Copy of Andreev Reflection in Superconducting Spin Polarised Devices

Interactions between electrons in solids are responsible for many of the most intriguing and exciting physical phenomena studied in modern physics. This project aims to investigate and exploit interactions between 2 such phenomena, intimately connected to the quantum nature of an electron's angular momentum, or spin, that at first sight appear to be incompatible and complementary. By using the latest experimental techniques for creating nanometre scale structures and carrying out measurements at temperatures less than a third of a degree, this work will develop our understanding of the most fundamental interactions that will be at the heart of the next epoch in electronics. This work builds on my experience in device materials, applying an understanding of materials science to help study problems in low temperature and condensed matter physics that can then be applied in useful electronic or magnetic devices.Electrons can be described in terms of a set of properties, for example, energy, momentum and angular moment, that quantum mechanics limits to certain specific values. For angular momentum or spin there are 2 values; 'up' and 'down'. Superconductivity, the property where a material will conduct electricity at low temperatures and magnetic fields without resistance, is associated in metals with pairing of electrons with opposite spins, one 'up' and one 'down'. Ferromagnetism, a material's having permanent magnetism below a critical temperature, is associated with the alignment of electrons in one direction. Electron spin can also be thought of as making the electrons act as little bar magnets; so aligning electron spins makes all the individual magnetic moments add up together.Of particular interest are the processes where electrons are transferred from a ferromagnet (parallel orientation) to a superconductor (anti-parallel orientation). Conventionally, a single electron entering a superconductor does so in a process known as Andreev reflection - it needs to take a second electron with it that must have opposite spin to form a pair in the superconductor. If the electrons are coming from a ferromagnet, there may not be enough suitable electrons due to the parallel alignment of spins.There is another possibility: most real magnetic materials do not form single domains with all the magnetic moments of the electrons aligned in the same direction. Instead, multiple domains with different orientations occur separated by domain walls where the magnetic moments twist from one orientation to the other. If the 2 electrons entering a superconductor are taken from different domains, it may be easier to pair the electrons up - a process known as cross Andreev reflection. To do this the two domains need to be placed within the superconducting coherence length - the distance over which the pairing of electrons happens. This distance is typically within 10s to 100s of nm, which is also the length scale of a domain wall width in many materials.These effects are only just within the range of experimental investigation. However, such effects have already been shown, theoretically at least, to form the basis for devices to manipulate and control the spin orientations of electrons. The use of an electron's spin, in addition to its charge, to carry information is the subject of much current research and the development of so called 'spintronics' is widely held to be key to opening up a new era in electronics as fundamental as the development of the transistor. This project both depends on and complements that research. The combination of spintronic elements in superconducting devices will give insight into the optimization of the materials and design of devices in spintronics research as well as elucidating the fundamental physics of the interaction of superconductivity and magnetic materials. In essence, this would be the equivalent in superconducting devices of the transition from passive magnetic devices to spin active devices.