We carried out investigation on the magnetic properties of MnAs thin film grown on GaAs(001) substrate by
using molecular-beam epitaxy. The magnetic Curie temperature was ~340 K which was higher than that of the
bulk. Magnetization (M) versus magnetic field (H) showed strong two-fold symmetric anisotropy. At low temperature
of 2 K, the M-H curve and temperature dependence of magnetization exhibit peculiar behavior that
has not been observed before. It should be noted that these unique results were only observed when a magnetic
field was applied. We also observed a weak-localization-like magnetoresistance (MR) at 2 K. As an origin of the
observed peculiar magnetic properties and MR, we suggest diamagnetic domains which are phase transited
from ferromagnetism at low temperature.