We observed two opposite spin reorientation transitions (SRT), perpendicular to in-plane magnetization and vice versa, in ultrathin Ni films grown on clean and preoxidized Cu(001) surfaces covered with Cu overlayers. The magneto-optical Kerr effect measurement shows that the Cu capping stabilizes perpendicular magnetization for the clean surface and in-plane magnetization for the preoxidized surface. Correspondingly, the x-ray magnetic circular dichroism measurement elucidates that upon Cu capping the Ni orbital moment is suppressed for clean Cu(001) and enhanced for preoxidized Cu(001). These clearly contrasting findings can be explained by the fact that oxygen atoms act as a surfactant; oxygen always locates at the surface even after Ni and Cu deposition. The present results clearly demonstrate that the modification of the in-plane orbital moment drives the SRT, which is sensitive to interfacial interaction.

Fig. 1　
Evolution of the LEED (low energy electron diffraction) pattern [(a)-(d)] and the AES (Auger electron spectroscopy) intensity (e) during the Ni growth on Cu(001).

(a) p(1×1) for a Cu(2 ML)-capped Ni(10 ML) film on clean Cu(001)
(b) (2√2×√2)R45° for preoxidized Cu(001)
(c) c(2×2) for a Ni(5.5 ML) film grown on O/Cu(001)
(d) c(2×2) for a Cu(2 ML)-capped Ni(5.5 ML) film on O/Cu(001), which is different from (a)
(e) O-KLL AES intensities as a parameter of Cu capping thickness for O/Cu(001) (black), Ni(10 ML) on O/Cu(001) (red), Cu(0.2 ML) on Ni/O/Cu(001) (green), Cu(2 ML) on Ni/O/Cu(001) (blue), and Cu(10 ML) on Ni/O/Cu(001) (pink). These results indicate that oxygen locates always at the surface. green), Cu(0.2 ML) on Ni/O/Cu(001) (green), .

Fig. ２　

(a) Polar MOKE intensity (perpendicular magnetization) for various Cu capping thicknesses onto the Ni film without oxygen measured in remanence. The data was taken on a wedged Ni film.
(b) The same as (a), but measured for the Ni film grown on the preoxidized surface.
These results imply that the Cu capping stabilizes perpendicular magnetization for clean Cu(001), while it unstabilizes perpendicular magnetization for preoxidized Cu(001).

Fig. ３

(a) Magnetic hysteresis loops using the longitudinal Kerr effect (in-plane magnetization) measured on Cu/Ni(5.5 ML)/Cu(001). The coercive field is reduced with the Cu capping.
(b) The same as (a), but on Cu/Ni(4.8 ML)/O/Cu(001). The coercive field is once enhanced and subsequently reduced with the Cu capping.
　

Fig. ４　
Ni L_2,3-edge x-rau magnetic circular dichroism spectra.

(a) Cu(xML)/Ni(5.5 ML)/Cu(001) taken at grazing x-ray incidence (30°). The intensity is monotonically reduced with the Cu capping.
(b) Cu(x ML)/Ni(10.5 ML) /Cu(001) taken at normal x-ray incidence (90°). The intensity does not vary with the Cu capping.
(c) Cu(x ML)/Ni(4.8 ML)/O/Cu(001) taken at grazing x-ray incidence (30°). The intensity is once enhanced and subsequently reduced with the Cu capping.

Fig. ５　

Comparison between the hysteresis loss changes (circles) from magneto-optical Kerr effect and the orbital magnetic moment (squares) from x-rau magnetic circular dichroism for (a) Cu(x ML)/Ni(5.5 ML)/Cu(001) and (b) Cu(x ML)/Ni(4.8 ML)/O/Cu(001). The plots demonstrate a one-to-one correspondence between the hysteresis loss and orbital magnetic moment.

Fig. ６　

Spin reorientation transitions for Cu/Ni/Cu(001) and Cu/Ni/O/Cu(001). Yellow arrows indicate the magnetic easy axes. It is noted that oxygen locates always at the surface.