A broadband thin-film SQUID magnetometer pumped at 10 GHz

A new microwave SQUID magnetometer is described which employs a thin-film sensor operable at 4.2 K. Both sensitivity and bandwidth are substantially improved over comparable SQUIDS pumped at RF. Flux sensitivity of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Φₒ/100,000</tex> rms Hz -1/2 has been attained. The corresponding energy sensitivity is <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Eₘᵢₙ=LᵢIₘᵢₙ²/2 = 2×10⁻³⁰</tex> J/Hz. measured at the terminals of the input coil. It is argued that this energy sensitivity is the most useful basis on which to compare SQUID magnetometers which differ in inductance. With a 10 cm diam pickup loop, the magnetic field sensitivity is approximately <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3×10⁻¹²</tex> G/Hz 1/2 . Signal bandwidths from dc to beyond 100 MHz are observed, limited only by the input circuit. Feedback loop bandwidths greater than one MHz have been achieved, which allow highly linear operation at frequencies greater than 100 kHz and slew rates approaching <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10⁷ Φₒ</tex> /Second.