Researchers in the US have developed a new technique for making small and powerful permanent magnets. These so-called ‘exchange-coupled nanocomposites’ contain two magnetic phases, which makes them stronger than conventional magnets made from single phase materials (H Zeng et al. 2002 Nature 420 395)
‘Exchange-spring magnets’ are promising candidates for advanced permanent magnetic applications, such as recording and storage devices, because they have a large energy product -the figure of merit for a magnet’s strength. A high energy product requires the material to have a large magnetization and a large ‘coercivity’ – the magnetic field needed to reduce the magnetization of a ferromagnetic material to zero.
Exchange spring magnets contain a magnetically hard phase, which has a high coercivity, and a soft phase, with low coercivity. These two phases interact by ‘exchange coupling’. The hard phase provides high anisotropy and the soft phase high magnetization. For the exchange coupling to be effective, however, the hard and soft phases must be controlled at the nanometre scale, which can be difficult.
Now, Hao Zeng and co-workers at the IBM TJ Watson Research Center in New York, with colleagues at Louisiana Tech University and the Georgia Institute of Technology have devised a novel nanoparticle self-assembly method. They use nanoscale iron-platinum and iron oxide (Fe3O4) particles as the ‘building blocks’ in the assembly. The components are mixed together and allowed to self-organise.
Optimum exchange coupling, and therefore the maximum energy product, can be obtained by changing the size and composition of the individual building blocks. The energy product of this two-phase material is 20.1 mega gauss oersteds, which is over 50% higher than the value for conventional iron-platinum magnets.
The workers now plan to compress this material to make high-density magnets and to improve the alignment of the axes of the hard phase grains to increase the magnetization value of the composite. They also hope to look at other magnetic materials such as samarium cobalt and neodymium iron boride.