Correlating a material's atomic-scale structure to its functionality is central to our understanding of the physical and chemical world, and hence to most technological development. Scanning transmission electron microscopy (STEM) now dominates high resolution materials characterisation in the physical sciences, routinely revealing structural details that are otherwise indiscernible. It excels in the analysis of aperiodic structures including defects, inhomogeneities and interfaces that are below the resolution of other microscopies and cannot be studied using diffraction. These structures are important because they often dominate a material's properties, for better or worse. Atomic-scale resolution also underpins the development of devices, which may now contain features of only a few tens of atoms in dimension, often to harness quantum effects that can only be controlled on the nanoscale. Frustratingly, many materials remain inaccessible to atomic resolution STEM. One example is that the magnetic fields used to focus a STEM instrument interfere with magnetic samples, so that their intrinsic behaviour cannot be studied. We propose to capitalise on our expertise to address this problem. First, we will exploit improved electron lens designs to provide a three-fold improvement in 'field-free' imaging resolution. We will be able to visualise a sample's own electromagnetic fields on the atomic scale, facilitating novel studies of magnetic, quantum, microelectronic and plasmonic technologies alongside geological and chemical samples with nano-magnetic properties. An improved sensitivity to magnetic structure will enable the analysis of challenging samples such as synthetic antiferromagnets and low moment materials, which are of technological importance. We will also enhance time resolution and sensitivity by integrating the latest noise-free electron detectors for imaging and spectroscopy, providing enhanced capabilities for high-speed, high sensitivity analysis, particularly of delicate, beam-sensitive materials. We have been at the forefront of development in both of these areas and are exceptionally well-placed to grow an acknowledged UK research strength.

Abstracts are not currently available in GtR for all funded research. This is normally because the abstract was not required at the time of proposal submission, but may be because it included sensitive information such as personal details.

Ferromagnetic materials are found throughout the electromagnetic technology upon which modern life depends. They range from the bulk materials found in motors and dynamos to thin films used to store data in hard disk drives. Within a ferromagnet each atom has a magnetic moment, like planet earth, with north and south poles. The magnetic moments of adjacent atoms are forced to point in the same direction by the exchange interaction (EI), a purely quantum-mechanical effect, which is the most powerful force in magnetism, generating effective magnetic fields up to one hundred million times as strong as the earth's magnetic field. Our everyday experience is that some ferromagnets remain permanently magnetized while others do not. In the latter case, the magnetic moments have parallel alignment within microscopic regions known as domains, but different domains have magnetic moments pointing in different directions, so that there is no net magnetic moment overall. Neighbouring domains are separated by domain walls, about 10 nm (100 atomic diameters) wide, through which the orientation of the magnetic moments gradually rotates in a helical structure. The finite width of the domain wall is a consequence of the EI and the wall stores exchange energy like a spring. The proposed project is concerned with exchange spring (ES) structures that form through the thickness of multilayered thin films. Alternate layers are termed hard and soft because it is easier to form the helical structure in the latter. The helical structure is induced either by applying a magnetic field or by changing the relative alignment of the magnetic moments in different hard layers so as to twist the magnetic moments in the soft layers in between. By studying the form of the ES structure, and its response to external stimuli, we can obtain information about how the strength of the EI varies through the structure. The EI present in perfect crystals can already be calculated accurately. However, the magnetic materials used in the strongest permanent magnets, or as recording media in hard disk drives, are far from perfect and consist of nanoscale crystallites that interact with each other through the EI at their grain boundaries. Furthermore, the next generation of magnetic recording technology will use the combined influence of a magnetic field and a short laser pulse to switch the orientation of the magnetic moments so as to represent binary information. Rather little is known about the EI within the grain boundary regions, or how the EI is modified immediately after application of a laser pulse. The aim of this project is to use ES spring structures to obtain new information about the EI in such circumstances. State of the art thin film deposition will be used to fabricate ES structures in which the atomic scale structure can be carefully controlled so that the relationship between magnetic and structural properties can be better understood. Microwave radiation will be used to excite the ES so that magnetic moments oscillate with characteristic frequencies that allow the strength of the EI within different regions of the ES to be deduced. In particular, x-rays will be used to detect the motion, since by tuning the energy of the x-ray photons obtained from a synchrotron, the response of different atomic species can be separately determined, providing more detailed information of the mode of oscillation. Finally, the ES will be excited with an ultrafast laser pulse to soften the magnetic moments within one or more hard layer so that the ES can unwind. This unwinding motion will provide information about how the magnetic parameters of the material, including the EI, are modified by the laser pulse, and the conditions required for the magnetic moments of the hard layer to switch their orientation will be explored. The potential of ESs as laser assisted recording media will hence be determined.

