  Considerable efforts have been devoted recently to synthesize diluted
magnetic semiconductors having ferromagnetic properties at room temperature
because of their technological impacts for spintronic devices. In 2001
successful growth of GaMnN films showing room temperature ferromagnetism and
p-type conductivity has been reported. The estimated Curie temperature was 940K
at 5.7% of Mn, which is highest among diluted magnetic semiconductors ever been
reported. However, the electronic mechanism behind the ferromagnetic behaviour
has still been controversial. Here we show experimental evidence using the
ferromagnetic samples that Mn atoms are substitutionally dissolved into the GaN
lattice and they exhibit mixed valences of +2 (majority) and +3 (minority). The
p-type carrier density decreases significantly at very low temperatures. At the
same time, magnetization dramatically decreases. The results imply that the
ferromagnetic coupling between Mn atoms is mediated by holes in the mid-gap Mn
band.
