Robert Fichtl
Ungewöhnliche Grundzustände in Thiospinellen
Betreuer: Prof. Dr. Alois Loidl [Experimentalphysik V]
Datum der mündlichen Prüfung: 13.07.2007
136 Seiten, deutsch , OPUS (Online-Publikations-Server) der Universitätsbibliothek Augsburg oder ISBN 978 3 86664 249 2
Spinels form a class of materials with very versatile properties. Probably most prominent among them is the so-called "Black Prince's Ruby" - a 140-carat gemstone in the royal British crown almost of the size of a chicken egg, which was mistaken for a ruby for a long time. The spinels' technical application ranges from paint production and solid-state lasers to magnetic storage devices. From a scientific point of view they offer very rich physics including superconduction, metallic conductivity, and insulating behaviour at partly rather complex magnetic properties. This work is about unusual ground states that were found in various sulphur spinel systems: CdCr2S4 and HgCr2S4 show magnetic and electric order phenomena that are coupled with each other in an exceptionally strong way and FeCr2S4 exhibits a transition into the newly-discovered state of an "orbital glass" at low temperatures. Electronic orbitals play an important role in the observed phenomena since they can communicate magnetic exchange interactions and determine ferroelectricity (-> "multiferroics") but also are responsible for structural Jahn-Teller distortions (-> orbital order). The results of polarization measurements and dielectric spectroscopy in single- and polycrystalline material are presented and discussed in this work. CdCr2S4 and HgCr2S4 show multiferroic behaviour with (anti-)ferromagnetic order in a relaxor ferroelectric state. The unusually strong coupling between the different kinds of order at relatively high temperatures brings forth colossal magnetocapacitance (CMC), which leads to a variation of the dielectric constant in a magnetic field of up to 500 per cent or 3000 per cent, respectively. Moreover, in HgCr2S4 colossal magnetoresistance (CMR) effects are observed, which result in a variation of the dc conductivity in magnetic fields by more than six orders of magnitude. In contrast to the described order phenomena in CdCr2S4 and HgCr2S4, the unusual behaviour of FeCr2S4 is the observed disordered ground state, despite for this system a transition into an orbitally ordered state is expected at low temperatures. In this work the first evidence of glassy freezing of orbital degrees of freedom is provided. The continuous slowing down of the "reorientational dynamics" of the electronic orbitals can be followed by dielectric spectroscopy over several orders of magnitude down to the millisecond-regime.