Magistrsko delo
Simon Mattiazzi (Author), Mojca Vilfan (Mentor), Nerea Sebastián Ugarteche (Co-mentor)

Abstract

V magistrskem delu obravnavamo feromagnetne nematike. Gre za relativno nov material, ki ga pripravimo kot suspenzijo feromagnetnih nanoploščic v nematskem tekočem kristalu. Pokažemo, da se osnovni statični parametri (elastični konstanti K1 in K3, dvolomnost $\Delta n $ ter anizotropija dielektričnosti $\Delta \epsilon$) ne razlikujejo bistveno od tistih v čistem tekočem kristalu, pri čemer se eksperimentalno opremo na električni Fréederickszov prehod, ki ga opišemo s Frankovo elastično teorijo. Povzamemo razširitev te teorije v makroskopski opis feromagnetnih nematikov, kjer pomembno vlogo igra sklopitvena konstanta γ med magnetizacijo in direktorjem. Izmerimo jo z magnetnim Fréederickszovim prehodom. V nadaljevanju predstavimo termično vzbujena lastna fluktuacijska načina orientacije direktorja v nematikih (pahljača-upogib ter zasuk-upogib) ter povzamemo nadgrajeno teorijo, ki opisuje takšne fluktuacije v feromagnetnih nematikih. Z razmeroma novo eksperimentalno metodo, križno diferenčno dinamično mikroskopijo (c-DDM), ki omogoča proučevanje bistveno hitrejše dinamike od navadne DDM, in ustrezno izbiro orientacije polarizatorja in analizatorja ter območja preiskovanih valovnih vektorjev lahko fluktuacijske načine ter njihove sestavne deformacije preučujemo izolirano in jim izmerimo relaksacijski čas. Meritve opravimo v odvisnosti od jakosti zunanjega magnetnega polja in pokažemo, da teorija dobro napove več aspektov eksperimentalnih rezultatov. Na največje težave pri primerjavi teorije in eksperimentov naletimo pri upogibni komponenti načina pahljača-upogib, izpostavimo pa tudi nekatere druge omejitve izbranega pristopa.

Keywords

fizika kondenzirane snovi;nematski tekoči kristali;feromagnetizem;Fréederickszov prehod;fluktuacije;križna diferenčna dinamična mikroskopija;

Data

Language: Slovenian
Year of publishing:
Typology: 2.09 - Master's Thesis
Organization: UL FMF - Faculty of Mathematics and Physics
Publisher: [S. Mattiazzi]
UDC: 538.9
COBISS: 198668291 Link will open in a new window
Views: 49
Downloads: 23
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Other data

Secondary language: English
Secondary title: The Influence of Magnetic Field on the Dynamics of Fluctuations in Ferromagnetic Nematic Liquid Crystals
Secondary abstract: The main focus of this Master’s thesis are ferromagnetic nematic liquid crystals. These relatively new materials are prepared as suspensions of ferromagnetic nanoplatelets in nematic liquid crystalline hosts. We rely on the electric Fréedericksz transition to experimentally demonstrate that their fundamental static parameters (elastic constants K1 and K3, birefringence $\Delta n $, and dielectric anisotropy $\Delta \epsilon$) do not differ significantly from those in pure liquid crystal. We present the extended Frank elastic theory for a macroscopic description of ferromagnetic nematics by introducing the coupling constant γ between magnetization and director. The coupling constant was measured by observing the magnetic Fréedericksz transition. We describe thermally excited fluctuation modes of director orientation in nematics (splay-bend and twist-bend) and outline an extended theory for describing such fluctuations in ferromagnetic nematics. Using a relatively new experimental method, the cross-differential dynamic microscopy (c-DDM), which enables studying significantly faster dynamics than conventional DDM, along with appropriate selection of polarizer and analyzer orientations and ranges of investigated wave vectors, we can isolate and study fluctuation modes and their constituent deformation components and measure their relaxation rates. The relaxation rates measured at different external magnetic fields match the theoretically predicted values in several aspects. The bend component of the splay-bend mode is shown to be the most problematic for comparison with the theory. Some further limitations of the described approach are pointed out at the end.
Secondary keywords: condensed matter physics;nematic liquid crystals;ferromagnetism;Fréedericksz transition;fluctuations;cross-differential dynamic microscopy;
Type (COBISS): Master's thesis/paper
Study programme: 0
Thesis comment: Univ. v Ljubljani, Fak. za matematiko in fiziko, Oddelek za fiziko
Pages: 60 str.
ID: 24348351