Magnetic order in disordered NiCr(BO ₃ )O

A novel oxyborate, NiCr(BO₃)O, is synthesized using a flux method. The material crystallizes in an orthorhombic warwickite structure, space group <i>Pbnm</i>(62), with lattice parameters <i>a</i> = 9.3438(13) Å, <i>b</i> = 9.0908(13) Å, and <i>c</i> = 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 <i>T</i>_N = 45 K, as characterized by maximum <i>dc</i> magnetization and <i>ac</i> 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 (<i>θ</i> ≈ -130 K), indicating strong predominance of antiferromagnetic coupling. The effective magnetic moment (<i>μ</i>_eff) is ≈4.9<i>μ</i>_B per f.u. Field-induced spin-orientation transition is observed below <i>T</i>_N for an external field applied perpendicular to the <i>c</i>-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.