Magnetism and specific heat of ludwigites Mn1.17Co1.83BO5 and Mn1.39Co1.61BO5

Mn1.17Co1.83BO5 and Mn1.39Co1.61BO5 ludwigites were synthesized by the flux technique and investigated by means of X-ray diffraction, X-ray fluorescence, DC and AC magnetic susceptibility, and specific heat analysis. The crystal structure of both ludwigites belongs to the Pbam space group with a = 9.25 angstrom, b = 12.41 angstrom, and c = 3.05 angstrom for Mn1.17Co1.83BO5 and a = 9.27 angstrom, b = 12.45 angstrom, and c = 3.05 angstrom for Mn1.39Co1.61BO5. The simultaneously observed negative values of the Curie-Weiss temperatures and ferromagnetic-type hysteresis loops allow us to assume that the ferrimagnetic ordering is realized in Mn1.39Co1.61BO5 below TF = 60.8 K, while in Mn1.17Co1.83BO5 in addition to the above mentioned experimental facts the frequency dependencies of the real and imaginary parts of the AC magnetization were observed assuming the presence of the canonical spin-glass state below TSG = 44.5 K. The observed difference in coercive forces of M-H curves at low temperatures can be associated with presence of two spin subsystems for Mn1.17Co1.83BO5.