Session P46: Complex Oxide Interfaces & Heterostructures -- Skyrmions & Novel Magnetism

Observation of Nanoscale Skyrmions in SrIrO3/SrRuO3 Bilayers and SrRuO3 Single Layers in Two Distinct Regimes

The advent of skyrmion imaging and electrical detection is an exciting avenue of research as skyrmions hold promise for next-generation magnetic storage. Oxide materials are a great platform to study in this regard owing to their highly tunable properties, pristine epitaxy, and stability. In this talk, I will show Hall detection and real-space imaging of nanoscale skyrmions in perovskite oxide heterostructures of SrIrO₃/SrRuO₃ (SIO/SRO) epitaxial films grown on SrTiO₃(100) substrates. We show regions of large topological Hall resistivity at low temperatures which coincide with the field where the magnetization reverses. Additionally, we present a new high-temperature topological Hall effect not seen before in both bilayer SrIrO₃/SrRuO₃ and single layer SrRuO₃ films. This high temperature phase manifests as a “coercive field switching” in the electrical data and, consequently, as a large topological Hall effect signal. To corroborate our topological Hall signals with skyrmions, we have collected real-space images of isolated skyrmions with low-temperature magnetic force microscopy and show nanoscale skyrmions with diameters as small as 10 nm. Remarkably, the region where skyrmion bubbles are present precisely coincides with the topological Hall peaks. Our results open a platform for tunable nanoscale skyrmions in functional oxide materials.
  

Observation of room-temperature polar skyrmions in oxide superlattice

Recent discovery of polarization flux closures, vortices and skyrmions in polar oxide superlattices suggests the presence of a complex, multi-dimensional system capable of exotic physical responses[i]^,[ii],[iii], where the exploration of novel emergent phenomena and exotic phases in condensed-matter physics is greatly facilitated. Here, we have observed a room-temperature polar skyrmions in a PbTiO₃/SrTiO₃ superlattices grown on SrTiO₃(001) substrate. X-Ray diffraction and large scale scanning transmission electron microscopy imaging confirmed the present of the polar skyrmions phase. In addition, phase-field modeling and second-principles calculations reveal that the polar skyrmions have a skyrmion number of +1. Moreover, special uniform chirality was revealed by resonant soft X-ray diffraction experiments showing obvious circular dichroism. Such nanometer-scale polar skyrmions are the electric analogs of magnetic skyrmions, and could advance ferroelectrics towards new levels of functionality.
[i] Das, S., Tang, Y. L. et. al, Nature (under reviw)；[ii] Yadav, A. K. et. al, Nature 530, 198-201 (2016).；[iii] Shafer, P., García-Fernández. P. et al., Proc. Natl. Acad. Sci. USA 115, 915 (2018).   

Electric field Controllable “Negative Capacitance” in Polar Skyrmions: Topological transition?

Complex topological configurations are a fertile arena to explore novel emergent phenomena and exotic phases in condensed-matter physics. For example, the recent discovery of polar skyrmions, vortices in superlattices of (PbTiO₃)_n/(SrTiO₃)_nsuggests the presence of a complex, multi-dimensional system capable of exotic physical responses. Here, we demonstrate electric field controlled room-temperature negative capacitance and topological phase transition in polar skyrmions. In epitaxially grown heterostructures of PbTiO₃ and SrTiO₃capacitance was found to be larger compared to its indivual constituent's capacitance SrTiO₃, PbTiO₃. This indicates the indicates the ferroelectric was stabilized in a state of negative capacitance at room temperature. This phenomenon could be controlled by electric field and temperature. The STEM measurement, Phase field and Second principle calculation confirms the stable negative capacitance is due to boundary of polar skyrmions.Such phenomena could advance ferroelectrics towards new levels of functionality.
Das, S, Tang, Y. L. et. al, Nature (under reviw);Yadav, A.K., Nelson, C.T. et al., Nature 530, 198-201 (2016);Shafer, P., García-Fernández. P. etal., Proc. Natl. Acad. Sci. USA115, 915 (2018).   

