  The influence of the in-plane anisotropy on the magnetization of a
nanogranular film with perpendicular anisotropy has been studied. It is shown
that if a magnetic field is tilted with respect to the film normal, a critical
transition from the inhomogeneous magnetic state of granules with noncollinear
directions of their moments to the homogeneous one with parallel orientation of
granular magnetic moments takes place. The in-plane anisotropy is found to
affect the angular dependence of the critical field. The ensemble of oriented
biaxial particles is theoretically described in the double-well potential
approximation. Despite the biaxial magnetic anisotropy of particles, their
ensemble, if in the inhomogeneous state, is divided into two subensembles, with
the magnetic moments of particles being collinear in each of them. In the
critical field, a transition from the inhomogeneous state with two subensembles
into the homogeneous one takes place. The results of theoretical calculations
are compared with experimental data for a nanogranular Co/Al2On film with
perpendicular anisotropy containing 74.5 at.% Co, which exceeds the percolation
threshold. The magnetic moment of this film is a sum of two contributions: from
nanogranules with biaxial anisotropy and a phase forming the percolation
cluster. The magnetic properties of nanogranules, whose contribution is
separated from the total film magnetization, agree well with the calculation
data.
