Magnetic order in disordered NiCr(BO3)O

Abstract

A novel oxyborate, NiCr(BO₃)O, is synthesized using a flux method. The material crystallizes in an orthorhombic warwickite structure, space group Pbnm(62), with lattice parameters a = 9.3438(13) Å, b = 9.0908(13) Å, and c = 3.0507(4) Å. Although Ni and Cr atoms are highly disordered over two inequivalent metal sites, the compound undergoes a magnetic phase transition at T_N = 45 K, as characterized by maximum dc magnetization and ac susceptibility and a λ-peak in heat capacity. Upon further cooling, another magnetic anomaly occurs at about 10 K. At high temperatures, magnetic susceptibility follows the Curie-Weiss law, with a highly negative Weiss temperature (θ ≈ -130 K), indicating strong predominance of antiferromagnetic coupling. The effective magnetic moment (μ_eff) is ≈4.9μ_B per f.u. Field-induced spin-orientation transition is observed below T_N for an external field applied perpendicular to the c-axis. Magnetic heat capacity was determined by subtracting the lattice heat capacity of the nonmagnetic analog. Debye temperature is evaluated to be 365 K. NiCr(BO₃)O is the first example demonstrating magnetic ordering in the highly disordered oxyborate family.