Magnetic materials have been used for storing information for more than half a century, from the first magnetic tapes to modern data servers. These technologies have in common the usage of ferromagnets, producing magnetic fields which are easily measurable. Researchers at the University of Nottingham are working with Diamond Light Source to develop new technologies based on a different class of magnetic material: an antiferromagnet, which does not produce a magnetic field, but which has a hidden magnetic order that can be used to store the ones and zeros of information.
Figure 1: Schematic of magnetic moment orientation for binary information storage using (left) a ferromagnet and (right) an antiferromagnet.
Looking at the atomic scale, each atom is like a small magnetic compass, having a small magnetic moment. In a ferromagnet, once the information is written, all those atomic moments remain oriented in the same direction. In antiferromagnets, each magnetic moment aligns exactly opposite to its neighbours, effectively cancelling them out (Figure 1). This arrangement has some important advantages for memory applications: magnetic bits do not interact with each other, so can be packed more closely; they do not interact with external magnetic fields; their resonant frequencies, which determine the speed that information can be written, is typically 1000 times larger than in ferromagnets. Antiferromagnets can therefore be useful, but how would you store and read information in a material whose total magnetic moment is always zero? Dr Peter Wadley, a researcher at the University of Nottingham, and Sonka Reimers, a joint Nottingham and Diamond PhD student, are trying to answer that question in their search for new technologies for information storage and processing.
The team from the University of Nottingham came to work together with Prof Sarnjeet Dhesi and Dr Francesco Maccherozzi at Diamond’s Nanoscience beamline (I06), where they demonstrated that an electric current can produce changes in the direction of the magnetic moments in an antiferromagnet. The material they used consisted of thin films of CuMnAs, in which the antiferromagnetic manganese (Mn) ions are arranged in a grid pattern. Due to the particular symmetry of the atomic lattice, the electric current results in a local magnetic field with alternating sign, which is particularly efficient at rotating the antiferromagnetic orientation.
The experiment combined PhotoEmission Electron Microscopy (PEEM) with X-ray Magnetic Linear Dichroism to give a unique sensitivity to antiferromagnetic order. This allows the magnetic moments to be mapped out at the nanoscale, so that the changes produced by the current pulses can be directly identified. They observed that they can reversibly move antiferromagnetic domain walls separating differently oriented regions by reversing the sign of the electrical current pulse. The results provide new insights into the relationship between the antiferromagnetic domain structure and the current-induced fields – which can be much more complex than in ferromagnetic materials. This is a crucial step for the development of fast and efficient memory technologies.
Wadley P, et al. Current-polarity dependent manipulation of antiferromagnetic domains. Nature Nanotechnology (2018). DOI: 10.1038/s41565-018-0079-1
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