Magnetic and structural properties of epitaxially grown FeTaN thin films

Epitaxial FeTaN films (∼1500 Å) were grown as a function of nitrogen flow rate on epitaxial Cu(001) buffer layers (∼2000 Å) on Si(001) single crystal substrates to investigate the effect of Ta and nitrogen on the magnetocrystalline anisotropy and magnetostriction. Detailed structural investigation by transmission electron microscopy and x-ray diffraction showed that the FeTaN films were epitaxial with the Pitsch orientation relationship of FeTaN(110)||Cu(001) and FeTaN〈111〉||Cu〈110〉, which allows four different in-plane variants to coexist in the film. It was found that the saturation magnetization did not change with nitrogen addition (∼1600 emu/cc) up to 2% lattice dilation. The values of K₁ and λ₁₀₀ of Fe decreased slightly (20% and 10%, respectively), while λ₁₁₁ increased with increasing nitrogen content and eventually changed sign at ∼2% normalized lattice dilation. These results qualitatively agree with our earlier findings on (001) oriented FeTaN epitaxial films on MgO single crystal substrates. Also, our calculated saturation magnetostriction for nanocrystalline samples agrees very well with published data on the same FeTaN composition. Based on Hoffmann's ripple theory the ripple constant is calculated for nanocrystalline films using the Doyle-Finnegan model for the local average anisotropy and our measured single crystal constants. It was found that the effect of nitrogen on reducing the average anisotropy through the fundamental constants is not significant, and therefore the major factor in achieving a low ripple constant (i.e., soft magnetic properties) is the grain size, the number of grains across the thickness and the thin film stress.