Symposium CK
Functional Magnetic Oxides

Session CK-1 - Magnetic Oxide Thin Films Interfaces and Heterostructures

CK-1:IL01  Engineering the Functional Properties of 2-dimensional Electron Gases at Oxide Interfaces
F. MILETTO GRANOZIO, CNR-SPIN, Napoli, Italy

2-Dimensional electron gases (2DEGs) at oxide interfaces hold promise to combine the specific physics of low-dimensional systems with extraordinary functional properties as superconductivity, magnetism and ferroelectricity, that are commonly found in transition metal oxides, and are typically missing in semiconductors. In this talk, the phenomenology of oxide 2DEGs will be first revised. We will show that the properties of the LaAlO3/SrTiO3 interface, as originally discovered in Bell labs over a decade ago, can be suitably engineered by modifying, resorting to atomically controlled growth, the layers present either on the nominally polar or on the nominally non-polar side if the interfaces. As a result, specific functionalities including magnetism can be tailored into this system, thus allowing for the potential implementation of novel magnetic device concepts.


CK-1:IL02  Tuning the Properties of Oxide Heterostructures by Interfacial Oxygen Octahedral Coupling
G. KOSTER, University of Twente, Enschede, The Netherlands

Diverse electronic phases in solid state materials such as superconductivity, topological insulating phases, ferroelectricity and ferromagnetism are intimately coupled to the crystal symmetry. In ABO3 perovskites, the crystal symmetry resides in the corner sharing oxygen octahedral (BO6) network. These symmetries, or oxygen octahedral rotation (OOR) patterns in perovskite hetero-structures, are usually engineered by epitaxial strain. Furthermore, the required connectivity of the octahedra across the heterostructure interface enforces a geometric constraint to the 3-dimensional octahedral network in epitaxial films. Such geometric constraint will either change the tilt angle to retain the connectivity of the corner shared oxygen octahedral network or guide the formation of a specific symmetry throughout the epitaxial film. Control of the OOC provides a large degree of freedom to manipulate physical phenomena in complex oxide heterostructures.  In my presentation, I will discuss the control of oxygen octahedral coupling by interface-engineering in manganite as well as nickelate heterostructures.


CK-1:IL03  Domain Wall Conduction at All-in-all-out Antiferromagnetic Iridate Heterointerface
MASAKI UCHIDA, University of Tokyo, Tokyo, Japan

Magnetic oxide films and heterointerfaces have been the object of profound and continued study of emergent phenomena originating from their elaborate spin ordering. Recent theoretical and experimental studies have suggested that pyrochlore oxides possessing “all-in-all-out” spin ordering with broken time-reversal symmetry host a new topological phase called Weyl semimetal. This ordering has two distinct types of magnetic domains (all-in-all-out or all-out-all-in), and a non-trivial metallic surface state has been expected to appear at the domain wall between them. I introduce observation of this metallic conduction at the single all-in-all-out/all-out-all-in magnetic domain wall formed at the heterointerface of two pyrochlore iridates. By utilizing different magnetoresponses of them with different lanthanide ions, the domain wall can be controllably inserted at the heterointerface, the surface state being detected as anomalous conduction enhancement with a ferroic hysteresis. This establishment paves the way for further investigation and manipulation of this new type of topological transport in magnetic oxides.


CK-1:IL04  Complex Magnetic Order in Rare-earth Nickel Oxide Multilayers
E. BENCKISER, Max Planck Institute for Solid State Research, Stuttgart, Germany

Perovskite rare-earth nickelates exhibit a metal-insulator transition and an unusual period-four antiferromagnetic order which are the subject of debates for decades. While previously experiments were mostly performed on ceramic samples, the progress in epitaxial thin film growth in the past years opened up new perspectives to study the properties of nickelates and the influence of extrinsic parameters, like epitaxial strain, electronic confinement and interfacial magnetic interactions. In my talk, I will present results from resonant x-ray scattering studies which we performed on different nickelate-based heterostructures and where we found non-collinear magnetic order induced in ultra-thin LaNiO3 layers which are non-magnetic otherwise, studied magnetic exchange interaction between rare-earth and transition-metal ions in LaNiO3-DyScO3 superlattices, and demonstrated that it is possible to stabilize collinear and non-collinear magnetic states in thin NdNiO3 slabs if the interaction between neighbouring magnetic sites is truncated along the [111] pseudocubic direction. These results provide important insight to the relevant interactions stabilizing the magnetic order in nickelates and open up new perspectives for their selective manipulation.


