Beating the recording quadrilemma using Curie temperature modulated structures

Heat-assisted 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 2 recording to storage densities of several tens of Tb/in . 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 L1 0 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 new exchange-coupled double layer prototype system suitable for HAMR/BPM and to demonstrate recording at densities beyond current limits with a viable 2 extrapolation to several tens of Tb/in . To achieve these goals, we will fabricate an optimized [Co/Ni]N/TbxFe1-x-yCoy bilayer system. The [Co/Ni]N-multilayer serves as a high Curie temperature, low- anisotropy write layer which also generates sufficient stray field for the readout process. The amorphous ferrimagnetic TbxFe1-x-yCoy layer serves as a high anisotropy storage layer. With the Co content the Curie temperature of the TbxFe1-x-yCoy layer can 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. Furthermore, the amorphous structure of the TbFeCo storage layer will be advantageous for a narrow switching field distribution in BPM. In summary, the project addresses a novel exchange-coupled ferro-/ferrimagnetic composite Curie temperature modulated bilayer system for HAMR/BPM. Unique experimental methods are available in the two experimental groups with a thorough background in magnetic thin film research. The experimental work will be supported by a theory group having a long-standing experience in the field of magnetic recording systems.

In 2019, 80% of the world`s data is stored on magnetic hard disks, which are widely used in personal computers and especially in data centers. The project investigated the physical principles of hard disks based on heat-assisted magnetic recording (HAMR). This technology has the potential to realize storage densities of more than 10 Tb/in, i.e. densities about 6 times greater than those of commercial hard disks in 2019. The first products using this technology are expected to be launched on the market at the end of 2020. HAMR was developed because high density conventional hard disks require materials with high magnetic anisotropy for the required thermal stability of the data. However, these materials have too large a coercive field to be magnetized with conventional write heads. By local heating with a laser pulse these hard magnetic grains can be magnetized. The physics of magnetization dynamics during the writing process is complex and state of the art methods are extremely computationally intensive. These methods solve the equation of motion for the magnetization on each atom. Within the project, methods were developed that allow to combine many atoms to a computational cell. The computational effort for the description of the processes during the data recording could be reduced significantly and thus a variety of new materials for data recording could be investigated and finally new highly functional material compositions could be found. In particular, the problem was solved that the magnetization decreases strongly during the high temperature write process that leads to write errors. Material compositions consisting of exchange-coupled magnetic layers with different Curie temperatures (Tc) were discovered and investigated in detail. These materials show sufficiently high magnetization even when heated by the laser pulse, thus improving the writability. It could be shown that storage densities of up to 10 Tb/in can be achieved with these composite materials. With the help of the experimentally produced composite layers at the project partners, the magnetic properties could be investigated in detail. Special magnetic materials were used (ferrimagnets), in which the magnetic properties are strongly dependent on temperature. Thus, by changing the temperature in a systemic way, the magnetic properties could be varied and the influence of these different magnetic parameters on the most important magnetic parameters such as the coercive field could be investigated.