Session Q05: Magnetic Topological Thin Films and Heterostructures III

Meron and Chiral Kondo lattices in topological insulators and heterostructures

In this presentation, we present two topological phenomena emerging from the interplay of strong spin-orbit coupling and correlation effects. Firstly, I will discuss the fate of the surface Dirac cone of a three-dimensional topological insulator subject to a superlattice potential. Due to the topological nature of the bulk, surface band gaps cannot open; instead, prominent van Hove singularities emerge around charge neutrality. The latter combined with the strong spin-orbit coupling and Coulomb repulsion, give rise to a topological meron lattice spin texture. Numerical calculations are supported by a Ginzburg-Landau theory that we employed to classify the different magnetic orders of the surface Dirac modes. Shifting gears, the investigation then turns to transition metal dichalcogenides heterobilayer systems. This highly tunable platform enables unprecedent control over the ground state, opening the door to the synthetic realization of an orbital selective Mott transition. Here, non-local spin exchange processes lead to a chiral Kondo coupling, adding a unique twist to heavy fermion physics. We discuss the distinctive features of this state, including a strong dependence of the Kondo temperature on electron density, an anomalous Hall effect induced by chiral exchange, and the emergence of a topological spin-Hall Kondo insulator at integer filling. Finally, we present a purely repulsive mechanism leading to superconductivity in the small Fermi surface regime where the local moments are magnetically ordered. The implications of these findings for future experiments are discussed.   

Current-induced torques in nanoscale Weyl semimetals with magnetic textures

Manipulation of magnetization has been one of the primary objectives in the field of spintronics. Magnetic Weyl semimetals can be an interesting platform to reach this objective by using transport currents due to their underlying  band geometry and topology. In this work, we consider disk-shaped Weyl semimetals with inhomogeneous magnetization, subject to transport current flow. We show that the transport current produces an axial magnetization current due to orbital magnetic moments of the Weyl electrons. Combined with the pseudomagnetic fields produced by the magnetic texture, the axial current can generate a torque acting on the localized magnetic moments. For the case of magnetic vortices, the pseudomagnetic field arising from the inhomogeneous magnetization can be used to produce current-induced torque on the boundary of a vortex, which could reverse its circulation.

  

Unusual transport phenomena in a topological pyrochlore antiferromagnet

Pyrochlore iridates are topological antiferromagnets renowned for their exotic electronic characteristics stemming from entwined spin-orbit coupling, electronic correlation, and geometrical frustration. They exhibit a magnetic Weyl semimetallic phase (WSM) that is intrinsically coupled with a unique all-in-all-out antiferromagnetic (AFM) order. However, in bulk, the cubic symmetry of pyrochlores strictly enforces the anomalous Hall effect to disappear, hindering the study of emergent transport properties arising from the interplay of the WSM and AFM phases. In this presentation, we report on unusual magneto-transport phenomena emerging in (111)-oriented thin films of pyrochlores iridates. We highlight previously unidentified exotic transport behavior originating from the Weyl nodes and the Fermi-arc surface states. Collectively, our findings suggest that (111)-oriented pyrochlore iridates thin films represent a promising avenue for in-depth exploration of the interplay of relativistic Weyl fermions and magnetism in the synthetic frustrated kagome-triangular lattices.   

Electronic nematicity at the interface of a pyrochlore heterostructure

Quantum materials with pyrochlore structures harbor a plethora of exotic electronic phases, such as magnetic Weyl semimetal, spin ice, and quantum spin liquid. An interface between two dissimilar pyrochlore compounds can feature a strong interplay between distinct many-body phases, supporting emergent interfacial phenomena. In this presentation, we report on the evidence for the emergence of electronic nematicity at the interface of rare-earth iridates and titanates pyrochlores. We will unveil experimental signatures for the unusual coupling between non-trivial band topology, electronic correlations, and geometrical frustration. Additionally, we will compare our system to other correlated topological materials with similar electronic nematicity. Our work suggests ingredients for designing emergent magnetic topological phenomena by pyrochlore heterostructures.    

Magnetic ground state of the pyrochlore iridate Ho2Ir2O7

Rare-earth pyrochlore iridates R₂Ir₂O₇ (e.g., R = Y, rare-earth elements) are an exciting class of quantum materials that exhibit emergent electronic and magnetic phenomena. A little-explored member of the family, Ho₂Ir₂O₇, has been reported as a spin-ice iridate; however, the presence of two distinct magnetic sublattices on Ho³⁺ (4f-electrons) and Ir⁴⁺ (5d-electrons) has opened intriguing questions about each sublattice contribution to the overall electronic and magnetic properties of this pyrochlore iridate. In this talk, we will present our recent results on the first successful synthesis of high-quality large-size (111)-oriented Ho₂Ir₂O₇ thin films and mapping the magnetic ordering of the Ir sublattice. Our results reveal the intriguing effects of the large Ho moments on the Ir ordering.   

