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
$H_{\mathrm {c}}$
.