Neutron diffraction studies under zero and finite magnetic fields of the 1/2 quantum magnetization plateau compound Ni₂V₂O₇

We performed powder neutron diffraction experiments on a 1/2 quantum magnetization plateau compound Ni2V2O7 for magnetic fields in the range 0-10 T. The incommensurate magnetic structures with the propagation vector (k(x) similar to 0.45, 0, 0) were determined at 0 T and at 6.0 K (between T-N1 = 6.7 K and T-N2 = 5.7 K) and 2.3 K. Consequently, the Ni1 and Ni2 moments were disordered and ordered at 6.0 K, respectively, indicating a partially disordered state. The magnetic structure of Ni2 moments was a spin density wave structure with moments ordered parallel to the b axis and perpendicular to the propagation vector. Both Ni1 and Ni2 moments exhibited ellipsoidal cycloidal incommensurate magnetic structures at 2.3 K. These moments lay in the ab plane. The maximum magnitudes of Ni1 and Ni2 moments [1.62(16) and 2.50(6) mu(B)] at 2.3 K were sufficiently smaller and slightly larger than the value (similar to 2.2 mu(B)) expected from the classical picture. Further, we evaluated the field-induced magnetic moments as follows: m(1) = 0.3 and m(2) = 1.9 mu(B) at Ni1 and Ni2 sites, respectively, at 10 T and 1.9 K, where the 1/2 quantum magnetization plateau was observed. The results indicating both the ordered moment at 0 T and the field-induced moment at 10 T being small and large at Ni1 and Ni2 sites, respectively, were consistent with results expected in the dimer+monomer model that was proposed by J. J. Cao et al. [Phys. Rev. B 106, 184409 (2022)].