Moving domain walls induce losses in superconductor/ferromagnet hybrid systems

Recently many research groups have aimed at developing magnetic memories which are based on writing and reading magnetic information with the help of electric current. Such systems typically require so large currents to switch the magnetization that it can affect thermal stability of the memory element. In order to reduce heating effects, the superconducting materials which can sustain dissipationless electric current would be quite useful.

Superconducting current is a flow of electrons bound in Cooper pairs and therefore is fundamentally different from the usual current in normal metals which is carried by the single electrons. Therefore, in order to develop the superconductor/ferromagnet memory element it has been necessary to understand how the superconducting current can affect the magnetic state.

In their paper the research group has found answers to the two fundamental questions: whether supercurrent can change magnetic states and whether it is possible to avoid electric losses during this process.

"We have developed the theory which describes how the superconductor can lose its fundamental property of having zero resistance in typical superconductor/ferromagnet device. This happens because of the induced magnetization dynamics in the attached ferromagnet. Although the force which drives the magnetization comes from the superconducting current, the system becomes inherently dissipative and in principle cannot sustain any amount of the superconducting current because of the voltage generated by the magnetization dynamics," explains Academy Research Fellow Mihail Silaev.

"We find the low-current resistance associated with the domain wall motion driven by the superconducting current. We suggest the finite slope of Shapiro steps as the characteristic feature of the regime with domain wall oscillations driven by the ac external current flowing through the junction," Silaev continues.