Oxygen Effect on the Surface Structure and Magnetism of Ultrathin CoNi/Cu(001) Alloy Films

We present a study of the structure and the corresponding magnetic property of Co
₅
Ni
₉₅
/Cu(001) ultrathin films during exposure to oxygen. For a film below the critical thickness (t
c
) of spin reorientation transition (SRT) from in-plane to a perpendicular direction, the SRT occurs while introducing oxygen for 25 Langmuir (L), followed by decreasing the perpendicular magnetization for further oxygen exposure. For a film above t
c
, the perpendicular magnetization is enhanced while introducing oxygen for 25 L and decreases during additional oxygen exposure. In both these cases, the interlayer distance (
d
) varies only 1.1% during the SRT or the enhancement of the perpendicular magnetization, whereas the
d
increases substantially when perpendicular magnetization disappears. This implies that the oxygen-induced SRT of the CoNi/Cu(001) alloy films is not materially affected by the
d
of the surface layer. This might indicate the formation of antiferromagnetic oxide that induces the SRT of CoNi/Cu(001) alloy films.