Anti-Site Mixing and Lattice Topology as Main Factors Governing the Magnetic State in NiCr(BO3)O

Large-size single crystals of NiCr(BO3)O were grown using a flux method. The crystal structure refinement revealed significant anti-site mixing that yields the structural formula [Ni0.635Cr0.365]M1[Ni0.365Cr0.635]M2(BO3)O. The magnetic structure was studied experimentally through the orientation-dependent DC magnetization and AC susceptibility measurements and theoretically using the group-theoretical analysis and DFT + U calculations. Total energy calculations of symmetry-allowed magnetic structures for a hypothetical half-occupancy crystal [Ni0.5Cr0.5]M1[Ni0.5Cr0.5]M2(BO3)O revealed a dense spectrum of antiferromagnetic orderings within a narrow 30 meV range, with a tiny splitting of ΔE ∼ 1.0 meV/f.u between low-energy ab- and c-magnetic orderings. The evaluated intrachain, intra-ribbon, and inter-ribbon superexchange interactions are comparable in magnitude (|J| ∼10.0 K). This competition among exchange interactions effectively shifts the spin system from the structurally imposed quasi-1D regime to a highly frustrated 3D one. For the real crystal, it is proposed that the antiferromagnetic transition at TN = 45 K involves only a small fraction of the total spin ensemble characterized by a lower degree of local anti-site mixing. The remaining spins continue to fluctuate upon cooling to 10 K, where they continuously release their residual entropy through the formation of a short-range order.