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The electromagnetic force, one of the fundamental forces of nature, arises from the coupling of electricity and magnetism. The flow of electric charges can generate a magnetic field, while a changing magnetic field induces an electric field. Light itself is an electromagnetic wave, a travelling oscillation in an electromagnetic field. Electromagnetic effects underpin the vast majority of today's technology, with electric power generators, induction motors and transformers all relying upon electromagnetism. However, electromagnets typically consist of coils of wire and are therefore cumbersome, bulky and hard to produce. A novel class of multifunctional materials called multiferroics exhibit strong coupling between electricity and magnetism, and may enable the manufacture of electromagnets on an atomic scale. In a multiferroic material an electric current can produce a magnetisation, and conversely a magnetic field can generate an electric polarisation. This remarkable behaviour looks set to revolutionise spintronic technology, such as magnetic data storage (hard disks) and computer memory. The last few years has seen a renaissance in research into multiferroics, with many high profile articles in journals such as Nature and Science. Complex new particles called electromagnons are thought to be created in multiferroics by the coupling between atomic vibrations and magnetism, and have tentatively been suggested to absorb light at low energies, in the terahertz frequency range. However, the properties of electromagnons and multiferroics are still poorly understood by scientists, and little is known about the speed of the dynamic coupling between electric polarisation and magnetisation. My vision for this Fellowship is to create a world-class research group in the ultrafast physics of multifunctional materials, an area of science vital for the UK's future technology base. In the fellowship I propose to investigate the dynamic properties of multiferroic materials on ultra-short timescales, from less than one picosecond to over one nanosecond. In traditional materials such as inorganic semiconductors the phenomena that are observable on this timescale include oscillations of crystal lattices, the scattering of charges, and particle creation and destruction. All of these occurrences can be observed using ultra-short pulses of light at terahertz frequencies. My experimental approach will be to perturb the equilibrium state of a multiferroic with pulses of light from a laser, and then use a synchronised pulse of low-energy light at terahertz frequencies to track the dynamic conductivity of electromagnons. This investigation is highly novel, as the change in both the refractive index and absorption of electromagnon modes will be obtained on picosecond timescales. A key scientific result of my research will be a better understanding of the coupling between magnetisation and electric polarisation, which I will obtain by assessing the universality of electromagnons in multiferroics, and how stable they are when perturbed by electric and magnetic fields. I will seek to discover the ultimate speed limit of multiferroic memory elements, by measuring how rapidly the electric polarisation can be switched. I recently demonstrated that nickelates, related materials with strong charge and spin ordering, exhibit low energy collective vibrations in their equilibrium state. In this research programme I will also investigate the ultrafast dynamics of these modes. The proposed study will advance substantially our knowledge of multiferroic materials, and will benefit the development of functional devices by industry. This fellowship will allow me to create a world-class research group in an area of science vital for the UK's future technology base.
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