Spin-reorientation transitions in ultrathin Co films on Pt(111) and Pd(111) single-crystal substrates

Spin-reorientation transition (SRT) behaviors in ultrathin Co films grown on both Pt(111) and Pd(111) single-crystal substrates are investigated with in situ magneto-optical Kerr effects. At the region of SRT from the perpendicular to in-plane magnetization with the increasing of Co thickness, canted out-of-plane orientations are observed, from which we can determine not only the effective second- (K₂) but also fourth- (K₄) order anisotropy constants. Both the constants, which can be separated into bulk K2b(4b) and interface anisotropy terms K₂ₛ₍₄ₛ₎ are essential to understand the stable canted out-of-plane magnetization. We found that the value of K₂ₛ for Co/Pt(111) is larger than that for Co/Pd (111), resulting in the later onset thickness and wider range of the observed SRT, while the negative small values of K₄ₛ in the both systems yield the stable canted phases. In addition, we observed different polarization effects of Pd and Pt near the interfaces which might also contribute the contrasting SRT behaviors.