  Mn$_{3-x}$Co$_{x}$Ga epitaxial thin films were grown on MgO substrates by
magnetron co-sputtering. Structures were tetragonal or cubic depending on Co
content. Composition dependence of saturation magnetization and uniaxial
magnetic anisotropy $K_u$ of the films were investigated. A high $K_u$ (1.2 MJ
m$^{-3}$) was achieved for the Mn$_{2.6}$Co$_{0.3}$Ga$_{1.1}$ film with the
magnetic moment 0.84$\mu_B$. Valence band spectra were obtained by hard X-ray
photoelectron spectroscopy. Sharp peaks in the cubic case, which were absent in
the tetragonal case, prove that a van Hove singularity causes a band
Jahn-Teller effect with tetragonal distortion. Observations agree well with the
first-principles calculations.
