Detection of magnetic moments using a nano-SQUID: limits of resolution and sensitivity in near-field SQUID magnetometry

We investigate the coupling efficiency of a localized magnetic moment placed at a distance z from a DC-SQUID magnetometer of loop radius a with nanobridges of cross section r(2). Using simple magnetostatic considerations, we show that there exist two detection regimes: the usual far-field regime (z greater than or similar to a) for which inductive coupling is achieved by the entire loop and a near-field regime (z << a) where nanobridges become the active detecting elements. Simulation shows that the greatest coupling efficiency is obtained in the near-field regime (z << a) when the magnetic moment sits directly on the nanobridge. The maximum coupling limit is given by: 1/2 mu M-0/r. Using nanoscale weak links and typical noise performance of nano-SQUID, we conclude that the limit of single molecular magnet detection can be obtained with r similar to 1 nm, a value reachable using carbon nanotube Josephson junctions.