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 TN = 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 (μeff) is ≈4.9μB per f.u. Field-induced spin-orientation transition is observed below TN 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.