  We report the observation of ferromagnetic resonance (FMR) and the
determination of the magnetocrystalline anisotropy in (100)-oriented
single-crystalline thin film samples of GaMnP with x=0.042. The contributions
to the magnetic anisotropy were determined by measuring the angular- and the
temperature-dependencies of the FMR resonance fields and by superconducting
quantum interference device magnetometry. The largest contribution to the
anisotropy is a uniaxial component perpendicular to the film plane; however, a
negative contribution from cubic anisotropy is also found. Additional in-plane
uniaxial components are observed at low temperatures, which lift the degeneracy
between the in-plane [011] and [01-1] directions as well as between the
in-plane [010] and [001] directions. Near T=5K, the easy magnetization axis is
close to the in-plane [01-1] direction. All anisotropy parameters decrease with
increasing temperature and disappear above the Curie temperature T_C. A
consistent picture of the magnetic anisotropy of ferromagnetic GaMnP emerges
from the FMR and magnetometry data. The latter can be successfully modeled when
both coherent magnetization rotation and magnetic domain nucleation are
considered.
