Upper Limit of Carbon Concentration in Ferromagnetic L1₀-Ordered FePt-C for Tb/in² Data Storage Density Heat-Assisted Magnetic Recording Media

• Published in: IEEE transactions on magnetics Vol. 57; no. 10; pp. 1 - 6
• Main Authors: Choi, Minyeong, Hong, Yang-Ki, Won, Hoyun, Mankey, Gary J., Yeo, Chang-Dong, Shah, Nayem M. R., Lee, Woncheol, Jung, Myung-Hwa, Thiele, Jan-Ulrich
• Format: Journal Article
• Language: English
• Published: IEEE 01.10.2021
• Subjects: Brillouin function; Carbon; FePt-C; first-principles calculation; Heat-assisted magnetic recording; heat-assisted magnetic recording (HAMR); Iron; Magnetic properties; Magnetic recording; Media; Perpendicular magnetic anisotropy
• ISSN: 0018-9464; 1941-0069
• DOI: 10.1109/TMAG.2021.3079188

In high-density magnetic recording media, magnetically isolated grains are required to increase the signal-to-noise ratio (SNR). Carbon can be used to isolate FePt grains enabling their grain size smaller than 4.3 nm. Carbon atoms segregate to the boundaries during growth and provide an exchange-breaking layer, however, some other carbon atoms remain dissolved in the magnetic alloy. To identify the upper limit of carbon concentration in <inline-formula> <tex-math notation="LaTeX">L 1_{0} </tex-math></inline-formula>-ordered (Fe 0.5 Pt 0.5 ) 100- x C x , first-principles calculations are performed based on the density functional theory (DFT). The Brillouin function and Callen-Callen empirical relation determine the temperature-dependent magnetization and magneto-crystalline anisotropy energy enabling the determination of magnetic properties and Curie temperature required by 4 Tb/in 2 heat-assisted magnetic recording (HAMR) media and beyond. The calculated magnetization (<inline-formula> <tex-math notation="LaTeX">M_{s} </tex-math></inline-formula>) of <inline-formula> <tex-math notation="LaTeX">L 1_{0} </tex-math></inline-formula>-ordered (Fe 0.5 Pt 0.5 ) 100- x C x decreases to 770 emu/cm 3 at <inline-formula> <tex-math notation="LaTeX">x =20 </tex-math></inline-formula> from 1030 emu/cm 3 at <inline-formula> <tex-math notation="LaTeX">x = 0 </tex-math></inline-formula> at 300 K, and the magnetocrystalline anisotropy constant (<inline-formula> <tex-math notation="LaTeX">K_{u} </tex-math></inline-formula>) to 2.05 MJ/m 3 at <inline-formula> <tex-math notation="LaTeX">x =20 </tex-math></inline-formula> from 15.48 MJ/m 3 at 300 K. It is striking to find that the Curie temperature (<inline-formula> <tex-math notation="LaTeX">T_{C} </tex-math></inline-formula>) increases to 728 K at <inline-formula> <tex-math notation="LaTeX">x =20 </tex-math></inline-formula> from 719 K at <inline-formula> <tex-math notation="LaTeX">x =0 </tex-math></inline-formula>. Regardless of carbon concentration, the magnetic anisotropy direction is the out-of-plane. Combining <inline-formula> <tex-math notation="LaTeX">M_{s} </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">K_{u} </tex-math></inline-formula> at 300 K with <inline-formula> <tex-math notation="LaTeX">T_{C} </tex-math></inline-formula>, the <inline-formula> <tex-math notation="LaTeX">M_{s} </tex-math></inline-formula>-<inline-formula> <tex-math notation="LaTeX">K_{u} </tex-math></inline-formula>-C concentration relation is plotted to guide the design of <inline-formula> <tex-math notation="LaTeX">L 1_{0} </tex-math></inline-formula>-ordered Fe-Pt film for Tb/in 2 recording media. It is found that the upper limit of carbon concentration is determined to be about 12 at.% to retain <inline-formula> <tex-math notation="LaTeX">M_{s} \ge800 </tex-math></inline-formula> emu/cm 3 , <inline-formula> <tex-math notation="LaTeX">T_{C} \ge430 </tex-math></inline-formula> K, and <inline-formula> <tex-math notation="LaTeX">K_{u} \ge 5 </tex-math></inline-formula> MJ/m 3 , which are necessary to achieve areal densities of 4 Tb/in 2 and beyond.