  A set of thin film Mn$_x$Si$_{1-x}$ alloy samples with different manganese
concentration x = 0.44 - 0.63 grown by the pulsed laser deposition (PLD) method
onto the Al$_2$O$_3$(0001) substrate was investigated in the temperature range
4 - 300 K using ferromagnetic resonance (FMR) measurements in the wide range of
frequencies (f = 7 - 60 GHz) and magnetic fields (H = 0 - 30 kOe). For samples
with x = 0.52 - 0.55, FMR data show clear evidence of ferromagnetism with high
Curie temperatures T$_\text{C}$ ~ 300 K. These samples demonstrate complex and
unusual character of magnetic anisotropy described in the frame of
phenomenological model as a combination of the essential second order easy
plane anisotropy contribution and the additional forth order uniaxial
anisotropy contribution with easy direction normal to the film plane. We
explain the obtained results by a polycrystalline (mosaic) structure of the
films caused by the film-substrate lattice mismatch. The existence of extra
strains at the crystallite boundaries leads to an essential inhomogeneous
magnetic anisotropy in the film plane.
