Diamond Annual Review 2023/24

MagneticMaterials Group Sarnjeet Dhesi, Science Group Leader T he Magnetic Materials Group (MMG) operates several polarised X-ray beamlines to develop new and novel quantummaterials. Our state-of- the-art beamlines cover probes as diverse as Resonant Elastic/Inelastic X-ray Scattering (REXS/RIXS), X-ray Magnetic Circular/Linear Dichroism (XMCD/XMLD) and PhotoEmission Electron Microscopy (PEEM). Together with the beamline teams, our research community has used the polarised X-ray probes to explore a wide range of new phenomena. In this contribution, we present research demonstrating how REXS on I10 and I16 can reveal now skyrmion monopoles unfold into skyrmion strings and how axion insulators can host more than one chiral magnetic structure. RIXS, combined with circularly polarised X-rays on I21, has been used to detect chiral phonons in quartz and determine the sense of chirality away fromthe zone centre. On I06, PEEMhas been used to image arrays of magnetic nanorings to understand how their complex emergent dynamics can be used for reservoir computing. Hysteretic behaviour and nonlinear dynamics in functional magnetic metamaterials provide an extensive state space with which to solve complex computational problems. On I06, PEEM has been used to image the domain wall configuration of permalloy nanoring arrays after moderate magnetic field cycling. Whilst the behaviour of each nanoring is stochastic, the interaction between all the nanorings gives rise to a repeatable global magnetic domain wall configuration whose state can be read using anisotropic magnetoresistance. These measurements can then be used to develop reservoir architectures that rely on the dynamic response of the nanoring arrays to specific input signals. Magnetic ordering in skyrmions is attributed to spontaneous symmetry breaking and evolves into two or three-dimensional networks from monopole point defects. On I10, REXS has been used to track the reversible transformation of monopole arrays in Cu 2 OSeO 3 single crystals before and after being covered by a ferromagnetic thin film. The results show that temperature cycling can be used to reversibly extend and shorten string-like skyrmion structures to monopole point defects. On I16, an unusual helimagnetic ordering has been discovered in the topological insulator, EuIn 2 As 2 . Using REXS, a unform phase with a helical magnetic structure was detected which ruled out a possible phase-separation scenario. Furthermore, temperature cycling can transform broken helix magnetic structures to pure helix structures and implies that the material satisfies the symmetry requirements of an axion insulator. Chiral phenomena can lead to magnetic ordering in non-magnetic materials such as quartz. On I21, circularly polarised X-rays couple to chiral phonons at specific points in reciprocal space which then allows a determination of the chiral dispersion of the lattice modes. In principle, this then leads to phononmagnetic moments that could create a net magnetisation in the material. In the past year, the MMG has focussed on developing a road map to implement the recommendations of the international review of the group in October 2022. A data analysis scientist is now in position to develop real-time data visualisation and evaluation tools, upgrades to sample environments are underway, continuous scanning on the diffractometers is being addressed and the group is working on rebranding its name to more closely align with the work carried out on the beamlines. The MMG has also been busy improving its suite of instruments. Beamline I21 has installed a polarisation analysis capability which significantly adds to the analysis and assignment of RIXS spectral features. Beamline I06 continues to operate the new laserPEEM facility with the high intensity continuous-wave laser producing first images following the installation of an in-house custom designed laser interlock system. The laserPEEM has also been used to train the user community in the principles of PEEM via one day workshops comprising lectures and hands-on practical sessions. On I16, the team has undergone significant changes and the team is now charged with developing its core activities, with the user community, following the recommendations of the international review of the MMG. On I10, a Halbach array is being developed for the RASOR diffractometer that will provide variable magnetic fields for REXS studies. The Materials Characterisation Laboratory (MCL) has had a very 32 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 3 / 2 4 Top L-R: Thomas Rice, Colin Brodie, Bottom L-R: Akhil Deve and Mark Sussmuth.