Performance of nanoSQUIDs for small spin cluster detection

In the present paper, performance of nano-superconducting-quantum-interference devices SQUIDs has been investigated in view of their employment in the detection of small spin populations. The analysis has been focused on nano-SQUID sensors having a square loop with a side length of 200 nm. We calculate the spin sensitivity and the magnetic response relative to the single Bohr magneton single spin , as a function of its position within the SQUID hole. The results show that the SQUID response depends strongly on the spin position; the ratio between the spin sensitivity evaluated in the center of the loop and the minimum one is as high as a factor of 3 for a spin at a reasonable distance z of 10 nm from the SQUID plane. Furthermore, the magnetic flux due to several hundred of spins has been evaluated by considering different random spin distributions within the SQUID hole. Due to the both nonuniform SQUID response and the random distribution process, the results show a statistical uncertainty which has been evaluated as a function of the spin number. The estimated informations are very useful to optimize the sensor performance in view of the most nanomagnetism applications.