Watching them spin
The evidence for this comes from an experimental single bit storage device that Dr Wadley and colleagues have been investigating at Diamond. It is a thin film of antiferromagnetic copper manganese arsenide (CuMnAs) grown by molecular epitaxy on a semiconductor base at the University of Nottingham. The team has shown that they can read and write to this device, but they needed more information on the behaviour of the affected regions. The lack of external magnetic field makes it very hard to see what is going on. “There is an electrical signal but it is very faint,” said Dr Wadley. Instead the team used X-ray magnetic linear dichroism photoemission electron microscopy (XMLD-PEEM) available at Diamond.
Dr Peter Wadley, Research Fellow at the School of Physics and Astronomy, University of Nottingham.
The CuMnAs film was exposed to a beam of highly tuned monochromatic polarised X-rays. This displaced electrons from the material which were then detected by an electron microscope. The resulting images, essentially black and white photos, showed the magnetism on the surface of the material (Fig. 2). By comparing images before and after writing a bit of information, the researchers clearly showed that the spin orientation of the domains changed in response to the pulse of current, supporting the electrical readouts from the storage cell. Further experiments were conducted under magnetic fields up to 12T (enough to completely wipe a conventional magnetic storage device) and at temperatures between 150K and 304K, and in both cases the information remained intact.
Figure 2: (a) Optical microscopy image of the device used for PEEM imaging. (b) XMLD PEEM image of the domain structure in the central region of the cross. Vertically aligned spin domains appear black and horizontally aligned domain appear white.