The demand for the ever-increasing speed and density of information storage has triggered an intense search for ways to control the magnetic state of tiny magnets, as also used in computer hard drives. Aiming to improve magnetic recording speed and spatial resolution, the researchers tried to switch magnetization in microstructures by using a femtosecond - one millionth of one billionth of a second - laser pulse. This led to an unexpected discovery…
Researchers at PSI spotted a curious black-and-white magnetic substructure on a five-by-five micrometre square – and were reminded of the stylised Batman logo. The black areas reveal where the magnetisation is pointing downwards, i.e. into the picture; the white ones where it is pointing upwards.
'Batman' shows the way
When the size of the magnetic microstructure was still pretty large, of the order of five thousandth of a millimeter, the laser light did not switch the structure homogeneously but formed a ‘batman’-like pattern (see Figure 1). This pattern showed features which were smaller than the wavelength of the light, showing that light-matter interaction strongly depends on interference of the incident and the reflected light waves. Thus, the switching pattern can be controlled by structure design. Using computational methods the authors confirmed this hypothesis and revealed the feasibility of nanoscale magnetic switching even for an unfocused laser pulse.
Novel data storage opportunities
Controlling the switching pattern, which in this particular case had an ironic ‘batman’-like shape, opens novel opportunities for very-high-density data storage, for example by recording several bits of information in a single magnetic structure.
Experiment and theory by comparison: the PSI researchers’ Dutch colleagues were able to illustrate the magnetic structures generated by laser beams effectively in computer simulations.
Prof. Theo Rasing of Radboud University says: ‘Since our group in Nijmegen discovered that femtosecond laser pulses are able to reverse magnetization, we started to work on how to minimize the size of the switched domain. You can in principle follow two approaches: make the structures smaller or focus the light to a smaller spot. By structuring the materials we discovered indeed that you can achieve sub-wavelength switching even on much larger structures. By controlling the laser pulse, this can be done in a controlled way. The ability to detect magnetic changes with sub-100 nm resolution was crucial for the whole project. Our collaborations through EU-networks with the main synchrotrons in Europe therefore played a decisive role for the success of this project.’