Big steps for tiny magnets

Magnets so small that they are invisible to the human eye are expected to drive the development of miniature microelectronic devices. One application will be 0in hard disk drives and other forms of computer memory, while in vehicle engineering, molecular magnets could form the basis of a new range of magneto rheological fluids used as […]

Magnets so small that they are invisible to the human eye are expected to drive the development of miniature microelectronic devices.

One application will be 0in hard disk drives and other forms of computer memory, while in vehicle engineering, molecular magnets could form the basis of a new range of magneto rheological fluids used as intelligent dampers and shock absorbers.

In the field of ferrofluidics (Technology News, 4 September) they could be used as ‘designer’ seals, and in medicine, to target and remove cancer cells.

Collaborative research between the Weizmann Institute in Israel and Oxford University has led to the development of molecular magnets much smaller than existing ones based on metal-organic compounds.

The research programme is headed by Professor Reshef Tenne, and continues his earlier pioneering work into buckyballs the popular term for carbon-based organic compounds called fullerenes, which took the chemical industry by storm with their potential as ‘designer’ materials.

These soccer ball-like molecules form the building blocks of a range of materials that can be tailored to suit specific applications, notably, in the case of organic compounds, as lubricants, because of their carbon content.

Tenne has shown that inorganic compounds can also become designer materials with their own special properties.

Earlier experiments with inorganic materials showed that it should be possible to produce molecular-size magnets from a material that is normally anti-ferromagnetic.

Inorganic compounds form a large family of materials that includes the transition metals. At the macroscopic level, these are characterised by having a planar structure arranged in a even number of layers.

Each layer has an opposing magnetic moment which cancels out the other, making the material anti-ferromagnetic.

In a single molecular layer, the material would become ferromagnetic. But this introduces instability because, says Tenne, the edges are reactive.

In the case of nickel dichloride, for example, there are fewer nickel atoms at the edge of the molecule compared with the inner layer. The outer edge is unsaturated and needs to bind with atoms to become stable.

Stability is essential where the magnetic material is used to store electronic data. It is particularly important in high-density applications where on-off switching operations must not affect the integrity of stored data elsewhere.

The molecular chain can be made stable by closing the chain, forcing it to become a cage or sphere.

Fullerenes and their extended form nanotubes are cage-like molecules made up of 60 atoms arranged as 12 pentagons.

Normally, planar materials such as graphite are arranged as hexagons. Tenne is using well-defined crystal engineering techniques which control temperature and gas flow to introduce the pentagons into the planar structure.

Pentagons have a bending effect on the planar structure, forcing the molecule into its characteristic soccer-ball shape, which gives it stability.

The single-layer molecular cage will have ferromagnetic properties, Tenne believes. The next step is to synthesise larger quantities in order to evaluate the magnetic properties of the material more fully.

Interestingly, the polarity of the material can be switched by magnetic, electrical or light wave excitation, which opens up new application possibilities.

Tenne’s research at the Weizmann Institute is assisted by graduate student Yaron Rosenfeld Hacohen and colleague Dr Enrique Grunbaum.

Electron microscopy work is being undertaken by Drs John Hutchison and Jeremy Sloan at Oxford University’s Department of Materials.