Investigating correlated charge, lattice and spin degrees of freedom in layered perovskite ruthenates

Abstract

This thesis explores the magnetic and dielectric properties of B-site substituted perovskite systems, specifically double-, triple-, and quadruple-perovskites, with an emphasis on ruthenium-based compounds. Perovskite oxides with ABO3 crystal structure, are well-known for their diverse physical and chemical properties, owing to their immense flexibility in accommodating a majority of elements in the periodic table as A- and B-site substitutions. These alterations can modify the structural geometry from the ideal cubic form, affecting various properties, including magnetism. In particular, the study focuses on systems with A = Ba/Sr and B = Ru, investigating how the arrangement of corner- and face-shared octahedra influences their magnetic interactions. Ruthenium-based perovskites are of great interest due to the plethora of unexpected physics they exhibit, such as quantum critical behavior, unconventional superconductivity, and altermagnetism. Ruthenium, with its 4d orbital character, has notable spin-orbit coupling (SOC) and spatially extended orbitals that enhance the exchange interactions and orbital overlap, resulting in complex magnetic behaviors. We employ a range of experimental techniques, including x-ray and neutron powder diffraction, magnetometry, electronic and thermal transport, and linear and nonlinear dielectric susceptibility measurements, in order to investigate these materials. Additionally, we detail the construction and functioning of a homemade automated dielectric susceptometer which allows us to explore the multifunctionality of these materials at criticality. The first section focuses on Ba2CoRuO6 (BCRO), a double-perovskite system which structurally mimics its triple-perovskite sibling. The presence of significant antisite disorder (about 30%) on the metal sites located within the face-shared octahedra, combined with the geometrically frustrated hexagonal structure, prevents long-range magnetic ordering. Instead, BCRO exhibits a cluster-glass (CG) ground state below ∼43 K, characterized by classic glassy behaviour such as frequency-dependent ac-susceptibility, ageing, and memory effects. These features differentiate BCRO from typical spin-glass systems. The presence of exchange anisotropy leads to locally dominant antiferromagnetic spin clusters, which persist even at high temperatures, as demonstrated by the absence of a pure paramagnetic behaviour in susceptibility measurements. In the second section, the nonlinear dielectric properties of three triple-perovskite ruthenates - Ba3CoR2O9, Ba3BiRu2O9, and Sr3CaRu2O9 - are explored. These materials show significant coupling between their spin, charge, lattice, and polar degrees of freedom. For instance, Ba2CoRu2O9 exhibits a frequency-dependent relaxation in χ2,3 just above a magneto-elastic transition around 100 K, followed by a loss of polarization. Ba2BiRu2O9 displays a more complex relaxation behavior, strongly influenced by the spin-gap opening at 175 K. Meanwhile, Sr2CaRu2O9 shows strong dispersion effects around its magnetic transitions, highlighting a robust interaction between magnetic and dipolar orders. The study of higher-order susceptibilities provides deeper insights into these systems’ dynamics, revealing information that is otherwise masked in the linear polarization responses alone. The final section examines the quadruple-perovskite Ba4NaRu3O12, which exhibits a spin-lattice coupled ground state with long-range magnetic order below 257 K. The magnetic properties of this system are strongly influenced by its structural motif, where RuO6 octahedra are linked to Ru2O9 dimers, separated by nonmagnetic NaO6 layers. Neutron diffraction at 13 K reveals antiferromagnetic order, with Ru2 and Ru3 dimer moments oppositely aligned, while the Ru1 triangular layers show a staggered arrangement along the c-axis. Specific heat measurements reveal a boson peak contribution associated with optical modes at low temperature, with a maximum vibrational energy of around 55cm-1. Charge transport below the magnetic transition follows a variable-range hopping (VRH) mechanism, though the power law energy dependence of the Coulomb gap with an exponent > 2 suggests the presence of multiparticle correlation effects. Overall, this thesis provides an in-depth exploration of the magnetic and dielectric properties of B-site substituted perovskites, highlighting the complex interplay of spin, charge, and lattice degrees of freedom in these systems. The findings underscore the richness of ruthenium-based perovskites in hosting diverse and unconventional physical phenomena.