James Loudon is a Senior Research Associate in the Electron Microscopy Group investigating magnetic skyrmions as part of The Skyrmion Project, a joint investigation with the universities of Durham, Warwick, Cambridge, Oxford and Southampton funded by the EPSRC. Prior to this he was a Royal Society University Research Fellow and Director of Studies in Physics and Materials Science at Homerton College. Before that he was a postdoctoral researcher (2006-2007) at Cornell University, Ithaca, NY, USA, investigating the structure and properties of functional magnetic oxide materials using high resolution scanning transmission electron microscopy and was a Junior Research Fellow at Homerton College, University of Cambridge (2003-2006).
His PhD (2000-2003, received in March 2004) is entitled 'An Investigation of the Unconventional Phases in the Lanthanum Calcium Manganite System'. The thesis describes an investigation of the unusual low temperature phase transitions which occur in (La,Ca)MnO3, a so-called colossal-magnetoresistive (CMR) material.
Skyrmions are vortex-like swirls of magnetisation that can occur in magnetic materials which have a broken symmetry. The concept of a skyrmion was introduced by Tony Skyrme in 1961 in the context of nuclear physics and magnetic skyrmions were predicted in 1989. They were discovered experimentally in 2009 and this has prompted recent intense research as they are promising objects for various spintronic applications, notably racetrack computer memories. We are using electron microscopy to investigate the detailed magnetic structure of skyrmions and their dynamics.
FLUX VORTICES IN SUPERCONDUCTORS
Following the method of Tonomura et al. (see Nature 360, 51, 1992 for example), we successfully imaged the flux lattice in superconducting BSCCO using transmission electron microscopy. This video shows vortices entering a magnesium diboride superconductor which has very few defects. As the magnetic field is increased, vortices form at the edges and shoot into the centre, faster than the frame rate of the video. The vortices repel one another and as more enter, they jostle to find the optimum position. As the magnetic field is increased, they form different competing arrangements until finally stabilising at the highest field.
ELECTRON DIFFRACTION FROM ANTIFERROMAGNETS
In a ferromagnet, all the magnetic moments of the atoms are aligned parallel to one another. However, in an antiferromagnet, the atomic moments are antiparallel on adjacent atoms and so an antiferromagnet produces no external magnetic field. Neutrons have a magnetic moment and so are sensitive to the magnetic fields between atoms and neutron diffraction has been used extensively to investigate the structure of antiferromagnets. Electrons should also be sensitive to the magnetic field between atoms as they feel the Lorentz force on passing through a magnetic field. Can the effects of antiferromagnetism be seen in an electron diffraction pattern? Find out here.
MICROSCOPIC ANALYSIS OF PHASE TRANSITIONS
Transmission electron microscopy provides a unique method to measure the order parameter of phase transitions on a local scale. This has allowed a clarification of the nature of several phase transitions, notably the structural transformation that accompanies the antiferromagnetic transition in SrFe2As2 and the first order ferromagnetic transition in La0.7Ca0.3MnO3. Not only does it help clarify whether the transitions are first or second order, but it elucidates the mechanism by which phase transitions take place.
CHARGE ORDERING AND PHASE COEXISTENCE
The charge ordering modulation that occurs in some manganite materials has been described as a localisation of Mn3+ and Mn4+ ions. This localisation produces superlattice reflections in a diffraction pattern indicating that the size of the unit cell has increased. It was originally thought that this type of localisation could only produce supercells that were an integer multiple of the undistorted unit cell. However, this investigation showed that the modulation was not composed of integer subunits and, as a consequence, the modulation is more likely to be the result of a charge density wave rather than a localisation of two Mn species.
To ascertain the extent of the valence modulation in charge ordered materials, electron energy loss spectra were acquired from individual 'stripes' - atomic planes originally supposed to contain localised Mn3+ or Mn4+ ions. The experiment was performed using the Technai F-20 electron microscope at Cornell University using a combination of high resolution scanning transmission electron microscopy and electron energy loss spectroscopy. No periodic valence modulation was observed, placing an upper limit of ±0.04 on any valence changes of the Mn ions.
The charge ordering transition as determined by resistivity and magnetisation measurements occurs at 230 K in La0.48Ca0.52MnO3. However, this investigation has shown that the superlattice modulation thought to be associated with the charge ordering transition is still present, although very much weaker, at 293 K.
The charge ordered phase is usually associated with antiferromagnetism. Electron holography and dark field imaging were used to image and measure the absolute value of the local magnetisation in both the ferromagnetic and 'charge ordered' phases in the same region of a La0.5Ca0.5MnO3 specimen. It was found that the structural modulation thought to be the result of charge ordering occurred in both ferromagnetic and non-ferromagnetic regions of the specimen.
James Loudon provided images of electron interence for the BBC production An Evening with the Stars presented by Brian Cox on 18th December 2011. The images showed electrons landing on a detector after passing either side of a positively charged wire (called an electron biprism) which brings two electron beams together, causing interference. At short exposures, electrons are detected as single points - and so appear to be particles - but at long exposures a wave pattern is seen showing that electrons behave both as waves and particles. This experiment is similar to the double-slit experiment first used by Thomas Young in 1803 to demonstrate that light could behave like a wave and more recently used to demonstrate the counter-intuitive results of quantum mechanics.
Akira Tonomura famously performed this experiment with a single-electron detector to demonstrate not only that electrons behave both as waves and particles but that each electron interfered with itself as it passed the biprism wire. Anton Zeilinger's group have shown that similar interference patterns can be acquired even with large molecules such as 'buckyballs' formed with 60 or 70 carbon atoms.
Department of Materials Science and Metallurgy
Electron Microscopy Group