Development of a Bespoke Hybrid Electron Paramagnetic Resonance (EPR) Spectrometer for Simultaneous In Situ Neutron Diffraction Studies

We present the design, construction, and commissioning of the first hybrid Electron Paramagnetic Resonance (EPR) spectrometer engineered to operate within a Neutron Powder Diffractometer (NPD), enabling simultaneous measurements of spin properties and crystal structure. Developed for the Polaris instrument at the ISIS Neutron and Muon Source (U.K.), the system was specifically designed to function within the constraints of a neutron beamline. Achieving in situ EPR capability within the diffractometer required several necessary compromises to the diffraction geometry. The finite gap between the two halves of the electromagnet body reduces detector visibility, limiting coverage for a proportion of scattering angles. The retained magnetic return-path introduce restricted regions in 2<inline-formula><tex-math notation="LaTeX">$\theta$</tex-math></inline-formula> that require reorientation of the instrument to access. Furthermore, the change in sample geometry from a conventional vertical cylindrical configuration to a skewed horizontal cylinder had a detrimental effect on diffraction peak shape and width. The aluminium microwave cavity, although thinned to 0.1 mm to maximise neutron transparency, introduces additional background Bragg reflections. Despite these trade-offs, simultaneous EPR and neutron diffraction data were successfully acquired. Key innovations include a bespoke electromagnet compatible with beamline access, a thinwalled X-band quasi-elliptical microwave cavity, a remotely tuneable waveguide coupling scheme, and a modular support structure optimised to minimise parasitic scattering. The system was benchmarked using standard paramagnetic reference samples, demonstrating EPR sensitivity comparable to commercial laboratory instruments. This work establishes a new class of multimodal instrumentation, enabling operando investigations of materials in which coupled spinlattice phenomena govern functionality, including batteries, catalysts, and quantum materials.