The magic drawing board

When a superconductor is placed in a strong magnetic field it loses the characteristic of superconductivity. This is an unfortunate property which acts as an obstacle to the development of applications based on current transfer. To rectify this, it is first necessary to understand this phenomenon and then to control it. Jérémy Brisbois and the team from the Physics Department of the University of Liege have succeeded in doing so by coating the superconductor with a thin layer of magnetic material. This trick meant that they were then able to “see” the movement of magnetic field lines inside the superconductor.

Superconductors have intrigued and interested physicists for a long time. This is due, in particular, to one of their characteristics: when they are placed in a magnetic field they lose their perfect conductivity. At least this is the case for so-called type 1 superconductors where relatively weak fields are concerned. In this case, beyond a certain value, the magnetic field penetrates the superconductor rather than being rejected by it and this destroys the property of superconductivity. There are also type 2 superconductors (often made up of alloys), which accept much stronger magnetic fields because they tolerate a local penetration of the magnetic field.  This has made possible the use of superconductivity in the enormous magnets of particle accelerators such as LHC (Large Hardon Collider) at CERN. But beyond certain magnetic field values, perfect conductivity also disappears. How can this phenomenon be explained? It can be explained by the movement of the magnetic lines in the material. These are grouped into miniscule cylinders that measure only a hundred nanometers known as vortices. Inside these cylinders the material is normal (non-superconducting) and in the absence of electrical current, or in a weak current, they do not move, which gives the material its superconducting properties. If the current becomes more intense however, these vortices move inside the material and dissipate energy, putting an end to the perfect conductivity state of the material.

Superconductivity is therefore an essential property for the applications of current transfer. And researchers obviously try to extend this limit as far as possible (that is to say, to maintain the superconductivity at higher and higher magnetic field levels). For this, more needs to be known about the behavior of the vortices and to find a way of immobilizing them in the material. One of the ways of doing this, and undoubtedly the most intuitive, is to drill miniscule holes in the material so that the vortices “fall” into them and remain immobile. While this method has had some success it is particularly inflexible: once the holes have been drilled there is no going back. If we want to study another configuration, a new piece of material must be “manufactured”.  “We therefore tried to find a more flexible method which involved covering the superconductor with a thin magnetic layer. This is exactly the same principle the magnetic drawing board is based on: it can be written on and erased at will by means of a “magic” pen. The board contains a magnet, which means that lines can be drawn, because the magnet attracts the magnetic particles that are on the board. In our case, the role of the drawing board is played by the thin layer of magnetic material that covers the superconductor and the role of the magic pen is played by the superconducting vortices, which are an extremely localized source of magnetic field. They also leave a trace of their passage by locally polarizing the magnetic layer”, explains Jérémy Brisbois, a research fellow at the FNRS in the Physics Department of the University of Liege (Experimental Physics of Nanostructured Materials, Professor Alejandro Silhanek) and first author of the article published in Scientific Reports (1). The system under study (figure 1) is composed of a silicon base upon which a layer of niobium with a thickness of 140 nm is placed. This layer of niobium is in turn covered with a layer of ferromagnetic material (in this case Fe₂₀Ni₈₀); the magnetic field H is applied to all of these layers. As can be seen in the sketch, the ferromagnetic layer does not have a regular form (square or rectangular) but has been designed in such a way as to directly show how the behavior of the vortices is influenced in accordance with the different angles.

Different scenarios

The first observation to be made when such a device is placed in a magnetic field: the vortices behave differently according to whether or not there is a magnetic layer above the superconductor. When there is no magnetic layer, the vortices enter the superconductor through the middle of the edges of the material in a symmetrical manner; when there is a magnetic layer, the penetration is enhanced at one of the edges, depending on the direction of the magnetization of the layer. 

“These observations had already been made previously”, explains Jérémy Brisbois, “But we obtained clearer images than those that had been available up to this point”. Encouraged by this success, the physicists from Liege then multiplied their experiments by varying the different parameters. One of these parameters was the thickness of the ferromagnetic layer varying from 50 to 450 nm. Another parameter was the temperature, a very important variable when we are dealing with superconductivity, because the phenomenon only appears below a critical threshold temperature. 

Among the different results obtained which contribute to a better understanding of the phenomenon of the formation and progression of the vortices in the superconductor, one of them is particularly remarkable. When the device is cooled to a very low temperature (4K or -269°C), the vortices enter abruptly into the superconductor and form branches of magnetic flux, also known as avalanches. In this case, when the temperature has increased to the point where the superconductivity has been destroyed completely, the traces of the vortices remain visible in the magnetic layer. “In other words”, explains Jérémy Brisbois, “we succeeded in marking out the trajectories of the vortices and keeping a trace of them before they disappeared”. This result is all the more interesting as the traces remained visible at room temperature which obviously facilitated their observation and the study of the behavior of the vortices.

Having succeeded in recording the trace of the vortices, the researchers tried to print other sources of magnetic field in the ferromagnetic layer, notably a magnetic structure (NdFeB) composed of squares with sides of 100 µm, alternatively magnetized in opposite directions. When this vary particular “chessboard” is placed in direct contact with the superconductor, it can clearly be seen that the magnetic field is printed in the ferromagnetic layer that covers the superconductor.  

“From now on”, continues Jérémy Brisbois, “the impression of the chessboard left in the magnetic layer can be used to influence the movement of vortices in the superconductor. The advantage of this technique as opposed to drilling holes in the material is that our chessboard is easy to handle: we can imagine a multitude of different geometries and they can easily be changed. One sample is sufficient to test everything: we keep the trace, we erase, and we start again, exactly like a child with their magic drawing board”! This flexibility enables us to make a giant leap forward with regard to the study of vortices and their behavior as well as controlling their movement because it allows the experimenters to easily master the maneuver. It is no longer a case of just looking, but also to guide the vortices in accordance with the aim of the research. 

(1) Imprinting superconducting vortex footsteps in a magnetic layer, Brisbois Jérémy et al., Scientific Reports 6, 27159 (2016).