We demonstrate that resonant excitation of CdMnTe self-assembled quantum dots creates an ensemble of spin-polarized magnetic polarons at B=0 T. The strong spatial confinement characteristic of quantum dots significantly increases the stability of magnetic polarons so that the optically induced spin alignment is observed for temperatures > 120 K. *Author to whom the correspondence should be addressed: electronic mail: seb@physics. Incorporation of magnetic ions into semiconductor QDs offers the possibility of studying the interaction between carriers and magnetic ions under strong spatial electronic confinement [8][9][10]. It has been shown that EMPs confined in magnetic QDs, show significant enhancement of their thermal stability [11]. In addition, the spin relaxation time in non-magnetic semiconductor QDs has been found to be considerably longer than in QWs, reaching several hundreds of picoseconds [12,13]. One might thus anticipate that it might be possible to polarize Mn spins within a DMS QD by a suitably polarized exciton. 2In this letter we demonstrate the observation of optically induced magnetization of the Mn spins embedded in CdMnTe QDs. We show that through the resonant excitation of spinpolarized excitons one can control the alignment of zero-dimensional EMPs in QDs and therefore the magnetization direction of large QD ensembles. Moreover, due to significant enhancement of the stability of spin-polarized EMPs in these zero-dimensional nanostructures this optically induced spin alignment is observed above 120 K.The samples containing magnetic CdMnTe QDs and non-magnetic CdTe QDs were grown by molecular beam epitaxy on (100) Spin-polarized excitons were created resonantly in the QDs by LO phonon-assisted absorption into the QD ground states by both σ + -and σ − -polarized light [14]. Continuous wave argon ion-pumped dye laser (Rhodamine 590) was used as a tunable excitation source. The sample was placed in a variable temperature (from 4 K to 120 K) continuous-flow helium cryostat. Polarization of both excitation and emission was controlled by Babinet-Soleil compensators and Glan-Thomson linear polarizers, which enables the measurement of the emission intensity in both circular polarizations as a function of the excitation polarization. The emission was dispersed by a triple monochromator and detected by a cooled CCD camera. 3In Fig. 1a we show resonantly excited photoluminescence (PL) spectra for non-magneticCdTe QDs measured at B=0 T. If one excites the inhomogeneously broadened QD ensemble resonantly, spectral lines are observed that are significantly sharper than the non-resonant spectrum (shown by the shaded region in Fig. 1a). These lines are related to LO phonon-assisted absorption in QDs [14]. It is important to note that spin-polarized excitons are excited directly into the ground states of QDs responsible for this enhanced emission. For the resonant spectra shown in Fig. 1a the excitation is σ + -polarized while we analyze both σ + (circles) and σ − (squares) -polarized emission. We find that ...

We study the magnetization dynamics in CdMnTe quantum dots using sub-wavelength optical microscopy imaging at B=0T. For continuous laser illumination each dot exhibits strong and unique circular polarization despite completely unpolarized ensemble emission. This implies that after an exciton recombines, the spontaneous ferromagnetic alignment of magnetic impurities persists for over 100 microseconds, which is million times longer than in CdMnTe quantum wells. The spin memory effect points towards qualitatively different picture of magnetization dynamics in zero-dimensional limit.

We report on low temperature polarization-resolved imaging of single magnetic self-assembled CdMnTe quantum dots (QDs) in the absence of magnetic field. Using longitudinal optical phonon-assisted absorption to photoexcite spin-polarized excitons into a QD ground state, we find that the magnetic impurities within CdMnTe QDs can be aligned ferromagnetically with a single emission lines exhibiting a circular polarization as large as 65%. These results demonstrate that the magnetization of a single magnetic QD can be optically controlled with a suitably polarized laser.

We observe a strong dependence of the exciton spin relaxation in CdTe quantum dots on the average dot size and the depth of the confining potential. For the excitons confined to the asgrown CdTe quantum dots we find the spin relaxation time to be 4.8 ns. After rapid thermal annealing, which increases the average dot size and leads to weaker confinement, we measure the spin relaxation tine to be 1.5 ns, resulting in smaller values of the absolute polarization of the quantum dot emission. This dramatic enhancement of the spin scattering efficiency upon annealing is attributed to increased mixing between different spin states in larger CdTe quantum dots. 1The manipulation of the exciton spin properties in semiconductor nanostructures requires precise control of the exciton energy levels. This may be achieved either by careful design of the structure or by applying electromagnetic fields. In the case of semiconductor quantum dots (QDs) the internal exciton structure is very sensitive to the size and shape of the QD [1].Depending on the symmetry of the QD potential the ground state of the exciton is either twofold degenerate (symmetric QD) or split by the exchange interaction (asymmetric QD) [2]. Moreover, energy levels of excitons in QDs are also affected by external magnetic fields via the Zeeman interaction [2,3], which leads to spin splitting determined by the effective g-factor. A number of studies have shown that the degeneracy of the exciton levels significantly impacts the exciton spin relaxation in semiconductor QDs. For the split energy levels (either by asymmetry [4][5][6] or by magnetic field [6][7][8]) the spin of the excitons is conserved throughout their lifetime. In contrast, when the exciton levels are degenerate, the exciton spin relaxation is usually extremely rapid [7].Recent theoretical models have determined the admixture of exciton states in QDs, mainly due to spin -orbit and the electron -hole exchange interactions, as the major mechanisms of the exciton spin relaxation in QDs [9][10][11]. Thus, the exciton spin relaxation in QDs also depends on the energy separation between the ground states and the excited states (such as light-hole exciton levels). Clearly, the separation between heavy-and light-hole energy levels decreases for larger QD sizes [12], which increases the mixing between states with different spin [11]. As a result, exciton spin relaxation in QDs should strongly depend not only on the symmetry of the QD potential or external magnetic fields but also on the QD size [11].In this letter, we demonstrate a way of tuning the exciton spin relaxation time in QDs through rapid thermal annealing. Using a recently developed method [13] to analyze the

We study exciton spin-relaxation in CdTe self-assembled quantum dots (QDs) by using polarized photoluminescence (PL) spectroscopy in magnetic field. Experiments on single CdTe QDs and on large QD ensembles show that by combining LO phonon-assisted absorption with circularly polarized excitation, spin-polarized excitons are photoexcited directly into the ground states of QDs. We find that for single symmetric QDs at B = 0 T, where the exciton levels are degenerate, the spins randomize very rapidly, so that no net spin polarization is observed. In contrast, when this degeneracy is lifted by only a fraction of a Kelvin through the application of small magnetic fields, optically created spin-polarized excitons maintain their polarization on a time scale much longer than the exciton recombination time. Similar behaviors are found in a large ensemble of CdTe QDs. Finally, due to strong electronic confinement in CdTe QDs, the large spin polarization of excitons shows no dependence on the number of LO phonons emitted between the virtual state and the ground state of the exciton.