Magnetic frustration and low-dimensional magnetism : in transition metal fluorophosphates and square-lattice intermetallic compounds

Abstract: Solids can display a variety of vastly different magnetic properties. Besides the generally well known ferromagnets, antiferromagnets with their antiparallel arrangements of magnetic moments can exhibit a wide range of complex magnetic behavior such as magnetic frustration or low-dimensional antiferromagnetism. Magnetic frustration emerges from competing magnetic interactions and typically leads to unusual magnetic ground states such as incommensurate or non-collinear magnetic structures or spin glasses. Low-dimensional magnetic behavior occurs if the magnetic interactions within a solid become negligible in at least one dimension in space. These magnetic phenomena are not restricted to certain compound classes but commonly linked to structural features such as the magnetic ion lattice geometry or the topology of the crystal structure. Effects of magnetic frustration are most pronounced in materials with high crystal symmetries and commonly observed in antiferromagnets with certain magnetic ion lattices such as triangular or square nets. Low-dimensional magnetic interactions may arise from a spacial separation of the magnetic ions within the unit cell. Furthermore, complex magnetic properties may also arise from intricate magnetic ion lattices with unusual topologies, as it is often the case in open framework materials. This thesis focuses on the magnetic properties of a series of transition metal (T) fluorophosphates (T = Fe, Co, Ni) that display a variety of crystal structure topologies, the cubic perovskite (NH4)CoF3 as well as the intermetallic phases LaMn2(Ge1-xSix)2 and LaMn2-xAu4+x in which the Mn atoms form square nets.Ionothermal reactions, a soft-chemistry approach based on ionic liquids (ILs), was employed to synthesize the above mentioned transition metal phosphates and fluoride. Ionic liquids are salts with melting points below 100 ◦C that typically contain organic cations. Task-specific ILs may be designed to fulfill multiple purposes within an ionothermal synthesis. Thus, an IL may be the solvent, mineralizer, fluorine source and structure directing agent all in one. Thanks to the unique properties of ILs, ionothermal syntheses enable the formation of a wide range of crystal structure topologies from low-dimensional motifs to open frameworks. Furthermore, kinetics plays an important role during the crystal structure formation in an ionothermal reaction and may lead to metastable phases.Nearly all presented compounds display some type of complex magnetic behavior including magnetic frustration on the triangular and square lattices, incommensurate and/or non-collinear magnetic structures, spin glass behavior and low-dimensional magnetism of a spin dimer or chain. The magnetic properties were studied using magnetization measurements in combination with other techniques such as powder neutron and X-ray diffraction (PND, PXRD) and specific heat measurements. Temperature dependent PND measurements were employed to determine the magnetic structures and phase transitions in selected frustrated systems. A number of incommensurate and/or non-collinear arrangements of the magnetic moments including the 120◦, canted ferromagnetic, helical, conical and the so-called hedgehog spin-vortex crystal (SVC) state were observed. Density functional theory (DFT) calculations were performed to determine the total energies of possible magnetic structure solutions that cannot be distinguished from PND. Furthermore, some magnetic phase diagrams were established.