CK-1:IL05  Carrier Density Controlled Topological Hall Effect in EuTiO3 Films
K. AHADI, S. STEMMER, Materials Department, University of California, Santa Barbara, CA, USA

The topological Hall effect (THE) is a hallmark of topologically nontrivial (chiral) spin textures, such as skyrmions, and can be observed as a distinct, additional contribution in Hall measurements that is superposed on the ordinary and anomalous Hall effects. Oxide films and interfaces that support topologically nontrivial spin textures and whose carrier density can be manipulated are interesting, because the potential for control by electric field effect and because proximity effects can be utilized to realize other exotic states within all-epitaxial heterostructures. In this talk, we discuss the role of carrier density and band structure in the topological Hall effect in thin films of the itinerant ferromagnet Eu1-xSmxTiO3. EuTiO3 and Eu1-xSmxTiO3 films were grown by molecular beam epitaxy. EuTiO3 film is insulating. The Hall resistivity of the Eu1-xSmxTiO3 films exhibits the anomalous Hall effect below the Curie temperature as well as a THE below 2 K. It is shown that the carrier density controls the sign and strength of both Hall effects. Furthermore, metamagnetic transtions are also observed. The results open up interesting possibilities for epitaxial hybrid heterostructures that combine topological magnetic states, tunable carrier densities, and other phenomena.


CK-1:L06  Giant Topological Hall Effect from Magnetic Skyrmion Bubbles in Correlated Manganite Thin Films
L. VISTOLI, A. SANDER, QIUXIANG ZHU, S.E FUSIL, A. BARTHELEMY, V. GARCIA, M. BIBES, Unité Mixte de Physique CNRS/Thales, Université Paris-Sud, Université Paris-Saclay, Palaiseau, France; WENBO WANG, WEIDA WU, Rutgers Center for Emergent Materials and Department of Physics and Astronomy, Rutgers University, Piscataway, NJ, USA; B. CASALS, R. CICHELERO, G. HERRANZ, Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus de la UAB, Bellaterra, Catalonia, Spain

CaMnO3 is a charge-transfer insulator whose peculiarity is that the chemical substitution of Ca by Ce induces a transition to a metallic phase at low doping, featuring a highly non-coplanar spin structure. In addition, with compressive strain the magnetoelastic anisotropy imposes a perpendicular easy magnetization axis. The combination of these characteristics makes this compound attractive to probe for sought-after topological effects, such as magnetic skyrmions, in correlated systems. In this presentation we will report the presence of an anomalous Hall effect (AHE) and a topological-like Hall signal (THE) in these thin films. Remarkably, both signals are very large and present up to low temperatures. Magnetic force microscopy reveals that the magnetization reversal nucleates magnetic bubbles, whose density directly relates to the THE signal. The magnitude of the THE is much higher than any other previously reported, and what one would expect from such a skyrmion bubble density. We point out how this can be ascribed to the strong correlations close to the metal-to-insulator transition.
This work has received funding from the French National Research Agency ANR as part of the Investissements d’Avenir program (Labex NanoSaclay, reference: ANR-10-LABX-0035).


Session CK-2 - Spin Transport in Magnetic Oxides

CK-2:IL01  Spin/Charge Interconversion in Oxide Two-dimensional Electron Gases
M. BIBES, F. TRIER, Unité Mixte de Physique CNRS/Thales, Palaiseau, France

The Rashba spin-orbit coupling (SOC) is a relativistic phenomenon occurring in systems with broken inversion symmetry that splits the electronic bands depending on their spins in proportion of the local electric field and the atomic spin-orbit constant. Accordingly, the Rashba SOC is large at surfaces of heavy metals such as Au, at interfaces between heavy elements (e.g. Bi/Ag, but is also sizeable in two-dimensional electron gases (2DGEs) based on InAs or SrTiO3. In this latter case, the multiorbital nature of the bands (formed by non-degenerate Ti t2g states) endows the system with rich spin-orbit physics whose manifestations show up for instance in weak-antilocalization experiments or in the complex anisotropic magnetoresistance response. Remarkably, the Rashba SOC of SrTiO3-based 2DEGs is also highly tunable by gate voltages, offering possibilities for electric-field control spintronics. In this presentation, we will show how the Rashba SOC in SrTiO3-based 2DEGs can be harnessed to achieve interconversion between spin and charge currents through the (inverse) Edelstein effect. We will discuss the amplitude of the conversion and its gate dependence on the basis of the electronic structure of the 2DEGs and give perspectives for novel oxide-based spintronics devices.


CK-2:IL02  Transport Phenomena in Heterostructures of Strong Spin-orbit Interaction Oxides
JOBU MATSUNO, RIKEN Center for Emergent Matter Science (CEMS), Saitama, Japan

A strong spin-orbit coupling (SOC) inherent to 5d Ir oxides recently emerged as a new paradigm for oxide electronics. Here, we focus on magnetic skyrmion in bilayers consisting of ferromagnetic SrRuO3 and nonmagnetic SrIrO3. We observed an anomaly in the Hall resistivity in addition to anomalous Hall effect (AHE); this is attributed to topological Hall effect (THE) [1]. Magnetic skyrmions of 10–20 nm are probably generated by Dzyaloshinskii-Moriya interaction, which might be caused by both broken inversion symmetry at the interface and strong SOC of SrIrO3. Even more surprising is that we can control both AHE and THE by electric field in the SrRuO3-SrIrO3 bilayers [2]. We observed the clear electric-field dependence only when SrIrO3 is inserted between SrRuO3 and a gate dielectric. The results established that strong SOC of SrIrO3 is again essential in electrical tuning of these Hall effects. We are also searching for spin-current-driven thermoelectric conversion; high conversion efficiency is expected by utilizing and controlling SOC in Ir oxides. We will report on the latest results of spin Seebeck effect at interfaces between magnetic oxides and nonmagnetic Ir oxides.
[1] J. Matsuno et al., Sci. Adv. 2, e1600304 (2016). [2] Y. Ohuchi, J. Matsuno et al., in preparation.


