Realization of magnetic Weyl semimetallic phase in pyrochlore iridates thin films

Abstract

Weyl semimetals (WSMs) are characterized by two linear non-degenerate band crossing inside the bulk of the Brillouin zone. The spin-degeneracy of the bands can be lifted by either broken inversion symmetry or broken time-reversal symmetry. This leads to two classes of WSMs; non-centrosymmetric or magnetic WSMs. For magnetic WSMs, the intrinsic magnetic order breaks the time-reversal symmetry. The advantage of magnetic WSMs (topological magnets) is that their electronic band topology can be manipulated through a magnetic field control. Historically, the WSM phase was first theoretically predicted in pyrochlore iridates, A2Ir2O7 (where A is Y or rare-earth elements). Due to the experimental difficulties of getting a sizeable single crystal of pyrochlore iridates, previously, most of the studies were done in bulk polycrystals and very few on single crystal and epitaxial thin films. Thin films not only provide directional growth, but manipulation of the epitaxial strain can lead to the observation of many non-trivial magnetotransport phenomena, which could remain hidden in their bulk counterpart. In this thesis work, we have done detail structural, magnetic and magnetotransport studies on single crystalline epitaxial thin films of pyrochlore iridates grown on YSZ (111) substrate. The thin films are grown by means of two steps solid phase epitaxy technique; firstly an amorphous layer is deposited using a pulsed laser deposition (PLD) technique followed by ex-situ annealing in a furnace. Our detailed magnetotransport measurements identify the presence of Weyl points and the WSM phase in the studied pyrochlore iridates thin films. This thesis is organized as follows: In chapter 1, a general introduction to the transition metal oxides, especially of pyrochlore iridates, has been discussed. We have discussed how the combined effects of different fundamental interactions lead to many nontrivial electronic and magnetic ground states. Finally, we discussed the possibility of unconventional magnetotransport phenomena. In chapter 2, a description of all the experimental techniques along with their basic working principle has been mentioned, which include the thin film deposition by pulsed laser deposition (PLD). This is followed by sample characterization, such as atomic force microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, and magnetization. After that, device fabrication by optical lithography, argon ion etching, metalization, and packaging have been discussed. Lastly, we discussed the physical properties measurements system (PPMS), where all the magnetotransport measurements have been performed. In chapter 3, detailed thickness-dependent structural and electrical transport studies have been performed on Eu2Ir2O7 (111) epitaxial thin films. The systematic reduction in the film’s thickness causes expansion in the out-of-plane lattice parameter, possibly due to the systematic changes in the Ir/Eu stoichiometry in the films. The temperature-dependent electrical resistivity shows a metal-semimetal transition. The inverse residual resistivity ratio (1/RRR) systematically increases with the decrease in film thicknesses. In addition, the field-cooled magnetoresistance measurements verify the all-in-all-out/all-out-all-in (AIAO/AOAI) anti1 ferromagnetic (AFM) domains of Ir4+ 5d moments. This work reveals that film thickness can be a tuning knob of the ground state of Eu2Ir2O7 , which is of interest in searching the predicted 3D WSM phase. In chapter 4, we demonstrate evidence of low temperature anomalous and topological Hall effect (THE) in Eu2Ir2O7 (111) epitaxial thin film due to the real space and momentum space Berry curvature. The THE reaches a large value ρ THE xy ∼ 10 µΩcm at 2 K. The magnetoresistance (MR) shows a prominent negative variation (MR∼ 10% at 2 K) below 10 K. The field dependence of MR below 5 K varies quadratically in the low field regime, and above 40 kOe, shows a linear trend. The quadratic to linear crossover of the MR is explained by the fieldinduced spin canting (spin chirality) of the static spins in the AIAO/AOAI spin structure. In the intermediate temperature region, 15–25 K MR exhibits a hysteretic response associated with field-induced domain switching of the AIAO/AOAI spin structure. This work highlights the interplay of magnetism and electronic band topology in a spin-orbit coupled correlated electron system and unravels the possibility of realizing the WSM phase in antiferromagnetic Eu2Ir2O7 (111) thin films. In chapter 5, the spontaneous Hall effect has been studied on Eu2Ir2O7 (111) thin films. The high-temperature correlated-metallic part of the resistivity reveals the finite effect of the electronic correlation on the transport properties. A fingerprint of the Weyl semimetallic (WSM) phase in Eu2Ir2O7 (111) thin films is confirmed by the observation of a sizeable spontaneous Hall effect (SHE) with minimal magnetization (M). Furthermore, the values and temperature window of SHE shows strong sample dependence, which might arise due to minute Ir/Eu offstoichiometry and associated modification of the electronic band. In chapter 6, we studied the strain-induced modification of the magnetotransport properties of Sm2Ir2O7 (111) thin films. MR studies show the Ir4+ AIAO/AOAI spin structure is stable enough against the applied magnetic field at low temperatures (below 10 K). However, from 10 K onward, applying a magnetic field causes a change in the spin structure (from AIAO to three-out-one-in) and a plastic domain deformation. The Hall resistivity data shows anomalous/spontaneous Hall effect in its AIAO/AOAI antiferromagnetic phase. We have found that an epitaxial strain changes the sign and the magnitude of the anomalous Hall effect, possibly by modifying the electronic band dispersion and associated Berry curvature. Observation of epitaxial strain-induced large spontaneous Hall effect (∼ 50 Ωcm, at 7 K) identifies the presence of Weyl nodes and the Weyl semimetallic phase of Sm2Ir2O7 . In chapter 7, magnetotransport studies have been performed on Nd2Ir2O7 (111) epitaxial thin films. MR measurements show a field-induced modification of Nd3+ 4f spin structure and a plastic deformation of Ir4+ domain distribution. In contrast, the application of field (H) along [001] and [¯1¯10] directions could not modify any domain distribution. A large spontaneous Hall (SHE) signal has been observed for the application of field (H) along [001], [¯1¯10], and [111] directions. The appearance of a large spontaneous Hall signal for the applied field along [001] and [¯1¯10] directions rule out the domain switching as the origin of the Hall effect and confirm the presence of the WSM phase in Nd2Ir2O7 (111) thin films. In addition to SHE, a topological-like Hall signal appeared, possibly due to the presence of multiple Weyl points in the electronic band structure near the Fermi level.

In chapter 8, all the experimental observations during the course of investigations have been summarized. A future scope of fundamental research and potential application of this work is also proposed that will help the readers of this thesis further to carry out the important research problems in this field.