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

Doctoral thesis, 2024

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.