At the heart of Quantum Materials: Magnetism as a means and an end from a muon perspective

Abstract: Functional materials are at the center of solid state physics research and technological innovation in our era. In order to create (semi)autonomous, high precision and lightning-fast devices it is necessary to explore, modify and control the intrinsic properties of matter. This thesis focuses on quantum materials with strongly correlated physical entities, where properties may also be defined by geometry and can not thoroughly be described by classical physics. The magnetic and electrical properties of these materials are predominantly studied at an atomic level in order to understand the evolution of interactions between electron spins, charges and structure as well as dependencies from external parameters. Magnetic interactions are considered not only in the research subjects of this thesis but also in the employed experimental technique. Therefore, Chapter 2 is dedicated to a brief introduction in certain aspects of magnetism. Positive muon spin rotation, relaxation and resonance (μ+SR) is the principal experimental technique used in the presented studies. In essence this technique employs elementary particles called muons as magnetic probes in the studied samples. The reaction of muons to the local magnetic fields, which arise from the electronic structure or the atomic nuclei of solids, conveys direct or indirect information of phase transitions, dynamics or specific processes in the materials. In Chapters 3 and 4 the background and practicalities of the μ+SR technique will be elaborated to a sufficient extend that covers the scope of this thesis. The presented research encompasses a diverse array of materials with distinct structural characteristics and magnetic or electronic properties, which have been investigated utilizing μ+SR and complementary experimental techniques. A detailed introduction to each material and a summary of the experimental results can be found in Chapter 5. Concerning magnetic materials, the studies comprised LaSr1−xCaxNiReO6 and CrCl3 polycrystalline samples, as well as CoFeB/Ru/Pt superstructures. In the case of the double perovskite, three-dimensional magnet LaSr1−xCaxNiReO6, a dense and dilute magnetic phase were discovered above the critical temperature of magnetic order. The difference in size between the Sr and Ca cations alters the magnetic interaction between Ni and Re sub-lattices and as a result, the magnetically ordered phase. The magnetic phase diagram of the layered, two-dimensional magnet CrCl3 was similarly studied. The μ+SR analysis identified a dynamic layered antiferromagnetic ordering phase followed by a short range ordered ferromagnetic phase. Both studies exemplify the substantial advantage of measurements in zero external magnetic field with μ+SR. An alternative configuration with low energy μ+SR and adjustable muon implantation depth was employed to study the magnetic phases of multilayered CoFeB/Ru/Pt superstructures. A magnetic-skyrmion phase was identified and the persisting dynamics down to 50 K were studied in relation to an unexpected weak dependence on externally applied magnetic fields. Regarding ionic motion in non-magnets, a customized muon study was conducted on hybrid, organic-inorganic perovskite (CH3NH3)PbX3 (X=Br, Cl) single crystals, which constitute promising photovoltaic materials. Molecular fluctuations were studied across the structural phases of the materials with and without illumination. μ+SR results in these two environments indicate that molecular fluctuations and the type of halide ion are defining parameters on structural stability and carrier lifetimes, both of which are essential in solar cell applications.