Photocarrier-induced spin heating and spin-lattice relaxation in diluted magnetic Stranski-Krastanov quantum dots

Using the photoluminescence as a monitor, the Mn spin heating via photoexcited carriers and the spin-lattice relaxation of the magnetic ions in (Cd,Mn)Se quantum dots are studied. Spin dissipation times in the $1\mu \mathrm{s}$ range are observed. Direct heating via spin exchange with the photocarriers plays a minor role. The dependence of the spin-lattice relaxation rate on Mn concentration, temperature, as well as magnetic field indicates that an Orbach process (in a locally heated environment) via Mn pairs is responsible for the dissipation of the magnetic ion spin.

Figure 1

(a) Photoluminescence from the DMQD ensemble of the sample with $x=0.07$ at $B=0$ and $3\mathrm{T}$ for two different excitation intensities. The arrow marks the energy position for the time-resolved SLR measurements (see Fig. 3). (b) Spectrally integrated DMQD PL intensity vs $I_{\mathrm{exc}}$ for $x=0.03$ and 0.07 at $B=0$.

Figure 2

(a) Magnetic-field induced shift of the DMQD ${\sigma }^{+}$ PL maximum for different excitation intensities $(x=0.07)$. (b) Derived spin temperatures for stationary heating by photoexcitation for Mn concentrations of $x=0.07$ and 0.03. Dashed lines represent a fit of the data by $I_{\mathrm{exc}}\sim T_S^4-T_b^4$. Note the logarithmic scale used for $I_{\mathrm{exc}}$.

Figure 3

Time-resolved PL of the DMQD structure with $x=0.07$ (solid line) at $B=3\mathrm{T}$. For comparison, the rectangular laser pulse is drawn by a dashed line. Inset: enlarged view of the normalized heating and cooling transients. Thick lines represent a single-exponential fit.

Figure 4

(a) SLR rate $1∕{\tau }_{\mathrm{SLR}}$ versus inverse spin temperature $1∕T_S$ for two different Mn concentrations. Lines are to guide the eye. (b) Magnetic field dependence of the SLR rate for $x=0.07$ at three different $T_S$.