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 that are sensitive to the single-ion-anisotropy (SIA) energy ( D ), and intrachain ( {J}₀ ) and interchain ( {J}_1,2^′ ) 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){({H}₂O)}₂{({NO}₃)}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}_N=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 {J}₀=5.107(7) K,D=2.79(1) K, {J}₁^′=0.00(5)K,{J}₂^′=0.18(3) K , and a rhombic anisotropy energy E=0.19(9) K . Mean-field modeling 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.