We used continuous wave photoluminescence (cw-PL) and time resolved photoluminescence (TR-PL) spectroscopy to compare the properties of magnetic polarons (MP) in two related spatially indirect II-VI epitaxially grown quantum dot systems. In the ZnTe/(Zn,Mn)Se system the holes are confined in the non-magnetic ZnTe quantum dots (QDs), and the electrons reside in the magnetic (Zn,Mn)Se matrix. On the other hand, in the (Zn,Mn)Te/ZnSe system, the holes are confined in the magnetic (Zn,Mn)Te QDs, while the electrons remain in the surrounding nonmagnetic ZnSe matrix. The magnetic polaron formation energies MP E in both systems were measured from the temporal red-shift of the band-edge emission. The magnetic polaron exhibits distinct characteristics depending on the location of the Mn ions. In the ZnTe/(Zn,Mn)Se system the magnetic polaron shows conventional behavior with MP E decreasing with increasing temperature T and increasing magnetic field B. In contrast, MP E in the (Zn,Mn)Te/ZnSe system has unconventional dependence on temperature T and magnetic field B; MP E is weakly dependent 2 on T as well as on B. We discuss a possible origin for such a striking difference in the MP properties in two closely related QD systems.
Studies of ferromagnetic MnAs in recent years have revealed a wide range of properties desirable for spintronic applications. Previously studied MnAs spin-light-emitting-diodes exhibited a low value of spin injection into the device active region. In this work, we have investigated injection of spin polarized electrons from MnAs into AlGaAs(n)/GaAs(i)/AlGaAs(p) n-i-p structures. The band-edge electroluminescence emitted from these devices has a saturation circular polarization of 26% at 7 K and B=2 T. Using optical pumping measurements the corresponding electron spin polarization was determined to be 52%. Emission persists up to room temperature, with a saturation circular polarization of 6% at B=2 T.
An efficient electron spin-relaxation mechanism has been observed in InAs quantum dots (QDs) that manifests itself as a sharp drop in the circular polarization of the light emitted by Fe spin-light emitting diodes, which incorporate a single layer of InGaAs QDs, for a narrow range of magnetic fields around 5 tesla. The underlying mechanism occurs when the QDs are occupied by three electron-hole pairs forming a tri-exciton (3X) and is a two step process. The first step involves the spin flip of one of the three electrons mediated by the spin-orbit interaction; in the second step the 3X relaxes to its ground state via phonon emission.
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