MICROSTRUCTURE AND MAGNETISM IN FETAN FILMS DEPOSITED IN THE NANOCRYSTALLINE STATE

As-deposited, magnetically soft nanocrystalline FeTaN films are succes sfully grown by dc-magnetron reactive sputtering. Growth conditions ar e instrumental in extending the solubility limit of Ta in the bcc FeTa alloy. Nitrogen incorporation in FeTa films is found to be much highe r than in Fe films and can be explained in terms of thermochemistry us ing a large Ta-N interaction coefficient. The influence of different a lloying elements is discussed theoretically, with regard to the metal- nitrogen affinity. A ''typical columnar microstructure'' associated wi th the sputtering process is identified and its evolution versus the e xtent of nitrogenation is described in detail. Stress, magnetostrictio n, resistivity, and magnetic properties are respectively described as a function of both Ta and N contents of the films. The magnetic behavi or of as-deposited nanocrystalline FeTaN is found to be very sensitive to both the dimension of the grains, their morph7ology and the nature of the grain boundary material which represents a non-negligible volu me fraction in nanocrystalline films. It is proposed that the columnar structure plays the key role in promoting perpendicular anisotropy co mponent (K-perpendicular to) and controls a ''Stripe Domain'' like beh avior observed at high N contents, which cannot be explained in terms of film stress in this material. The contribution of the magnetoelasti c anisotropy is also described. In summary, by breaking the columnar s tructure? the incorporation of nitrogen first decreases K-perpendicula r to below the critical limit for formation of stripe domains. In thes e conditions, N acts as a ''grain refiner'' and excellent soft magneti c behavior is reported and explained in terms of ''vanishing magnetocr ystalline anisotropy.'' The good thermal stability of such soft films is confirmed. By contrast, higher nitrogen incorporation increases K-p erpendicular to above the critical limit, leading to a stable stripe d omain-like structure which does not allow for soft magnetic properties . This phenomenon has been found to be reversible at low temperature w here a complete restoration of the soft magnetic behavior has been obs erved. This anomalous result is explained by the transformation of the grain boundary material into a ''low Curie temperature phase'' for la rge N contents. (C) 1996 American Institute of Physics.