Broadband DC SQUID NMR spectrometry on metals.
This Thesis describes the development of a broadband pulsed NMR
spectrometer, based on a sensitive DC SQUID amplifier with wideband electronics, to
observe directly the free precession of nuclear spins in bulk metallic samples (with
broad NMR linewidths) at Larmor frequencies cß/2 ,r below 1 MHz.
The sample is located inside a pickup coil, which forms a superconducting
flux transformer with the input coil of the SQUID. The SQUID amplifier operates in a
flux-locked-loop (FLL), hence it is sensitive to signals from DC up to the bandwidth
of the FLL electronics.
A modified commercial DC SQUID amplifier, with modulated feedback
electronics, observed NMR signals from bulk platinum samples (T2 - 1.1 ms), at
1.5 K. The SQUID amplifier had a 50 kHz bandwidth, a dead-time - 50 μs, and a
coupled energy sensitivity cc - 500h. The measurements showed that it is important to
minimise the time-constant of eddy current decay in the sample, which scales with
r2, as expected, where r is the sample dimension.
A DC SQUID amplifier with additional positive feedback and wideband
electronics configured using the direct offset integration technique, observed NMR
signals from a bulk aluminium sample (T2 - 30 μs) at 20 mK. This SQUID amplifier
had a 7.5 MHz bandwidth, the dead-time was 55 μs for small transmitter pulses and
e,; - 600h. The use of a strongly coupled input coil with the SQUID necessitated
damping across the coil to smooth out the SQUID flux-voltage characteristicThe NMR measurements showed that eddy current decay is less important if
the NMR signal size is enhanced by cooling the sample. Measurements also
confirmed that the NMR signal from bulk metal is proportional to 4c0, and that a
reasonable estimate of the signal size is made by assuming the signal is due to spins
within half the skin-depth of the surface