Concentration transition in Cu2(Mn,Ga)BO5 solid solutions

The effects related to the conversion of magnetic structures of a Cu2MnBO5 ferrimagnet to a Cu2GaBO5 antiferromagnet under the Mn3+ & RARR; Ga3+ substitution were studied. The properties of four single crystal samples with the gallium concentration x = 0.04, 0.11, 0.17, 0.25 were examined. The chemical composition and valence state of the samples were determined by X-ray absorption spectroscopy element-sensitive technique. Distribution of the 3d metal ions over crystallographic positions was studied using the first principle calculations by Wien2k program. The evolution of the magnetic properties was investigated by the measurements of the temperature and field dependences of magnetization, dc and ac magnetic susceptibility. In agreement with the previous study of the structure evolution of Cu2Mn1-xGaxBO5 solid solutions, the critical concentration upon a change in the magnetic ordering type was found in the range x = 0.17-0.25. The non-monotonic behavior of magnetization in terms of the gallium content was found in the concentration range of 0.04-0.17. A complex pattern of magnetic phase transitions belonging to the concentration phase boundary was obtained in the compounds with gallium being within the range x = 0.17-0.25. One could observe the splitting of the peaks of the real part of ac-susceptibility of the high-temperature phase transition in the Cu2MnBO5-like phase (x = 0.04, 0.11) and that of the low-temperature one at the concentration boundary (x = 0.17, 0.25) with different frequency and orientational dependence. The type and origin of the multiple phase transitions observed in the studied samples were analyzed.