Abstract
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 M_z=±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.