Imaging the Structure and Dynamics of Flux Vortices in High Tc Superconductors

Lead Research Organisation: University of Cambridge
Department Name: Materials Science & Metallurgy


Superconductors have two main properties: their electrical resistance is zero and they expel magnetic field. Not all superconductors expel field completely, however. Type II superconductors allow magnetic field to penetrate along channels called 'flux vortices'. Each of these channels contains the smallest amount of magnetic field allowed by the laws of quantum mechanics and they can be treated as quantum particles just like electrons or photons. To emphasise this, they are sometimes called 'fluxons'.The behaviour of flux vortices is crucial to determining the properties of a superconductor. When an electrical current is passed through a type II superconductor, it generates a magnetic field and this field produces flux vortices. When these vortices move, energy is dissipated as though the superconductor had a non-zero resistance. This leads to heating which is detrimental for equipment such as superconducting magnets which require high electrical currents in order to operate. If, however, the vortices can be prevented from moving by being pinned by defects within the crystal structure of the superconductor, higher currents can be carried with a lower power loss.In this investigation we shall use transmission electron microscopy to image individual flux vortices. This technique was first employed to image vortices in niobium in 1992. It has only been successfully applied by one laboratory in the World until very recently when we used it to image vortices in Bi-Sr-Ca-Cu-O, a high temperature superconductor. It is superior to other magnetic imaging techniques as it has a better resolution and magnetic fields can be measured quantitatively. We intend to use this technique to study the detailed structure of fluxons and their interactions with one another and with different types of pinning site as well as their response to being confined in nanoscale superconducting samples of different geometries.Conventional type II superconductors have vortices which are cylindrical channels but in other materials, like high temperature superconductors, the vortex structure can be very different. We shall study the vortex structures produced in different superconductors by comparing experimental images of vortices with theoretical simulations. Electron microscopy is uniquely suited to this study as it is the only technique where the magnetic field within the specimen is measured rather than just the surface field.We shall also investigate how fluxons move in response to changes in magnetic field or temperature by recording images at video rate and studying the pinning of vortices by crystal defects. These defects can be simultaneously characterised in the electron microscope. This will enable us to determine the sort of defect that pin vortices most effectively. As well as naturally occurring defects, we shall investigate the effect of defects which are artificially created by ion beam irradiation using our focussed ion beam microscope. We shall also study the effect of pinning by magnetic nanostructures patterned on top of the sample using lithography where the pinning force comes from magnetic interactions rather than crystal defects.In very small superconducting samples, the arrangement and nature of flux vortices is different to that observed in bulk samples. We plan to study the novel effects that result from this geometrical confinement such as multiply quantised vortices and symmetry induced antivortices. There has been recent interest in the 'ratchet' mechanism where specially shaped specimens cause fluxons to move preferentially in a particular direction. It has been suggested that this effect could be used to reduce the electrical noise in superconducting devices. We shall extend this research by patterning different types of ratchet device and investigating whether a similar ratchet effect can be achieved by patterning magnetic nanostructures on the specimen surface.
Description Superconductors have no electrical resistance and expel magnetic flux. If a magnetic field is applied to an ideal (type I) superconductor, no flux enters until a critical field is reached at which point the material becomes normal. However, in type II superconductors, magnetic flux penetrates in discrete jumps by flowing along filaments known as flux vortices where superconductivity is suppressed. Each vortex contains one quantum of flux. The behaviour of flux vortices is crucial to the performance of almost all superconducting devices as energy is lost when they move. To prevent movement, defects can be introduced to pin the vortices. Determining the optimum defect to pin the vortices, and the nature of the defect-vortex interaction, is a key challenge. This research addressed this by imaging the structure and dynamics of vortices directly using transmission electron microscopy (TEM). TEM is an ideal tool as it has a high spatial resolution, can measure the internal magnetic fields and vortex motion can be observed in real time. There were severe technical challenges to overcome including the preparation of large (30 um), flat, thin (~200 nm) samples suitable for imaging the vortices. Outside of this team, only two groups (both in Japan) had been able to image vortices this way. Images were first acquired from Bi2Sr2CaCu2O8+d (BSCCO). However, the large vortex size (penetration depth) means the magnetic fields are weaker than almost any other superconductor and image contrast is very low. Nevertheless, this enabled analysis of vortex ordering and magnetic structure. Through a collaboration with ETH, Zurich, we obtained single crystals of YBa2Cu3O7-d (YBCO), MgB2, and iron-based superconductors. These samples do not cleave easily and a focussed-ion beam microscope (FIB) was used to thin the samples. Vortices in YBCO and, for the first time, MgB2, were imaged. The images from MgB2 enabled us to derive the internal magnetic structure of individual flux vortices and a paper is in preparation. The FIB thinning produces linear defects (thickness undulations) and, although only a few nm in height, are able to pin the vortices, profoundly affecting ordering and movement. A collaboration was begun with the femtosecond electron diffraction group at EPFL, Lausanne to investigate vortex formation and annihilation at femtosecond timescales.

As a complementary investigation, a structural phase transition in the iron-based superconductors was studied to elucidate the mechanism by which superconductivity arises. According to one theory, the transition should become second-order as the superconducting phase is approached. However, there was controversy over the order of the phase transition even in the end-member, SrFe2As2. We used electron microscopy to show that not only that the transition was first order but the new phase grew via the movement of transformation dislocations. Another end member, BaFe2As2, showed a second order transition.

Further investigation of fluxon dynamics was continued by Dr James Loudon as part of a Royal Society University Research Fellowship.
Exploitation Route The findings of this research showed primarily that vortex structure and motion could be observed clearly and quantitatively using a standard transmission electron microscope equipped with a cold stage and a standard energy filter. This enables a great fundamental understanding of the vortex-defect pinning which in turn should lead to a better understranding an performance of materials and devices that rely on the behaviour of type II supeconductors (e.g. SQUIDs, magnets,etc). Moving forward, this opens up new possibilities for microscopic imaging and analysis of vortices at low applied magnetic fields, complementing the work undertaken at much higher fields and by other, diffraction-based, techniques that require generally far larger samples for analysis.
Sectors Electronics,Energy,Transport

Description This research was to investigate whether we could use the TEM to study the structure and dynamics of flux vortices in Type II superconductors. This we achieved using a standard commercial instrument (for the first time) and were able to quantify the vortex packing and motion. The work had two immediate impacts. Firstly, it showed how the TEM could be used as a useful tool to investigate vortex pinning, a key parameter for successful commercial use of superconductors in e.g. magnets. Secondly, it showed how the vortex behaviour could be studied without the need for a dedicated and expensive electron microscope but using a combination of a standard TEM with a cold stage, energy filter and careful image processing, quantitative results could be achieved. We developed a method of thinning the high Tc materials using the FIB without introducing severe artefacts from the thinning process (e.g. Ga implantation) which might have a strong influence on the superconducting properties. This work has been caried forward by the post-doc employed on the grant, who won a Royal Society Research Fellowship, based on this and previous work.
First Year Of Impact 2009
Sector Electronics,Energy,Transport
Impact Types Societal,Economic

Description Royal Society of London
Amount £470,810 (GBP)
Funding ID UF071166 
Organisation The Royal Society 
Sector Charity/Non Profit
Country United Kingdom
Start 10/2007 
End 09/2012