Magnetic structure, excitations, and field-induced transitions in the honeycomb lattice compound Er₂Si₂O₇

Despite their promising optical properties, little is known of the polymorphic rare-earth disilicates (RE₂Si₂O₇) due to their complex structural phase diagram. For the monoclinic D-type Er₂Si₂O₇, in which the magnetic ions are arranged in a distorted honeycomb structure, initial predictions of a magnetic order below 1.8 K with a proposed q = 0 AFM magnetic structure were confirmed via powder neutron diffraction. The highly anisotropic magnetisation curves were understood by the use of simple Monte-Carlo simulations however, the presence of a magnetic feature at 1/3 disagreed with the theoretical model. Here, both polycrystalline and single crystals of D-type Er₂Si₂O₇ have been synthesised and investigated using neutron scattering techniques. Our H‖a magnetisation measurements reveal a highly temperature-sensitive plateau at 1/3 of magnetic saturation, accompanied by a significant increase of the magnetic unit cell. This manifests in a dramatic change in the single crystal neutron diffraction patterns when the narrow plateau stabilises. Magnetic diffuse features along the k and l directions suggest shorter-range magnetic correlations on the plateau along b* and c*. We also report that our magnetic excitations in the ordered state, observed via inelastic neutron scattering, suggest an almost ideal Ising system away from the plateau. Via a culmination of field and temperature-based studies of the magnetic transitions, we have constructed the intricate magnetic phase diagram of D-type Er₂Si₂O₇.