Low-Noise YBa$_2$Cu$_3$O$_7$ NanoSQUIDs for Performing Magnetization-Reversal Measurements on Magnetic Nanoparticles

TL;DR

This paper reports the fabrication and characterization of low-noise YBa2Cu3O7 nanoSQUIDs, demonstrating their potential for detecting magnetization reversal in magnetic nanoparticles with high sensitivity and exploring their noise properties.

Contribution

The study introduces YBCO nanoSQUIDs with exceptionally low flux noise and demonstrates their application in measuring magnetization reversal in a magnetic nanowire.

Findings

Flux noise below 50 nΦ0/Hz^{1/2} at 4.2K

Detection of magnetization reversal in a 39 nm Fe nanowire

Observation of frequency-dependent excess noise up to 7 MHz

Abstract

We fabricated YBa$_2$Cu$_3$O$_7$ (YBCO) direct current (dc) nano superconducting quantum interference devices (nanoSQUIDs) based on grain boundary Josephson junctions by focused ion beam patterning. Characterization of electric transport and noise properties at 4.2$\,$K in 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 $<50\,{\rm n}\Phi_0/{\rm Hz}^{1/2}$ in the thermal white noise limit, which yields spin sensitivities of a few $\mu_{\rm B}/{\rm Hz}^{1/2}$ ($\Phi_0$ is the magnetic flux quantum and $\mu_{\rm B}$ is the Bohr magneton). We observe frequency-dependent excess noise up to 7$\,$MHz, which can only partially be eliminated by bias reversal readout. This indicates the presence of fluctuators of unknown origin, possibly related to defect-induced spins in the SrTiO$_3$ substrate. We demonstrate the potential of using YBCO nanoSQUIDs for the investigation of small spin systems, by placing a 39$\,$nm diameter Fe nanowire, encapsulated in a carbon nanotube, on top of a non-optimized YBCO nanoSQUID and by measuring the magnetization reversal of the Fe nanowire via the change of magnetic flux coupled to the nanoSQUID. 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.