Magnetic systems are increasingly being used in Nanotechnology. They may represent a very useful source of transducer systems (Lowe, 1999) . For instance, a magnetoresistance biosensor has been used to measure interactions between single molecules (Baselt et al., 1998) and magneto-immunoassays (Kriz et al., 1998) , based ferromagnetic iron coated with dextran, and simple magnetic detectors based on coils in a Maxwell bridge, are being used more frequently. In addition, there is a strong driving force to produce smaller and smaller magnets for this market. This research is pushing the development of magnetic particles toward the area of Nanotechnology. However, there is a lower limit to the size of magnetic particles and for many materials, at room temperature, this size is 10-100nm. Despite these problems, there is a great deal of interest in single molecule magnets and a driving force to make such material bio-compatible for use in medical applications (Aubin et al., 2001; Boskovic et al., 2001; Gatteschi, 2001) .

In the study of DNA associated protein and enzymes the magnetic tweezer technique has been found to be a powerful tool, it comprises a single molecule anchored at one to a surface and, at the other end to a micron-scale magnetic bead. Magnets whose position and rotation can be controlled are used to pull and rotate the microbead, this stretching and twisting the molecule. The vertical magnetic force causes the DNA to extend and provides a restoring force to restrict the bead’s transverse Brownian fluctuations.

A wide range of magnetic materials can be produced down to micron level particle sizes by a variety of standard liquid and aerosol means. To achieve particle sizes much below a micron it is necessary to take a different approach directed at single-molecule or nanotube devices. Much of the research into small magnetic particles has been driven by the magnetic storage industry. This has meant that most of the work looking at small magnetic structures has dealt with films. Generally, ion beam machining or other techniques have been used to remove unwanted material leaving small magnetic “quantum dots”, wires or other simple geometric structures. These have been produced on colloidal surfaces as well as hard substrates so it is possible to remove and manipulate such dots. Another approach, developed at the National Physical Laboratory (Participant No. 2), is to use nano-particles created by soaking porous carbon blocks in magnetic salt solutions. These are then heated to around 1800º Centigrade where catalytic graphitisation occurs and particles of the magnetic material are left in the carbon, surrounded by a thin layer of graphite. This layer protects the magnetic particle from spontaneous oxidation, which is a serious problem with small particles. By this method, particles of magnetic elements such as nickel and cobalt can be made down to dimensions around 10nm. The graphite coating should also aid attachment of the particles to the DNA strands. A similar technique is to use carbon nanotubes, which can then be filled with the magnetic material. Work at the IBM Thomas J. Watson and Almaden Research Centres (E J Lerner, The Industrial Physicist, AIP, June 2000) has reported success in growing iron-platinum particles of about 3nm size. These are produced in heated solution from platinum acetylacetonate and iron carbonyl by reduction. To prevent oxidation and clumping of the particles they were immersed in emulsifying oils such as oleic acid and oleyl amine. The mixture was then spread on a plate and heated to around 560°C forming a carbon char around the particles. The technique is under investigation for magnetic storage applications but could equally apply to this work. Once again, the carbonation of the surfaces may well aid attachment to the DNA strands.

There are also a number of organic magnetic materials based on Fe8 and Mn12 structures with organic side arms. These may be investigated as they exhibit super-paramagnetism and might be more easily connected to DNA via their organic side arm groups.

Thermal energy gives rise to unwanted magnetisation reversal leading to a time dependence of the magnetisation. This thermal loss rate determines how small particles can be whilst still having sufficient magnetisation stability for use in magnetic media. The same situation will almost certainly apply for particles for use in DNA based devices. Such devices are hoped to operate at or near body temperature at least and at this sort of temperature the magnetic material will need to be chosen such as to maximise its thermal magnetisation-reversal energy. Likely candidates would be any high remanent field materials with high Curie points. Nickel, Cobalt, Iron, Cobalt Samarium, Neodymium-Iron-Boron-based materials or other permanent magnets might be suitable although there is a wide range of alternatives. Paramagnetic materials are not precluded if adequate detection methods can be demonstrated. It should be noted that a thermally-driven fluctuating magnetisation can serve as a detectable signal, under suitable conditions, just as readily as a time-dependent magnetisation. Several combinations of magnetic particle and readout will be considered and selection of materials is likely to be device and operating condition dependent with the pre-condition of determining which materials can be successfully attached to DNA strands.

The dynamic magnetic behaviour of nano-magnets is commonly described by the Landau-Lifshitz-Gilbert equation:

dJ/dt = -goJ ´ Heff + (a / Js)J ´ dJ / dt

This equation describes the physical path of the magnetic polarisation J towards equilibrium where the effective field Heff is the negative derivative of the total magnetic Gibb’s free energy which in turn is composed of the sum of the exchange energy, magnetocrystalline anisotropy energy, magnetostatic energy and the Zeeman energy. In this equation a is a damping constant describing the relaxation toward equilibrium. All contributions to Heff are only locally dependent on the magnetic polarisation or its derivatives except, of course, for the demagnetizing field. Various methods of solving this equation are suggested in (Suess, et al., 2001). This paper, as many others in the field is most concerned with magnetic switching times and the effects on them of damping, as this is one of the most important factors for magnetic storage. Magnetic switching could also be applied to the nano-devices under consideration in this project as this could imply magnetic switching of biological or hybrid systems.