Emergence of Spiral Spin Liquid on Corrugated Honeycomb Lattice

CaMn₂P₂ can be understood in terms of a frustrated J₁-J₂-J₃ Heisenberg model on a corrugated honeycomb magnetic lattice. Previous studies show that below the Neel temperature (T_N), the system forms a cycloidal 6×6 magnetic unit cell that conforms with various magnetic space groups. Here, we present single-crystal neutron-diffraction studies in large reciprocal space volumes to confirm the cycloidal magnetic structure and unravel additional characteristics. We find evidence of three magnetic domains. The analysis also narrows the possible magnetic space groups. At T_N, the insulator exhibits a strong first-order phase transition. Above T_N, the spin structure transforms into a spiral spin liquid phase manifested by a continuous ring of scattering with degenerate wavevectors corresponding to a collection of short-range spiral spin configurations. These degenerate states emerge as thermal fluctuations reduce the effective J₃ interaction.