Advances in SQUID magnetometers

The operation and noise limitations of de and RF SQUID's are outlined, and recent advances in their sensitivity are discussed. A model of the dc SQUID predicts an energy noise level per hertz referred to the SQUID of approximately
8 k_{B}TL/R \approx 8 k_{B}T(\piLC)^{1/2}
, where
L, R,
and
C
are the SQUID inductance and the shunt resistance and capacitance of each Josephson junction. Some examples of dc SQUID's are described to show that their performance is generally in reasonable agreement with the model. The noise energy has improved from about 2 × 10
^-30
J. Hz
^-1
for a device with
L = 1
nH and a tunnel junction area of 10
⁴
µm
²
to about 2 × 10
^-33
J . Hz
^-1
for a device with
L = 0.1
nH and a microbridge resistance of 40 Ω. Further improvements axe expected in the near future. The model of the RF SQUID predicts a noise energy per hertz referred to the SQUID of
[(\pi\alpha^{2}\phi\min{0}\max{2}/2L) + 2 \pi \alpha k_{B}T\min{a}\max{(eff)}]/\omega_{RF}
, where α is the intrinsic width of the distribution of flux transitions,
T\min{a}\max{(eff)}
is an effective amplifier noise temperature, and ω
RF
is the pump frequency. With one exception, the performance of the seven types of RF SQUID listed is in reasonable agreement with the model. The noise energy ranges from about 1.5 × 10
^-29
J . Hz
^-1
for a 20-MHz toroidal SQUID to 3.5 × 10
^-31
J . Hz
^-1
for 9-GHz reentrant toroidal SQUID; a somewhat better sensitivity has been reported for a 430-MHz device, apparently in conflict with the theory. In both dc and RF SQUID's, 1/
f
noise (
f
is frequency) is likely to extend to higher frequencies as the white-noise level is decreased.