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theses

The discovery of magnetism and further understanding of underlying exotic properties has played a critical role in human advancement that dates back from the earliest use of the magnetic compass as a navigational tool to the more recent advanced applications including spintronic and quantum computing. This drives the experimental and theoretical condensed matter community to continue studying magnetism and related phenomena with the aim of understanding the fundamental physics behind material’s magnetic properties and more importantly to discover new materials with exotic properties and ways to manipulate those properties. Magnetism in a material is essentially associated with its crystal structure as well as its electronic structure. By controlling the interplay between structural, electronic, and magnetic interactions in a material a variety of diverse magnetic phenomena can be realized. Some of the design approaches in discovering new magnetic materials consists of selective use of crystal symmetries, dimensionality, charge balance concepts, valence electron rules and introducing spin orbit coupling effects. Likewise, magnetic properties can be further tuned using different methods such as chemical pressure, external stimuli like high pressure, applied field or temperature. In this dissertation, we focus on chemical pressure tuning of magnetism in two structural families due its simple and low-cost fabrication process yet its remarkable ability to manipulate both the structural and magnetic properties in these systems. Chemical pressure effects can be versatile based on the characteristics of the substituent atom used in a host structure. When the substituent element is smaller in size than the host element, a positive pressure is exerted in the system with a subsequent bond contraction that is reminiscent of applying external pressure in a structure. In contrast, when a larger atom is substituted, a negative (tensile) pressure is applied on a system which would otherwise be inaccessible by external means. This simply sums up the power of chemical pressure in accessing new platforms of magnetic states with subtle changes to the chemical environment that are otherwise challenging to accomplish.
Motivated from the low dimensional CeCoIn5 structure type hosting exotic magnetic and superconducting states, we chose to work on its previously underappreciated anti-CeCoIn5 type structure. The study aims to discover new classical magnetic materials in the itinerant extreme, in particular high temperature ferromagnets where transition metals act as the major magnetic component in the system with a general formula MT5X (M: transition metal, T: Pd/Pt and X: P/As/Se) while heavy Pd/Pt are responsible for the spin orbit coupling effects in the system. Partial and complete chemical substitutional studies are conducted to understand the magnetic property evolution across the family. Signature results of the room temperature ferromagnet MnPd5P and its magnetic evolution from the antiferromagnet MnPt5P and MnPd5Se system with low temperature spin-reorientation are discussed in the first half of this thesis in Chapter 3 and 4.
The latter half of this thesis is based on another avenue of magnetism, which is frustrated magnetism that opens the door to non-conventional magnetic behaviors. Frustrated magnetism is caused by competing magnetic forces in a material which often emerge due to the geometrically frustrated crystal lattices. The highly degenerate ground states in frustrated magnets are an excellent platform to discover new magnetic states with exotic properties, for example quantum spin liquids (QSL) or spin ice. In these materials, the long-range anti-ferromagnetic or ferromagnetic order is prevented by the competing interactions, yet showing highly entangled spin fluctuations at very low temperatures that could be beneficial in quantum computing technologies. The alkali metal rare-earth dichalcogenide (ARECh2) materials are of great interest mostly due to their frustrated triangular geometries that host the magnetic rare-earth elements in the structure. The family is known for rich structural variation with plausible frustrated magnetic states, in particular quantum spin liquid states when spin ½ Yb3+ ¬is introduced into the system. The material series AYbSe2 (A: Na – Rb) has been extensively studied and identified as potential QSL candidates. However, LiYbSe2 has not been investigated despite the strong interest and active search for QSL in this family. Thus, by utilizing solid state synthetic strategies, we discovered the missing member in this family that was found to crystalize in a new cubic structure, different from the low dimensional crystal symmetries observed before, introducing 3D frustrated magnetism in this family which is discussed in Chapter 5. Not only is LiYbSe2 the first member found to crystallize in this crystal structure in the family, but our preliminary magnetic characterization also exhibits exotic magnetic properties at very low temperatures suggesting its nearly ideal QSL behavior that makes it a rare case among the few materials showing QSL state instead of a classical spin ice state in a pyrochlore magnetic lattice. Furthermore, the material shows signs of entering a superconducting state at high pressure. Future neutron diffraction and high-pressure studies on the new LiYbSe2 with the pyrochlore sublattice will provide an avenue to expand the frontiers of our knowledge on quantum materials into new areas.
Motivated from the new LiYbSe2 discovery that exhibits 3D frustrated magnetism, our attempts to tune its pyrochlore magnetic sublattice by applying chemical pressure to the system via non-magnetic Ga and In doping is presented in Chapter 6. The study reveals how the chemical pressure involved leads to a symmetry breaking in the 112 system, consequently tuning the frustrated magnetism from 3D to 2D in the Yb3+ sublattice. This behavior is different from the 151 family where the parent structure is maintained with subtle changes in magnetic properties across the family with chemical substitutions.

http://dissertations.umi.com/gsnb.rutgers:12459

Ph.D.

Includes bibliographical references

doi:10.7282/t3-mzbr-sd38

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