Ultralong spin memory of optically excited single magnetic quantum dots

We study the magnetization dynamics in CdMnTe quantum dots using subwavelength optical microscopy imaging at B=0 T. For continuous laser illumination each dot exhibits strong and unique circular polarization despite completely unpolarized ensemble emission. This implies that after an exciton recombines, the spontaneous ferromagnetic alignment of magnetic impurities persists for over 100 mu s, which is a million times longer than in CdMnTe quantum wells. The spin memory effect points toward a qualitatively different picture of magnetization dynamics in the zero-dimensional limit. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2993331]