Session G54: Emergent Properties of Complex Oxides Bulk, Thin Films, and Heterostructures II

Authors: Jian Liu, University of TennesseeCorrelation-topology Interplay in pseudospin-half square-lattice in artificial iridate superlattice.

Fundamental interests arise in systems where both electronic correlation and electronic topology play significant roles. Their interplay could provide not only new routes to unresolved problems but also opportunities for stabilizing novel quantum states. However, a key challenge in this extensive and largely unexplored regime is the fact that topological properties, such as Berry phase, concern extended electronic wavefunctions described in momentum space while electrons are localized in real space in correlated systems. This dilemma calls for toy-model materials where phenomena driven by the topology-correlation interplay can be captured and controlled. In this talk, I will discuss our recent work on experimental realization of artificial iridate superlattices that simulate the single-orbital square-lattice Hubbard Hamiltonian with engineered complex hopping. While the correlated pseudospin-half electrons have an antiferromagnetic Mott insulating ground state, their intermediate coupling strength allows significant longitudinal spin fluctuations, which manifest as anomalous magnetoresistance and anomalous Hall effect through the SU(2) symmetry-preserving and breaking components of the complex hopping, respectively. The latter also leads to an exceptionally large Ising anisotropy captured as a giant magnon gap beyond the superexchange approach. These unusual phenomena of a 2D Mott insulator highlight the rich interplay of electronic topology and electronic correlation in the intermediate-coupling regime.

    

Spontaneous Magnetization Reversal in La0.7Sr0.3MnO3 and LaMnO3 Thin Films

Utilizing many techniques (bulk magnetometry, neutron reflectometry and x-ray magnetic circular dichroism), we have discovered and explored spontaneous magnetization reversal in high-quality complex oxide La_0.7Sr_0.3MnO₃ and LaMnO₃ thin films grown via pulsed laser deposition, with reflection high energy electron diffraction (RHEED) monitoring to verify layer-by-layer growth. The spontaneous magnetization reversal occurs at low applied fields and originates from the competition between different types of magnetic order. An early explanation for this strong negative magnetization was increased Sr content at the interface, however, this is not present in our films, particularly in LaMnO₃. While the overall effect is observed across many sample thicknesses and oxygen growth pressures, these parameters affect the behavior systematically. Despite excellent film quality, magnetocaloric measurements suggest that these films have comparable relative cooling powers to their nanoparticle partners.   

Orbital-ordered ferromagnetic insulating state in tensile-strained SrCoO3 thin films

At ambient pressure, bulk SrCoO₃ (cubic perovskite) is a ferromagnetic (FM) metal. By contrast, magnetic properties of epitaxial SrCoO₃ thin films, especially at high tensile strain (ε ≥ 3%), remain unclear: Previous calculations had predicted antiferromagnetic (AFM) states more energetically favorable in this regime [1], but recent experiments suggested an FM insulating state [2]. In this work, using first-principles calculations, we perform extensive search for the structural, spin, magnetic, and orbital states of SrCoO₃ thin films. Our calculations indicate that for 0 < ε ≤ 2.5%, SrCoO₃ thin films favor an FM half-metallic state with intermediate-spin (IS, t_2g⁵e_g¹-like) Co exhibiting d⁶L character (Co³⁺ accompanied by O 2p holes). For ε ≥ 2.5%, an FM insulating state with high-spin (HS, t_2g⁴e_g²-like) Co prevails. This FM insulating state is achieved via complicated orbital ordering, cooperative Jahn–Teller distortion, and octahedral tilting about all crystal axes.

[1] J. H. Lee and K. M. Rabe, Phys. Rev. Lett. 107, 067601 (2011).

[2] Y. Wang et al., Phys. Rev. X, 10, 021030 (2020).   

Giant spin-charge conversion in an all-epitaxial single-crystal-oxide Rashba LaTiO3+δ/SrTiO3 heterostructure

The two-dimensional electron gas formed at interfaces between SrTiO₃ (STO) and other insulating oxide materials has attracted much attention for large spin-charge conversion due to the sizable Rashba spin-orbit interaction. However, those insulating layers grown on STO prevent the propagation of the spin current injected from an adjacent ferromagnetic layer. Here, instead of the insulating layers, we use a strongly correlated polar-metal LaTiO_3+δ (LTO), demonstrating a giant conversion efficiency λ_IEE up to ~190 nm, which is the highest value among those reported for all materials [1].

For spin-pumping experiments, we have grown an all-epitaxial (La,Sr)MnO₃ (30 u.c.)/LTO (3 u.c.) heterostructure on an STO (001) substrate. Our spin-pumping measurements show λ_IEE up to 193.5 nm at 15 K. Our tight-binding calculation, in which the band parameters are optimized to reproduce the band structure observed by angle-resolved photoemission spectroscopy, can quantitatively explain the temperature dependence of λ_IEE. This highly efficient conversion highlights the new hidden inherent possibilities of oxide interfaces.

