Breating the recording quadrilemma using Curie temperature modulated structures

热辅助磁记录（HAMR）与位模式介质（BPM）相结合是克服当前磁记录限制的候选技术之一，并有可能将磁记录的存储密度扩展到几十Tbit/in2。BMP是为了减少颗粒状记录介质中出现的过渡抖动噪声，在颗粒状记录介质中，由于每个比特需要大约30个不规则的记录介质磁性颗粒，因此比特过渡总是不规则的。为了确保磁性信息随时间的稳定性，设计了高各向异性，从而产生了大的矫顽力。反过来，在写入过程中需要热辅助来提高温度，从而将介质矫顽力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，这大大增加了热诱导的记录误差。共同申请者之一（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是为数不多的具有超高磁各向异性且具有足够热稳定性的材料体系之一。然而，这类材料的制备需要高温外延生长或在高温下退火才能获得L10相。此外，约750 K的居里温度对记录介质和写头的热管理策略都具有挑战性。本提案的目标是开发一种适用于HAMR/BPM的新型交换耦合双层原型系统，并演示在超过电流限制的密度下记录，并可行地外推到几十Tbit/in2。为了实现这些目标，我们将制作一个优化的[Co/Ni]/TbFeCo双层体系。[Co/Ni]多层作为一个高居里温度、低各向异性的写入层，也为读出过程产生了足够的杂散场。非晶铁磁TbFeCo层作为高各向异性的存储层。随着Co含量的增加，TbFeCo层的居里温度可以在400 ~ 600 K范围内调节，因此比l10 -有序FePt层的居里温度要小得多。这样可以降低写入温度，从而降低写入错误率，增加写入头近场传感器的使用寿命，并且通常简化了热管理问题。此外，非晶TbFeCo薄膜的阻尼参数取决于Tb含量，达到0.5的值明显大于FePt体系。根据我们的初步工作，大的阻尼参数对于可靠的磁化开关过程是必不可少的。由于含有TbFeCo的Tc调制结构中的大阻尼，BPM的热写入误差预计将从约5%降低到接近于零。

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.