The Strain Manipulation of Nanoscale Magnetic Structures

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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.