We observed three magnetic states of an ultrathin, atomically well-ordered Co film grown on a cleaved GaAs(110) substrate. For a Co thickness less than 3.4 monolayers (ML), we find a ferromagnetically dead layer associated with the formation of interfacial Co2GaAs. For thicknesses greater than 4.1 ML, the Co film grows with a bcc structure that contains 6 at. % Ga. The films are ferromagnetic with an easy axis along the  direction. This magnetic state persists up to a thickness of 7 ML, at which point an abrupt in-plane spin-reorientation transition reorients the magnetization along the  direction.
Physical Review B (Condensed Matter and Materials Physics)