Exotic magnetic phenomena in transition metal materials

Although the macroscopic properties of magnetic materials have been used for applications already for decennia if not centuries, magnetism remains one of the fundamental areas of interest within materials science. The ongoing investiga- tive quest remains extensive and intense, as still to date revolutionary new e®ects related to the magnetic interactions in materials are revealed. The intermutual competing couplings between microscopic lattice, charge, spin and orbital degrees of freedom in materials are entwined in a complex equilibrium that can result in fascinating, novel phenomena in special cases. Some recent examples are the phenomena of giant magnetoresistance and high temperature superconductivity. The research described in this dissertation focuses on a number of these special cases, in which the magnetism of the studied materials exhibits unconventional behavior. In general, the scienti¯c e®orts presented here are divided among two main topics. The ¯rst of these, which deals with thermally- and optically-switchable magnetism in inorganic molecular-based materials, is addressed in Chapters 2 and 3. The second topic, geometrical magnetic frustration, is explored in Chapters 4 through 6, focusing on a number of speci¯c pyrochlore and triangular systems. Chapter 2 discusses a speci¯c member of family of so-called Prussian Blue Analogues, a class of inorganic molecular crystals with extraordinary thermo- and photomagnetic properties. Focusing initially on polycrystalline material, SQUID magnetometry, X-ray photoemission spectroscopy and Raman spectroscopy were employed to establish, follow and quantify the thermal transition, which was ascribed to an electron transfer between the two 3d metal constituents, Fe and Mn. Quantitative comparison between conversion e±ciencies obtained using bulk- or surface-sensitive techniques revealed the intrinsic incompleteness of the charge transfer process to be primarily due to (electronic) surface reconstruction of the material. Additionally, analysis and comparison of a partially substituted sample showed the robustness of the charge transfer capability of metal centers upon inclusion of a neighboring impurity. Moreover, at low temperatures Raman spectroscopy discloses a remarkable photo-induced e®ect. (Summary.)