Novel properties of the layered material Ca2Mn3O8

Abstract

Manganese oxides and related mixed manganates have long been the subject of research interest, due to their diverse properties, such as cation exchange and molecular absorptive characteristics, in addition to excellent electrochemical and magnetic properties. Some of this research interest has stemmed from the observation of magneto-electric coupling (i.e. change in magnetic state via application of an electric field) in manganite perovskites. However, magneto-electric coupling originating as a consequence of magnetic frustration, has given rise to considerable research attention being focussed upon a range of compounds showing complex geometrically frustrated lattices (i.e. the impossibility to satisfy all magnetic spins simultaneously due to structural topology). The most obvious example of a geometrically frustrated magnetic system, is that based upon a triangular-lattice antiferromagnet. A family of materials possessing this arrangement are Delafossites, where a number of materials falling under this category have been found to possess multiferroic properties. A material possessing similar structural properties to Delafossites, i.e. a layered structure with magnetic species adopting a triangular arrangement, is Ca2Mn3O8. This material has received a lack of research attention, with the primary research focus being based upon its catalytic properties, however the potentially interesting magnetic characteristics have been largely ignored. The aims of this thesis are to contribute an enhanced understanding of the magnetic behaviour of Ca2Mn3O8, via performing an in-depth structural, morphological and magnetic characterization (chapter 3 and 4). This work shows that Ca2Mn3O8 possesses an isosceles-type of triangular lattice, leading to magnetic frustration, in addition to exhibiting a collinear commensurate four-sublattice ???? magnetic structure (chapter 4). Ca2Mn3O8, however, does not possess a full triangular lattice, with ordered vacancies present within the MnO6 layer, and thus can be considered to be a cation deficient Delafossite. Doping strategies to realise a full triangular lattice, via vacancy filling with magnetic and non-magnetic species, have also been investigated (chapters 5 and 6). Manganese oxides and related mixed manganates have long been the subject of research interest, due to their diverse properties, such as cation exchange and molecular absorptive characteristics, in addition to excellent electrochemical and magnetic properties. Some of this research interest has stemmed from the observation of magneto-electric coupling (i.e. change in magnetic state via application of an electric field) in manganite perovskites. However, magneto-electric coupling originating as a consequence of magnetic frustration, has given rise to considerable research attention being focussed upon a range of compounds showing complex geometrically frustrated lattices (i.e. the impossibility to satisfy all magnetic spins simultaneously due to structural topology). The most obvious example of a geometrically frustrated magnetic system, is that based upon a triangular-lattice antiferromagnet. A family of materials possessing this arrangement are Delafossites, where a number of materials falling under this category have been found to possess multiferroic properties. A material possessing similar structural properties to Delafossites, i.e. a layered structure with magnetic species adopting a triangular arrangement, is Ca2Mn3O8. This material has received a lack of research attention, with the primary research focus being based upon its catalytic properties, however the potentially interesting magnetic characteristics have been largely ignored. The aims of this thesis are to contribute an enhanced understanding of the magnetic behaviour of Ca2Mn3O8, via performing an in-depth structural, morphological and magnetic characterization (chapter 3 and 4). This work shows that Ca2Mn3O8 possesses an isosceles-type of triangular lattice, leading to magnetic frustration, in addition to exhibiting a collinear commensurate four-sublattice ???? magnetic structure (chapter 4). Ca2Mn3O8, however, does not possess a full triangular lattice, with ordered vacancies present within the MnO6 layer, and thus can be considered to be a cation deficient Delafossite. Doping strategies to realise a full triangular lattice, via vacancy filling with magnetic and non-magnetic species, have also been investigated (chapters 5 and 6).