Diamond Annual Review 2020/21

56 57 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 0 / 2 1 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 0 / 2 1 MagneticMaterials Group Sarnjeet Dhesi, Science Group Leader T he Magnetic Materials Group (MMG) at Diamond Light Source uses and develops a range of polarised X-ray probes including Resonant Elastic X-ray scattering (REXS), PhotoEmission ElectronMicroscopy (PEEM), X-ray Absorption Spectroscopy (XAS) and Resonant Inelastic X-ray Scattering (RIXS). Over the last year, our research community has gained fundamental insights into the electronic andmagnetic degrees of freedomunderpinning the physical properties of ahost ofmaterials using these probes. In this contribution, we present research demonstrating how PEEM can unveil the complex antiferromagnetic domain structure in CuMnAs thin films following intense pulsed electrical switching. We also present results reporting how soft X-ray scattering combined with soft X-ray imaging can reveal the tubular structure of magnetic Skyrmions in FeGe thin films. The power of REXS combined with neutron scattering has been deployed to understand the driving force behind helical magnetic ordering in parity and time odd systems such as the Fe-based langasites. Finally, the versatility of high-resolution RIXS is showcased in a remarkable study of lithium-ion (Li-ion) battery cathodes, demonstrating how metal migration and molecular oxygen formation degrade the charge-discharge cycle. The results demonstrate how polarised X-rays can uncover a wealth of electronic and magnetic detail to aid the development of advanced materials in applications ranging from low-power consumption electronics to next-generation energy materials. On the Nanoscience beamline (I06), PEEM combined with X-ray Magnetic Linear Dichroism (XMLD) has been developed into a versatile and powerful probe of antiferromagnetic ordering in thin films and single crystals. The electrical switching of an antiferromagnet, via a spin-orbit torque, can be detected using anisotropic magnetoresistance and was first directly observed in CuMnAs using XMLD-PEEM imaging on I06. A new development is the extremely large changes in resistance in CuMnAs after intense current pulsing or irradiation using focused picosecond laser light that has been reported recently. Using XMLD-PEEM on I06, the origin of the large change in resistance has been linked to a very high degree of domain fragmentation leading to a very high density of domain walls. On I10, the Beamline for Advanced Dichroism Experiments (BLADE), REXS has been used to explore the phase diagramof the Skyrmion structures formed in FeGe thin films. The tubular structure of Skyrmions has been studied using X-ray Magnetic Circular Dichroism (XMCD) depth-profiling on I10 previously, but in a new twist, researchers have applied a field in the plane of the film and created horizontal Skyrmions in the plane of the film that could be imaged using X-ray Scanning Transmission Microscopy (STXM). This opens an exciting route to directly image the destruction and formation of Skyrmions using polarised soft X-ray imaging and holography. On the Materials and Magnetism beamline (I16), REXS and non-resonant magnetic scattering have been used to explore the relationship between the chiral crystal structure in an iron-based langasite and the helical magnetic ordering of the frustrated triangular antiferromagnetic lattice. By conducting a detailed analysis of the REXS scattering as a function of photon energy, the crystal handedness could be determined. On the other hand, an analysis of the X-ray scattering intensity of the magnetic reflections with analyser angle, for left and right polarised light, revealed that the magnetic chirality follows the crystal chirality. The results were independently confirmed on separate samples using neutron scattering. The results pave the way to explore electric field control of chiral magnetic domains by combining the newly developed X-ray probe of magnetic chirality with high spatial resolution. On the RIXS beamline (I21), the high-energy resolution of the spectrometer has been used to understand the charging and discharging cycle in Li-ion battery cathodes. The high-energy resolution was key to measuring the small energy spacing changes between sharp inelastic peaks, which indicate the formation of molecular oxygen during the redox reaction. The results were combined with nuclear magnetic resonance data to show that the molecular oxygen is physically trapped in the lattice rather than chemically bonded in any nanovoids. The cycling of molecular oxygen in the lattice then leads to irreversible structural changes in the cathode materials, which reduces the practical energy density for storage. Significant gains can be made for energy storage materials if the formation and trapping of molecular oxygen can be supressed. RIXS has also been used to study charge ordering Figure 1: RIXS dispersion map along h, for l=1.0. The orange squares show a non-dispersive bimagnon and the green circles show the dispersion of the acoustic plasmon. Taken from A. Nag et al. Phys. Rev. Lett. 125 , 257002 (2020). Our objective is to operate a suite of state-of-the-art polarised X-ray beamlines with leading edge data acquisition, data logging and data analysis software. in p- and n-type cuprates in considerable detail, but the study of collective excitations or plasmons is less well established. In this respect, acoustic plasmons have been observed in n-type cuprates, but their observation in p-type cuprates has remained elusive. The high-resolution of I21 has allowed the detection of acoustic phonons in p-type La 1.84 Sr 0.16 CuO 4 (LSCO) and Bi 2 Sr 1.6 La 0.4 CuO 6+δ (Bi2201) for the first time. Figure 1 shows RIXS maps with momentum transfer along h, for l=1.0 for LSCO. The dispersion and loss of amplitude towards the zone-centre is indicative of the presence of acoustic plasmon. The same behaviour was found for Bi2201 implying that acoustic plasmons are a general feature of cuprates. The MMG has continued to innovate and develop the capabilities of its X-ray research facilities. The new Aberration-Corrected PEEM on I06, capable of operating at a sample temperature of 20 K, is currently being installed and will be available in the summer of 2021. This latest upgrade to the suite of instruments of the MMG allows higher-resolution spatial imaging using polarised soft X-rays and will be equipped with sample preparation and sample storage facilities as well as a purpose-built electromagnet.The existing Spectroscopic PhotoEmission and Low Energy Electron Microscope (SPELEEM) that has been in operation on I06 since January 2007 will be moved to a new purpose-built laboratory and combined with a continuous wave laser funded by the Engineering and Physical Sciences Research Council (EPSRC). I21 has upgraded its monochromator optics so that the beamline now reaches ~2.8 keV, essential for work on the ruthenates and irridates. In the coming year, the MMG beamlines will implement a NeXus file structure, which will enable faster data acquisition as well as a more detailed metadata structure. The MMG is dedicated to continually improving its facilities and would welcome further input from our user community. We organise regular workshops to explore new scientific and technical opportunities together with our user community. Our objective is to operate a suite of state-of-the-art polarised X-ray beamlines with leading edge data acquisition, data logging and data analysis software.