Low Noise NanoSQUIDs Based on Deep Submicron Josephson Tunnel Junctions

In the recent years, it has been shown that the magnetic response of nano-objects (nanoparticles, nanobeads, and small cluster of molecules) can be effectively measured by using a Superconducting Quantum Interference Device (SQUID) with a small sensitive area. Here, we present high sensitivity nanoSQUIDs based on deep submicrometer Josephson tunnel junctions fabricated by a Focused Ion Beam (FIB) sculpting method. Two different configuration have been realized: a vertical configuration where the nanosensor loop is perpendicular to the substrate plane and a planar one having the SQUID loop parallel to the substrate plane. In both configurations the flux capture and the junction areas are 0.2 and 0.09 μm², respectively. At liquid helium temperature the nanosensor have shown non-hysteretic current-voltage characteristics and large voltage modulation depths as well as high voltage responsivities. A spectral density of magnetic flux noise as low as 700 nΦ₀/Hz^1/2 has been measured in small signal mode using a low noise direct coupling electronics. The computed spin sensitivity has provided a value less than 10 Bohr magneton per bandwidth unit.