Spin-structure driven Topological properties in Weyl Semimetallic Pyrochlore Iridates: Structural and Magnetotransport studies

Abstract

Over the past decade, topological magnetic materials have captivated the scientific community, spurring extensive research into their unique properties. In this thesis, we explore pyrochlore iridates RE2Ir2O7 (where RE represents rare-earth elements), known for their distinctive crystal structure composed of interconnected three-dimensional corner-sharing tetrahedra occupied by RE3+ and Ir4+ ions. This complex arrangement often leads to significant geometrical frustration due to the competition between neighboring spins of the magnetic ions. This geometrical frustration leads to unique magnetic structures in pyrochlore iridates systems. Consequently, RE2Ir2O7 undergoes an antiferromagnetic ordering of Ir4+ ions at its metal-insulator transition temperature. The presence of 5d−based iridium in pyrochlore iridates creates a rich landscape for discovering exotic electronic ground states, driven by the interplay of strong spin-orbit coupling and electron correlations. Pyrochlore iridates have recently garnered significant attention due to theoretical predictions of the Weyl semimetal (WSM) phase. WSMs are characterized by two linear non-degenerate band crossings in the momentum space. The spin-degeneracy of these bands can be lifted by either broken inversion symmetry or broken time-reversal symmetry (TRS), resulting in two classes of WSMs: non-centrosymmetric and magnetic WSMs. In magnetic WSMs, intrinsic magnetic order breaks TRS, allowing their electronic band topology to be manipulated through magnetic control. This unique control offers potential applications for Weyl fermions in developing advanced quantum computing technologies and novel electronic devices. The all-in-all-out (AIAO) ordering in pyrochlore iridates breaks TRS, fulfilling a crucial requirement for the emergence of the WSM phase. Additionally, the noncoplanar spin structure of pyrochlore iridates can induce spin chirality, leading to the formation of a real-space Berry phase and the manifestation of the topological Hall effect (THE). In this study, we selected Gadolinium (Gd) and Dysprosium (Dy) from the rare-earth family to investigate the impact of their high magnetic moments on the materials’ properties. We mainly focused on realizing the WSM state in our single crystalline thin films through magnetotransport studies. viiSpin-structure driven Topological properties in Weyl Semimetallic Pyrochlore ....... In Chapter 1, we have provided a comprehensive introduction to pyrochlore iridates, exploring their geometric frustration and the various magnetic structures possible within this family of materials. We have discussed the interactions present in these systems and how the interplay between them leads to the emergence of diverse electronic ground states. Our focus centers on the topological Weyl semimetallic (WSM) state. We have briefly introduced the WSM state and discussed the methods for experimentally observing the WSM phase. As our aim is to detect the signature of the WSM state through magnetotransport studies, we have mentioned the relation between magnetotransport and the Berry curvature contribution in momentum space. Finally, we outline the motivation behind our thesis. In Chapter 2, we have detailed the experimental techniques and their fundamental working principles. This includes the solid-state synthesis method for polycrystalline sample preparation and the pulsed laser deposition (PLD) method for thin film deposition. Wethendescribe the sample characterization techniques, such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and magnetization measurements. For structural studies of polycrystalline samples, we discuss Raman spectroscopy measurements. Following this, we cover the device fabrication process for thin film samples using optical lithography. Lastly, we explain the physical properties measurement system (PPMS), where all the magnetotransport measurements have been conducted. In Chapter 3, we performed a detailed examination of the impact of the RE-site magnetic moment on the structural, electrical transport, and magnetic properties of pyrochlore iridates. We prepared polycrystalline samples of Dy2Ir2O7, (Dy0.5Gd0.5)2Ir2O7, and Gd2Ir2O7 and diluted the higher magnetic moment of Dy3+ through partial and complete substitution of Dy3+ ions with Gd3+ ions. Temperature-dependent resistivity and magnetization measurements revealed a transition from semiconductor to Mott-insulator as the compounds transformed from paramagnetic to an antiferromagnetic all-in-all-out (AIAO) state at TN, due to Ir4+ ordering. This ordering builds a spin-phonon coupling in the systems through f-d exchange interaction. In Dy2Ir2O7, irreversible magnetization data suggest the presence of magnetostriction below TN. A study of temperaturedependent lattice constants and Raman spectroscopic investigations confirm this. Conversely, Gd3+-substituted samples did not exhibit magnetostriction, but an increase in the full width at half maximum (FWHM) of Raman modes near TN suggests the presence of weak spin-phonon coupling. Spin-structure driven Topological properties in Weyl Semimetallic Pyrochlore ....... In Chapter 4, we explored the magneto-transport properties of epitaxial thin films of pyrochlore iridates grown by pulsed laser deposition (PLD). We verified the magnetic ordering of Ir4+ moments in (111)-oriented Gd2Ir2O7 single crystalline thin films, which coincides with the metal-to-semimetal transition. Low-temperature resistivity measurements reveal gapless semi-metallic behavior, while high-temperature transport data indicate non-Fermi liquid behavior, underscoring the significance of electron correlations in this 5d system. The samples showed anomalous Hall effect (AHE) and topological Hall effect (THE) at low temperatures. Manifestation of AHE without spontaneous magnetization in the samples suggests an intrinsic origin, which we attribute to the momentum space Berry curvature associated with Weyl nodes in the electronic band structure. Additionally, a THE signal of large amplitude at 7 K indicates non-zero spin chirality due to magnetic field-induced domain modifications in the Ir4+ sublattice. Large magnitudes of negative magnetoresistance (MR) and antisymmetric MR analysis support the spin chirality contribution. In Chapter 5, we investigated (111)-oriented Dy2Ir2O7 epitaxial thin films to explore the effect of the higher magnetic moment of Dy3+ on the magneto-transport properties. As the sample cools, Ir4+ orders into the AIAO spin structure around 100 K, creating a f ictitious field on Dy3+ moments and aiding their ordering into the same AIAO structure around 10 K. Field-dependent resistivity analysis indicates that at a critical field H′, the Zeeman energy is sufficient to induce a spin-flip in the Dy3+ spins, transforming the AIAO/AOAI structure to the 3I1O/1I3O structure. The transition facilitates domain imbalance of the Ir4+ moments through f-d exchange interaction, resulting in hysteresis behavior revealed in the MR study. The introduction of strain in the thin film breaks the cubic symmetry in the crystal structure and leads to a spontaneous Hall signal, a signature of the WSM state in the sample. Hall data revealed a topological-like Hall signal, attributed to the domain imbalance of Ir4+ moments and modification of Dy3+ spin structure to the 3I1O/1I3O configuration. In Chapter 6, we have comprehensively summarized the experimental results and key findings from our research on the role of RE−site magnetic moment in the structural properties in bulk samples and the realization of Weyl semimetallic state through magnetotransport properties in the epitaxial thin films of pyrochlore iridates. We have also proposed future research directions and potential applications of this work to guide the readers of this thesis for further investigations in this field.