Structure and magnetism of Co-Cr thin films with an artificial superlattice structure

Cobalt-chromium (Co-Cr) thin films deposited in an alternating multilayer structure by the magnetron sputtering method have an artificial superlattice hexagonal closed pack (hcp) structure with the c-axis normal to the film plane, where the c-plane of hcp-Co grows epitaxially on the (110) plane of the bcc-Cr. The film, composed of monatomic layers of Cr and then several atomic layers of Co in an alternating fashion, has an especially excellent hcp structure and shows a clearly separated spot pattern in reflection high-energy electron diffraction analysis. The artificial superlattice structure of the film is stable thermally to temperatures up to 500 °C. Small-angle x-ray diffraction analysis confirmed the layering of monatomic layers of Co and Cr, where the periodicity of layers was found to be about 5 Å. Transmission electron microscopic observation proved the artificially layered structure in these films. The films showed ferromagnetism over a wide Cr composition range, where the thickness ratio of Co and Cr layers was varied from 4/1 to 1/4. The perpendicular magnetization hysteresis loop was obtained in films with a Co layer thinner than about 12.5 Å for various thickness ratios of Co and Cr layers. It is concluded that the origins of the perpendicular magnetization in the Co-Cr films with artificial superlattice structures are the magnetocrystalline anisotropy and the shape anisotropy related to the columnar structure.