<p>Thin metal films have found their use in many magnetic devices. They form pseudo two-dimensional systems, where the mechanisms for the magnetic interactions between the layers are not completely understood. Layered crystal structures have an advantage over such artificial systems, since the layers can be strictly mono-atomic without any unwanted admixture. In this study, some model systems of layered magnetic crystal structures and their solid solutions have been investigated by x-ray and neutron diffraction, Mössbauer and electron spectroscopy, heat-capacity and magnetic measurements, and first-principle electronic structure calculations, with the goal of deepening our understanding through controlled chemical synthesis.</p><p>The compounds TlCo<sub>2</sub>S<sub>2</sub>, TlCo<sub>2</sub>Se<sub>2</sub> and their solid solution TlCo<sub>2</sub>Se<sub>2-x</sub>S<sub>x</sub>, all containing well separated cobalt atom sheets, order with the moments ferromagnetically aligned within the sheets. In TlCo<sub>2</sub>S<sub>2</sub>, the net result is ferromagnetism, while TlCo<sub>2</sub>Se<sub>2</sub> exhibits antiferromagnetism. The inter-layer distance is crucial for the long-range coupling, and it was varied systematically through Se-S substitution. The incommensurate helical magnetic structure found for TlCo<sub>2</sub>Se<sub>2</sub> (x = 0) prevails in the composition range 0 ≤ x ≤ 1.5 but the pitch of the helix changes. The accompanying reduction in inter-layer distance on sulphur substitution varies almost linearly with the coupling angle of the helix. An additional competing commensurate helix (90°) appears in the medium composition range (found for x = 0.5 and 1.0).</p><p>The systems TlCo<sub>2-x</sub>Me<sub>x</sub>Se<sub>2</sub> show helical magnetic ordering for Me = Fe or Cu, while a collinear antiferromagnetic structure occurs for Me = Ni. Magnetic order is created by iron substitution for copper in the Pauli paramagnetic TlCu<sub>2</sub>Se<sub>2</sub>, but now with the moments perpendicular to the metal sheets.</p><p>TlCrTe<sub>2</sub> forms a quite different crystal structure, with intra-layer ferromagnetic alignment and net collinear antiferromagnetism. In contrast to the other phases, the values of the moments conform well to a localised model for Cr<sup>3+</sup>.</p>
| Identifer | oai:union.ndltd.org:UPSALLA/oai:DiVA.org:uu-5973 |
| Date | January 2005 |
| Creators | Ronneteg, Sabina |
| Publisher | Uppsala University, Department of Materials Chemistry, Uppsala : Institutionen för materialkemi |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Doctoral thesis, comprehensive summary, text |
| Relation | Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, 1651-6214 ; 97 |
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