  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 3$d$ 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$_z$=$\pm$4 can be controlled by resonant optical pumping and a spin
relaxation time in the 20 $\mu$s range is obtained at T=4.2 K. This spin memory
at zero magnetic field is limited by the interaction with phonons.
