Co5/3Nb1/3BO4: a new cobalt oxyborate with a complex magnetic structure

Abstract Needle-shape single crystals of Co5/3Nb1/3BO4 warwickite were grown using the flux technique. X-ray diffraction measurements have revealed an orthorhombic structure (Sp. Gr. Pbnm) where the octahedral M1 site is occupied by a mixture of Co2+/Nb5+ ions and the M2 site is exclusively filled by Co2+ ions. Using dc magnetization measurements it was established that the new material undergoes two magnetic transitions: an antiferromagnetic transition at TN1=27 K and a ferrimagnetic one at TN2=14 K, below which a hysteresis cycle opens. Both magnetic transitions are marked by anomalies in the specific heat. High magnetic anisotropy with c-axis as a hard magnetization direction was detected.