  A range of high quality Ga1-xMnxN 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 (Tg) between 700 and 590 {\deg}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 Tg. 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 Tg
reduced to 590 {\deg}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 (Tc) correlate with local Tg. It has been found that a typical 10
{\deg}C variation of Tg across 1 inch substrate can lead to 40% dispersion of
Tc. The established here strong sensitivity of Tc on Tg turns magnetic
measurements into a very efficient tool providing additional information on
local Tg, 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 Tc by two different methods, each sensitive to
different moments of Tc distribution, may serve as a tool for quantification of
spin homogeneity within the material.
