SECTION 9.4. THE SQUID MAGNETOMETER 89
Coils mounted on the poles of the magnet pick up the signal resulting from the motion of the
sample. This ac signal at the vibration frequency is proportional to the magnitude of the
moment of the sample. However, since it is also proportional to the vibration amplitude and
frequency, the moment readings taken simply by measuring the amplitude of the signal are
subject to errors due to variations in the amplitude and frequency of vibration. In order to
avoid this difficulty, a nulling technique is frequently employed to obtain moment readings
that are free of these sources of error. These techniques (not included in the diagram shown
in the figure) make use of a vibrating capacitor for generating a reference signal that varies
with moment, vibration amplitude, and vibration frequency in the same manner as the signal
from the pickup coils. When these two signals are processed in an appropriate manner, it is
possible to eliminate the effects of vibration amplitude and frequency shifts. In that case,
one obtains readings that vary only with the moment of the sample.
9.4. THE SQUID MAGNETOMETER
The influence of magnetic flux on a Josephson junction may be employed for measur
ing magnetic fields or magnetizations. The basic element of a Superconducting Quantum
Interference Device (SQUID) magnetometer is a ring of superconducting metal containing
critical current of an array of two Josephson junctions is periodic in field units of
one or two weak links. The name quantum interference is derived from the fact that the
due
to interference effects of the electron-pair wave functions. A so-called dc SQUID is built
with two Josephson junctions and a dc current is applied to this device. The effect of a radio
frequency (RF) field on the critical current is used to detect quasi-static flux variations. The
RF SQUID is a simple ring with only one Josephson junction. Variation of the flux in the ring
results in a change of impedance. This change in impedance results in detuning of a weakly
coupled resonator circuit driven by an RF current source. Therefore, when a magnetic flux
is applied to the ring, an induced current flows around the superconducting ring. In turn, this
current induces a variation of the RF voltage across the circuit. With a lock-in amplifier this
variation is detected. A feedback arrangement is used to minimize the current flowing in
the ring, the size of the feedback current being a measure of the applied magnetic flux. The
method is capable of measuring magnetic moments in the range
accuracy of 1%. Custom-designed dc SQUIDs can have a few orders of magnitude higher
sensitivities. For a detailed treatise on the operation principles and design considerations
of dc and RF SQUID sensors, we refer to the book of van Duzer and Turner (1981) or the
chapter by Clarke (1977).
detection by means of a SQUID is extremely sensitive. In commercial magnetometers the
with an
References
Clarke, J. (1977) in B. B. Schwartz and S. Foner (Eds) Superconductor applications: SQUIDS and machines,
New York: Plenum Press.
van Duzer, T. and Turner, C. W. (1981) Principles of superconductive devices and circuits, New York: Elsevier.
Zijlstra, H. (1967) Experimental methods in magnetism, Amsterdam: North-Holland Publishing Company.