  The extensive experimental and computational search for multifunctional
materials has resulted in the development of semiconductor and oxide systems,
such as (Ga,Mn)N, (Zn,Cr)Te, and HfO2, which exhibit surprisingly stable
ferromagnetic signatures despite having a small or nominally zero concentration
of magnetic elements. Here, we show that the ferromagnetism of (Zn,Cr)Te, and
the associated magnetooptical and magnetotransport functionalities, are
dominated by the formation of Cr-rich (Zn,Cr)Te metallic nanocrystals embedded
in the Cr-poor (Zn,Cr)Te matrix. Importantly, the formation of these
nanocrystals can be controlled by manipulating the charge state of Cr ions
during the epitaxy. The findings provide insight into the origin of
ferromagnetism in a broad range of semiconductors and oxides, and indicate
possible functionalities of these composite systems. Furthermore, they
demonstrate a bottom-up method for self-organized nanostructure fabrication
that is applicable to any system in which the charge state of a constituent
depends on the Fermi-level position in the host semiconductor.
