Considerable efforts have been devoted recently to synthesizing 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 was reported.
The estimated Curie temperature was 940 K at 5.7% of Mn, which is the highest
among diluted magnetic semiconductors ever reported. However, the electronic
mechanism behind the ferromagnetic behaviour has still been controversial. Here
we show experimental evidence using ferromagnetic samples that Mn atoms are
substitutionally dissolved into the GaN lattice and they exhibit mixed valences of
+2 (majority)
and +3. 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.
Room temperature ferromagnetism of GaMnN thin films is awaked by a mild
hydrogenation treatment of samples synthesized by molecular beam epitaxy. Local
environment of Mn atoms is monitored by Mn-L2,3 near edge x-ray absorption fine
structure (NEXAFS) technique. Doped Mn ions are present at substitutional sites
of Ga both before and after the hydrogenation. No secondary phase can be
detected. Major valency of Mn changes from +3 to +2 by the hydrogenation. The
present result supports the model that the ferromagnetism occurs when Mn2+ and
Mn3+ are coexistent and holes in the mid- gap Mn band mediate the magnetic
coupling.Comment: 12 pages, 5 figure
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