Effect of N₂ Addition on Nanostructure and Magnetic Properties of Sputtered FePt-BN Granular Films During Deposition for Heat Assisted Magnetic Recording

Magnetic properties and nanostructure for FePt-30 vol% BN granular films deposited while flowing N ₂ gas with ratio between 0% and 5% with respect to the total flow of N ₂ + Ar are investigated. Here is a list of the results: 1) A maximum coercivity ( $H_{\mathrm {c}}$ ) of 18.9 kOe, a saturation magnetization ( $M_{\mathrm {s}}$ ) of 600 emu/cm ³ , a perpendicular magnetic anisotropy ( $K_{\mathrm {u\bot }}$ ) of $1.7\times 10^{\mathrm {7}}$ erg/cm ³ , and a magnetic anisotropy field ( $H_{\mathrm {k}}$ ) of 58 kOe, with smaller grain size and higher degree of grain separation are obtained when N ₂ gas with a ratio of around 2% is added; 2) higher fraction of N ₂ gas ratio leads to an increase of the amount of magnetic grains with $c$ -axis parallel to the film plane; 3) lattice constant $a$ remains constant, a minimum value of lattice constant $c$ and the ratio $c/a$ are also observed for a N ₂ ratio of around 2%; 4) there are two phenomena observed in nanostructure and magnetic properties defined by the N ₂ ratio. For N ₂ ratios under 2%, changes in the $c/a$ ratio, the degree of order, and the agglomeration suppression of grains enhance $M_{\mathrm {s}}$ , $K_{\mathrm {u\bot }}$ , and $H_{\mathrm {k}}$ . This indicates that the segregation of B into grain boundaries as the result of the N ₂ addition. For N ₂ ratios over 2%, the change of tetragonal into cubic FePt magnetic grains, the reduction of the degree of order, and the decrease of $M_{\mathrm {s}}$ and $K_{\mathrm {u\bot }}$ suggest that excess nitrogen dissolves into the FePt magnetic grains; and 5) another consequence of the 2% N ₂ ratio is the enhancement of the magnetic grain decoupling along with the increase of $H_{\mathrm {k}}$ , this results in the maximization of $H_{\mathrm {c}}$ .