Phenomenological Insights into Exotic Interfacial Phenomena in a Novel Pyrochlore Heterostructure

Pyrochlores are known to show exotic phases of matter, such as topological Mott insulators, Axion insulators, Weyl semimetals, and spin liquids, etc. The interface of two pyrochlore materials can feature a strong interplay of topology, magnetism, and geometrical frustrations. In a recent study by Wu et al., the authors report the emergence of nematicity at the interface of a novel pyrochlore heterostructure. Our study theoretically described the observed phenomena, offering qualitative insights that help elucidate the observed experimental anomalies. In this presentation, I will show that the interface of the pyrochlore heterostructure realizes a Kondo interaction between the two parent compounds, and by treating both the two dissimilar phases of matter across the interface and the coupling between them concomitantly, one could capture most of the experimental observations. Our findings spotlight a promising approach for describing new synthetic quantum materials with complex electronic dynamics unattainable in bulk crystals.   

Optical magnetic circular dichroism in all-in-all-out antiferromagnet Eu2Ir2O7

Pyrochlore iridate, R₂Ir₂O₇ (R = rare earth elements), is a good platform to study various interesting electronic phases due to the strong spin-orbit coupling. One example is the Weyl semimetal (WSM) state, which was firstly predicted in pyrochlore iridates, can be present in the all-in-all-out (AIAO) spin configuration of the Ir⁴⁺ sites. The WSM hosts linear crossings in the momentum space where Berry curvature accumulate, which can give rise to a large intrinsic anomalous Hall effect. Recently, a finite anomalous Hall conductivity is observed in (111)-oriented epitaxial film of Eu₂Ir₂O₇. In this work, we report magnetic circular dichroism (MCD) with 800 nm optical frequency in the AIAO state of Eu₂Ir₂O₇ thin films.  It indicates a net magnetization after field cooling. The reverse sign of MCD under opposite cooling fields reveals the magnetic origin and selective formed domains between AIAO and all-out-all-in (AOAI). The net magnetization agrees with previous reported lattice distortion in the thin film form. Our work stimulates studies of topological states in pyrochlore iridates and applications in the thin films.   

Nonlinear and nonreciprocal transport effects in untwinned thin films of ferromagnetic Weyl metal SrRuO3

In topological Dirac and Weyl semimetals, nontrivial conical bands with Fermi-arc surfaces states give rise to negative longitudinal magnetoresistance due to chiral anomaly effect and unusual thickness dependent quantum oscillation from Weyl-orbit effect, which were demonstrated recently in ferromagnetic Weyl metal SrRuO₃ (SRO) thin films. In this work, we report the experimental observations of large nonlinear and nonreciprocal transport effects for both longitudinal and transverse channels in an untwinned Weyl metal of SRO thin film grown on a SrTiO₃ substrate. From rigorous measurements with bias current applied along various directions with respect to the crystalline principal axes, the magnitude of nonlinear Hall signals from the transverse channel exhibits a simple sinα dependence at low temperatures, where α is the angle between bias current direction and orthorhombic [001]_o, reaching a maximum when current is along orthorhombic [1-10]_o. On the contrary, the magnitude of nonlinear and nonreciprocal signals in the longitudinal channel attains a maximum for bias current along [001]_o, and it vanishes for bias current along [1-10]_o. The observed α-dependent nonlinear and nonreciprocal signals in longitudinal and transverse channels reveal a magnetic Weyl phase with an effective Berry curvature dipole along [1-10]_o, accompanied by 1D chiral edge modes along [001]_o.   

Intrinsic anomalous Hall conductivity and field-enhanced diamagnetism in untwinned Weyl metal SrRuO3 thin films

The anomalous Hall effect (AHE) manifests in solids with broken time-reversal symmetry. In a ferromagnetic metal, intrinsic and extrinsic AHE can contribute to the Hall signal. In this report, we studied the low-temperature magnetotransport properties of ferromagnetic untwinned SrRuO₃ (SRO) thin films grown on SrTiO₃ (001) substrates with low residual resistivity for various thicknesses (ts) ranging from 3.9 nm to 37.1 nm. The magnitude of the Hall conductivity |σ_xy| at zero magnetic fields in the low-temperature regime was found to be nearly t-independent and approaching a constant value of about 2×10⁴ Ω^-1m^-1, which is close to the estimated intrinsic anomalous Hall conductivity due to Berry curvatures of the bulk band of about e²/hc_o ≈ 5×10⁴ Ω^-1m^-1 (c_o being the lattice constant). The negligible variation of |σ_xy| with σ_xx in the low-temperature regime at zero magnetic field reveals that the |σ_xy| is nearly independent of the electron scattering lifetime. This suggests a small contribution of extrinsic skew scattering effect to the AHE, and the intrinsic AHE dominates the zero-field Hall signals in SRO at low temperatures. Interestingly, a rigorous magnetization measurement on a SRO film using a SQUID magnetometer reveals a field-enhanced diamagnetic response that increases as the temperature drops. The intrinsic value of the |σ_xy|~ e²/hc_o and the field-enhanced diamagnetic response in the SRO film in the low-temperature regime strongly support the presence of the Weyl metal phase in SRO. A systematic study on the temperature and thickness-dependent Hall conductivity and magnetization will be presented and discussed.   

