Evidence for orbital correlations and incipient orbital magnetism in ultrathin Co/Ni films

We present experimental evidence for orbital magnetism in heterostructures including ultrathin (111)-oriented Co/Ni bilayers. Magnetoelectronic measurements based on the anomalous Hall effect (AHE) show a nonlinear feature which appears on the background of paramagnetic response above the Curie point T_c of ferromagnetic films. The dependence on the magnetic field and temperature associated with this feature is well-described by the Landau theory of phase transition, with the transition temperature above T_c. The amplitude of the feature is maximized for equal thicknesses of Co and Ni.

We utilize the Hubbard model complemented with two-order-parameter mean-field calculations to show that our observations can be explained by ferromagnetic orbital correlations between 3d electrons associated with orbitally-selective electron hopping confined to the film plane. Such correlations are maximized for 3d shell population of one hole per site realized in thin Co/Ni films with equal thicknesses of Co and Ni. Our model shows that ferromagnetism in thin films can be substantially enhanced by orbital correlations. These correlations allow one to control magnetic anisotropy and magnetoelectronic phenomena associated with spin-orbit coupling such as anomalous and spin Hall effects.