  We report on Gd ion implantation and magnetic characterization of GaN films
on sapphire substrates and of bare sapphire and Si substrates to shed light on
the mechanism underlying the induced magnetism upon Gd ion implantation. For
all three hosts, giant magnetic moments per Gd ion were observed at
temperatures of 5 through 300 K. The maximum moment per Gd in GaN was 1800
mu_B, while the moments in Gd-implanted Si and sapphire were only slightly
smaller. The apparent induced ferromagnetic response was found to be
metastable, disappearing after on the order of 50 days at room temperature,
except for the implanted sapphire. We argue that our findings support a
defect-based picture of magnetism in Gd-implanted semiconductors and
insulators.
