Sarnjeet Dhesi

My work focusses on understanding the properties of novel materials in order to propel the development of new energy-efficient devices. In particular, my research activity concentrates on manipulating magnetism via strain, electrical current pulses or ultrafast laser pulses and understanding the resulting changes on the nanoscale using polarised x-ray spectroscopy, microscopy and diffraction. For instance, multiferroics are novel materials where the coupling of electric, magnetic, and elastic orders could lead to a new generation of memory devices combining non-volatile storage with efficient switching mechanisms. However, their technological potential can only be realised if multiferroic domains within thin films can be precisely defined and controlled. Here, the combination of intense polarised x-rays with a PhotoEmission Electron Microscope (PEEM) is key to understanding how strain in thin films affects the formation of antiferromagnetic domains. In thin films of BiFeO3 PEEM imaging combined with X-ray Magnetic Linear Dichroism (XMLD) has demonstrated the existence of nanoscale monoclinic domains which then pin exactly one antiferromagnetic domain each. These insights have directly led to the development of new BiFeO3 films, grown on different substrates, that avoid the formation of the disruptive monoclinic domains.

Synthetic multiferroics can be fabricated by growing ferromagnetic thin films on ferroelectric substrates leading to large strain-mediated magnetoelectric effects. Ni thin films grown on the piezoelectric PMN-PT is one such example for which electrically driven ±90° magnetisation rotations have been observed which is consistent with a normal strain induced along a particular crystallographic axis. Looking closer with the PEEM combined with X-Ray Magnetic Circular Dichroism (XMCD) however, has revealed that shear strain components are crucial in understanding the nanoscale separation of the ferromagnetic film into areas that rotate by significantly less than ±90°.

Another way to control magnetism in thin films is to inject current pulses into a thin film which rotates the local magnetisation through a Néel spin-orbit torque. In thin films of CuMnAs we have injected electric current pulses and used XMLD-PEEM to observe rotations of the nanoscale magnetic domains. Furthermore, the imaging results from the PEEM were directly related to anisotropic magnetoresistance measurements which indicated that substantial areas of the thin film did not switch. Current research activity now focusses on improving the performance of the CuMnAs thin films.