Current doctoral projects

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We have two DPhil projects starting in October 2020

Harnessing the power of topology in oxide electronics for future IT components

This project will be supervised by Prof. Paolo G. Radaelli

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In spite of its extraordinary success in fuelling the IT revolution, silicon (CMOS) technology is intrinsically energy-inefficient, because it relies on the movement of electrical charge, which is associated with Joule heating. One of the front runners among ‘beyond-CMOS’ technologies is spintronics, which relies on spins rather than charges to transfer and process information; however, much of the energy efficiency of spintronics is lost if spin flipping – the elementary spintronic operation – must in itself be performed by electrical currents. For this reason, voltage control of magnetic components is widely considered to be the key to large-scale commercialisation of spintronics [1-4]. The field of oxide electronics emerges precisely from the consideration that oxides, especially those containing magnetic transition metal ions such as Co, Mn and Fe, can display a multitude of intriguing electrically-controlled multi-functional properties in their insulating states, whilst integration with CMOS is already a reality. The potential of oxide electronics can be further enhanced by exploiting the power of topology, which involves, quite literally, tying spins into ‘magnetic knots’. In work recently published in Nature Materials [4], an international team of collaborators lead by Professor Paolo G. Radaelli (Oxford Physics) presented a major breakthrough in this field [5,6]: they created, for the first time, small-scale hybrid oxide/metal topological magnetic objects, consisting of tightly-coupled spin vortices in antiferromagnetic iron oxide (-Fe2O3) and ferromagnetic metallic cobalt (Co). One particularly appealing feature of this system is that it employs cheap and readily available materials (-Fe2O3 is the most abundant constituent of common rust!) and relatively simple fabrication, raising hopes that these systems could be deployed on a commercial scale in the future (for extended lay descriptions, see the Oxford Physics Newsletter – Autumn 2018 and this Diamond Research Highlight).

This EPSRC-funded DPhil project will give the successful candidate the opportunity to develop this line of research in different directions, both fundamental and applied:
• Identify and grow new oxides with topological magnetic states, study their fundamental properties and image the topological structures at the nanoscale using state-of-the-art microscopy techniques.
• Experiment with novel ways to control topological magnetic states, exploiting either intrinsic magnetoelectric properties or interactions with active substrates
• Design and test prototype devices, built using electron beam lithography and other clean room processes.
This project is likely to involve a combination of experimental techniques, such as:
Growth of thin films and devices – currently in collaboration with the groups of Prof. Thorsten Hesjedal (Oxford Physics), Prof. Chang-Beom Eom (Univ. of Wisconsin – Madison) and Prof. Venky Venkatesan (National University of Singapore).
Advanced microscopy. To image multi-functional domains, which are the fundamental unit of information storage in oxides, we employ spectral microscopy (PEEM – we invented and continue to develop many of the relevant data analysis methods at the Diamond synchrotron), Magnetic Force Microscopy (MFM – in house) Magneto-Optical Kerr Effect magnetometry/microscopy (new in house development) and Nitrogen Vacancy Centre Microscopy (new collaboration with the Max Planck Institute for Solid State Research in Stuttgart)
X-ray coherent imaging: this is a new direction, to be developed in collaboration with Prof. Laurent Chapon at the Diamond Light source, which should enable imaging of ferro- and antiferromagnetic domains with an unprecedented resolution of ~4 nm.
Dielectric and transport measurements (in house)
Elastic neutron scattering. We will perform experiments on bulk and films samples predominantly at the ISIS facility at Rutherford Appleton Laboratory.
X-ray scattering, including resonant and non-resonant magnetic X-ray diffraction with hard and soft X-rays. We run state-of-the-art laboratory instrumentation in the Clarendon Laboratory, but we perform most of our high-end experiment at the Diamond Light source.
Nanofabrication. In collaboration with National University of Singapore, we will be using electron beam lithography and other clean-room methods to design and build prototype oxide quantum materials devices.
Depending on the candidate's interests, the project may involve development of one or more of the above techniques (particularly the new techniques such as MOKE and N-V microscopy and X-ray coherent imaging) and may also include a computational element. In collaboration with Prof. Feliciano Giustino at the Oden Institute, Univ. of Texas, we employ Density Functional Theory methods and other computational techniques to model the functional properties of oxides and to predict their behaviour in different architectures. We also employ micromagnetic simulations to study the formation and dynamics of topological strcutures such as vortices and merons. There will also be an opportunity of extended stays at one of the collaborating institutes to acquire new skills.
For more information email Prof. Paolo Radaelli and visit the group webpages.
[1] Matsukura, F., Tokura, Y. & Ohno, H. Control of magnetism by electric fields. Nature Nanotechnology 10, 209–220 (2015).
[2] Eerenstein, W., Mathur, N. D. & Scott, J. F. Multiferroic and magnetoelectric materials. Nature 442, 759–65 (2006).
[3] Ramesh, R. & Spaldin, N. A. Multiferroics: progress and prospects in thin films. Nature Materials 6, 21–9 (2007).
[4] S. Manipatruni, et al, Nature 565, 35 (2019).
[5] F. Chmiel et al., Nature Materials 17, pages 581–585 (2018)
[6] M. Fiebig, Nature Materials 17, pages 567–568 (2018)

