Soft magnetic dot as a linear flux amplifier

It has been demonstrated that the critical current versus the applied flux relationship of an Al superconducting loop enclosing a soft Permalloy magnetic dot has a variable field-dependent phase. It is, therefore, possible to use this structure as a magnetic flux amplifier for applications in superconducting quantum interference devices. The dimensions of a single-domain Permalloy dot were selected in such a way that the preferential orientation of the magnetization is rotated from the perpendicular direction. By increasing an applied magnetic field, the magnetization of the dot rotates towards the out-of-plane direction. This magnetization rotation provides a flux gain and an enhancement of the field sensitivity. In the low-field regime, where the structure was investigated, the magnetization of the dot changes linearly with the applied flux and the flux gain is constant. As a result of a pronounced shape anisotropy, the flux gain can be tuned by adjusting the dimensions of the dot.