Low-noise YBa₂Cu₃O_7-δ direct-current superconducting quantum interference device magnetometer with direct signal injection

We have fabricated several low-noise direct-current superconducting quantum interference device (SQUID) magnetometers from single layers of YBa₂Cu₃O_7-δ on 10 mm×10 mm bicrystal substrates. The magnetometer design consists of a single-turn pickup loop that is directly coupled to the SQUID inductance. At 77 K, these magnetometers exhibit large voltage modulation with applied flux of over 40 μV. The minimum flux noise, measured at 77 K using conventional flux-locked loop electronics with bias current reversal, is 3.5×10^-6 Φ₀/Hz above 10 kHz and 6.5×10^-6 Φ₀/Hz at 1 Hz. The field-to-flux conversion efficiency is measured to be 10 nT/Φ₀, resulting in a white magnetic field noise of 35 fT/Hz above 10 kHz, increasing to 65 fT/Hz at 1 Hz.