Magnetic properties of a staggered $S=1$ chain with an alternating single-ion anisotropy direction

Materials composed of spin-1 antiferromagnetic (AFM) chains are known to adopt complex ground states which are sensitive to the single-ion-anisotropy (SIA) energy (D), and intrachain (J₀) and interchain (J'ᵢ) 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 Ni(pyrimidine)(H₂O)₂(NO₃)₂, a chain compound where the tilting of Ni octahedra leads to a 2-fold alternation of the easy-axis directions along the chain. Muon-spin relaxation measurements indicate a transition to long-range order at TN=2.3\,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 J₀ = 5.107(7)\,K, $D = 2.79(1)$\,K, J'₂=0.18(3)\,K and a rhombic anisotropy energy $E=0.19(9)$\,K. Mean-field modelling reveals that the ground state structure hosts spin canting of φ≈6.5∘, which is not detectable above the noise floor of the elastic neutron diffraction data. Monte-Carlo simulation of the powder-averaged magnetization, $M(H)$, is then used to confirm these Hamiltonian parameters, while single-crystal $M(H)$ simulations provide insight into features observed in the data.