Diamond Light Source - Annual Review 2022/23

34 35 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 2 / 2 3 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 2 / 2 3 MagneticMaterials Group Sarnjeet Dhesi, Science Group Leader T he Magnetic Materials Group (MMG) develops and operates a suite of polarised X-ray beamlines to understand new and novel material properties. The beamlines are home to a variety of extremely sensitive material probes including Resonant Elastic/Inelastic X-ray Scattering (REXS/RIXS), X-ray Magnetic Circular/Linear Dichroism (XMCD/XMLD) and PhotoEmission Electron Microscopy (PEEM). Over the last year, our research community has used the polarised x-ray probes to explore a wide range of advancedmaterials to discover fascinating newphenomena. In this contribution, we present researchdemonstratinghowREXS on I16 and I10 can reveal the long-range ordering of vortex domains in ferroelectric materials. On I21, RIXS has been used to discover electronic ordering in the parent compound of a new class of superconducting materials, further establishing the similarity between Mott insulators, such as the cuprates, and nickelates. On I06, a Mott insulator was imaged through its metal- insulator transition using time-resolved PEEM demonstrating the possibility of electronically generating new, non-equilibriummetallic phases of matter following ultrafast optical pulse excitation. Ferroelectric materials play a key role in low-power nanoscale electronic devices and are commonly used in everyday applications such as sensors and actuators. The unique properties of these materials arise from their non-centrosymmetric crystal structure which enables electric polarisation under stress and vice versa. In recent years, topological vortex structures have been discovered in ferroelectric PbTiO 3 /SrTiO 3 superlattices using high- resolution Transmission Electron Microscopy (TEM) which revealed picoscale swirling atomic displacements in thinned samples. On I10, these PbTiO 3 /SrTiO 3 superlattices were further investigated using polarised soft x-ray REXS to map the long-range distribution and structure of the vortices. The study explored the three-dimensional structure of the polar vortices by comparing circularly polarised light scattering with theoretical simulations developed at Diamond. The combined study allowed a detailed 3D picture of the chiral electric polarisation structures to be built for the first time. On I16, ferroelectric thin films of PbTiO 3 , sandwiched between SrRuO 3 , were investigated using the power of hard x-ray REXS. Reciprocal space maps recorded on I16 were combined with high-resolution TEM and theoretical calculations to demonstrate the presences of long-range vortex structures in the sandwiched PbTiO 3 thin films which were interpreted as arising from the electric analogue of the magnetic Dzyaloshinskii–Moriya interaction. Mott insulators have very little energy difference between different phases so that magnetic fields, optical pulses and temperature can be used to drive such materials across electronic and magnetic phase boundaries. On I06, ultrafast optical pulses have been used to launch a metal-to-insulator transition inV 2 O 3 with PEEM imaging showing that a newmetastable, metallic phase retains nanoscale monoclinic distortions usually associated with the insulating phase. The aim in the future is to combine ultrafast optical pulses with electric fields to achieve low-power, reversible control of the metal-to- insulator transition. Doped Mott insulators have been studied extensively to understand the pairing mechanism in high-T c superconducting materials such as the cuprates. Decades after the discovery of high-T c superconductivity, the pairing mechanism and the influence of charge-density wave order remains ill-understood. On I21, the parent compound of infinite-layer nickelate superconductors has been investigated using RIXS. Interestingly, the charge- density waves competing or cooperating with superconductivity in the cuprates is found to exist in the layered nickelates too. The work indicates that, despite the differences between cuprates and nickelates, similar underlying electronic states and layered structures are important for superconductivity. In the past year, the MMG has undergone an international review with several commendations regarding the capabilities of the team and the instruments. The review panel recommended that the MMG develop new sample environments, new sample management systems, wider data analysis workflows and that programmatic science themes across the beamlines should be explored and developed. In parallel, a range of recommendations were made to upgrade the beamlines as well as implementing new capabilities such as coherent diffraction imaging. In this sense, the MMG has been busy improving its suite of instruments. Beamline I21 now operates at a higher energy range (~ 3 keV) giving access to complex ferroelectric and magnetic ordering in 4 d transition metals. Recently, a new multilayer grating has been installed with a factor >30 increase in the detected photon flux during the first RIXS commissioning experiments performed at the Ru L 3 edge. Future plans involve a second multilayer coating to cover the S K-edge which will be pivotal to research into energy materials on I21. Beamline I06 has started user operations on the new laserPEEM facility with first experiments investigating local rotational orientations in 2D materials. The laserPEEM facility is equipped with a high intensity continuous wave laser allowing magnetic contrast imaging and also hosts surface preparation and characterisation facilities including Scanning Tunnelling Microscopy, Low Energy-Electron Microscopy (LEEM) and spatially resolved Low-Energy Electron Diffraction on the sub-micron scale. In the past year, the laserPEEM has been used to train the user community in the principles of PEEM and LEEM via one day workshops comprising lectures and hands-on practical sessions. On I16 a new low-vibration cryostat is being commissioned for REXS studies of micron sized samples and on I10 an in-plane, rotatable magnetic field for the RASOR diffractometer is being developed. The MMG has also established a Materials Characterisation Laboratory (MCL) to screen and align samples ahead of beamtime. The laboratory hosts a SuperNova Diffractometer, VSM SQUID, Atomic Force Microscope, Magnetic Force Microscope and sputter deposition facilities. The new Diamond-II Coherent Soft X-Ray Imaging and Diffraction (CSXID) flagship beamline has been the focus of considerable activity over the last year with the user working group reviewing the technical design report and plans to clear the area around straight I17 being progressed. In the coming year, the MMG will focus on harmonising the user experience across the beamlines and explore new strategies for sample management and metadata structures that further enable automated analysis. TheMagnetic Materials Group enables competitive, world-leading research in the UK. (Findings of the International Review of the Magnetic Materials Group) The Materials Characterisation Laboratory (MCL) hosts facilities for x-ray diffraction, imaging and magnetometry. For more information contact [email protected]. The new laserPEEM facility on I06 is available for sample testing, user training and peer reviewed proposals. For more information contact [email protected].