Interface Coupling Transition in a Thin Epitaxial Antiferromagnetic Film Interacting with a Ferromagnetic Substrate

We report experimental evidence for a transition in the interface coupling between an antiferromagnetic film and a ferromagnetic substrate. The transition is observed in a thin epitaxial NiO film grown on top of Fe(001) as the film thickness is increased. Photoemission electron microscopy excited with linearly polarized x rays shows that the NiO film is antiferromagnetic at room temperature with in-plane uniaxial magnetic anisotropy. The anisotropy axis is perpendicular to the Fe substrate magnetization when the NiO thickness is less than about 15 Å, but rapidly becomes parallel to the Fe magnetization for a NiO coverage higher than 25 Å.

Figure 1

PEEM images of XMLD asymmetry obtained at the Fe (top) and Ni (bottom) $L_2$ edge on a $\mathrm{NiO}/\mathrm{Fe}(001)$ wedged thin film. The indicated crystallographic directions refer to the Fe substrate. The NiO thickness increases from the left to the right, with an average value in the left, central, and right panels equal to about 12, 18, and 24 Å, respectively. The NiO thickness variation in each image is about 3 Å.

Figure 2

On the left: Fe (top) and Ni (bottom) $L_2$ edge asymmetry PEEM images obtained over a region showing two Fe magnetic domains with magnetizations forming an angle equal to 180°. On the right: Asymmetry profiles obtained from the lines indicated in the corresponding PEEM images. The indicated crystallographic directions refer to the Fe substrate.

Figure 3

Pictorial representation of the spin arrangements discussed in the text. In each panel, increasing NiO thickness is from left to right. The shading is a reminder of the magnetic contrast observed in PEEM images. (a) Domain nucleation, (b) shearlike rotation, (c) rigid rotation.