Initial work with magnetic tweezers will utilise commercially available magnetic beads, coated with streptavidin, which are just in the sub-micron size range and will allow detection by conventional means (e.g. sub-micron Hall bars).  However the program of work will depend upon production of smaller particles with strong magnetic properties.  Therefore, development of small ferromagnetic particles to be coated with avidin or streptavidin will be a key next stage to allow progress in other Workpackages.  In addition magnetic carbonaceous nanoparticles will be produced, including Fe, Co or Ni filled carbon nanotubes and fullerenes.  Later stages will involve the investigation of superparamagnetic organic molecules (such as Fe8 or Mn12 or Ferritin) or magnetic endo-fullerenes to form ultra small size magnetic beads.  In parallel with these developments novel bead detection methods will be developed, based on detection of Larmor precession of bead magnetic moment using scanned probe techniques, to enable detection and spatial localisation of single nanoscale beads. It should be emphasised here that detection of a time dependent signal (arising either from the precessing spin of a single paramagnetic molecule (as a r.f. component in the tunnelling current to the tip of a scanned probe microscope placed above the particle) or from the fluctuating thermally-driven superparamagnetic moment) is just as valid a detection technique as is detection of a static ferromagnetic polarisation.  An array of non-scanning MFM tips to sense motion along micro-channel, micron-sized Hall effect sensors, flux gates or resonant force detection of the magnetisation could also be investigated. Sub-micron sized SQUID detection offers the possibility of a non-intrusive measurement method that could be “on-chip”.

Outcomes:

Ports have made a variety of sub-micron sized magnetic particles, that are being analysed for uniformity of size, using AFM and uniformity of magnetic properties, using MFM (Ports and NPL).

Paper in  preparation.