Breating the recording quadrilemma using Curie temperature modulated structures

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Heat-assisted magnetic recording (HAMR) combined with bit-patterned media (BPM) is one of the candidate technologies to overcome present limits in magnetic recording and to possibly extend magnetic recording to storage densities of several tens of Tbit/in2. BMP are required to reduce the transition jitter noise that would be present in granular recording media, where the bit transitions are invariably irregular given that each bit requires about 30 irregular magnetic grains of the recording media. To ensure stability of the magnetic information over time, high anisotropy is engineered which gives rise to a large coercivity. In turn heat assistance is necessary to raise the temperature during writing thereby reducing the medium coercivity to levels that can be written. However, elevated temperatures also lower the magnetization which substantially increases thermally induced recording errors. One of the co-applicants (D. Suess) has proposed a composite media structure, consisting of two exchange-coupled layers with different Curie temperatures, to overcome the above limitations. L10-ordered FePt is one of the few material systems with ultra-high magnetic anisotropy providing sufficient thermal stability. However, the preparation of materials of this class requires either high-temperature epitaxial growth or annealing at elevated temperatures to obtain the L10 phase. Moreover, the Curie temperature of about 750 K requires challenging heat management strategies for both the recording media as well as for the write-head. The goals of this proposal are to develop a novel exchange-coupled double layer prototype system suitable for HAMR/BPM and to demonstrate recording at densities beyond current limits with a viable extrapolation to several tens of Tbit/in2. To achieve these goals, we will fabricate an optimized [Co/Ni]/TbFeCo bilayer system. The [Co/Ni]-multilayer serves as a high Curie temperature, low-anisotropy write layer which also generates sufficient stray field for the read-out process. The amorphous ferrimagnetic TbFeCo layer serves as a high anisotropy storage layer. With the Co content, the Curie temperature of the TbFeCo layer can be tuned within the interval from 400 K to 600 K and is hence considerably smaller than that of L10-ordered FePt. This allows lower writing temperatures that reduce writing error rates, increases the lifetime of near field transducers of the write heads, and generally simplifies heat management issues. Further, the damping parameter in amorphous TbFeCo films depends on the Tb content reaching values of to 0.5 significantly larger than those obtained in the FePt system. According to our preliminary work, a large damping parameter is essential for a reliable magnetization switching process. Due to large damping in the Tc modulated structure containing TbFeCo thermally written in errors in BPM is expected to decrease from about 5 % to close to zero.