IT IS BECOMING increasingly common to use the prefix "nano" to describe developments in science and technology in which nano-scale effects are, to be polite, of fleeting importance. Nanomagnetism, however, is different, because the fundamental properties of magnets are defined at nanometre length scales.
Nanomagnets can measure anything from just under a micron to a few nanometres in size, and have applications that range from medical imaging and drug delivery to sensors and computing. Moreover, nano-scale ferromagnetic particles have played a central role in the enormous advances made within the data-storage industry over the past 50 years, and look set to continue this role for decades to come.
Ferromagnetic materials consist of tiny individual domains in which the magnetic moments of all the component atoms or molecules point in the same direction. This direction varies from one domain to the next, but what makes ferromagnets useful is the fact that all the domains remain aligned in an external magnetic field -- even after the field has been switched off. A computer disk, for example, contains a 2D ferromagnetic thin film on which information is stored in sub-micron-sized "bits" made of hundreds of domains. The magnetic moments within different domains are forced to align with the magnetic field produced by the read head, which consists of a current-carrying coil wound round a magnetic yoke. Since the moments in the magnetic domains remain stable, the material "remembers" whatever information has been recorded.
Nanomagnetism basically involves studying how such ferromagnetic materials behave when they are geometrically restricted in at least one dimension.Apart from 2D thin films, such objects can be 1D "nanowires", or zero-dimensional "magnetic islands". Thanks to new high-resolution fabrication techniques, these objects are now relatively easy to make. Indeed, physicists have been able to create nanomagnets with structures that range from relatively large micron-sized domains to individual atomic chains.
In the July issue of Physics World Denis Koltsov and Mark Perry of the Nanoscale Science Laboratory at the University of Cambridge in the UK describe this work in more detail.