title: Structure-property relationships in magnetic layered oxide materials
Creators: name: McPhillips, Holly Louise
Publisher: name: University of Kent
Year: 2024

description: Layered oxide materials remain a popular research topic due to their compositional flexibility and rich physical properties. This has led to their wide range of applications in areas like catalysis, energy technologies and multiferroic materials. The latter has potential uses as multiple state memory storage and low power electronic devices when the electric and magnetic order parameters coexist, and these order parameters are (magnetoelectrically) coupled (ME). Geometric magnetic frustration, when the underlying lattice topology or geometry prevents all pairwise magnetic exchange interactions from being satisfied, has been observed in some magnetic layered oxide materials, which exhibit exotic magnetic and ME phases. This work explores the structure-property relationships of two systems, Ca2Mn3O8 and A2Mo3O8 (A2+ = Mn, Fe, Co), which contain triangular connectivity in their structural motifs that can give rise to magnetic frustration. The delafossite-type structure, Ca2Mn3O8 (C2/m, no. 12) contains a geometrically frustrated bow-tie (triangular-like) sublattice motif of edge-sharing Mn4+ octahedra. It was only recently that its magnetic structure was comprehensively studied, revealing two magnetic phase transitions at T* = 130 K and TN = 58 K. At T*, the observed magnetic behaviour coincides with non-linear thermal expansion in the structure and the symmetry is maintained. Preliminary neutron scattering measurements also indicated the onset of short-range correlations between Mn ions at T*. Thus, local structural changes may be responsible for the onset of this behaviour, which is yet to be studied. Furthermore, in published structure-property studies, it was suggested that Ca2Mn3O8 exhibits a three-dimensional collinear commensurate canted four sub-lattice-like (⇈⇊) spin structure below TN. However, correlations between Mn ions across the bow-tie sublattice motif are not fully understood. Considering the magnetic behaviour at T* and TN, a local structure and magnetism investigation for Ca2Mn3O8, is presented in chapter 3. The remainder of this work covers structure and magnetism of the polar hexagonal layered molybdenum oxide system, A2Mo3O8 where A2+ = Fe, Mn, Co (P63mc, no. 186). The structure comprises two networks that are stacked along the c-axis: a honeycomb motif of A2+ in octahedral and tetrahedral coordinated environments, and a Kagomé-like motif of Mo4+ in which the triangular connectivity results in S = 0 spin- iv singlet trimers. The magnetic behaviour of A2Mo3O8 is driven by the A2+ ions only. When the A2Mo3O8 materials are cooled, they undergo a transition in the temperature range between 40 K and 60 K and become multiferroic. A delicate balance of competing superexchange interactions within the honeycomb motif is responsible for various spin structures (antiferromagnetic or ferrimagnetic) depending on the identity of A2+. Published variable field measurements also show that these ordered phases are easily converted into one another. Compared to the Fe and Co analogues, understanding the structure and magnetism of the Mn2Mo3O8 is limited and is the subject of chapter 4 of this work. A similar study for A-site doped structures, MnCoMo3O8, MnFeMo3O8 and MnZnMo3O8 is also presented in chapter 5. Finally, a variable pressure study of the structure of A2Mo3O8 is presented in chapter 6 to give an insight into the structural stability of these materials.