Session M37: Complex Oxide Thin Films: Novel Strain, Voltage, and Quantum Transport Phenomena

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A defining feature of emergent phenomena in complex oxides is the competition and cooperation between ground states. In manganites the balance between metallic and insulating phases can be tuned by the lattice[1]; extending the range of lattice control would enhance the ability to access other phases. Freestanding complex oxide membranes[2] provide unique opportunities to realize previously unobserved strain-induced responses. Oxide membranes can be mechanically coupled to an external platform without an epitaxial relationship, of which strain states and symmetries can be freely chosen by design. Moreover, the critical fracture strain is often greatly enhanced for small materials length scales, allowing tensile strain states larger than the typical values ceramics withstand. Here we stabilize uniform tensile strain in La_0.7Ca_0.3MnO₃ membranes, exceeding 8% uniaxially and 5% biaxially[3]. Uniaxial and biaxial strain suppresses the ferromagnetic metal at distinctly different strain values, inducing an insulator that can be extinguished by magnetic field. This highly-tunable strained membrane approach provides a broad opportunity to design and manipulate correlated electron states.

[1] Y. Tokura, Reports on Progress in Physics 69, 797 (2006).
[2] D. Lu et al., Nature Materials 15, 1255 (2016)
[3] S.S. Hong et al., Science 368, 71 (2020)   

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Application of a strong electric stimulus, voltage or current, to ferromagnetic (FM) oxide (La,Sr)MnO₃ (LSMO) triggers the metal-insulator transition producing a low- to high-state resistive switching. This switching occurs by the nucleation and growth of an insulating paramagnetic (PM) barrier perpendicular to the current flow, in contrast to conventional filamentary percolation parallel to the current. The voltage-induced barrier formation results in an unusual FM/PM/FM configuration leading to a dramatic change of magnetic anisotropy. Before the PM barrier nucleation, the devices have an easy-plane anisotropy showing nearly square hysteresis loops in any in-plane direction. After the barrier nucleation, the direction perpendicular to the barrier becomes the hard anisotropy axis, which is evidenced by the complete suppression of the remanence and coercivity and the strong enhancement of the saturation fields in the hysteresis loops. Our work shows that employing resistive switching is a viable strategy to achieve voltage-controlled magnetism.   

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Electrolyte gating has proven remarkably effective in controlling electronic, magnetic, and optical materials properties. Both electrostatic and electrochemical mechanisms occur [1], one example of the latter being the voltage-driven perovskite (P) to brownmillerite (BM) transition found in SrCoO_3-d [2], which realizes a ferromagnetic metal/antiferromagnetic insulator transition. In this work, we provide the first complete doping- and strain-dependent study of the ion-gel-gating-induced P to BM transition over the entire La_1-xSr_xCoO_3-δ phase diagram, via transport, magnetometry, and operando synchrotron X-ray diffraction. The electrically-actuated P to BM transition is found to occur over a wide x range, modulating the Curie temperature over ~220 K. Most significantly, the variation in threshold voltage with x and strain point to electrochemical activity controlled by the Co⁴⁺/Co³⁺ ratio, interpreted in terms of trends in thermodynamic rather than kinetic (i.e., oxygen vacancy diffusion) parameters. These results provide global insight into the electrochemical control of this model oxide.
1. Leighton, Nat. Mat. 18, 13 (2018).
2. Lu et al., Nature 546, 124 (2017).
3. Chaturvedi et al., in prep.   

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Recent advances in complex oxide heterostructures have realized extraordinary control over oxygen vacancies (V_O), including strain-tuned V_O order, and electric-field-controlled transformations between perovskite and V_O-ordered structures. Perovskite cobaltites such as La_1-xSr_xCoO_3-δ provide a prime example, recent work demonstrating that strain engineering of V_O ordering induces large (~10⁷ erg/cm³) perpendicular magnetic anisotropy. Here we show that V_O-ordered epitaxial La_0.5Sr_0.5CoO_3-δ films exhibit not only strong magnetic anisotropy, but also a giant form of anisotropic magnetoresistance (AMR) [1]. This has magnetic field, temperature, and angular dependencies in quantitative accord with conventional AMR, but with AMR ratios up to an extraordinary 40.3%, 20-times enhanced over bulk cobaltites, and ~10-100 times larger than typical transition metals. This giant AMR has no strong dependence on heteroepitaxial strain (between -2.1% and +1.8%) or thickness, and is instead ascribed to symmetry lowering associated with V_O ordering. The AMR ratios thus obtained are among the largest reported in the over 160-year history of this phenomenon, despite the absence of heavy elements.
[1] Walter et al., Phys. Rev. Mater. 4, 091401(R) (2020)   

