The University of Jyväskylä, Finland, has been involved as part of an international collaboration that has identified a way to completely suppress superconductivity in superconducting and ferromagnetic junctions. The results published in Nature Communications are key to the development of non-volatile superconducting random access memories and could enable more energy-efficient information and communication technologies.

Superconductivity is a state of matter observed in some materials at low temperature, characterized by a vanishing resistance below the so-called critical temperature. A superconducting switch is a mechanism for switching superconductivity in the material on and off, analogous to an electrical switch, which interrupts current flow in an electrical circuit.

De Gennes' superconducting switch showed the way 60 years ago

In 1966, Pierre-Gilles de Gennes proposed a mechanism for switching superconductivity on and off in a magnetic device. This involved a superconductor sandwiched between ferromagnetic insulators, in which the net magnetic exchange field could be controlled through the magnetization orientation of the ferromagnetic layers.

The critical temperature of the superconductor is suppressed for a parallel alignment of the magnetization of the ferromagnetic layers. Conversely, if the magnetization of the ferromagnetic layers aligns antiparallel, the influence of the two ferromagnetic layers on the superconductivity cancels out.

"Although these structures have demonstrated a sensitivity of the superconducting critical temperature to the magnetization orientations, the corresponding shifts in the critical temperature were lower than predicted," explains Postdoctoral Researcher Alberto Hijano from the University of Jyväskylä.

First absolute superconducting switch

Researchers recently realized the first de Gennes' superconducting switch where superconductivity is completely suppressed. Europium sulfide (EuS) was chosen as the insulating ferromagnet, and niobium (Nb) the superconductor. The key element of this switch is an additional gold layer between the EuS and Nb, which boosts the large proximity exchange field induced in the sample. This enables absolute on/off switching of superconductivity.

"This achievement could pave the way for the development of non-volatile superconducting random access memory devices. The absolute superconducting switch would also save energy and could be used in low energy electronics to drastically reduce the energy consumption. The magnetic switch would eliminate the continuous heat load present in current thermal switches," says Hijano.

The work is a collaboration between the University of Cambridge, the University of the Basque Country and the University of Jyväskylä.