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Synopsis

Rare-earth-based intermetallic systems display a wide variety of distinct magnetic properties. Besides the well-known ferromagnetic and antiferromagnetic orderings, specific arrangements of magnetic moments in the lattice can exhibit a wide range of complex magnetic behaviours, such as magnetic frustration, low-dimensional antiferromagnetism, quantum spin liquids and so on. These magnetic phenomena are not restricted to a certain class of materials but are commonly linked to a structural motif. It has been shown that the effects of magnetic frustration are often pronounced in materials with high crystal symmetries and are commonly observed in antiferromagnets with certain lattice geometries, such as 2D square lattices, Kagome (corner-sharing triangles) and triangular lattices (edge-sharing triangles) and pyrochlore (corner-sharing tetrahedra) and face-centred cubic (edge-sharing tetrahedra) structures. The systems studied in this thesis include CeCd3P3 and RV6Sn6, which adopt a hexagonal crystal structure with 2D triangular lattices of 4f moments possessing trigonal and hexagonal point symmetry, respectively, and RNi4Cd, where the 4f  moments form a 3D face-centred cubic lattice. These systems serve as platforms for exploring the magnetic properties of 4f moments in different environments, with a particular emphasis on geometrically frustrated lattices. This thesis is based on the characterization of their crystal structures and their thermodynamic and transport properties.

This thesis also addresses the role of carrier density in 4f magnetism, which is often overlooked. In metals, conduction electrons are mediators for indirect Ruderman–Kittel–Kasuya–Yosida (RKKY) exchange interactions. In Ce- and Yb-based metallic systems, the interplay between intersite 4f–4f coupling and onsite Kondo coupling with conduction electrons determines the ground state, as described by Doniach’s phase diagram. However, the carrier density, which is determined by the number of conduction electrons and holes, can impact these interactions and potentially influence the established phase diagram. Therefore, it is necessary to study the phase diagram for low-carrier density systems. While numerous examples of metallic systems exist, those with low carriers are scarce. CeCd3P3 and Ce3+xRu4Sn13−x are low-carrier systems, as evident from their large room-temperature-resistivity values (>1 mΩ cm). CeCd3P3 with negligible hybridization provides an opportunity to study frustrated magnetism on Ce triangular lattices and to investigate RKKY physics in lowcarrier density cases. Ce3+xRu4Sn13−x, with two crystallographic sites for Ce ions, represents a rare example, where magnetic and electronic properties can be systematically controlled by the occupancy of Ce-ion on the 2a site, denoted as x. The electrical resistivity of Ce3+xRu4Sn13−x displays a transition from Kondo lattice behaviour to semiconductor-like behaviour with increasing x. Investigating the ground state of Ce3+xRu4Sn13−x for various x may elucidate the role of low carriers on the magnetic properties of heavy fermion systems.

Keywords: Rare-earth-based intermetallic compounds, geometrical frustration, low-carrier-density,

magnetism, crystalline electric field effect, RKKY, Kondo effect