  We have investigated the room temperature ferromagnetic order that develops
in Al-substituted magnesium oxide, Mg(Al)O, nanoparticles with Al fractions of
up to 5 at.%. All samples, including undoped MgO nanoparticles, exhibit room
temperature ferromagnetism, with the saturation magnetization reaching a
maximum of 0.023 emu/g at 2 at.% of Al. X-ray photoelectron spectroscopy
identifies the presence of oxygen vacancies in both doped and undoped MgO
nanoparticles, with the vacancy concentration increasing upon vacuum annealing
of Mg(Al)O, resulting in two-fold enhancement of the saturation magnetization
for 2 at.% Al-doped MgO. Our results suggest that the oxygen vacancies are
largely responsible for room temperature ferromagnetism in MgO.
