We have been able to observe two new magnetic phases of ultrathin Co by growing atomically well ordered Co films on cleaved GaAs(110) substrates. 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, we find a new magnetic phase of bcc Co with an easy axis along the direction. This phase persists up to a thickness of 7 ML, at which point an abrupt in-plane spin-reorientation transition reorients the magnetization along the  direction. The new bcc-Co phase provides a new magnetic structure for potential spintronics applications.
Physical Review B (Condensed Matter and Materials Physics)