Resolving the magnetic ground state of the 5d1 W5+ system, WOPO4

Recent studies on 5d1 systems highlight their potential for hosting unconventional magnetic phases due to strong spin-orbit coupling and multipolar interactions. One such 5d1 system, WOPO4, crystallizes in the monoclinic P21/m space group with edge-sharing WO6 octahedra forming chains interconnected via PO₄ tetrahedra. Through specific heat, magnetic susceptibility, magnetization, resistivity, and synchrotron single-crystal X-ray diffraction measurements, we find magnetic transitions at 21 K and 16 K, a broad maximum at 40 K, a reduced magnetic moment, and a charge density wave (CDW) at 275 K with q = (0, 1/4, 0). These features suggest a complex ground state shaped by intertwined spin, charge, and lattice degrees of freedom.

To explore this complexity, we propose an inelastic neutron scattering experiment on MERLIN to probe the magnetic ground state in WOPO₄ and to determine the spin dynamics across the ordered phases. This will require measurements at 5, 20, 30, and 200 K to capture the temperature evolution of dynamical structure factor. The data will be modeled using linear spin wave theory to elucidate the magnetic Hamiltonian. This experiment forms part of a larger study involving neutron diffraction, resonant elastic X-ray scattering, and RIXS, aiming to comprehensively understand the magnetic ground state, the role of multipolar interactions, and the validity of the jeff = 3/2 state in this material.