Ultralow temperature NMR detection using dc SQUIDs

Abstract

Nuclear magnetic resonance (NMR) experiments at ultralow temperatures place high demands on superconducting quantum interference device (SQUID) amplifiers. We have developed a suitable direct-coupled readout SQUID electronics system, which utilizes additional positive feedback (APF), and have achieved a system bandwidth of 5 MHz, maximum slew rate of 1.3×10⁷ Φ₀/s at 14 kHz and a white noise level of 3.4 μΦ₀/$\text{Hz}$. We have also investigated improving the signal-to-noise ratio by using a low noise series array of dc SQUIDs and by placing the SQUID amplifier closer to the ultralow temperature experiment. We will discuss how we have configured our SQUID amplifiers for operation at T≪1 K.