New route to synthesizing thin film and heterostructures of magnetic pyrochlore iridates.

This presentation introduces a new method for growing high-quality pyrochlore iridate thin films. Towards this goal, we have successfully achieved quality film growth by integrating our previously developed SPE technique [1] with a spin-ice pyrochlore buffer layer. The film's structural and electronic quality including the interface are examined using advanced techniques such as STEM imaging, HAADF/EELS elemental mapping, and a detailed HAADF/ADF comparative study. Furthermore, we assess the surface quality through x-ray reflectivity and atomic force microscopy. The enhanced growth efficiency of these pyrochlores thin films is attributed to two key factors: the lattice compatibility between the two pyrochlore structures of the iridate and titanate layers and our recently developed layer-by-layer spin-ice synthesis along the [111] direction [2]. These novel heterostructures pave the way for further in-depth inquiry into the topological electronic and magnetic properties inherent to the magnetic pyrochlore iridates.

    

Two-Stage Growth Mechanism for Synthesizing Pyrochlore Iridate Thin Films

To harness the unique topological and magnetic characteristics of pyrochlore iridates, we introduce a new two-stage growth strategy to fabricate stoichiometric and epitaxial thin films. The first synthesis stage focuses on achieving an amorphous continuous network, subsequently transitioning to the critical solid phase epitaxy (SPE) stage. In the solid epitaxy (SPE) stage, we employ an in-situ post-annealing method in a pure oxygen environment to ensure the preservation of stoichiometry and structural uniformity across the film's depth. A distinguishing feature of this SPE stage is the use of directional planar laser-heat annealing, drastically enhancing the transformation kinetics from the glassy to the crystalline phase. Also, this work draws interesting parallels with other pyrochlore iridates and titanates fabrication techniques, setting a new approach for the synthesis of epitaxial films of pyrochlores with the platinum group metals   

Magnetic and Transport Properties of Epitaxial Thin Films of Kagome Ferrimagnet RMn6Sn6 (R = Er, Tb)

Kagome ferrimagnets RMn₆Sn₆ family is a highly tunable system for studying the interplay between magnetism and non-trivial band structures (e.g., Dirac cones, flat bands). In particular, a large intrinsic anomalous Hall effect has been demonstrated in out-of-plane magnetized TbMn₆Sn₆ bulk crystals, making it a promising candidate material for realizing high-temperature quantum anomalous Hall effect. Looking forward, the growth of RMn₆Sn₆ thin films is highly desired for both research and applications. Here we discuss the molecular beam epitaxy (MBE) growth, magnetic properties, and transport properties of ErMn₆Sn₆ and TbMn₆Sn₆ thin films. The ErMn₆Sn₆ and TbMn₆Sn₆ thin films are grown on Pt(111) buffer layers using atomic-layer MBE. The structures of the thin films are confirmed by a combination of reflection-high energy electron diffraction (RHEED), X-ray diffraction (XRD), and atomic force microscopy (AFM). Our SQUID measurements show that TbMn₆Sn₆ thin films have uniaxial anisotropy below room temperature, while ErMn₆Sn₆ thin films favor in-plane magnetization. We have also developed a semiclassical theory based on the Fuchs-Sondheimer model to analyze the transport properties of RMn₆Sn₆/Pt bilayers.   

Synthesis and physical properties of epitaxial Mn3Sn thin films.

The large Anomalous Hall Effect (AHE) and electrical switching capabilities of Mn₃Sn thin films at room temperature provides opportunities for a variety of technological applications. The current understanding of the static magnetic structure and domain switching in these thin films is inferred from transport measurements and neutron diffraction from single crystals. To gain further insight into the physical properties of Mn₃Sn in a device like format, we have grown and characterized epitaxial Mn₃Sn thin films with thicknesses up to 100 nm and performed X-ray diffraction and AHE measurements to probe the structural and electronic properties. We describe optimization of the film growth process and the dependence of the electronic structure on film thickness. We achieve high crystalline quality films with sample volumes that enable future neutron diffraction experiments.