The Strain Manipulation of Nanoscale Magnetic Structures

现代技术设备中的许多部件，如计算机、通信设备(例如移动电话)和传感器，都是由磁性材料以非常小的规模制造的。例如，现代计算机硬盘驱动器和磁性随机存取存储器(MRAM)包含几十纳米大小的磁性元件。在这样的设备中，磁性元件的磁化方向被用来存储信息。控制磁化的方向是通过使用电流在局部产生磁场，或者通过使用称为自旋转移扭矩的效应使电流通过设备来实现的。这些技术具有以下缺点：施加电流时所消耗的能量、小型化的限制(由于需要将产生磁场的组件与其他磁性设备集成在一起)以及由于杂散磁场而难以寻址单个元件。这些问题的一个解决方案是创造出通过施加电压来控制磁状态的装置。在这个项目中，我将采用一种新的方法来实现这一点，在混合装置中将磁性材料和压电材料结合起来。压电材料具有这样一种特性，即当施加电压时，它会发生物理膨胀或收缩。这可以用来将应变传递给磁性材料。某些磁性材料具有很大的磁致伸缩性能，这意味着如果它们被应变，那么磁化方向就会旋转。例如，我将研究磁致伸缩过渡金属合金FeCo，FEPD和FePT。我将使用超导量子干涉器件(SQUID)磁学和磁力显微镜(MFM)等现代表征技术来研究这些材料的块状和纳米级的磁性，并将使用最先进的生长和制造技术(例如溅射沉积和电子束光刻)来制造几十纳米大小的器件。通过在GHz频率下进行电传输实验(与现代计算技术中使用的频率相当)，我的目标是演示通过施加超快(皮秒)电压脉冲来实现设备磁状态的超快切换。这些纳米尺度的设备还将用于研究自旋转移扭矩等现象的基本物理。我将研究的另一类设备是纳米机电系统(NEMS)，它由纳米级的振动梁和悬臂组成。这类设备具有作为高灵敏度称重秤的潜在应用，对于量子物理和经典物理之间重叠的更基础研究也很有趣。利用磁致伸缩铁磁材料制备NEMS将为探测和驱动机械振荡提供新的手段。这一提议为研究新材料系统中的基本物理现象提供了令人兴奋的机会，并有望产生关于纳米器件中新现象和新功能的知识。这项工作的结果将有助于未来计算、通信和传感器技术的设计。

Many of the components in modern technological devices such as computers, communications devices (e.g. mobile phones) and sensors are made on a very small scale from magnetic materials. For example, modern computer hard drives and magnetic random access memory (MRAM) contain magnetic elements that are a few tens of nanometres in size. In such devices the direction of the magnetisation of the magnetic elements is used to store information. Controlling the direction of magnetisation is achieved by using electrical current to generate a magnetic field locally or by passing an electrical current through the device using an effect called spin transfer torque . These techniques have disadvantages arising from the energy dissipated in applying electrical currents, the limits on miniaturisation (due to the need to integrate the components which generate the field with other magnetic devices) and the difficulty in addressing individual elements due to stray magnetic fields. A solution to these problems would be to create devices in which the magnetic state is controlled by applying electrical voltages. In this project I will do this by adopting a novel approach, combining the magnetic material with piezoelectric material in hybrid devices. Piezoelectric material has the property that it will physically expand or contract when an electrical voltage is applied to it. This can be used to transfer strain to the magnetic material. Certain magnetic materials have large magnetostrictive properties, which means that if they are strained then the magnetisation direction will rotate. For example, I will study the magnetostrictive transition metal alloys FeCo, FePd and FePt. I will study the magnetic properties of these materials in the bulk and on the nanoscale using modern characterisation techniques such as Superconducting Quantum Interference Device (SQUID) magnetometry and Magnetic Force Microscopy (MFM), and I will use state of the art growth and fabrication techniques (e.g. sputter deposition and electron beam lithography) to fabricate devices a few tens of nanometres in size. By conducting electrical transport experiments at GHz frequencies (comparable to the frequencies used in modern computing technology) I aim to demonstrate ultra-fast switching of the magnetic state of the devices by applying ultra-fast (picosecond) voltage pulses. The nanoscale devices will also be used to study the fundamental physics of phenomena such spin transfer torque . Another class of devices that I will study are nano-electro-mechanical systems (NEMS) which consist of nanoscale oscillating beams and cantilevers. Such devices have potential applications as highly sensitive weighing scales and are also interesting for more fundamental studies of the overlap between quantum and classical physics. The use of magnetostrictive ferromagnetic materials to fabricate NEMS will offer new means to detect and drive the mechanical oscillations.This proposal presents exciting opportunities to study fundamental physical phenomena in new material systems and promises to produce knowledge of new phenomena and new functionalities in nanoscale devices. The results of this work will contribute to the design of future computing, communications and sensor technologies.