Hysteretic NanoSQUID Sensors for Investigation of Iron Oxide Nanoparticles

Magnetic nanosensors based on niobium nano superconducting quantum interference device (nanoSQUID) have been fabricated and employed to investigate the magnetic properties of iron oxide nanoparticles. The nanoSQUIDs, fabricated by electron beam lithography, consist in a square loop interrupted by two nanometric constrictions (Dayem bridges). The flux capture area is 0.5 μm² while the Dayem nanobridges have a width and a length ranging from 50 to 90 nm and 70 to 80 nm, respectively. Measurements of current to voltage (I-V) and critical current to magnetic flux (I_C-Φ) characteristics have been reported for different nanobridge dimensions. At T = 4.2 K the nanosensors have shown a hysteretic I-V characteristic and a triangular shaped I-Φ pattern suggesting a strong deviation from sinusoidal current-phase relationship. Due to the hysteretic behavior, the devices have been employed as a magnetic flux to current transducer. In such a configuration an overall magnetic flux resolution of about 0.3 mΦ₀ has been estimated. These nanosensors have been successfully employed to measure the field dependence and the time relaxation of the magnetization of iron oxide nanoparticles having a mean diameter of 4.2 nm. The experimental curves undoubtedly prove that the nanoSQUIDs reported here can be successfully employed to investigate the magnetic properties of very small nanoparticles.