Breaking symmetry with light: ultra-fast ferroelectricity and magnetism from non-linear phononics

A collaboration between Prof. Paolo G. Radaelli and Prof. Andrea Cavalleri, who holds a joint appointment between the Clarendon Laboratory and the Max Planck Institute for the Structure and Dynamics of Matter. (Hamburg).

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The use of light to control the structural, electronic and magnetic properties of solids is emerging as one of the most exciting areas of condensed matter physics. One promising field of research, known as femto-magnetism, has developed from the early demonstration that magnetic ‘bits’ in certain materials can be ‘written’ at ultra-fast speeds with light in the visible or IR range [1]. More radically, it has been shown that fundamental materials properties such as superconductivity can be ‘switched on’ transiently under intense illumination [2]. Recently, the possibilities of manipulating materials by light have been greatly expanded by the demonstration of mode-selective optical control, whereby pumping a single infrared-active phonon mode results in a structural/electronic distortion along the coordinates of a second, anharmonically coupled Raman mode – a mechanism that was termed ‘nonlinear phononics’. Crucially, the Raman distortion is partially rectified, meaning that it oscillates around a different equilibrium position than in the absence of illumination. Recently it was realised that, under appropriate conditions, the rectified Raman distortion can transiently break the structural and/or magnetic symmetry of the crystal and hypothesised that such symmetry breaking would persist for a time corresponding to the carrier envelope of the pump, which can be less than a picosecond, and can give rise to the ultra-fast emergence of ferroic properties such as ferromagnetism and ferroelectricity [4]. Even more recently, this effect was experimentally demonstrated for the first time in our collaborators’ laboratory in Hamburg. Surprisingly, photo-ferroicity persisted for a significantly longer time than the carrier envelope (100s of ps). Although this is not yet fully understood, the most likely explanation is that magnetisation is being transferred to slower electronic/magnonic excitations.

This DPhil project will give the successful candidate the opportunity to pioneer this new field of research. Initial experiments on the ‘photo-ferroic’ materials that we have already characterised will be performed at the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg, Germany. As a mode-selective pump, we are employing coherent laser radiation in the THz or far-IR range with sub-ps carrier envelopes, while the transient emergence of the ferroic properties will be probed with second-harmonic generation (SHG), Faraday rotation and dichroic absorption of visible/near-infra-red light. Later on in the project, changes in the crystal and magnetic structures of the materials will be probed with X-rays at free electron laser sources such as the European XFEL in Hamburg. Meanwhile, the candidate will develop search strategies for new classes of ‘photo-ferroic’ materials, based on symmetry and time-dependent density functional theory calculations. He/she will develop the materials specifications in collaborations with crystal growers in Oxford and elsewhere, and will be involved hands on in all aspects of the design and realisation of the experiments and the data analysis.

The experimental part of this project will be predominantly based in Hamburg, so it is essential for the candidate to be willing and able to be based in Germany for extended periods during the DPhil.

[1] “Femtomagnetism: Magnetism in step with light”. Uwe Bovensiepen, Nature Physics 5, 461 - 463 (2009) abstract pdf
[2] See for example M. Mitrano,et al., “Possible light-induced superconductivity in K3C60 at high temperature”, Nature, 530, 461–464 (2016). [more at this link].
[3] “Nonlinear phononics as an ultrafast route to lattice control”, M. Först et. al., Nature Physics, 7, 854–856 (2011).
[4] “Breaking Symmetry with Light: Ultra-Fast Ferroelectricity and Magnetism from Three-Phonon Coupling”, P. G. Radaelli, Phys. Rev. B 97, 085145 (2018).