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Controlling the atomistic defect in perovskite transition metal oxides facilitates tailoring of their emergent functionality, especially in terms of opto-electromagnetic and energy properties. Pulsed laser epitaxy (PLE) offers a deliberate control of the formation of defects in perovskite oxides, leading to a modulation of the charge, spin, lattice, and orbital degrees of freedom, sometimes separately, and in other times collectively. Perovskite EuTiO₃ (ETO) is a promising candidate for studying the strong coupling between the lattice, electronic, and magnetic ground state by introducing defect and/or epitaxial strain, because it exhibits a magneto-electric phase transition from a paraelectric (PE) antiferromagnetic (AFM) to a ferroelectric (FE) ferromagnetic (FM) phase. Here, we systematically induced defects in ETO thin films using PLE. The lattice unit cell (u.c.) volume was modulated by defect engineering, which was closely related to the FM ordering in the thin film. The change in the electronic structure further supported the FM phase with weaken Eu-Ti-Eu superexchange interaction.   

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Controlling ionic distribution and stoichiometry in complex oxide heterostructures has been utilized to significantly alter and tune functional properties. Recently, deposition of a strong oxygen getter on top of an oxide thin film has emerged as a novel way to tailor oxygen stoichiometry and nanoscale functional properties. In this talk, I will focus on Gd/La_0.67Sr_0.33CoO₃ (LSCO) heterostructures due to the high oxygen ion conductivity, as well as the coupled magnetic and electronic properties of LSCO, which are strongly dependent on the oxygen stoichiometry. This combination of properties enable the ionic control of the functional properties of LSCO thin films through the presence of oxygen getter layers such as Gd. We utilize x-ray nanodiffraction to directly image the nanoscale morphology of LSCO thin films as they are progressively transformed from the equilibrium perovskite phase to the metastable brownmillerite (BM) phase with increasing Gd thickness. Our studies show the coexistence of perovskite and BM phases with a critical oxygen vacancy concentration threshold which leads to the formation of extended BM filaments. Our studies provide an unprecedented nanoscale survey of the phase separation in the cobaltites and shed light on the formation of the metastable BM phase.   

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Resistive switching effects offer new opportunities in the field of conventional memories as well as in the booming area of neuromorphic computing. The tunneling electroresistance, usually observed in ferroelectric tunnel junctions, allows for large and fast resistance variations triggered by voltage pulses. We have recently demonstrated that similar tunnel resistance switching effects can be produced in judiciously chosen metal/oxide junctions by an electrochemical (redox) mechanism (1). Here we show that this mechanism can be exploited in junctions based on NdNiO₃, an interesting material due to its first-order metal-insulator transition that adds an additional degree of freedom to the studied devices. We find that the studied NdNiO₃/MoSi junctions present large tunnel electroresistance (several orders of magnitude), and that, depending on the temperature, the lifetime of the resistance states can be largely tuned. Furthermore, the resistance switching is accompanied by a shift of the metal-insulator transition temperature, which further supports the redox scenario.

(1) Rouco, V., et al. Quasiparticle tunnel electroresistance in superconducting junctions. Nature communications, 11(1), 1-9 (2020)   

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We explore the electronic and magnetic transitions of strained thin film Eu_1-xLa_xTiO₃ (0< x <1) grown by oxide molecular beam epitaxy (MBE). In bulk, undoped EuTiO₃ (Ti⁴⁺) is insulating and antiferromagnetic with a Néel temperature of ~ 5.5 K, while LaTiO₃(Ti³⁺) is a Mott insulator with a Néel temperature of ~ 160K. Previous experiments of doping EuTiO₃ or LaTiO₃ thin films show novel emergent states, such as antiferromagnetic or ferromagnetic, anomalous Hall effect, and metal-insulator transition. We show the properties of high-quality thin films of Eu_1-xLa_xTiO₃ from x = 0 to x = 1. With a different ratio of Eu²⁺/La³⁺, we observe a series of transitions in the films. We will complete the phase diagram of this system.

