Experimental observation of the surface anomalous Hall effect in CoNi3 (001) epitaxial films*

Although many attempts have been made to study the anomalous Hall effect (AHE), the contributions from the surface and the bulk have rarely been entangled. In this work, we have disentangled contributions from the surface and the bulk to the AHE in CoNi3 (001) epitaxial and CoNi3 polycrystalline films. The surface and the bulk anomalous Hall angles are found to have opposite signs. Due to the competition of the surface and the bulk effects, the anomalous Hall angle at a fixed temperature changes from negative to positive as the film thickness increases. Furthermore, when the temperature increases, the anomalous Hall angle decreases in thick films and increases in thin films. In particular, the scaling law of the bulk AHE is analyzed. The values of the scattering independent parameter in epitaxial and polycrystalline CoNi3 films are close to each other and the side-jump mechanism is negligible. Moreover, since the skew scattering parameters in polycrystalline films are much larger than those of epitaxial films and therefore strongly depend on the mechanism of the electron scattering. The experimental results are successfully replicated by first-principles calculations.