We demonstrate detection of a single core-shell magnetite–silica nanoparticle (outer diameter ∼120 nm, moment ∼104μB) using an Nb dc superconducting quantum interference device (SQUID) with the loop size of 350 nm operational at T < 10 K. The system noise was minimized down to 0.2 µΦ0 Hz−1/2 using a cryogenic SQUID series array pre-amplifier. Initial measurements of an individual magnetic nanoparticle were performed and a clear change of the noise spectra of the nano-SQUID was detected at low frequencies in the presence of the nanoparticle. Similar behaviour was confirmed with an FePt nanoparticle with a larger magnetic moment (diameter ∼150 nm, moment ∼106μB). Thus, we demonstrate a magnetic sensor based on a dc nano-SQUID and enabling detection of small moments (potentially down to a few electron spins). Such a sensor is of considerable significance for nanomagnetic metrology and quantum information processing based on spin systems.
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Cox et al. (2010) studied this question.
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