Fundamental Magnetic Properties and Structural Implications for Nanocrystalline Fe-Ti-N Thin Films

TL;DR

This study investigates the magnetic properties and structural changes in nanocrystalline Fe-Ti-N thin films, revealing how nitrogen content influences anisotropy, lattice structure, and magnetic behavior through comprehensive experimental analysis.

Contribution

It provides new insights into the relationship between nitrogen doping, structural transitions, and magnetic anisotropy in Fe-Ti-N thin films, including quantitative analysis of anisotropy energy scaling.

Findings

Nitrogen induces a structural transition from bcc to bct in the films.

Uniaxial anisotropy increases linearly with nitrogen content.

Cubic anisotropy energy scales with grain size, matching theoretical predictions.

Abstract

The magnetization (M) as a function of temperature (T) from 2 to 300 K and in-plane field (H) up to 1 kOe, room temperature easy and hard direction in-plane field hysteresis loops for fields between -100 and +100 Oe, and 10 GHz ferromagnetic resonance (FMR) profiles have been measured for a series of soft-magnetic nano-crystalline 50 nm thick Fe-Ti-N films made by magnetron sputtering in an in-plane field. The nominal titanium concentration was 3 at. % and the nitrogen concentrations (xN) ranged from zero to 12.7 at. %. The saturation magnetization (Ms) vs. T data and the extracted exchange parameters as a function of xN 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 xN. Films with xN > 1.9 at. % show an overall uniaxial anisotropy, with an anisotropy field parameter Hu that increases with xN. The corresponding dispersion averaged uniaxial anisotropy energy density parameter <Ku> = HuMs/2 is a linear function of xN, with a rate of increase of 950 erg/cm3 per at. % nitrogen. The estimated uniaxial anisotropy energy per nitrogen atom is 30 J/mol, a value consistent with other systems. For xN below 6 at. %, the scaling of coercive force Hc data with the sixth power of the grain size D indicate a grain averaged effective cubic anisotropy energy density parameter <K1> that is about an order of magnitude smaller that the nominal K1 values for iron, and give a quantitative <K1> vs. D response that matches predictions for exchange coupled random grains with cubic anisotropy.