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Dr James Loudon

PhD (Clare College, Cambridge University)
MSci in Experimental and Theoretical Physics (Clare College, Cambridge University)
MA in Natural Sciences (Clare College, Cambridge University)

Department of Materials Science and Metallurgy
University of Cambridge
27 Charles Babbage Road
Cambridge CB3 0FS

Office: 0_026
Tel: +044 (0)1223 760779
Fax: +044 (0)1223 334567
Email: j.c.loudon@gmail.com

Research
Recent Publications

James Loudon is a Royal Society University Research Fellow at Cambridge University investigating the behaviour of flux vortices in high temperature superconductors by real-time imaging using electron microscopy. He is also Director of Studies in Physics and Materials Science at Homerton College.

Prior to this 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. Before that, he 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. To download a pdf version of the thesis (14MB) click here.

James also gives lectures in electron microscopy for the PhD and MPhil courses in Nanotechnology and his lecture notes can be downloaded from here.

James Loudon's Research

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Summary of James Loudon's research:

FLUX VORTICES IN SUPERCONDUCTORS




Following the method of Tonomura et al. (see Nature 360, 51, 1992 for example), the flux lattice in superconducting BSCCO was successfully imaged on 17th January 2006 using transmission electron microscopy. We have also imaged vortices in YBCO and MgB2. The pictures above show vortices in MgB2 taken at 10.8 K with the B-fields indicated. The flux vortices appear as black-white features. As the field goes negative, antivortices appear and it can be seen that the contrast of the vortices reverses.

ELECTRON DIFFRACTION PATTERNS 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? The diffraction pattern above shows that antiferromagnetic reflections (indicated by arrows) can be observed in NiO. This observation could open the way to imaging the structure of antiferromagnets on a nanometre scale.

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

The charge ordering modulation that occurs in some manganite materials has been described as a localisation of Mn3+ and Mn4+. 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.

Spectroscopy of Charge Ordered Stripes

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. This work was published in Physical Review Letters (see publications below).

Charge Order at Room Temperature

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.

Charge Ordered Ferromagnetic Phase

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.

Coexistence of Charge Ordered and Monoclinic Phases

As the calcium doping, x, is increased beyond about 0.6, the low temperature structure of La1-xCaxMnO3 changes from the modulated 'charge ordered' structure associated with a CE type antiferromagnetic phase to a monoclinic phase associated with a C type antiferromagnet. We have used electron microscopy to investigate this phase coexistence, elucidating the size of the coexisting phases and the mechanism by which one phase forms within another.

Direct Evidence of Coexisting Ferromagnetism and Charge Order

For manganites with 0.2 < x < 0.5, La1-xCaxMnO3 is ferromagnetic and for 0.5 < x < 0.9 it is antiferromgnetic at low temperature. In a narrow composition range near x = 0.5, these two totally dissimilar phases are thought to coexist. This hypothesis was directly confirmed by using electron holography to show that these phases coexisted on a micron scale in La0.5Ca0.5MnO3 at 90 K.

Outreach


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 below show 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. The exposure time is increased from 0.01s for the first image to 40s for the final image. 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. Click here for more information. 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.

James Loudon's Recent Publications

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Journal Papers 

2014

Monchesky TL, Loudon JC, Robertson MD, Bogdanov AN
Comment on "Robust Formation of Skyrmions and Topological Hall Effect Anomaly in Epitaxial Thin Films of MnSi" 108k
Phys. Rev. Lett. 112, 059701, 2014

Blackburn AM, Loudon JC
Vortex beam production and contrast enhancement from a magnetic spiral phase plate 108k
Ultramicroscopy 136, 127, 2014

2013

Loudon JC, Bowell CJ, Zhigadlo ND, Karpinski J, Midgley PA
The Magnetic Structure of Individual Flux Vortices in Superconducting MgB2 Derived Using Transmission Electron Microscopy108k
PHYSICAL REVIEW B 87: art. no. 144515, APR 2013
Preprint available at arXiv:1303.5642

2012

Loudon JC
Antiferromagnetism in NiO Observed by Transmission Electron Diffraction108k
PHYSICAL REVIEW LETTERS 109: art. no. 267204, DEC 2012.
Preprint available at arXiv:1206.6734

2011

Loudon JC, Bowell CJ, Zhigadlo ND, Karpinski J, Midgely PA
Imaging Flux Vortices in MgB2 using Transmission Electron Microscopy108k
PHYSICA C 474:18-20, MAR 2012
Preprint available at arXiv:1109.1969
Click here to view the supplementary information.

