  We report the effect of dilute alloying of the anion sublattice with S on the
in-plane uniaxial magnetic anisotropy and magnetization reversal process in
Ga1-xMnxP as measured by both ferromagnetic resonance (FMR) and superconducting
quantum interference device (SQUID) magnetometry. At T=5K, raising the S
concentration increases the uniaxial magnetic anisotropy between in-plane <011>
directions while decreasing the magnitude of the (negative) cubic anisotropy
field. Simulation of the SQUID magnetometry indicates that the energy required
for the nucleation and growth of domain walls decreases with increasing y.
These combined effects have a marked influence on the shape of the
field-dependent magnetization curves; while the direction remains the easy axis
in the plane of the film, the field dependence of the magnetization develops
double hysteresis loops in the [011] direction as the S concentration increases
similar to those observed for perpendicular magnetization reversal in lightly
doped Ga1-xMnxAs. The incidence of double hysteresis loops is explained with a
simple model whereby magnetization reversal occurs by a combination of coherent
spin rotation and noncoherent spin switching, which is consistent with both FMR
and magnetometry experiments. The evolution of magnetic properties with S
concentration is attributed to compensation of Mn acceptors by S donors, which
results in a lowering of the concentration of holes that mediate
ferromagnetism.
