Günter Obermeier «  Zakir Seidov »  Andre Wobst
Zakir Seidov
ESR investigations of TlMX2 (X = Fe, Ga, In; X = S, Se)
Supervisor: Prof. Dr. Alois Loidl [Experimental physics V]
Date of oral examination: 07/11/2003
109 pages, english , 3-8322-1811-4, Shaker Verlag Aachen 2003
The aim of the present work was the investigation of electric and magnetic properties of TlMX2 chalcogenide compounds, where M denotes Fe, Ga, or In and X is the chalcogen S or Se. Special attention was devoted to the local structural and electronic properties using electron-spin resonance (ESR). The main results in the magnetic TlFeX2 compounds concerned the temperature dependence of the magnetic susceptibility, which above the Neel temperature increases linearly and not show any tendency for saturation. The ESR intensity of the strongly exchange-narrowed resonance detected at g=2, which locally probes the spin susceptibility of Fe3+, supprort these results. A similar behavior spin susceptibility is known from one-dimensional organic compounds, where the linear increase is the signature of one-dimesional metallic conductivity. Hence, both TlFeS2 and TlFeSe2 are probably intrinsic one dimesional metals, although their macroscopic resistivity increases to low temperatures like in semiconductor due to breaks and imperfections in the chain. In the non-magnetic TlMX2 (M = Ga, In, X = S, Se) compounds the microscopic nature of the commensurate and incommensurate phases was investigated. About 1%25Fe3+ doped into the MX4 tetrahedra provided the appropriate ESR - probe. The complicated spectra and their angular dependence could be explained by strong rhombic crystal field originating from the local distortion of the MX4 tetrahedra. In TlGaSe2 and TlInS2 the spectra can be detected in commensurate phase, but disappear on approaching the incommensurate phase and do not recorved in the paralectric phase, wheareas is non ferroelectric TlGaS2 the spectra appear in the whole temperature region. From these results it follows that the local distortion of the MX4 tetrahedra which rise to the strong crystal field appear independently from the ferroelectricity. An incommensuration modulation of lattice yields a broad distortion of resonance fields, which makes the signal undetectable.