Superconducting magnetometers with sensitivities approaching 10 -10 gauss

Abstract

As will be described elsewhere in this conference by Mercereau, when a superconducting film ring of appropriate geometry is placed in a time varying magnetic field, there will be a quantized Faraday induction signal in the ring which will be periodic in the applied flux with a period ϕ0 = h/2e = 2.07 × 10-7 gauss.cm2. Experimentally the induced emf is detected by inductively coupling the ring to a tank circuit resonant at radio frequencies (1-30 MHz) which has a Q factor of 200 to 1,000. At a pump frequency of 30 MHz, for optimum coupling of the ring device to the tank circuit, the amplitude of the detected voltage across the tank circuit is of the order of 10 to 50 microvolts. The periodic behavior of this detected voltage with applied magnetic flux has been used to make a number of very sensitive and accurate magnetometers. For the measurement of very small magnetic fluxes (Δϕ ≲ ϕ0), the tank circuit containing the ring device is connected to a feedback or mulling circuit. A small amplitude modulation field at frequency ωmod (ωmod < ωpump/1/2 Q) is applied to the device and the detected signal across the tank circuit at frequency ω mod is amplified, phase detected and the output signal applied to a coil surrounding the device. In this manner, the device is maintained or "locked" to some specific value of magnetic flux and the current in the feedback coil is a measure of the change of the magnetic flux at the device subsequent to the closing of the feedback loop. This type of circuit is capable of maintaining the field at the device constant to 10-4 ϕ 0 rms for a 1 s time constant, which for devices with diameters 21/2 mm corresponds to a minimum detectable field of 4 × 10-10 gauss rms. A discussion of the sources of noise that will ultimately limit the sensitivity of these devices will be given. The measurement of large changes in magnetic flux (Δϕ > ϕ0) can be accomplished by making use of the periodic response of the devices to ambient fields. A small amplitude modulation field at frequency ωmod is again applied to the device and the detected signals across the tank circuit to which the device is coupled at the fundamental frequency ωmod and the harmonic frequency 2ωmod are detected and processed by suitable logic circuits to produce an output which indicates digitally the change in the magnetic flux in units of ϕ0/4 as well as the sign of the field change. The maximum counting rate of such a circuit is limited by the maximum modulation frequency that can be used which must satisfy the 2ωmod ≲ ω pump/Q/2. For a 30 MHz and Q = 300 the maximum counting rate achieved was about 104 per s. For a 1 mm diameter device, this circuit can track magnetic field changes as fast as 0.06 gauss per s digitally in units of 6 × 10-6 gauss. Much higher counting rate should be achieved by increasing the pump frequency to microwave frequencies. For example, for 1010 Hz, and Q = 1,000, modulation frequencies as high as 107 Hz can be used and thus, counting rates approaching this value should be realized assuming an adequate signal to noise ratio for this bandwidth can be obtained. Although these devices are intrinsically sensitive to changes in the ambient magnetic flux, it is possible to modify either of the above configurations to determine absolute flux. If the device can be rotated exactly 180° about an axis normal to the axis of the cylinder on which the film ring is deposited, the absolute value of the magnetic field is obtained by appropriate averaging of the output readings of the circuits for the 0 and 180° orientations of the device