A neutron diffraction study of the magnetic structure and phase transition in Ni₃¹¹B₇O₁₃Cl boracite

The nuclear and magnetic structures and the magnetic phase transition of Ni₃¹¹B₇O₁₃Cl [Ni-Cl] boracite have been investigated by means of high resolution and high flux neutron powder diffraction between 1.5 and 30 K The temperature dependence of the magnetic intensities shows an unsharp onset at T_N = 9 K, indicating the existence of a second-order magnetic phase transition- Refinements of the magnetic structure give rise to the magnetic space group Pc'a'2₁' for the magnetic phase below T_V, with the magnetic modes C_x, F_l and A_z for the three Ni sublattices. The three Ni sites display two kinds of magnetic moment with different values and orientations: Ni(1) has a moment parallel to the main antiferromagnetic axis along [100]_or, while Ni(2) and Ni(3) have their antiparallel moments along [010]_or-directions. The total corresponds to a coplanar two-dimensionally canted antiferromagnetic arrangement, which is characteristic for the magnetic structure of Ni-Cl. The refined magnetic moment values at 1.5 K are: μ_1x = 1.65(2) μ_B for Ni(1), and μ_2y = -μ_3v for Ni(2) =0.79(1) for Ni(2) and Ni(3), respectively, showing a weak magnetic superexchange interaction compared with the spin only moment 2μ_BNi²⁺ observable by neutrons. The magnetic ordering process and the possible frustration effects due to the interwoven configurations between the three Ni sites and the formation of isolated Ni₃Cl groups, are discussed based on the proposed magnetic structure.