The magic drawing board

The magic drawing board
11/17/16

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.

Ardoise magique illu2

Images of the magnetic field obtained by means of the magneto-optical imaging technique. The black zones correspond to a zero magnetic field while the white zones indicate a strong magnetic field. The image (a) maps the magnetic field in a square-shaped superconductor film similar to the one shown in figure 1, and shows that the penetration takes place preferentially through the middle of the edges. When the superconductor is partially covered by a magnetic layer with a thickness of 450 nm, as in image (b), the vortices penetrate more easily from the left side whereas they are held back elsewhere. The side where penetration is facilitated is controlled by the direction of the magnetization of the magnetic layer as represented by the orange arrow.

“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.

Ardoise magique illu3

The image (a) maps the penetration of the vortices when the magnetization of the magnetic layer is in the direction of the orange arrow. In contrast with figure 2, the flow here enters abruptly in the form of straight branches called avalanches. When the device is heated above the critical temperature, the superconductivity disappears and the image (b) shows the traces left by the vortices in the magnetic layer which has a thickness of 50nm. These are only visible on the side where the vortices entered in the opposite direction to the magnetization because they are the result of the reversal of the magnetization by the flux entering the Permalloy. Image (c) indicates an excellent correspondence between the traces in the magnetic layer (in red) and the penetration of the vortices (in blue); the areas where blue and red overlap are colored in white.

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.

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Written by : Henri Dupuis Translated by: Enda Breen Based on the work of:


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