Photoinduced magnetism in CdF2 with bistable donors:: The clue for the negative U?

We report measurements of photoinduced magnetisation of CdF2 bulk crystals doped with In or Ga. At temperatures at which metastable photo transformation of donors occurs, magnetic susceptibility increases and it follows a Curie law. Low magnetic field data are dominated by spin-spin ferromagnetic coupling of d(5) electrons of Mn codopants with donor electrons of In or Ga metastably phototransferred from a localised to the hydrogenic shallow donor state. Enhancement of magnetisation is due to effective spin increase resulting from this coupling. At high magnetic fields (above 2 T) and temperatures below 5K, saturation of the photoinduced magnetisation is observed. This provides first direct proof of a negative-U character of the donors in CdF2.