Magnetic ground state and spin excitations in the two-dimensional trimerized collinear-II lattice antiferromagnet Li₂Ni₃P₄O₁₄

We report the magnetic ground state, spin excitations, and spin Hamiltonian of the two-dimensional (2D) spin-1 trimerized Heisenberg antiferromagnet (AFM) Li₂Ni₃P₄O₁₄. Below the magnetic ordering temperature T_N = 14.5 K, the compound exhibits a canted long-range antiferromagnetic order with a propagation vector k = (0 0 0), consistent with the magnetic space group P2₁/c.1 (No. 14.75). The ground-state magnetic structure consists of ferromagnetic (FM) spin trimers of Ni²⁺ ions. The spin trimers are coupled antiferromagnetically along the c axis and ferromagnetically along the a axis. Inelastic neutron scattering (INS) reveals gapped and dispersive magnon excitations below the T_N, and gapless quasielastic scatterings at higher temperature. The linear spin-wave theory simulations reveal the essential features of the excitation spectrum by a spin Hamiltonian composed of ferromagnetic intratrimer exchange interaction J₁ and intertrimer exchange interactions J₂ (FM) and J₃(AFM) within the bc plane. The J₂ and J₃ are along the b axis and c axis, respectively, with strengths of J₂/J₁ = 0.79 and J₃/J₁ ∼ = −0.91. In addition, a weak interplanar ferromagnetic exchange interaction J₄ (|J₄/J₁| ∼ 0.12 is found along the a axis. The determined exchange constants reveal a 2D trimerized collinear-II spin lattice within the bc plane. The analysis of INS spectra by linear spin-wave theory also yields a moderate single-ion anisotropy (D/J₁ = 0.48), which accounts for the observed spin gap below T_N as well as the metamagnetic transition near 44 kOe in dc magnetization (M vs H) curves. These findings identify Li₂Ni₃P₄O₁₄ as a rare realization of a two-dimensional trimerized spin system and offer the direct experimental confirmation of theoretically predicted magnon excitations, unveiling the fundamental characteristics of the expected excitation spectrum.