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 μm 2 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 …