Co_(5/3)Nb_(1/3)BO_4: A new cobalt oxyborate with a complex magnetic structure

摘要

Needle-shape single crystals of Co_(5/3)Nb_(1/3)BO_4 warwickite were grown using the flux technique. X-ray diffraction measurements have revealed an orthorhombic structure (Sp. Gr. Pbnm) where the octahedral Ml site is occupied by a mixture of Co~(2+)/Nb~(5+) ions and the M2 site is exclusively filled by Co~(2+) ions. Using dc magnetization measurements it was established that the new material undergoes two magnetic transitions: an antiferromagnetic transition at T_(N1) = 27 K and a ferrimagnetic one at T_(N2) = 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.