This work was supported by Grants-in-Aid for Scientific Research, CREST and PRESTO of JST, Spin-RNJ, and ANRI fellowship.

[1] S. Kaneta-Takada et al., Nat. Commun. 13, 5631 (2022).   

Magnetism and proximity-induced Rashba effect at Mn-3d bands of asymmetric BaMnO3|KTaO3 heterojunction

Rashba-like spin-orbit interaction (SOI) at oxide heterostructures emerges as a much sought-after feature in the context of oxide spintronics and spin-orbitronics. KTaO₃ (KTO) is one of the best substrates available for the purpose, owing to its strong SOI and alternating +1|-1 charged layers along the (001) direction. We visualize the Rashba-like interaction in KTO (001) surface with the help of spin texture plotted directly from density functional theory (DFT) calculations along with the isoenergetic contours, providing a confirmatory test of the presence of only linear Rashba interaction. We use DFT calculations to examine the asymmetric BaMnO₃|KTaO₃ (BMO|KTO) oxide heterostructure where the inequivalent bottom and top interfaces break the inversion symmetry due to their opposite polar discontinuities. We observe Rashba-like splitting for the bands of Mn-3d near the Fermi level of C-type antiferromagnetic (BMO|KTO) owing to the proximity to Ta atoms from the 5d series. We comprehensively analyze Rashba-like SOI with the help of three-dimensional band dispersion, isoenergetic contours, and projected spin textures for Rashba-like Mn-3d bands. Our results reveal reasonably strong linear Rashba interaction in the heterostructure. The rigorous analysis of spin textures of the antiferromagnetic heterostructure presented here may be crucial for future developments in spintronics.

    

Linear-in-temperature resistivity in PdCrO$_2$ from magneto-elastic interactions

The kindred delafossite metals PdCrO$_2$ and PdCoO$_2$ exhibit a strikingly different temperature dependence of the in-plane resistivity at intermediate temperatures. The magnitude of the resistivity in the former displays a sublinear approach to a pronounced T-linear scaling. This difference, apparently linked to antiferromagnetic correlations in PdCrO$_2$, cannot be fully accounted for by conventional scattering mechanisms. Motivated by this puzzle, we consider a particular coupling of electrons to magneto-elastic (EME) interactions in a 3D system composed of alternately stacked layers of itinerant electrons and antiferromagnetic Mott insulators. We show that EME interactions can explain the anomalous transport properties of PdCrO$_2$ and offer several experimental predictions to showcase the effects of the EME coupling.   

Length Scale Dependence of Anisotropic Magnetoresistance in Phase-Separated (La1-yPry)1-xCaxMnO3 Thin Film Microstructures

Electronic phase coexistence occurs in perovskite manganites such as (La_1-yPr_y)_1-xCa_xMnO₃ (LPCMO) between ferromagnetic metallic (FMM) and charge-ordered insulating (COI) phases, leading to unique electronic and magnetic properties. For example, in thin films of LPCMO grown on (110) NdGaO₃ (NGO) substrates, anisotropic strain leads to uniaxial, in-plane magnetic anisotropy.  We fabricated micrometer scale Hall bars of nominal size 120 µm x 20 µm, 240 µm x 40 µm, and 540 µm x 100 µm on a thin film (120 nm thickness) through photolithographic methods in order to isolate the properties of several FMM regions which can have sizes on the order of tens of micrometer.  Furthermore, we designed a home-built apparatus which could accommodate a 10.5 mm by 7.5 mm chip carrier (needed for the microfabricated Hall bar sample) and perform Planar Hall effect (PHE) and anisotropic magnetoresistance (AMR) measurements.  The PHE measurements confirmed uniaxial magnetic anisotropy that had been seen previously in magnetization measurements.  Taking advantage of the length scale of our microstructures, we observed the length scaled dependence of PHE and AMR of LPCMO at different magnetic fields and temperatures.  We expect that the competition between the shape anisotropy of the FMM regions that are pinned by the microstructure’s dimensions and the magnetic anisotropy due to lattice strain will allow us to tune the magnetic anisotropy of phase-separated manganite thin films.   

Probing Hysteresis Across the Topotactic Perovskite-Brownmillerite Transformation in Electrolyte-Gated La0.5Sr0.5CoO3-δ

Reversible voltage control of electronic, magnetic, and optical properties over extraordinary ranges has recently been demonstrated in electrolyte-gated perovskite cobaltite films. This is achieved by reversibly cycling between perovskite (P) and oxygen-vacancy-ordered brownmillerite (BM) phases via electrochemical control. Most studies focus on the end states alone, however, i.e. P and BM, and little has been reported on the detailed gate voltage (V_g) dependence. Here, using ion gel gating of 10 unit-cell-thick La_0.5Sr_0.5CoO_3-δ films, we reveal that probing hysteresis loops around the P - BM transformation provides a wealth of new information. We combine source-drain current and gate current hysteresis loops with operando synchrotron X-ray diffraction, and transport and magnetotransport measurements. These reveal asymmetric hysteresis, non-monotonic transformation rates, and limits on reversibility, which we discuss and explain in detail. Minor hysteresis loops further enable the rational design of an optimized hysteresis cycle, resulting in ~10⁵ ON/OFF ratios with high reversibility.   

