Possible Noncollinear Magnetism and Different Valence State of Manganese Ions in Quasi-Low-Dimensional Ludwigites of (Ni,Cu,Mn)(3)BO5

The content of transition-metal ions and their valence state for three compositions of NixCuyMn3-x-yBO5 are estimated by the methods of X-ray absorption spectroscopy. In all the three compositions, the copper content does not exceed 0.25. One of the compositions has manganese ions both in the trivalent and bivalent states according to the K-edge position. The calculations of the total energies of various cation-ordered structures using the software package Wien2K show that copper ions predominately occupy one site, namely, 2(2d). The measurements of the magnetic properties show two types of magnetic transitions in all the three compositions, with one of them being in the range of 60-75 K and the other one at 10 K. Based on the exchange interactions calculated in the framework of the empirical model, magnetic ordering temperatures are estimated for two- and three-sublattice structures and the magnetic transition in the range of 60-75 K is supposed to be connected with magnetic moment ordering in sites 2 and 3.