2010

Loudon JC, Bowell CJ, Gillett J, Sebastian SE, Midgley PA
Determination of the Nature of the Tetragonal to Orthorhombic Phase Transition in SrFe2As2 by Measurement of the Local Order Parameter108k
PHYSICAL REVIEW B 81: art. no. 214111 JUN 2010
Preprint available at arXiv:1002.1639v2

2009

Loudon JC, Midgley PA
Imaging Flux Vortices in Type II Superconductors with a Commercial Transmission Electron Microscope 108k
ULTRAMICROSCOPY 109: 700-729 JAN 2009
Preprint available at arXiv:0807.2401v2

2007

Loudon JC, Fitting Kourkoutis L, Ahn JS, Zhang CL, Cheong S.-W., Muller D.A.
Valence Changes and Structural Distortions in "Charge Ordered" Manganites Quantified by Atomic-Scale Scanning Transmission Electron Microscopy108k
PHYSICAL REVIEW LETTERS 99: art. no. 237205 DEC 2007


2006

Loudon JC, Midgley PA
Comparison of the Ferromagnetic Phase Transitions in La0.7Ca0.3MnO3 and Single Crystal Nickel by Micromagnetic Imaging 108k
PHILOSOPHICAL MAGAZINE 86 (20): 2941-2956 JUL 2006


Loudon JC, Midgley PA
Micromagnetic Imaging to Determine the Nature of the Ferromagnetic Phase Transition in La0.7Ca0.3MnO3 108k
PHYSICAL REVIEW LETTERS 96: art. no. 027214 JAN 2006


2005

Loudon JC, Midgley PA
Real Space Imaging of Coexisting 'Charge-Ordered' and Monoclinic Phases in La1-xCaxMnO3 (x = 0.67 and 0.71)108k
PHYSICAL REVIEW B: RAPID COMMUNICATIONS 71 art. no. 220408(R) 2005


Loudon JC, Cox S, Mathur ND, Midgley PA
On the Microstructure of the Charge Density Wave Observed in La1-xCaxMnO3 108k
PHILOSOPHICAL MAGAZINE 85 (10): 999-1015 APR 2005


Loudon JC, Cox S, Williams AJ, Attfield JP, Littlewood PB, Midgley PA, Mathur ND
Weak Charge-Lattice Coupling Requires Reinterpretation of Stripes of Charge Order in La1-xCaxMnO3 108k
PHYSICAL REVIEW LETTERS 94: art. no. 097202 MAR 2005
Also available at arXivcond mat/0308501

2002

Loudon JC, Mathur ND, Midgley PA
Charge-ordered ferromagnetic phase in La0.5Ca0.5MnO3108k
NATURE 420 (6917): 797-800 DEC 26 2002
Also available at arXiv:cond-mat/0209436v1

Lloyd SJ, Loudon JC, Midgley PA
Measurement of magnetic domain wall width using energy-filtered Fresnel images108k
JOURNAL OF MICROSCOPY 207: 118-128 Part 2 AUG 2002

2001

Lloyd SJ, Mathur ND, Loudon JC, Midgley PA
Magnetic domain-wall width in La0.7Ca0.3MnO3 thin films measured using Fresnel imaging 108k
PHYSICAL REVIEW B 64 (17): art. no. 172407 NOV 1 2001

Conferences

2011

International Conference on Strongly Correlated Electron Systems, Cambridge, UK. 108k

Loudon JC, Bowell CJ, Midgley PA
Quantitative Imaging of Flux Vortices in Superconductors
J. Phys.: Conf. Ser. 391, 012117, 2012

Loudon JC, Bowell CJ, Gillett J, Sebastian SE, Midgley PA
Determination of the Nature of the Structural Phase Transitions in 122 Pnictide Systems
J. Phys.: Conf. Ser. 391, 012134, 2012

2010

American Physical Society March Meeting, Portland, Oregon, USA. 108k

Determination of the Nature of the Tetragonal to Orthorhombic Phase Transition in SrFe2As2 by Measurement of the Local Order Parameter

2007

American Physical Society March Meeting, Denver, Colorado, USA. 108k

Lorentz Imaging of Superconducting Flux Vortices with a Commercial Transmission Electron Microscope

2005

American Physical Society March Meeting, Los Angeles, USA. 108k

On the Nature of the Charge Ordered Phase in La1-xCaxMnO3 (14.5Mb Powerpoint)
Overheads from talk given at the conference.

Coexistence of 'Charge Order' Modulation, Tweed Microstructure and Needle Twins in La1-xCaxMnO3 (x?=?0.67 and 0.71) (1Mb Powerpoint)
Poster Presentation.

2003

International Conference on Magnetism, Rome, Italy. 108k

Loudon JC, Midgley PA, Mathur ND
Direct Evidence of Phase Coexistence in La0.5Ca0.5MnO3 108k
J MAGN MAGN MATER 272-76: 13-14 Part 1 Sp. Iss. SI MAY 2004

2001

Electron Microscopy and Analysis Group Conference, Dundee. 108k

Makin OS, Saxton WO, Loudon JC, Lloyd SJ, Midgley PA
Direct specimen plane wave recovery for strong objects
INSTITUTE OF PHYSICS CONFERENCE SERIES (168): 139-142 2001

Loudon JC, Lloyd SJ, Midgley PA, Mathur ND
The magnetic domain wall width of La0.7Ca0.3MnO3 determined from a Fresnel defocus series
INSTITUTE OF PHYSICS CONFERENCE SERIES (168): 449-452 2001



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