Magnetic transitions and different valence states of manganese ions in solid solutions (Ni,Cu,Mn)$_3$BO$_5$

The content of metal ions and their valence state for three Ni$_x$Cu$_y$Mn$_{3-x-y}$BO$_5$ compositions were estimated by EXAFS and XANES spectroscopy methods. In all three compositions, the copper y content did not exceed 0.25; in the third composition, some manganese ions entered the compound not only in the trivalent, but also in the divalent state. A detailed study of the magnetic properties showed that two magnetic transitions are observed in all compositions: one in the region of 60–75 K, and the second in the region of 10 K. Based on the exchange interactions calculated within the framework of the empirical model, the magnetic ordering temperatures for two- and three-lattice magnets were determined and it was assumed that the magnetic transition in the region of 60–75 K is associated with the ordering of magnetic moments in positions 2 and 3.