Probing Magnetism in Complex Transition Metal Oxides

Abstract

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.

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