  Cross-control of a material property - manipulation of a physical quantity
(e.g., magnetisation) by a nonconjugate field (e.g., electrical field) - is a
challenge in fundamental science and also important for technological device
applications. It has been demonstrated that magnetic properties can be
controlled by electrical and optical stimuli in various magnets. Here we find
that heat-treatment allows the control over two competing magnetic phases in
the Mn-doped polar semiconductor GeTe. The onset temperatures $T_{\rm c}$ of
ferromagnetism vary at low Mn concentrations by a factor of five to six with a
maximum $T_{\rm c} \approx 180$ K, depending on the selected phase. Analyses in
terms of synchrotron x-ray diffraction and energy dispersive x-ray spectroscopy
indicate a possible segregation of the Mn ions, which is responsible for the
high-$T_{\rm c}$ phase. More importantly, we demonstrate that the two states
can be switched back and forth repeatedly from either phase by changing the
heat-treatment of a sample, thereby confirming magnetic phase-change- memory
functionality.
