Design of multiloop input circuits for high-T c superconducting quantum interference magnetometers

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Abstract
A technique is developed for the design of input circuits of superconducting quantum interference devices (SQUIDs) including those based on high transition temperature (Tc) superconductors. A multitransformer SQUID circuit is reduced to a single‐loop one with effective values of its loop inductance and applied flux. A numerical model of a multitransformer magnetometer is formed and analyzed. The planar transformer coupling coefficient is evaluated. The multitransformer SQUID structures as a whole are optimized with respect to their performance, taking into account the limits set by fabrication technology. Three gradiometer‐type device designs are discussed. The most complex SQUID treated is a second‐order gradiometer for magnetoencephalography use which needs only a two superconducting films technology to be realized. Designs are based on available high‐Tc superconductor technology. Very competitive performance with high energy resolution and magnetic‐field gradient sensitivity is predicted at nitrogen temperature using a multitransformer coupling circuit.