Investigation of magnetic properties and electronic structure of layered-structure borides AlT2B2 (T=Fe, Mn, Cr) and AlFe2-xMnxB2

The ternary phases AlT2B2 Mn, Cr) and quaternary phases AlFe2-xMnxB2 have been synthesized by arc-melting and characterized by powder X-ray diffraction, magnetic measurements, Mo ssbauer spectroscopy, and electronic band structure calculations. All the compounds adopt the AlFe2B2-type structure, in which infinite zigzag chains of B atoms are connected by Fe atoms into [Fe2B2] slabs that alternate with layers of Al atoms along the b axis. The magnetic measurements reveal that AlFe2B2 is a ferromagnet with T-c=282 K while AlMn2B2 and AlCr2B2 do not show magnetic ordering in the studied temperature range of 1.8-400 K. A systematic investigation of solid solutions AlFe2-xMnxB2 showed a non-linear change in the structural and magnetic behavior. The ferromagnetic ordering temperature is gradually decreased as the Mn content (x) increases. The Mossbauer spectra reveal the presence of nonmagnetic (NM) and ferromagnetic (FM) spectral components in all Mn-containing samples, with the amount of NM fraction increasing as the Mn content increases. While for the AlFe2-xMnxB2 samples with x=0.0 and 0.4 the hyperfine splitting of the FM spectral component collapses at temperatures close to the Curie temperatures determined from the magnetic measurements, for the x=1.2 and 1.6 samples the FM fraction exhibits a sizable unquenched hyperfine splitting at room temperature, a finding that is inconsistent with the observed magnetic properties. Along with the increase in the amount of the NM fraction, this observation suggests formation of Fe-rich and Mn-rich regions in the structure of the solid solutions. Quantum-chemical calculations and crystal orbital Hamilton population analysis provide a clear explanation of the distinction in properties for this series of compounds and also reveal the importance of electronic factors in modifying the magnetic properties of these materials. (C) 2014 Elsevier Inc. All rights reserved.