Breating the recording quadrilemma using Curie temperature modulated structures

热辅助磁记录（HAMR）与位图案化介质（BPM）相结合是克服磁记录中的现有限制并可能将磁记录扩展到几十Tbit/in 2的存储密度的候选技术之一。需要BMP来减少将存在于颗粒记录介质中的转换抖动噪声，在颗粒记录介质中，如果每个比特需要记录介质的大约30个不规则磁性颗粒，则比特转换总是不规则的。为了确保磁信息随时间的稳定性，设计了高各向异性，这产生了大的磁化率。反过来，在写入过程中需要热辅助来升高温度，从而将介质的介电性降低到可以写入的水平。然而，升高的温度也降低了磁化强度，这大大增加了热致记录误差。其中一个共同申请人（D。Suess）提出了一种复合介质结构，由两个具有不同居里温度的交换耦合层组成，以克服上述限制。L10有序FePt是少数具有超高磁各向异性的材料系统之一，提供足够的热稳定性。然而，这类材料的制备需要高温外延生长或在升高的温度下退火以获得L10相。此外，大约750 K的居里温度需要对记录介质以及写入头两者具有挑战性的热管理策略。本提案的目标是开发一种新的交换耦合双层原型系统，适合HAMR/BPM，并证明记录的密度超过目前的限制与一个可行的外推到几十Tbit/in 2。为了实现这些目标，我们将制作一个优化的[Co/Ni]/TbFeCo双层系统。[Co/Ni]-多层用作高居里温度、低各向异性写入层，其还产生用于读出过程的足够杂散场。非晶亚铁磁TbFeCo层用作高各向异性存储层。随着Co含量的增加，TbFeCo层的居里温度可以在400 K至600 K的区间内调节，因此比L10有序FePt的居里温度小得多。这允许降低写入错误率的较低写入温度，增加写入头的近场换能器的寿命，并且通常简化热管理问题。此外，在非晶TbFeCo膜的阻尼参数取决于Tb含量达到0.5的值显着大于在FePt系统中获得的。根据我们的初步工作，一个大的阻尼参数是必不可少的一个可靠的磁化切换过程。由于包含TbFeCo的Tc调制结构中的大阻尼，BPM中的热写入误差预期从约5%降低到接近零。

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.