Hole- Cr + nanomagnet in a semiconductor quantum dot

We study a new diluted magnetic semiconductor system based on the spin of the ionized acceptor Cr +. We show that the negatively charged Cr + ion, an excited state of the Cr in II-VI semiconductor, can be stable when inserted in a CdTe quantum dot (QD). The Cr + attracts a heavy-hole in the QD and form a stable hole-Cr + complex. Optical probing of this system reveals a ferromagnetic coupling between heavy-holes and Cr + spins. At low temperature, the thermalization on the ground state of the hole-Cr + system with parallel spins prevents the optical recombination of the excess electron on the 3d shell of the atom. We study the dynamics of the nano-magnet formed by the hole-Cr + exchange interaction. The ferromagnetic ground states with Mz=±4 can be controlled by resonant optical pumping and a spin relaxation time in the 20 µs range is obtained at T=4.2 K. This spin memory at zero magnetic field is limited by the interaction with phonons.