Non-Equilibrium Spin Dynamics of Magnetically Frustrated Systems

Abstract

Magnetic frustration represents a captivating phenomenon in condensed matter physics, occurring when the spin interactions within a system cannot simultaneously be satisfied due to geometric constraints or competing exchange interactions. A prominent example of such frustration is found in the three-dimensional pyrochlore lattice, composed of corner-sharing tetrahedra containing magnetic ions, which is a hallmark structure in both spinel and pyrochlore oxides. These materials serve as ideal models for investigating the complex interplay among spin, orbital, charge, and lattice degrees of freedom, with the added complexity of chemical disorder and randomness. Spinel oxides typically consist of two sublattices: a pyrochlore sublattice formed by octahedrally coordinated B-sites and a diamond-shaped sublattice composed of tetrahedral A-sites. The intrinsic frustration in these systems often hinders the development of long-range magnetic order, leading to unconventional ground states characterized by nearly degenerate spin configurations. The introduction of nonmagnetic ions, such as Zn, Ge, and Ti, in ordered systems, exemplified by the substitution in Co3O4, creates additional bond or site frustration, thereby modifying the magnetic and electronic properties of these materials.This thesis investigates a range of frustrated oxide systems, focusing on the magnetic behavior and spin dynamics influenced by chemical disorder and diluted interactions. The experimental studies explore magnetic transitions, non-equilibrium spin dynamics, and free-energy landscapes through memory, aging, and rejuvenation experiments. Specific systems under investigation include GeCo2O4 and its Zn-substituted variant, ZnGeCoO₄, which exhibit a transition from antiferromagnetic order to a cluster spin-glass state, characterized by complex relaxation dynamics. Further dilution in ZnTiCoO4 results in competing exchange interactions and enhanced frustration, while ZnMnCoO4 shows short-range ferrimagnetism followed by cluster spin-glass behavior, all of which are analyzed using advanced magnetometry and heat capacity measurements. A detailed structural and magnetic study of rare-earth pyrogermanates, such as Gd2Ge2O₇ and GdNdGe2O7, reveals distinct magnetic ground states and pressure-driven structural transitions, with glassy dynamics emerging upon Nd substitution. The chapters of this thesis encompass the synthesis, characterization techniques, theoretical calculations, and magnetic analyses, ultimately providing a comprehensive understanding of frustrated magnetism in these oxide materials and also present a layout for scope to the future research work in the field of magnetic-frustration in strongly correlated spinel oxides.