(Ga, Mn)N layers were grown by radio-frequency plasma-assisted molecular beam epitaxy with Mn concentrations of similar to5 or 50 atom %. In the material doped with 5 atom % Mn, the growth conditions were adjusted to produce second phases in one case, detectable by X-ray diffraction. Under our conditions, the dominant second phases are GaxMny-related and not MnxNy. All three types of material (single-phase or GaMn second phases at different concentrations) exhibit 300 K hysteresis in magnetic moment vs. field (M-H) plots, but the temperature dependence of the field-cooled (FC) and zero-field cooled (ZFC) magnetization is quite different in each case. A clear difference in FC-ZFC magnetization at 300 K in the single-phase material is observed. (C) 2003 The Electrochemical Society.