H. Shin, et al., PHYS. REV. B 101, 214105 (2020)   

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Emergence of the magnetic Weyl semimetal state has recently been reported in ultrahigh-quality SrRuO₃ thin films [1,2], providing a new platform for exploring novel physics related to a pair of Weyl nodes in epitaxial heterostructures. However, there is a dearth of knowledge on the thickness dependence of the quantum transport properties peculiar to the magnetic Weyl semimetal state in it, and hence, we investigated magnetotransport properties of SrRuO₃ with various thicknesses (t = 1–60 nm). Signatures of Weyl fermions—unsaturated linear positive magnetoresistance accompanied with a quantum oscillation having a non-trivial Berry phase—were observed for t ≥ 10 nm. The residual resistivity increases with decreasing film thickness, indicating more disorder near the interface. This thickness-dependent scattering affects magnetic properties as well. The Curie temperature decreases and the coercive field increases with decreasing thickness. These results provide a guideline for designing SrRuO₃-based functional heterostructures in which Weyl fermions play an essential role.
[1] K. Takiguchi, Y. K. Wakabayashi et al., Nat. Commun. 11, 4969 (2020).
[2] Y. K. Wakabayashi et al., APL Mater. 7, 101114 (2019).   

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Controlling the electronic and magnetic properties in strongly correlated electron systems has been widely investigated through engineering the lattice by chemical pressure effects. Here, we present a systematic study on the isovalent substitution in the growth of epitaxial SrRu_1-xSn_xO₃ (0 < x < 1) thin films by pulsed laser deposition. Since the quaternary members of this system is unstable in bulk, we used the thin film approach to stabilize the phase at different concentrations of x. X-ray diffraction, transport, and magnetometry measurements were performed to build a phase diagram relating to the functional properties at certain x concentrations. As the evolution of x increase from SrRuO₃ (a metallic ferromagnetic) to SrSnO₃ (an insulating diamagnetic), we observed a metal-insulator transition. This talk will focus on the structure, electronic, and magnetic properties due to the influence of chemical doping, highlighting doping as a route to tune functional properties in 4d transition metal oxides.   

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Research on quantum transport in high electron mobility oxides started from such conventional semiconductors as ZnO and SrTiO₃ (STO). To extend the research further, EuTiO₃ (ETO) is an ideal system by adding a knob of magnetic control (local 4f⁷ moment on Eu²⁺). We employ a metalorganic gas source molecular beam epitaxy (gas source MBE) at very high substrate temperatures that enabled us to observe quantum Hall effect in STO quantum well [1]. In the case of strained films of La doped ETO, we observed additional terms in the anomalous Hall effect during the magnetization process, which is not proportional to the magnetization [2]. The mobility in ETO films for strain-free films on STO substrates has reached to 3,000 cm²V^-1s^-1 at 2 K [3]. Our samples exhibited strong Shubnikov–de Haas (SdH) oscillations in a forced-ferromagnetic state above 4 T at 2 K, making them distinct from the conventional magnetic semiconductors whose lack of high carrier mobility is known to suppress such quantum oscillations. Our band calculation suggests that the high mobility electrons populate in the bands, which are originally derived from Ti 3d orbitals and are fully spin polarized and modified by the f-d coupling between Eu 4f and Ti 3d orbitals. These new findings strongly suggest that the electron-doped ETO film with high mobility is an ideal magnetic semiconductor to explore novel magneto ‘quantum’ transport phenomena.

References:
[1] Y. Matsubara, K. S. Takahashi et al., Nat. Commun. 7, 11631 (2016).
[2] K. S. Takahashi et al., Sci. Adv. 4, eaar7880 (2018).
[3] K. Maruhashi, K. S. Takahashi et al., Adv. Mater. 32, 1908315 (2020).