Picosecond Dynamics of Magnetic Exchange Springs

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.

The proposed project will provide understanding of the magnetic exchange interaction (EI) within nanostructured materials so that new technologies can provide solutions to societal problems. We would all benefit if global warming could be slowed by the introduction of more efficient motors and dynamos containing stronger permanent magnets, while the Big Data revolution could bring widespread societal benefits, e.g. in pre-emptive healthcare, if hardware capacity can keep pace with growth in information content. Continued scaling of data storage technology is essential and requires new recording methods and evolution of magnetic storage media. Both the strongest permanent magnets and the latest magnetic storage media are composed of nanoscale grains. Control of the EI at the grain boundaries is critical to their performance, and will be advanced by the results of our research.

The global market for permanent magnets was estimated to be $15.3B in 2012, increasing to $28.7B by 2019, with production being dominated by China. The danger of over-reliance on a single source has stimulated research into alternatives, for example at Toyota in Japan, and the Fraunhofer Institute in Darmstadt. The present project will develop UK expertise and the environment required for inward investment in permanent magnet materials. Seagate Technology, a project partner in the proposal, has targeted heat assisted magnetic recording (HAMR) for the delivery of increased data density in hard disk recording. Seagate's manufacturing facility in Northern Ireland already produces ~30% of the world supply of recording head transducers and provides thousands of jobs that will be safeguarded by the success of HAMR. Little is known about how the magnetic anisotropy and EI are modified following laser excitation and the present project will address these questions. However it will also examine whether ES media could be written using light alone. We already collaborate with HGST/WD and the organizations mentioned above, so are confident that our ideas will be heard. We will also organize a conclave to bring together managers from industry and international leaders in academic research to discuss the underlying science and technological opportunities that they foresee. This will take place at the end of year 1, so that we can develop the relationships that are forged throughout the grant. A second workshop on the science and applications of the EI will be held in month 33 to disseminate research results and stimulate industrial take up.

High frequency measurements are increasingly important across a wide range of information technologies as bandwidth and bit rates increase. The use of state of the art microwave, optical and x-ray measurement apparatuses will equip the postdoctoral researchers within the project for a career in academia, high-tech industry, or at national laboratories. Numerical modelling is now an essential part of high value manufacturing since repeated prototyping is prohibitively expensive, and so a PhD student will be well prepared to conduct modelling in either an academic or industrial context. The project overlaps strongly with the interests of the Centre for Doctoral Training (CDT) in Electromagnetic Metamaterials at Exeter, providing opportunities for CDT students to interact with the project, while the project will inform the training delivered within the CDT. The investigators have an excellent record of sending students and postdocs into employment related to their studies, demonstrating that their skills are valued and in demand. Finally, it is important that members of the general public understand the broad principles by which technology works so that they engage and experience its benefit. We will use our involvement with the science and technology to excite the interest of a general audience, encourage young people to work in this field, and stimulate thinking about how new technology may be used.