Low-Noise YBa2Cu3O7 Nano-SQUIDs for Performing Magnetization-Reversal Measurements on Magnetic Nanoparticles

We fabricate YBa2Cu3O7 (YBCO) direct-current nano-superconducting quantum-interference devices (nano-SQUIDs) based on grain-boundary Josephson junctions by focused-ion-beam patterning. Characterization of electric transport and noise properties at 4.2 K in a magnetically shielded environment yields a very small inductance L of a few pH for an optimized device geometry. This, in turn, results in very low values of flux noise < 50 n Phi(0)/Hz(1/2) in the thermal white-noise limit, which yields spin sensitivities of a few mu(B)/Hz(1/2) (Phi(0) is the magnetic flux quantum, and mu(B) is the Bohr magneton). We observe frequency-dependent excess noise up to 7 MHz, which can be eliminated only partially by bias reversal readout. This behavior indicates the presence of fluctuators of unknown origin, possibly related to defect-induced spins in the SrTiO3 substrate. We demonstrate the potential of using YBCO nano-SQUIDs for the investigation of small spin systems, by placing a 39-nm-diameter Fe nanowire encapsulated in a carbon nanotube on top of a nonoptimized YBCO nano-SQUID and by measuring the magnetization reversal of the Fe nanowire via the change of magnetic flux coupled to the nano-SQUID. The measured flux signals upon magnetization reversal of the Fe nanowire are in very good agreement with estimated values, and the determined switching fields indicate magnetization reversal of the nanowire via curling mode.