Microstructure and magnetism in FeTaN films deposited in the nanocrystalline state

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Abstract
As-deposited, magnetically soft nanocrystalline FeTaN films are successfully grown by dc-magnetron reactive sputtering. Growth conditions are instrumental in extending the solubility limit of Ta in the bcc FeTa alloy. Nitrogen incorporation in FeTa films is found to be much higher than in Fe films and can be explained in terms of thermochemistry using a large Ta-N interaction coefficient. The influence of different alloying elements is discussed theoretically, with regard to the metal-nitrogen affinity. A ‘‘typical columnar microstructure’’ associated with the sputtering process is identified and its evolution versus the extent of nitrogenation is described in detail. Stress, magnetostriction, resistivity, and magnetic properties are respectively described as a function of both Ta and N contents of the films. The magnetic behavior of as-deposited nanocrystalline FeTaN is found to be very sensitive to both the dimension of the grains, their morphology and the nature of the grain boundary material which represents a non-negligible volume fraction in nanocrystalline films. It is proposed that the columnar structure plays the key role in promoting a large perpendicular anisotropy component (K⊥) and controls a ‘‘Stripe Domain’’-like behavior observed at high N contents, which cannot be explained in terms of film stress in this material. The contribution of the magnetoelastic anisotropy is also described. In summary, by breaking the columnar structure, the incorporation of nitrogen first decreases K⊥ below the critical limit for formation of stripe domains. In these conditions, N acts as a ‘‘grain refiner’’ and excellent soft magnetic behavior is reported and explained in terms of ‘‘vanishing magnetocrystalline anisotropy.’’ The good thermal stability of such soft films is confirmed. By contrast, higher nitrogen incorporation increases K⊥ above the critical limit, leading to a stable stripe domain-like structure which does not allow for soft magnetic properties. This phenomenon has been found to be reversible at low temperature where a complete restoration of the soft magnetic behavior has been observed. This anomalous result is explained by the transformation of the grain boundary material into a ‘‘low Curie temperature phase’’ for large N contents.

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