This thesis ranges within the vast framework of experimental condensed matter physics. Several different systems, and physical phenomena, are presented here from a structuralist standpoint. In fact, we show how, in solid condensed matter, the underlying arrangement of atoms, the symmetry of their structure, and their mutual interactions, underpin the form and the nature of their collective emergent properties. Our effort in this work was focused on unveiling complex magnetic ground states in newly synthesized materials, as well as in the clarification of unconventional symmetry breaking phenomena in highly debated systems. In all cases, we could understand the physics of such systems only when we elucidated the details, and temperature dependent evolution, of their structures.

About the choice of target materials for our investigations, our starting point has not only been fundamental condensed matter physics, but also forward looking towards a sustainable future. Here we considered both the development of energy efficient spintronics and quantum computing, as well as the need for efficient conversion and storage of clean energy. Therefore, this project is concerned with the advanced characterization of novel ”multifunctional” materials, that constitute a unique playground for fundamental scientific research, but also lend themselves to potential novel technical applications. Such materials might indeed display high temperature dynamical properties, which make them suitable for rechargeable batteries and heat conduction applications. At the same time, they are also strongly correlated electron systems at lower temperatures, and their fundamental magnetic and electronic properties are relevant for the development of quantum devices. To explore these properties, extensive experimental studies using large-scale research facilities were employed. In this project, several unique and powerful state-of-the-art high-resolution neutron scattering, X-ray scattering, and muon spin rotation techniques were used.

Transition metal oxides (TMOs) exhibit a wide range of electronic and magnetic properties, making them essential in condensed matter physics. In magnetic TMOs, the ability to tune the magnetic properties offers valuable insights into correlated electron systems and potential functionalities in next-generation materials.

This Licentiate thesis investigates how antiferromagnetic (AFM) ordering can be tuned in powder AReO₄ (A = Mg, Zn), and LiFePO₄ using large-scale facility techniques. We explore how the application of hydrostatic pressure or the substitution of the non-magnetic ion affects the magnetic structure and ordering temperatures.

The work utilises muon spin spectroscopy and resonance ($\mu^+$SR) and neutron powder diffraction (NPD) to probe the magnetic properties of these materials. For LiFePO₄, high-pressure μ⁺SR experiments reveal that compressive strain enhances AFM ordering, contrary to theoretical predictions. For AReO₄, NPD and μ⁺SR suggest two possible AFM spin structures. Our measurements show a remarkably low ordered magnetic moment for both MgReO₄ and ZnReO₄. Bond valence sum (BVS) analysis supports a Re⁶⁺ oxidation state in both compounds, and we attribute the low magnetic moment to strong spin-orbit coupling (SOC).

This thesis demonstrates how NPD and μ⁺SR serve as complementary techniques for investigating complex magnetic systems and how a local probe, the muon, sensing only its immediate environment, can provide insight into macroscopic magnetic properties. The findings contribute to a deeper understanding of the magnetic phase of LiFePO₄ and Re⁶⁺ magnetism in octahedral coordination with oxygen.