Magnetic properties of iron oxide nanoparticles investigated by nanoSQUIDs

Magnetization measurements of Fe3O4 nanoparticles have been performed by using a nanosized superconducting quantum interference device (nanoSQUID). The nanosensor consists of a niobium loop having an area of 0.5 μm2 interrupted by two Dayem nanobridges. The device fabrication procedure is based on the electron beam lithography, thin film deposition and the lift-off technique. The characterization of the nanodevice at T = 4.2 K includes measurements of current-voltage, critical current vs. magnetic flux characteristic and flux noise. A proper feedback circuit has been employed to increase the dynamic range of the nanosensor. The magnetic nanoparticles under investigation have a diameter of 4 nm and 8 nm and were synthesized by thermal decomposition of metallorganic precursors in the presence of oleic acid and oleylamine as surfactants and organic solvent with high boiling point. Measurements of magnetization as a function of the external magnetic field for both nanoparticle diameters are reported at liquid helium temperature. In both cases, it can be observed an evident magnetic hysteresis indicating a blocking temperature well above 4.2 K. The reliability and the clarity of the reported measurement demonstrates that a low noise nanoSQUID is a powerful tool to investigate the properties of magnetic nano-objects.