  We have performed a depth profile study of thermally diffused Mn/GaAs (001)
interfaces using photoemission spectroscopy combined with Ar$^+$-ion
sputtering. We found that Mn ion was thermally diffused into the deep region of
the GaAs substrate and completely reacted with GaAs. In the deep region, the Mn
2$p$ core-level and Mn 3$d$ valence-band spectra of the Mn/GaAs (001) sample
heated to 600 $^{\circ}$C were similar to those of Ga$_{1-x}$Mn$_x$As,
zinc-blende-type MnAs dots, and/or interstitial Mn in tetrahedrally coordinated
by As atoms, suggesting that the Mn 3$d$ states were essentially localized but
were hybridized with the electronic states of the host GaAs. Ferromagnetism was
observed in the dilute Mn phase.
