Ferromagnetic ordering in a new nickel polyborate NiB₁₂O ₁₄(OH)₁₀

A new nickel polyborate, NiB₁₂O₁₄(OH)₁₀ was synthesized using boric acid as a flux. This material has two-dimensional borate layers with a quasi-square lattice of Ni²⁺. The Ni ²⁺ ions locate in the plane of the two-dimensional layer, bridged through B_nO_n+1 chains in the plane and connected with the three-membered ring borate groups out of the plane. The dc and ac magnetic susceptibility, magnetization and specific heat measurements show that this material undergoes a weak ferromagnetic phase transition at T_c = 5.8 K. At T < T_c, a metamagnetic phase transition is observed at about 5 T, associated by a spin-flop, suggesting the ferromagnetic ordering is induced by an antiferromagnetic interaction. A broad maximum in the ac susceptibility at T_M = 23-24 K indicates an intermediate short-range ordering.