Over the last few decades most of the major technological advances that have shaped our society have been underpinned by improvements in our understanding of how the structure of materials at the atomic scale shapes their behavior in the observable world. Nowhere is this more evident than in the storage, processing and use of data, driven by complex magnetic phenomena such as Giant Magneto Resistance.
Prof Andrew Boothroyd nominated Dr Johnson, who is currently a Royal Society University Research Fellow based in the Department of Physics at the University of Oxford. He has also previously been an instrument scientist, working on the WISH diffractometer at ISIS. He says, “Roger's skill is in exploiting advanced neutron diffraction and complementary techniques to solve complex crystal and magnetic structures, and in using physical insights from his structure solutions to work out the magnetoelectric coupling mechanisms. He has the rare ability to see through the layers of complexity that shroud real-world materials and find simple explanations for how they work."
In recent years Dr Johnson has produced a series of groundbreaking publications on this highly complex class of materials, based on neutron diffraction experiments. This included his 2016 paper solving the magnetic structure of CaMn7O12 through the observation of a series of higher order Fourier components in single crystal and powder neutron diffraction at ISIS, and is considered to be one of the most complex magnetic structures ever solved. Quite remarkably, however, out of this complexity emerged an intuitive understanding based on the theories developed by Goodenough in the 1950's – pointing towards a universal magneto-orbital coupling in this class of materials.
In another recent publication, Dr Johnson discovered another novel magnetic phase of the manganites, again employing neutron powder diffraction at ISIS. This discovery was a triumph, as it unified the phase diagram of complex magneto-orbital textures, revealed in Dr Johnson's prior research on complex multiferroic phases, with that of simple perovskites studied since the1950s.
Prof Boothroyd says, “Dr Johnson's work has defined new benchmarks in the complexity of magnetic structures solvable by neutron diffraction. By unravelling the mechanisms that couple electronic and magnetic degrees of freedom his research has led to a resurgence of interest in magnetic phenomena in manganites, paving the way to their exploitation in technological applications."
The prize is named in honour of the late Professor B T M Willis, founding chairman of the Institute of Physics and the Royal Society of Chemistry Neutron Scattering Group. Prof Felix Fernandez-Alonso is the current chair. He says, “Terry Willis was a pioneer of neutron diffraction techniques for the investigation of the structures of complex materials, including complex manganese oxides. It would be fitting for the award to go to a young scientist who has taken the technique to the next level. Dr Johnson is a very worthy recipient of the prize."
References
Johnson et al. (2012), Phys. Rev. Lett. 108, 067201
Johnson et al. (2011), Phys. Rev. Lett. 107, 137205
Johnson et al. (2016), Phys. Rev. B 93, 180403(R)
Perks et al. (2012), Nature Comms. 3, 1277
Johnson et al. (2018), Phys. Rev. Lett. 120, 257202