Resonant X-ray diffraction study of chiral polar skyrmions in PbTiO3/SrTiO3 superlattices

Emergent topologies in ferroelectric heterostructures—the polar analogs of magnetic vortices and skyrmions—have become a recent topic of interest for their potential to host exotic functionalities (e.g., emergent chirality and negative capacitance). These topologies can be stabilized in low-dimensional ferroelectrics; namely, superlattices of PbTiO₃/SrTiO₃. Polar skyrmion structures have been observed in PbTiO₃/SrTiO₃ superlattices grown on SrTiO₃ substrates with a combination of scanning transmission electron microscopy, X-ray diffraction, and second-principles calculations. Using resonant soft X-ray diffraction, we study the chirality of these polar skyrmions and show that the skyrmions have a preferred handedness. The origin of the circular dichroism is shown to be a chiral configuration of the titanium orbitals in the skyrmions.   

Living on the edge: multiple phase competition and topological order in pyrochlore oxides

Pyrochlore oxides, A2B2O7, are one of nature's best tricks - ubiquitous minerals, capable of incorporating an extremely wide range of metal cations. Over the past decade these materials have risen to prominence in the community studying frustrated magnetism, for their ability to instantiate so many different novel, and topological phases of matter. Best known among these is spin ice, celebrated for its magnetic monopole excitations, but small changes in chemistry can lead to a wide range of other exotic phonomena, including both classical and quantum spin liquids.

In this talk, we explore one particular avenue in this lively field, namely the multiple-phase competition which occurs in magnetic oxides with strong spin-orbit coupling. Considering Yb2Ti2O7, Er2Ti2O7 and Er2Sn2O7 as examples, we show how the seemingly diverse physics of these systems can be understood through a single unifying principle, namely the interplay between different competing forms of order [1]. We also explore how this conceptual framework can be used to "design" new forms of spin liquid, including phases which realise fracton topological order.

Finally, we pose the question, could different modes of growth give us access to pyrochlore oxides with even more exotic, or controlable, properties ?

[1] Han Yan, Owen Benton, L.D.C. Jaubert and Nic Shannon, Phys. Rev. B 95, 094422 (2017)   

Anisotropic magnetoresistance in multiorbital systems

Magnetotransport is a useful probe for studying magnetism, electron-electron interactions, as well as the underlying symmetries of a crystal. We study the impact of an in-plane magnetic field on 2D multiorbital electron gases as a function of electron density, scattering length, temperature, and spin-orbit interactions. These parameters affect the shape of the different spin-split Fermi surfaces, which in turn is shown to set the amplitude of the anisotropic magnetoresistance and its dominant symmetry components. We comment on the validity of the relaxation-time approximation and the necessity to resort to the full Boltzmann equation.   

Imaging, controlling and harnessing non-collinear magnetism in perovskite oxides

In magnetic perovskites, first-neighbour antiferromagnetic super-exchange interactions usually dominate, but may coexist with other terms such as ferromagnetic double-exchange or Dzyaloshinskii-Moriya interactions. This often produces non-collinear spin configurations leading to weak ferromagnetism or to spatially modulated spin structures. A prototypical non-collinear magnetic oxide is multiferroic BiFeO₃ that shows a cycloidal order with a 64 nm period in the bulk. In this talk, I will present real-space images of the cycloidal structure and its manipulation by electric field [1]. In a second part, I will report the observation of a very large topological Hall effect (THE) in thin films of a lightly electron-doped manganite. Magnetic force microscopy reveals the presence of small magnetic bubbles, whose density vs. magnetic field peaks near the THE maximum, as is expected to occur in skyrmion systems. The THE critically depends on carrier concentration and diverges at low doping, near the metal-insulator transition, suggesting its amplification by strong correlations, in the vicinity of the Mott transition [2].
[1] I. Gross et al, Nature 549, 252 (2017).
[2] L. Vistoli et al, Nature Phys. (2018) http://dx.doi.org/10.1038/s41567-018-0307-5   

High-throughput Design of Interfacial Perpendicular Magnetic Anisotropy at Heusler/MgO Heterostructures

Thin magnetic tunnel junctions (MTJs) with a large interfacial perpendicular magnetic anisotropy (K_i) have great applications in spin transfer torque magnetoresistance random access memories. A large K_i is required for achieving a high thermal stability of the MTJ, and thus to design an interfacial perpendicular magnetic anisotropy at the interface between ferromagnetic electrodes and oxide barriers is of great interests. Here we show that, by modelling Heusler/MgO heterostructures using high-throughput first-principles electronic structure calculations, we are able to rapidly identify a series of Heusler/MgO heterostructures with a large K_i value. This work shows an effective way to design novel magnetic materials via high-throughput electronic structure calculations.