Spin direction, quantum size effects, layer stacking, and microstructure of ultrathin Co films on W(110)

The spin direction in atomically flat ultrathin Co layers with well-defined thickness is studied experimentally with spin-polarized low-energy electron microscopy and theoretically with first-principles calculations. Spin-orbit coupling and atomic positions at the interface affect the spin direction over a wide thickness range. Spin-dependent quantum size effects produce a layer stacking-dependent oscillatory evolution of the spin direction with thickness not seen in other ultrathin ferromagnetic films. This difference is explained by the theoretical model, which gives a broader understanding of the spin direction in ultrathin ferromagnetic films in general.