We are able to measure hysteresis and noise in a single magnetic particle with diameter of the magnetic core in the range 10-120 nm and the moment down to 10⁴ µ_B at 8K.

The detection of ever-smaller magnetic particles is of crucial technological and scientific importance, driven both by the needs of the IT and telecom communities but also by the medical and biological requirements, in addition to improved understanding of the basic physics of small number of coupled spins. SQUIDs, the most sensitive detectors for a wide range of physical parameters, are also exquisitely suitable for the measurements on magnetic nanoparticles.  It was proposed that, if a small enough SQUID is used, its sensitivity should be sufficient to detect the reversal of a single Bohr magneton moment. In our studies we deal with optimisation of the SQUID design with respect to noise, coupling to the nanoparticle and operation in varying d.c.  magnetic field. 

Fig.1 - Left: TEM image of a magnetite-silica core-shell nanoparticle. Bar scale is 20 nm. Right: SEM image of a collection of magnetite-silica nanoparticles and a sharpened carbon fibre tip with a single nanoparticle attached to its apex, ready to be placed on a nano-SQUID. Inset shows a magnified image of the nanoparticle located at the end of the tip. Bar scale is 3 μm.

We have developed the FIB-based technique allowing successful nanomanipulation of a nanoparticle with the size down to 100 nm (Fig. 1) ^[1-2].  The nanoparticle can be accurately positioned with respect to the SQUID loop and then removed without affecting the SQUID properties and performance. We use an ultra-low noise nano-SQUID to measure the hysteretic magnetisation behaviour of a single FePt nanoparticle at a temperature of around 7 K in a magnetic field of only ~10 mT (Fig. 2) ^[2]. 

Fig.2 - Hysteresis plots for a single FePt nanoparticle for a range of applied magnetic fields.

Fig. 3: The flux noise for the nano-SQUID with (red) and without (black) a single magnetic nanoparticle (a) magnetite-silica (diameter ~120 nm, moment ~10⁴ μ_B, T_b ~ 220 K) and b) FePt (diameter ~150 nm, moment ~10⁶ μ_B, T_b ~ 30 K) measured at T=7.8 K. The green line represents the difference between two traces. The dashed black line is the 1/f noise fit to the difference line at low frequencies, i.e. a) f< 20 Hz and b) f<80 Hz. .

We also study the noise associated with a single magnetic particle. We demonstrate a clear change of the noise spectra of the nano-SQUID detected at low frequencies in the presence of the nanoparticle. Similar behaviour was confirmed for a single core-shell magnetite-silica nanoparticle (diameter ~120 nm, moment ~10⁴ μ_B) and an FePt nanoparticle with a larger magnetic moment (diameter ~150 nm, moment ~10⁶ μ_B) (Fig. 3) ^[1]. 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.

References:

1. Magnetic nanoparticle detection using nano-SQUID sensors
   O. Kazakova, L. Hao, D. Cox, P. See, and J. Gallop,
   J. Phys. D. 43,  474004 (2010).
2. Detection of Single Magnetic Nanobead with a nano-SQUIDs
   L. Hao, J.C. Gallop, O. Kazakova, D. Cox, C.  Aßmann, F. Ruede, D. Drung, and Th. Schurig,
   Appl. Phys. Lett. 98, 092504 (2011).
3. Readout of NanoSQUID Sensors Using a SQUID Amplifier
   F. Ruede, S. Bechstein, L. Hao, C. Aßmann, Th. Schurig, J. Gallop, O. Kazakova, J. Beyer, and D. Drung
   IEEE Trans. Appl. Supercond., 21, 408 (2011).