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Materials composed of spin-1 antiferromagnetic (AFM) chains are known to adopt complex ground states that are sensitive to the single-ion-anisotropy (SIA) energy ( $D$ ), and intrachain ( $J_0$ ) and interchain ( $J_{1,2}^{\prime }$ ) exchange energy scales. While theoretical and experimental studies have extended this model to include various other energy scales, the effect of the lack of a common SIA axis is not well explored. Here we investigate the magnetic properties of $\mathrm{Ni}\left(\mathrm{pyrimidine}\right){\left({\mathrm{H}}_2\mathrm{O}\right)}_2{\left({\mathrm{NO}}_3\right)}_2$ , a chain compound where the tilting of Ni octahedra leads to a twofold alternation of the easy-axis directions along the chain. Muon-spin relaxation measurements indicate a transition to long-range order at $T_{\text{N}}=2.3\mathrm{K}$ and the magnetic structure is initially determined to be antiferromagnetic and collinear using elastic neutron diffraction experiments. Inelastic neutron scattering measurements were used to find $J_0=5.107\left(7\right)\mathrm{K}, D=2.79\left(1\right)\mathrm{K},J_1^{\prime }=0.00\left(5\right)\mathrm{K}, J_2^{\prime }=0.18\left(3\right)\mathrm{K}$ , and a rhombic anisotropy energy $E=0.19\left(9\right)\mathrm{K}$ . Mean-field modeling reveals that the ground state structure hosts spin canting of $\varphi \approx 6.5^{\circ }$ , which is not detectable above the noise floor of the elastic neutron diffraction data. Monte Carlo simulation of the powder-averaged magnetization, $M\left(H\right)$ , is then used to confirm these Hamiltonian parameters, while single-crystal $M\left(H\right)$ simulations provide insight into features observed in the data. Published by the American Physical Society2025 

We investigate the magnetic properties of $S=1$ antiferromagnetic diamond-lattice, $\mathrm{Ni}{X}_2{\left(\mathrm{pyrimidine}\right)}_2$ $(X=\mathrm{Cl}, \mathrm{Br})$ , hosting a single-ion anisotropy (SIA) orientation which alternates between neighboring sites. Through neutron diffraction measurements of the $X=\mathrm{Cl}$ compound, the ordered state spins are found to align collinearly along a pseudo-easy axis, a unique direction created by the intersection of two easy planes. Similarities in the magnetization, exhibiting spin-flop transitions, and the magnetic susceptibility in the two compounds imply that the same magnetic structure and a pseudo-easy axis is also present for $X=\mathrm{Br}$ . We estimate the Hamiltonian parameters by combining analytical calculations and Monte Carlo (MC) simulations of the spin-flop and saturation field. The MC simulations also reveal that the spin-flop transition occurs when the applied field is parallel to the pseudo-easy axis. Contrary to conventional easy-axis systems, there exist field directions perpendicular to the pseudo-easy axis for which the magnetic saturation is approached asymptotically and no symmetry-breaking phase transition is observed at finite fields. Published by the American Physical Society2024