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A range of high quality Ga 1−x Mn x N layers have been grown by molecular beam epitaxy with manganese concentration 0.2  x  10%, having the x value tuned by changing the growth temperature (T g ) between 700 and 590 °C, respectively. We present a systematic structural and microstructure characterization by atomic force microscopy, secondary ion mass spectrometry, transmission electron microscopy, powder-like and high resolution X-ray diffraction, which do not reveal any crystallographic phase separation, clusters or nanocrystals, even at the lowest T g . Our synchrotron based X-ray absorption near-edge spectroscopy supported by density functional theory modelling and superconducting quantum interference device magnetometry results point to the predominantly +3 configuration of Mn in GaN and thus the ferromagnetic phase has been observed in layers with x > 5% at 3 < T < 10 K. The main detrimental effect of T g reduced to 590 o C is formation of flat hillocks, which increase the surface root-mean-square roughness, but only to mere 3.3 nm. Fine substrates' surface temperature mapping has shown that the magnitudes of both x and Curie temperature (T C ) correlate with local T g . It has been found that a typical 10 o C variation of T g across 1 inch substrate can lead to 40% dispersion of T C . The established here strong sensitivity of T C on T g turns magnetic measurements into a very efficient tool providing additional information on local T g , an indispensable piece of information for growth mastering of ternary compounds in which metal species differ in almost every aspect of their growth related parameters determining the kinetics of the growth. We also show that the precise determination of T C by two different methods, each sensitive to different moments of T C distribution, may serve as a tool for quantification of spin homogeneity within the material.

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