  Ferrimagnetic Mn$_2$Fe$_x$Ga $(0.26 \leq x \leq 1.12)$ thin films have been
characterised by X-ray diffraction, SQUID magnetometry, X-ray absorption
spectroscopy, X-ray magnetic circular dichroism and M\"{o}ssbauer spectroscopy
with the aim of determining the structure and site-specific magnetism of this
tetragonal, D0$_{22}$-structure Heusler compound. High-quality epitaxial films
with low RMS surface roughness ($\sim 0.6$ nm) are grown by magnetron
co-sputtering. The tetragonal distortion induces strong perpendicular magnetic
anisotropy along the $c$-axis with a typical coercive field $\mu_0 H\sim 0.8$ T
and an anisotropy field ranging from $6$ to $8$ T. Upon increasing the Fe
content $x$, substantial uniaxial anisotropy, $K_\mathrm{u} \geq 1.0$ MJ/m$^3$
can be maintained over the full $x$ range, while the magnetisation of the
compound is reduced from $400$ to $280$ kA/m. The total magnetisation is almost
entirely given by the sum of the spin moments originating from the
ferrimagnetic Mn and Fe sublattices, with the latter being coupled
ferromagnetically to one of the former. The orbital magnetic moments are
practically quenched, and have negligible contributions to the magnetisation.
The films with $x=0.73$ exhibit a high anomalous Hall angle of $2.5$ % and a
high Fermi-level spin polarisation, above $51$ %, as measured by point contact
Andreev reflection. The Fe-substituted Mn$_2$Ga films are highly tunable with a
unique combination of high anisotropy, low magnetisation, appreciable spin
polarisation and low surface roughness, making them very strong candidates for
thermally-stable spin-transfer-torque switching nanomagnets with lateral
dimensions down to $10$ nm.
