StatusThe thesis was presented on the 24 December, 2010
Approved by NCAA on the 24 February, 2011
Abstract– 0.85 Mb / in romanian
The dissertation has been completed in Chishinau in 2010, written in the Romanian language, and contains introduction, 5 chapters, conclusions, 147 references, 112 pages of the text body, 64 figures, 6 tables.
The main aim of the work was to prepare homogeneous poly- and single crystals of the ternary compounds AB2X4 (A=Fe, Co, Hg; B=Cr, Al, Co; X=O, S) with spinel structure and to investigate their structural, magnetic, and thermodynamic properties.
The single phase crystals of FeCr2S4 with magnetic and structural properties similar to single crystals were obtained using the novel method of Spark Plasma Sintering. The perfect single crystals of Fe1-xCuxCr2S4 with reproducible properties were obtained with the chemical transport reactions method. The experimental studies revealed the fact that with the increase of substitution concentration the lattice parameter decreases but both the magnetic moment and the Curie temperature increase. The reduced values of the Curie temperature of single crystals compared to polycrystals are attributed to the presence of the Cl ions, that influence the super-exchange magnetic interactions.
The perfect single crystals of HgCr2S4 were grown using chemical transport reactions. Their magnetic and thermodynamic properties were studied. The presence of strong magnetic frustrations due to competing antiferromagnetic and ferromagnetic exchange interactions was established. The phase transition to antiferromagnetic state at the temperature of 22.5 K was found, accompanied by the anomaly in the specific heat. The appearance of ferromagnetic correlations at the temperatures below 60 K was demonstrated, but the ground state was shown to be antiferromagnetic.
The structural, magnetic, and thermodynamic properties of the Co(Al2-2xCo2x)O4 system were studied in the whole concentration range of substitution of Al ions by Co ions in the octahedral positions for the first time. A continuous range of solid solutions between CoAl2O4 and Co3O4 was revealed. It was demonstrated that in the studied range of substitution the samples exhibit a large negative Curie-Weiss temperature of the order of -110K. This effect is ascribed to a high multiplicity of the magnetic superexchange interactions, whereas the inversion is considered as negligible. It was shown that with the decrease of Co concentration in the octahedral positions the ground state becomes spin-liquid , which is characterized by the temperature dependence of the magnetic specific heat Cm~T2.5 at low temperatures. The samples with x<0.2 manifest the spinglass behaviour, and their heat capacity follows the temperature dependence Cm~T2, attributed to the structural disorder caused by inversion. The mechanism of frustration in the Co(Al2-2xCo2x)O4 system is related to the bond frustrations due to competing magnetic exchange interactions between Co2+ ions in the tetrahedral positions.
The main results of the thesis were presented in 15 scientific publications (6 articles and 9 conference abstracts).
Keywords: magnetic semiconductors, spinel structure, inversion, Spark Plasma Sintering,
chemical transport reactions, magnetic susceptibility, specific heat, magnetic frustrations,
ferrimagnetic ground state, antiferromagnetic ground state, spin-glass, spin-liquid, competing
ferromagnetic and antiferromagnetic exchange interactions.