Exchange spring magnetic thin films: The future recording media

数据存储正是数字时代的核心，也是纳米技术的核心开发者和用户。以极小的成本按需存储和检索海量信息的能力彻底改变了社会的运转方式。这场革命的核心设备是硬盘驱动器(HDD)，在过去的50年里，硬盘驱动器的数据密度增加了100,000,000倍，现在单个3.5英寸设备可以提供1TB的存储容量。实现这种容量爆炸的关键磁性部件是记录头和存储介质。作为记录头换能器基础的研究工作，巨磁电阻(GMR)效应得到了2007年诺贝尔物理学奖的认可。在这里提出的工作中，将探索基于薄膜交换弹簧的新型、高度工程化的磁性介质的潜力。这些材料允许定制介质热稳定性和切换场之间的关系，使得具有足够各向异性以避免热激活反转的薄膜仍然可以被来自写头的可用场反转。为了充分利用这些材料，迫切需要解决薄膜交换弹簧磁体中激动人心的基本问题。具体地说，对于技术相关厚度(~10 nm)的薄膜，在保持可寻址能力的同时实现最大热稳定性的最佳交换弹簧结构是什么；在致密填充的颗粒状材料中，晶间交换弹簧或对于图案化结构的岛间交换耦合如何改变反转行为和热稳定性；在反转过程中弹簧结构的细节是什么；交换弹簧薄膜对杂散场的反转有多大抵抗力；薄膜交换弹簧还可以在哪些应用领域提供增强的功能。我们的研究目标是为围绕薄膜交换弹簧磁体的重要问题提供定量的答案。我们计划通过在PI开发的创新的矢量磁学测量协议的基础上实现这一点，以确定特殊设计的样品的磁性，其中我们将通过选择材料、耦合层和光刻处理来系统地控制薄膜交换弹簧。建议的测量方案充分利用了矢量磁强计跟踪磁化矢量的位置和力矩的能力，同时以任意角度施加磁场并将样品保持在设定的温度。这一能力使反转过程能够被准确地表征，例如，可以量化成核和磁化壁过程之间的关系。我们计划的一个关键部分是对薄膜交换弹簧磁体的行为进行建模，从而从我们独特的数据中获得最大的科学产出。建立了一种基于动力学蒙特卡罗方法的模拟框架，用于计算薄膜交换弹簧介质的能垒和剩余磁滞回线。这种对现有模拟程序的扩展将使研究交换和静磁相互作用对磁化反转行为的影响成为可能。利用这一新开发的功能，将计算不同场角和不同晶间交换作用下交换弹簧介质的剩余磁滞回线，并直接与实验结果进行比较。

Data storage is right at the centre of the digital age and a core developer and user of nanoscale technology. The ability to store and retrieve vast amounts of information on demand, and at miniscule cost, has revolutionised the way society functions. The device at the heart of this revolution is the hard disk drive (HDD) where over the last 50 years data densities have increased by a factor of 100,000,000 so that 1TB of storage capacity is now available in a single 3.5 inch device. The key magnetic components that have enabled this explosion in capacity are the recording head and the storage medium. The research work which forms the basis of recording head transducers, the giant magnetoresistive (GMR) effect was recognised with the 2007 Nobel prize in physics.In the work proposed here, the potential of new, highly engineered magnetic media based on thin film exchange springs will be explored. These materials allow the relationship between medium thermal stability and switching field to be tailored, so that thin films with sufficient anisotropy to avoid thermally activated reversal can still be reversed by the fields available from a write head. In order to take full advantage of these materials there is a pressing need to address the exciting fundamental questions in thin film exchange spring magnets. Specifically, what is the optimum exchange spring structure for thin films at technologically relevant thicknesses (~10 nm) that achieves maximum thermal stability whilst retaining addressability; in dense packed granular materials how does intergranular or, for patterned structures, inter-island exchange coupling modify the reversal behaviour and the thermal stability; what are the details of the spring structure during reversal; how resistant are exchange spring thin film to reversal from stray fields; what other application areas can thin film exchange springs provide enhanced functionality. The goal of our research is to provide quantitative answers to the important questions surrounding thin film exchange spring magnets. We plan to achieve this by building on the innovative vector magnetometry measurement protocols developed by the PI to determine the magnetic properties of specially designed samples, where we will systematically control the thin film exchange spring by choice of materials, coupling layers and lithographic processing. The proposed measurement programme makes full use of the vector magnetometer's ability to track the position and moment of the magnetisation vector whilst applying a field at an arbitrary angle and maintaining the sample at a set temperature. This capability allows the reversal process to be accurately characterised so that, for example, the relationship between nucleation and domain wall processes can be quantified.A critical part our programme is the work to model the behaviour of thin film exchange spring magnets and so obtain the maximum scientific output from our unique data. A simulation framework based on a kinetic Monte Carlo scheme to compute energy barriers and remanent hysteresis loops will be developed for thin film exchange spring media. This expansion of the existing simulation code will make it possible to study the effect of exchange and magnetostatic interactions on the magnetization reversal behavior. Using this newly developed capability, remanent hysteresis loops of exchange spring media will be computed for different field angles and different intergrain exchange interactions and compared directly with the experimental results.