Magnetoresistance anomaly during electrical triggering of a metal-insulator transition

Combing resistive switching and spintronic functionalities in a single device is an exciting opportunity to bring together the advantages of charge- and spin-based electronics, enriching the design space for practical applications, and to further the basic understanding of interactions between electrical and magnetic properties of materials. Here we explored magnetotransport in ferromagnetic oxide (La,Sr)MnO₃ (LSMO) during electrical triggering of the intrinsic metal-insulator transition (MIT), which produces volatile resistive switching. This switching occurs in a characteristic spatial pattern, the formation of a paramagnetic insulating barrier perpendicular to the current flow, in contrast to the conventional filamentary percolation parallel to the current. At the threshold voltage of the MIT triggering, we observed strong anomalies including large magnitude increase and flipping of the sign of anisotropic and colossal magnetoresistances. The computational analysis revealed that these anomalies are due to the coupling between the voltage induced paramagnetic insulating barrier formation and intrinsic magnetoresistance. Our work demonstrates a novel approach to manipulate magnetic properties by electrically driving the material into the out-of-equilibrium resistive switching state.   

Ferromagnetic Resonance study of Spin-phonon Coupling in La0.7Sr0.3MnO3 /La0.7Sr0.3FeO3 superlattices

Epitaxial La_0.7Sr_0.3MnO₃ (LSMO)/La_0.7Sr_0.3FeO₃ (LSFO) superlattices are ideal perovskite oxide platforms for investigating intriguing interfacial phenomena such as charge transfer, strain relaxation, and ferromagnetic/antiferromagnetic coupling. As a result, the functional properties of LSMO/LSFO superlattice may be tunable via careful substrate engineering. In this work, we utilize the fact that the SrTiO₃ substrate has been shown to present structural phase transitions as a function of temperature to study the magnetic properties of epitaxial LSMO and LSFO thin films as well as LSMO/LSFO superlattices using ferromagnetic resonance (FMR) spectroscopy. We identify strong spin-phonon coupling as evidenced by the gap opening at the crossing points of magnon and phonon bands. Combined with the optical second harmonic generation, we further investigate the impact of the structural phase transitions in the SrTiO₃ substrate on the spin-phonon coupling in LSMO/LSFO superlattices.    

Intrinsic exchange bias from interfacial reconstruction in NiCo2O4/Al2O3 thin films

Exchange bias that occurs from a magnetic/non-magnetic interface, called intrinsic exchange bias, is an intriguing topic for the nominal simplicity and potential applications. We have studied the epitaxial NiCo₂O₄ (111) films grown on Al₂O₃ (0001) substrates and discovered intrinsic exchange bias up to ≈ 1 kOe; the magnetization can even be pinned along only one direction. Thickness-resolved reflection high energy electron diffraction (RHEED) indicates an interfacial reconstruction that mimics CoO, which can serve as the antiferromagnetic layer for the exchange bias. The thickness of the interfacial layer is sensitive to the O₂ background pressure of the thin film growth, which in turn changes the exchange bias. These results suggest highly tunable interfacial structures and magnetic properties in NiCo₂O₄ films.

    

Electronic properties of the interface between perovskite and brownmillerite phases in La1-xSrxCo3O3-δ

The family of La_1-xSr_xCoO_3-δ(LSCO) oxides are promising materials to realize low-power neuromorphic devices due to a metal to insulator transition (MIT) between different phases that can be efficiently induced by, e.g. varying the oxygen vacancy content (δ). We previously carried out a series of studies^1–3 to unravel the MIT in LSCO as a function of δ, and we investigated the transformation from a metallic perovskite (P) to an insulating brownmillerite (BM) phase. In this study, we examined the electronic properties of several LCO-P/L(S)CO_3-δ heterostructures using DFT+U method implemented in the Quantum Espresso Code. We identified values of δ, x and strain conditions under which a local closing of the gap occurs at the interface between two insulating phases. We find that when the position of the valence band edge in the L(S)CO_3-δ phase is higher than that of the perovskite phase, and concomitantly the out-of-plane Co-O-Co bond angle is larger than a threshold angle 163º, then a charge transfer to the P phase can occur. These conditions are in turn favored by higher δ values, in-plane tensile strain and Sr doping. A necessary condition for the closing of the local gap is δ>=0.375, leading to the transfer of an extra electron to the e_g orbital of the P phase, localized in proximity of the interfacial layer. The presence of metallic interfacial layers presents an opportunity to engineer atomic resistive switching states in La_1-xSr_xCoO_3-δ similar to those that one would utilize in a Mott field-effect transistor.

^1-3 S. Zhang et al., npj Comput. Mater 2020; Chem. Mat. 2021; Chem. Mat. 2022.