We report the magnetic ground state, spin excitations, and spin Hamiltonian of the two-dimensional (2D) spin-1 trimerized Heisenberg antiferromagnet (AFM) $\mathrm{L}{\mathrm{i}}_2\mathrm{N}{\mathrm{i}}_3{\mathrm{P}}_4{\mathrm{O}}_{14}$. Below the magnetic ordering temperature $T_{\mathrm{N}}=14.5$ K, the compound exhibits a canted long-range antiferromagnetic order with a propagation vector $k$  = (0 0 0), consistent with the magnetic space group $P{2}_1/c.1$ (No. 14.75). The ground-state magnetic structure consists of ferromagnetic (FM) spin trimers of $\mathrm{N}{\mathrm{i}}^{2+}$ 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 $T_{\mathrm{N}}$, 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 $J_1$ and intertrimer exchange interactions $J_2$ (FM) and $J_3$(AFM) within the $bc$ plane. The $J_2$ and $J_3$ are along the $b$ axis and $c$ axis, respectively, with strengths of $J_2/J_1=0.79$ and $J_3/J_1\sim =-0.91$. In addition, a weak interplanar ferromagnetic exchange interaction $J_4 \left(\right|J_4/J_1|\sim 0.12$ 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 ($D/J_1=0.48$), which accounts for the observed spin gap below $T_{\mathrm{N}}$ as well as the metamagnetic transition near 44 kOe in dc magnetization ($M$ vs $H$) curves. These findings identify $\mathrm{L}{\mathrm{i}}_2\mathrm{N}{\mathrm{i}}_3{\mathrm{P}}_4{\mathrm{O}}_{14}$ 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.

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• Magnetic order
• Magnetic phase transitions
• Magnetism
• Spin dynamics
• Antiferromagnets
• Crystal structures
• Magnetization measurements
• Neutron diffraction
• Neutron scattering
• Spin wave theory