Magnetic properties and structural implications for nanocrystalline Fe-Ti-N thin films

The magnetization and ferromagnetic resonance (FMR) profiles have been measured for a series of soft-magnetic nanocrystalline Fe-Ti-N films. Magnetization (M) measurements were made as a function of temperature (T) from 2 to 300 K and in-plane field (H) up to 1 kOe. Additionally, room temperature easy and hard direction in-plane field hysteresis loops were measured for fields between +/- 100 Oe, and 10 GHz FMR measurements were performed. The 50 nm thick films were made by magnetron sputtering in an in-plane field. The nominal titanium concentration was 3 at. % and the nitrogen concentrations (x(N)) ranged from 0 to 12.7 at. %. The saturation magnetization (M-s) vs T data and the extracted exchange parameters as a function of x(N) are consistent with a lattice expansion due to the addition of interstitial nitrogen in the body-centered-cubic (bcc) lattice and a structural transition to body-centered-tetragonal (bct) in the 6-8 at. % nitrogen range. The hysteresis loop and FMR data show a consistent picture of the changes in both the uniaxial and cubic anisotropy as a function of x(N). Films with x(N)>= 3.9 at. % show an overall uniaxial anisotropy, with an anisotropy field parameter H-u that increases with x(N). The corresponding dispersion averaged uniaxial anisotropy energy density parameter < K-u >=HuMs/2 is a linear function of x(N), with a rate of increase of 950 +/- 150erg/cm(3) per at. % nitrogen. The estimated uniaxial anisotropy energy per nitrogen atom is 30 J/mol, a value consistent with other systems. For x(N) below 6 at. %, the scaling of coercive force H-c data with the sixth power of the grain size D indicate a grain averaged effective cubic anisotropy energy density parameter < K-1 > that is about an order of magnitude smaller that the nominal < K-1 > values for iron, and give a quantitative < K-1 > vs D response that matches predictions for exchange coupled random grains with cubic anisotropy.