gnetic nano-sensors based on niobium dc SQUID (Superconducting Quantum Interference Device) for nanoparticle characterization are presented. The SQUIDs consists of two Dayem bridges of 90 nm x 250 nm and loop area of 4, 1 and 0.55 mu m(2). The SQUIDs were designed to have a hysteretic current-voltage characteristic in order to work as a magnetic flux-current transducer. Current-voltage characteristics, critical current as a function of the external magnetic field and switching current distributions were performed at liquid helium temperature. A critical current modulation of about 20% and a current-magnetic flux transfer coefficient (responsivity) of 30 mu A/Phi(0) have been obtained, resulting in a magnetic flux resolution better than 1 m Phi(0). In order to show the effectiveness of sensor for nanomagnetism applications, we performed measurements with and without magnetic nanoparticles on the SQUID loop applying a magnetic field parallel to the SQUID plane. In this configuration the magnetic flux coupled to the SQUID is mainly due to the presence of magnetic nanoparticles. The magnetic nanoparticles can be easily detected and their response to magnetic field studied. Measurements has been performed on Fe3O4 nanoparticles prepared by thermal decomposition method with a nominal particle size of 8 nm. Some examples of magnetization measurements were recorded at low temperature after Zero Field Cooling.
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