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 was Director of Studies in
Physics and Materials Science at Homerton College from
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 gave lectures in electron microscopy for the PhD and MPhil courses in Nanotechnology and his
lecture notes can be downloaded from here.
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. We have also imaged
vortices in YBCO and magnesium diboride. The pictures above show
vortices in magnesium diboride 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.
The video below 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? 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
and the first order ferromagnetic transition in La0.7Ca0.3MnO3. Not
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
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.
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.
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