Diamond Annual Review 2019/20

38 39 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 1 9 / 2 0 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 1 9 / 2 0 MagneticMaterials Group Sarnjeet Dhesi, Science Group Leader T he Magnetic Materials Group comprises scientists, engineers and technicians, working across beamlines I06, I10, I16 and I21, developing and exploiting a diverse range of sensitive polarised X-ray probes including resonant X-ray scattering, PhotoEmission Electron Microscopy (PEEM), X-ray Absorption Spectroscopy (XAS) and Resonant Inelastic X-ray Scattering (RIXS). Over the last year, our research community has investigated a wide range of new materials to gain fundamental insights into the electronic and magnetic degrees of freedom underpinning their physical properties. In this contribution, we present research from the beamlines demonstrating how X-ray Magnetic Circular Dichroism (XMCD) can isolate surface magnetism in topological insulators and how nanoscale X-ray magnetic imaging can reveal localised shear strain effects that remain hidden in macroscopic measurements. We also present the first results from a new nickelate superconducting material demonstrating hybridisation between two-dimensional (2D) and three-dimensional (3D) electronic states and X-ray scattering work that hints at covalency in uranium sesquinitride. 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 nuclear fuels. Nickelates have long held the promise of superconductivity, so that its recent discovery in an in nite-layer nickelate has created considerable interest. There are a number of similarities between the nickelates and other 2D transition-metal oxide superconductors, such as the cuprates for which superconductivitywasfirstobserved inthe1980s.However,thereareanumber of differences that are expected to lead to a distinctly different mechanism for superconductivity in the nickelates. Firstly, the O to Ni charge transfer energy in nickelates is ~3 times that of the O to Cu charge transfer energy in the cuprates which leads to a much weaker hybridisation between the O and Ni electronic states. The large charge transfer energy also implies that the Ni to Ni super-exchange energy, mediated by the O sites, is lower in the nickelates. Thesetwoeffectsthenhave immediate consequences for the electronic and magnetic structure of the infinite-layer nickelate. On the Inelastic X-ray Scattering beamline (I21), Hepting and co-workers have used XAS and RIXS to build a detailed picture of the underlying electronic structure at the O and Figure 1 (a) Experimental and (b) calculated low energy-transfer RIXS maps showing the spin-flip excitations. The solid white lines represent the NiO XAS spectrum. The main peak (MP) and satellite peak (SP), at which the RIXS spectra show the single and double magnon structure with a maximum intensity, are indicated by the broken lines 1 . Ni sites of an infinite-layer nickelate thin film. The results demonstrate the similarity of Ni valence to the Cu sites in cuprates case, but with negligible hybridisation between the oxygen 2 p states and the Ni 3 d states and much stronger hybridisation between the Nd 5 d states and the Ni 3 d states leading to electron pockets at specific points in the 3D Brillouin zone and a self-doping effect. This first study was performed in the normal state of the material and future studies will have to address how superconductivity emerges by introducing doped charge carriers via Ni-Nd hybridisation. The high-efficiency collection geometry and spectral resolution on I21 have also enabled many- body spin-flip excitations in anti-ferromagnet NiO to be explored for the first time. Double spin-flip excitations have been known to be present in NiO for some time, but the distinction between double magnons arisings from two excitations on two single sites or on the same site has proved difficult to determine. On I21, RIXS at the Ni L 3 edge combined with theoretical calculations has now shown that the double-magnon excitations arise mainly from spin-flip excitations on the same magnetic site (Fig. 1). Uranium dioxide is commonly used in nuclear reactor rods, but uranium mononitride could be the material of choice due to its higher thermal conductivity and strength. Unfortunately, U 2 N 3 is known to form on the surface of UN and its solubility in water hinders safe disposal of any spent fuel. On the Materials and Magnetism beamline (I16), hints on the formation of U 2 N 3 have been gained by understanding its magnetic properties using resonant X-ray scattering. In general, diffraction peaks from U 2 N 3 contain contributions from two different U sites with different symmetries, but by a judicious choice of reflections and experimental geometry one of the sites could be proposed to have no 5 f contribution implying the presence of a U(VI) valent ion. The high valence at one of the U sites can then be directly related to the rapid rate of corrosion in U 2 N 3 and its solubility in water. Topological insulators are well known for an insulating bulk phase with topologically protected surface states that resist any perturbation by disorder. Introducingmagnetic impurities in the surface region can result in the opening of a tuneable band gap, but little is known about the change in the properties of magnetic topological insulators close to the surface. On BLADE: X-ray Dichroism and Scattering beamline (I10), XMCD has been used to measure the magnetic properties of Cr doped Bi 2 Se 3 sheets inserted at different depths in a thin film. The results show that the surface maintains an ordered magnetic phase, even though the bulk becomes non-magnetic, with a Curie temperature ~15K higher than the bulk. The magnetoelectric effect was predicted and discovered over half a century ago, but only recently have composite materials with large effects been realised. The control of the local magnetisation vector in ferromagnetic thin films grown on ferroelectric substrates has therefore been an area of intense study, but recently PEEM studies of Ni films grown on PMN-PT has unveiled a new twist to the story. Through a nanoscale analysis of the changes in the local magnetisation vector of the Ni film, after applying a strain via the PMN-PT, a picture emerges in which both the normal and shear components of the strain have to be taken into account to understand the ~62° rotation of the magnetisation vector. The discovery then opens up the possibility of independently storing non-volatile binary information using up to six possible states. The Magnetic Materials Group has continued to innovate and develop the capabilities of its X-ray research facilities. The new 1.6 T electromagnet, operating at a sample temperature less than 8 K, is available for the user community on I10 with proposals being accepted via standard and rapid access modes. This latest addition to the suite of instruments of the group is specifically designed for rapid characterisation of samples using XMCD as well as X-ray magnetic linear dichroism. I16 has added further polarisation control of the incoming X-rays, using a double-bounce phase retarder system, and also implemented a versatile polarisation analysis system for the scattered X-rays. I21 has had an upgrade of the monochromator optics to maintain a world- leading throughput and energy resolution and is currently implementing polarisation analysis. In the coming year, the Nanoscience beamline (I06) will have a major upgrade of the PEEM facility to an aberration-corrected PEEM with additional in situ sample storage and sample magnetisation facilities. The Magnetic Materials Group 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. 1. Nag A. et al. Many-Body Physics of Single and Double Spin-Flip Excitations in NiO. Phys. Rev. Lett. 124 , 067202 (2020). DOI: 10.1103/ PhysRevLett.124.067202