Magneto-Electric Coupling in van der Waals Multiferoic CuCrP₂S₆ Explored via Single Crystal Neutron Diffraction

The van der Waals (vdW) multiferroic CuCrP₂S₆ exhibits strong magneto-electric coupling, expected to persist down to or near the two-dimensional (2D) limit. An electrical polarization sets in at 150K with the displacement of Cr³⁺ octahedra along the c-axis. An A-type antiferromagnetic (AFM) order sets in with a Néel temperature of T_N = 34K. Previous work with magnetic succeptibility and polarization measurements have indicated that in-plane magnetic fields (H) can modulate the polarization, causing a spin-flop transition at 0.3T that enters a forced ferromagnetic state upon saturation at 6.1T. We present single crystal neutron diffraction with and without applied field from WAND² and HB3A at the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory (ORNL) with which we monitor directly the effects of the applied field on the magnetic moment, showing the suppression of the AFM order as a function of H on magnetic Bragg reflections as well as an enhancement on nuclear peaks as it enters the forced ferromagnetic state. We also present results from our non-field neutron diffraction on single crystals of a size and quality unprecedented in literature for this material allow for an exploration of the magnetic symmetry in terms of both the magnetic space group and irreducible representations. Such measurements for this and many related materials have proven prohibitively difficult in the past due to the vdW c-axis coupling being very prone to stacking faults, limiting previous neutron diffraction to polycrystalline studies where 3D information is lost.