The magic drawing board
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. |
|
|||||||||||||||||||||||
© 2007 ULi�ge
|
||