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

摘要

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 aniso-tropy 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 >=H_uM_s/2 is a linear function of X_N, with a rate of increase of 950 ± 150 erg/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.