  We show that it is possible to tune the N\'eel temperature of
nickel(II)-cobalt(II) oxide films by changing the Ni to Co ratio. We grow
single crystalline micrometric triangular islands with tens of nanometers
thickness on a Ru(0001) substrate using high temperature oxygen-assisted
molecular beam epitaxy. Composition is controlled by adjusting the deposition
rates of Co and Ni. The morphology, shape, crystal structure and composition
are determined by low-energy electron microscopy and diffraction, and
synchrotron-based x-ray absorption spectromicroscopy. The antiferromagnetic
order is observed by x-ray magnetic linear dichroism. Antiferromagnetic domains
up to micrometer width are observed.
