Study of obliquely deposited thin cobalt films

Thin cobalt films, 40 nm and 100 nm in thickness, were deposited by thermal evaporation at an incidence angle of 45° in a system with a base pressure of approximately 10−5 mbar, simultaneously on unheated glass substrates and NaCl crystals. The magnetic microstructure of the films was investigated with the conventional Bitter pattern technique and the Fresnel mode of transmission electron microscopy (TEM), while the morphological structure was observed using TEM, atomic force microscopy (AFM) and scanning electron microscopy (SEM). The films were found to have uniaxial in-plane magnetic anisotropy. The magnetic microstructure of the films 40 nm and 100 nm thick consisted of domains running (and magnetized) predominantly in the direction perpendicular and parallel to the incidence plane, respectively. For the films studied, some correlation between the magnetic microstructure and the morphological structure was detected.

Research highlights▶ The paper reports simultaneously on the magnetic domain structure of obliquely deposited thin cobalt films (40 nm and 100 nm thick) and their morphological structure. Such studies are in fact rare (Refs. [5], [6], [7], [8], [9] and [10] cited in the paper). ▶ Moreover, to our knowledge, observations of the morphological structure of these films have not yet been carried out simultaneously by transmission electron microscopy (TEM) and atomic force microscopy (AFM). ▶ The films of both thicknesses were found to have uniaxial in-plane magnetic anisotropy. ▶ The magnetic microstructure of the films 40 nm thick was composed of domains running and magnetized predominantly in the direction perpendicular to the incidence plane of the vapor beam. ▶ As the film thickness was changed from 40 nm to 100 nm, the magnetic anisotropy was observed to change from the direction perpendicular to parallel with respect to the incidence plane. ▶ Thanks to the application of TEM and AFM, complementary information on the morphological structure of the films could be obtained. ▶ In comparison with TEM images, AFM images revealed grains larger in size and slightly elongated in the direction perpendicular rather than parallel to the incidence plane. ▶ These experimental findings clearly show that surface diffusion plays an important role in the process of film growth. ▶ For the films 40 nm thick, the alignment of columnar grains in the direction perpendicular to the incidence plane was observed. ▶ This correlates well with the magnetic domain structure of these films. ▶ For the films 100 nm thick, the perpendicular alignment of columnar grains could also be found, although in fact with larger difficulty. ▶ TEM studies showed that the films consisted mainly of the hexagonal close-packed (HCP) crystalline structure, but no preferred crystallographic orientation of the grains could be detected for the films of both thicknesses. ▶ For the films 100 nm thick, the alignment of magnetic domains in the direction parallel to the incidence plane indicates that the crystallographic contribution to the magnetic anisotropy is dominant over the shape anisotropy.