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

Kim Kong Tham,Shin Saito,Takashi Saito,Ryosuke Kushibiki

doi:10.1109/tmag.2022.3146934

Abstract

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 ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$H_{\mathrm {c}}$ </tex-math></inline-formula> ) of 18.9 kOe, a saturation magnetization ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$M_{\mathrm {s}}$ </tex-math></inline-formula> ) of 600 emu/cm 3 , a perpendicular magnetic anisotropy ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K_{\mathrm {u\bot }}$ </tex-math></inline-formula> ) of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.7\times 10^{\mathrm {7}}$ </tex-math></inline-formula> erg/cm 3 , and a magnetic anisotropy field ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$H_{\mathrm {k}}$ </tex-math></inline-formula> ) 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$c$ </tex-math></inline-formula> -axis parallel to the film plane; 3) lattice constant <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$a$ </tex-math></inline-formula> remains constant, a minimum value of lattice constant <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$c$ </tex-math></inline-formula> and the ratio <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$c/a$ </tex-math></inline-formula> 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$c/a$ </tex-math></inline-formula> ratio, the degree of order, and the agglomeration suppression of grains enhance <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$M_{\mathrm {s}}$ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K_{\mathrm {u\bot }}$ </tex-math></inline-formula> , and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$H_{\mathrm {k}}$ </tex-math></inline-formula> . 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$M_{\mathrm {s}}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K_{\mathrm {u\bot }}$ </tex-math></inline-formula> 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$H_{\mathrm {k}}$ </tex-math></inline-formula> , this results in the maximization of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$H_{\mathrm {c}}$ </tex-math></inline-formula> .

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