  Tetragonal Mn$_x$Ga$_{1-x}$ (x=0.70, 0.75) thin films grown on SrTiO$_3$
substrates at different temperatures and thicknesses exhibit perpendicular
magnetic anisotropy with coercive fields between 1-2 T. Transmission electron
microscopy (TEM) and X-ray diffraction (XRD) reveal that 40nm samples grown at
300-350$^{o}$C lead to polycrystalline films with the tetragonal c-axis
oriented primarily perpendicular to the film plane but with some fraction of
the sample exhibiting the c-axis in the film plane. This structure results in a
secondary magnetic component in the out of plane magnetization. Growth at
300$^{o}$C with a reduced thickness or Mn concentration significantly decreases
the presence of the tetragonal c-axis in the film plane, thus improving the
magnetic properties. TEM is of critical importance in characterizing these
materials, since conventional XRD cannot always identify the presence of
additional crystallographic orientations although they can still affect the
magnetic properties. Our study points to ways that the microstructure of these
thin films can be controlled, which is critical for utilization of this
material in spintronic devices.
