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 mu m(2) 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.