ferromagnetism of the unified system, as evidenced by its hysteresis loop, can be tuned. In particular, the 2DHG spin system becomes polarized in the effective magnetic field from the p-d exchange 5,8 . Recently 10 , it was shown that in addition to this equilibrium 2DHG polarization there is an alternative mechanism involving spin-dependent capture of carriers from the SC into the FM. For ferromagnetic (Ga,Mn)As on top of an (In,Ga)As QW, electron capture induces electron spin polarization in the QW, representing a dynamical effect in contrast to the exchange-induced equilibrium polarization.Here we study a different FM/QW hybrid, consisting of a Co layer and a CdTe II-VI semiconductor QW, separated by a nanometrethick barrier. Owing to the negligible hole tunnelling through the barrier, this hybrid combination shows mostly a quasi-equilibrium proximity effect due to p-d exchange interaction between magnetic atoms and holes in the QW. Surprisingly, howev...

Integration of magnetism into semiconductor electronics would facilitate an all-in-one-chip computer. Ferromagnet/bulk semiconductor hybrids have been, so far, mainly considered as key devices to read out the ferromagnetism by means of spin injection. Here we demonstrate that a mn-based ferromagnetic layer acts as an orientation-dependent separator for carrier spins confined in a semiconductor quantum well that is set apart from the ferromagnet by a barrier only a few nanometers thick. By this spin-separation effect, a non-equilibrium electronspin polarization is accumulated in the quantum well due to spin-dependent electron transfer to the ferromagnet. The significant advance of this hybrid design is that the excellent optical properties of the quantum well are maintained. This opens up the possibility of optical readout of the ferromagnet's magnetization and control of the non-equilibrium spin polarization in non-magnetic quantum wells.

We report on magnetic field-induced oscillations of the photon echo signal from negatively charged excitons in a CdTe/(Cd,Mg)Te semiconductor quantum well. The oscillatory signal is due to Larmor precession of the electron spin about a transverse magnetic field and depends sensitively on the polarization configuration of the exciting and refocusing pulses. The echo amplitude can be fully tuned from the maximum down to zero depending on the time delay between the two pulses and the magnetic-field strength. The results are explained in terms of the optical Bloch equations accounting for the spin level structure of electrons and trions.

No abstract

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.The spin orientation of electrons is studied in ferromagnet (FM)-semiconductor (SC) hybrid structures composed of a (Ga,Mn)As ferromagnetic layer, which is placed in the direct vicinity of a non-magnetic SC (In,Ga)As quantum well (QW). It is shown that the polarization of carriers in the SC QW is achieved by spin-dependent tunnelling into the magnetized ferromagnetic layer. This leads to dynamical spin polarization of the electrons, which can be directly observed by means of time-resolved photoluminescence. We find that the electron spin polarization grows in time after excitation with an optical pulse and may reach values as large as 30%. The rate of spindependent capture grows exponentially steeply with decreasing thickness of the spacer between ferromagnetic layer and QW, and it persists up to the Curie temperature of the (Ga,Mn)As layer. From time-resolved pump-probe Kerr rotation data, we evaluate a value of only a few μeV for the energy splitting between the electron Zeeman sublevels due to interaction with the ferromagnetic (Ga,Mn)As layer, indicating that the equilibrium spin polarization is negligible.Schematic presentation of electron spin orientation in a semiconductor quantum well (QW) under linearly polarized excitation due to spin-dependent capture of electrons in the ferromagnetic layer (FM). The arrows in the FM box indicate the orientation of the magnetization M. The effect is detected by appearance of a circular polarization degree of photoluminescence after pulsed optical excitation (right). The data are shown for a spacer thickness of 5 nm.