Optically-induced magnetization of CdMnTe self-assembled quantum dots

Abstract

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 ...

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