Recent developments in magnetic nanotechnology have seen new device concepts emerge that could challenge traditional silicon-based microelectronics in certain applications. A key advantage of magnetic devices over alternative technologies is that they generally do not require power to retain data. In specific cases, magnetic nanotechnology devices may also offer higher device density, lower power consumption, improved reliability or additional functionality compared with their rivals. Some of these magnetic devices are made using thin ferromagnetic layers separated by a non-magnetic metal spacer layer just a few atoms thick. The upper and lower layer will have different magnetisation directions and the electrical resistance of the overall device depends on their relative orientation due to an effect known as 'giant magnetoresistance' (GMR). Already, these devices are widely used as magnetic field sensors in many applications, e.g. in computers and automotive products. Other technologies are being developed based upon networks of planar magnetic nanowires, usually with just a single magnetic layer and no spacer layers. The geometry of the wires is important, since this restricts magnetisation to lie in one of two directions along the wire axis. This provides a simple system for representing the binary numbers of digital information. Opposite magnetisation directions can meet, and where this happens, they are separated by a transition region known as a 'domain wall'. Domain walls can be easily created or removed and made to propagate through a nanowire network using magnetic fields or electrical currents in the nanowires. In this way, information is written, deleted and sent through a circuit, be it a sensor, memory or logic device. However, for these devices to be commercially successful, we must have read-out of the magnetic data in form compatible with modern electronics. There have not been any demonstrations of this to date. In this collaborative research programme, we will address this deficiency by developing a nanoscale device to read data in magnetic nanowires. Our recent calculations have shown that the magnetic field from domain walls is very high close to the nanowires. We will use this field to change the magnetic configuration of a nearby sensor and detect these changes using GMR. This will be a significant step for magnetic nanowire technologies since it will allow nanowire devices to be fully integrated as stand-alone integrated circuits. We will also use these sensors for scientific measurements to improve our understanding of the behaviour of domain walls in magnetic nanowires.The applicants for this project bring together world-leading experience in nanofabrication, magnetic nanowires, GMR materials and computer modelling of nanoscale magnetic systems, making this the ideal team to undertake such a challenging project.

The smooth operation of the modern world depends on our ability to store and access data reliably. Almost everything we need, from the accurate management of bank accounts to the flexibility of digital entertainment, requires the reading and interpretation of strings of binary '1's or '0's. At the heart of data storage, such binary numbers usually exist in the form of either the polarity of electrical charge (in DRAM, Flash or FRAM) or the orientation of magnetisation (in magnetic hard-drives). Charge-storage devices and magnetic storage devices both have negative aspects about their architectures or operation, and so for some years there has been interest in developing a memory element that combines the positive features of each, allowing 'writing' of information to be done electrically, and 'reading' to be done magnetically. Materials that are both ferromagnetic and ferroelectric would be highly desirable for such applications and, as a result, so-called 'multiferroics' have become a topic of great recent research interest. Unfortunately, there are very few known multiferroic systems and none has been discovered to date which can readily be made and simultaneously displays both large polarisation and magnetisation. The first element of this proposal is therefore to explore two relatively new groups of multiferroics (birelaxors and lattice strained EuTiO3) to see if they can offer properties that are superior to the best known multiferroic currently available (bismuth ferrite). The use of a multiferroic in a memory element requires the manipulation of magnetic and ferroelectric regions, known as domains. While a great deal is known about domain behaviour in ferromagnets and in ferroelectrics separately, much less is known about the static and dynamic behaviour of multiferroic domains. Exploration of domains in meso and nanoscale objects (dimensions relevant to high density memory) will be performed on small scale single crystals, cut from high purity bulk material using a Focused Ion Beam-based methodology uniquely developed by the applicants. To date this has given extremely clear information on ferroelectrics and should be ideal for fundamental investigations into multiferroic domain properties. In addition to interest in multiferroic memory, researchers have become increasingly excited by the potential use of multiferroics in more exotic applications - the domain walls in bismuth ferrite have been found to act as planar conductors and large photovoltaic effects have been displayed. To date, such effects have only been probed in thin films grown by pulsed laser deposition. While this is a useful and flexible growth technique it has a tendency to introduce significant levels of defects that can lead to properties which are extrinsic, rather than intrinsic to the material. We wish to examine the properties of single crystal thin films of bismuth ferrite (and later birelaxors) made using the established Focused Ion Beam process mentioned above for such exotic domain wall and photovoltaic effects. Importantly, using this approach should allow a different view, which may corroborate or conflict with information to date only obtained through PLD grown films.