Current-Driven Domain Wall Motion in Multilayer Nanowires

The study of spin-transfer torque at a magnetic domain wall continues to be one of the most vibrant areas of research in spintronics, motivated by the prospect of novel memory and logic systems and devices. At heart, the phenomenon is based on a fundamental law of nature: conservation of angular momentum. As an electron moves, as part of a flow of electrical current, through a magnetic domain wall, the direction of magnetisation around it will rotate from that in the first domain to that in the second. The magnetic moment on that electron, which arises from its spin angular momentum, will have to rotate accordingly. This results in a change of angular momentum on the electron by a single quantum unit. This change is compensated for by an equal change in the magnetisation of the metal that is carrying the current. The outcome is that if enough electrons pass through a domain wall, the 'electron wind' will push the wall along, just as a sail is blown along by wind in the atmosphere. The potential for using this effect to write and manipulate data represented magnetically in the next generation of nanoelectronics has lead to proposals for device architectures such as IBM's racetrack memory. At present, research in the field is overwhelmingly dominated by a single material and sample architecture: the lithographically patterned Permalloy nanowire. (Permalloy is a magnetically soft alloy of nickel and iron.) This is in spite of the fact that such nanostructures will probably not form the basis of any eventual device: the domain walls within them are too wide, too complex, and insufficiently rigid. Very high current densities, within an order of magnitude of the point of wire breakdown through electromigration, are needed to move them. From the point of view of basic research, it is clear that only a very restricted number of the possibilities for domain walls in nanowire systems has been investigated with any rigour. We will carry out a wide-ranging study of nanowires fabricated from multilayer films, drawing on years of experience in the preparation and study of such materials. Our attention will be focussed on two main classes of magnetic multilayer. The first class is the so-called synthetic antiferromagnet. Here two magnetic layers sandwich a thin metal spacer layer, through which they are coupled so that their magnetic moments prefer to lie in opposite directions. The lack of a net magnetic moment means that such structures are impervious to moderate magnetic fields and can be packed densely together on a chip without interacting, both attractive for spintronic technologies. Moreover, we have carried out preliminary micromagnetic simulations, which predict narrow, simple domain walls in such structures. The second class is multilayers in which the magnetisation lies perpendicular to the film plane. Recent results that we (and others) have obtained on these systems show that the efficiency of the spin-torque effect is roughly one hundredfold larger in these materials than in Permalloy - but that the defects in the materials lead to wall pinning effects that are larger by the same amount, so that huge current densities are still required. Here we will study the nature of the defects and so learn how to eliminate them, allowing such devices to operate with currents up to one hundred times smaller, leading to ten thousand times less power consumption. We will also investigate the control of the spin-torque effect using local electrical gates, making use of another recent discovery: the fact that in such thin perpendicular layers, a suitable structure incorporating an interface with a dielectric can give rise to electric fields acting as effective magnetic fields on moving electrons, giving rise to a new spin-torque effect through spin-orbit interactions. This will give control of domain wall pinning with a fine spatial and time resolution using voltages, giving the prospect of novel device architectures.

The field of magnetism has been revitalised by the emergence of ultrathin film and multilayer structures. One of the most appealing aspects of this field is the very close relationship between the discovery of new basic physics and applications in multi-billion dollar industries: this was the basis of the award of the Nobel Prize for Physics in 2007 for the discovery of the giant magnetoresistance. As a result, in the past decade, magnetic nanotechnology and spintronics has had a profound impact on the information technology industry, and the uses to which IT is put by society at large. Ultrahigh density hard disks are not only found in computers, but in high-end camcorders and mobile phones, and have enabled technologies like the iPod and Sky+. Cloud computing, search engine, video sharing, and social networking applications require vast quantities of storage found in server farms around the world. These two growth areas, portable data storage and server farms, are both ones where power consumption is critical. Hard disks consume power because they contain motors to provide the high speed rotation of the disk. Moreover, these motors take up space and weight in mobile applications. Hard disks retain a dominant position in the marketplace because they are at least a factor or ten cheaper, per bit stored, than any other storage medium presently available. Some so-called solid-state hard disks are available based on flash memory, but these are still expensive and there are issues connected to the fact that flash bit cells may only be re-written a finite-albeit large-number of times. Magnetic random access memories based on spintronic devices have recently entered the marketplace, but are also still high cost, niche products. Nevertheless, there are a number of proposals for 'storage class' magnetic memories that could form a solid state replacement for a hard disk at equivalent cost, which will repeat the success story of spintronics in the data storage industry. Many of these work on the principle of moving magnetic domain walls around using spin-polarised current pulses. One that has been widely publicised is the 'racetrack memory' concept from IBM. The advances we will make in this project will help to bring this about, by reducing the current densities needed to move domain walls, making devices lower power and less liable to fail. This will have impact on the microelectronics and data storage industries, and hence on the IT and consumer electronics industries that use these components. Development of such technologies requires industry to employ individuals with the skills in which we shall train the researchers working on this project. In order to ensure this impact is brought about we will, of course, publish our work in peer-reviewed journals and present it at the leading international conferences in the field. We shall carry out public outreach activities that we shall use to explain how this work, and the broader area of spintronics, have lead to the ubiquitous technologies that we rely on in our information society. We shall make use of our Universities' press offices to issue press releases and otherwise publicise newsworthy breakthroughs. Beyond these standard pathways to impact, we will undertake two special activities to open up new pathways to impact. The first of these is a workshop to be held towards the end of the project to which we will invite leading international scientists from academia and industry to exchange knowledge on the latest prospects for domain wall technologies. The second is the close relationship with Hitachi, a key player in the data storage and microelectronics industries, which we shall develop through carrying out part of the project in their Cambridge Laboratory.