Epitaxial growth of Co on GaAs~001! and its in-plane magnetic anisotropy are studied using reflection
high-energy electron diffraction, a high-resolution transmission electron microscope, and the magneto-optical
Kerr effect. In the initial and final stages of growth, Co exists in single-crystalline body-centered-cubic ~bcc!
and hexagonal-closed-packed ~hcp! phases, respectively, while in the middle stage the coexistence of the bcc
and hcp structures is observed. For the bcc Co thin films on GaAs~001!, a fourfold in-plane magnetic anisotropy
with easy axes along the ^100& directions is realized and discussed.