CK-2:IL03  Spin Seebeck Effects in Magnetic-oxide-based Multilayers
R. RAMOS, Advanced Institute for Materials Research, Tohoku University, Sendai, Japan; T. KIKKAWA, Advanced Institute for Materials Research and Institute for Materials Research, Tohoku University, Sendai, Japan; A. ANADON, I. LUCAS, Fundacion Instituto de Nanociencia de Aragon and Departamento de Fisica de la Materia Condensada, Universidad de Zaragoza, Zaragoza, Spain; R. IGUCHI, National Institute for Materials Science, Tsukuba, Japan; S. Daimon, Advanced Institute for Materials Research and Institute for Materials Research, Tohoku University, Sendai, Japan; K. UCHIDA, National Institute for Materials Science, Tsukuba and PRESTO, Japan Science and Technology Agency, Saitama, Japan; H. ADACHI, Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Japan; P.A. ALGARABEL, Departamento de Fisica de la Materia Condensada and Instituto de Ciencia de Materiales de Aragon, Universidad de Zaragoza and Consejo Superior de Investigaciones Cientificas, Zaragoza, Spain; L. MORELLON, Fundacion Instituto de Nanociencia de Aragon and Departamento de Fisica de la Materia Condensada, Universidad de Zaragoza, Zaragoza, Spain; M.H. AGUIRRE, Fundacion Instituto de Nanociencia de Aragon and Departamento de Fisica de la Materia Condensada and Laboratorio de Microscopias Avanzadas, Universidad de Zaragoza, Zaragoza, Spain; S. MAEKAWA, Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Japan; M.R. IBARRA, Fundacion Instituto de Nanociencia de Aragon and Departamento de Fisica de la Materia Condensada and Laboratorio de Microscopias Avanzadas, Universidad de Zaragoza, Zaragoza, Spain; E. SAITOH, Advanced Institute for Materials Research and Institute for Materials Research and Center for Spintronics Research Network, Tohoku University, Sendai and Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Japan

The spin-Seebeck effect (SSE) refers to the thermal generation of spin currents in a ferromagnet and their electrical detection in an adjacent heavy metal, offering a new paradigm for heat to electricity conversion. However, the low magnitude of the thermoelectric voltage represents an obstacle for the development of potential applications. One possible approach to overcome this limitation is the use of magnetic-oxide/metal multilayers, where an enhancement of the SSE thermoelectric voltage is expected due to an increase of the spin current in the multilayers. I will present results in n x [Fe3O4/Pt] multilayers, where large SSE voltage and power enhancements have been observed. Its reciprocal effect, the spin Peltier effect, has been also measured showing a clear increase of the spin-current-induced temperature difference in the multilayers, consistent with the SSE results. Finally, temperature dependent measurements of the SSE show that the enhancement factor can be further improved as the temperature is decreased, due to the larger length-scale for spin current propagation, indicating the possibility to improve the SSE thermoelectric performance by selection of suitable materials.

 
Session CK-3 - Electronic Structure and Correlation Effects

CK-3:IL01  Correlated Electronic Structure of Oxide Heterostructures
F. LECHERMANN, Institut für Theoretische Physik, Universität Hamburg, Hamburg, Germany

The recent possibilities for designing materials beyond nature's original conception in the context of oxide heterostructures, open a fascinating new chapter in condensed matter physics (e.g. [1]). Oxide interface architectures enable a novel way to disturb bulk-known electronic states with the option for emerging exotic phases. Insulators of band or Mott kind appear most susceptible, as the effective doping via an interface gives rise to a plethora of physics. Furthermore the classification boundaries between weak- and strong-correlation problems may abrogate in such doping scenarios. In this talk a theoretical effort to tackle these modern challenges on a realistic level is presented. By means of the combination of density functional theory (DFT) with explicit many-body methods, such as e.g. dynamical mean-field theory (DMFT), it becomes possible to address the intricate interplay between band-structure effects and electronic correlations on an equal footing, even for those latest materials developments [2]. Main focus will be on titanate heterostructures of various kinds, where the competitions between different electronic interface phases pose demanding problems.
[1] C. A. Jackson, et al. Nat. Commun. 5, 4258 (2014). [2] F. Lechermann, Sci. Rep. 7, 1565 (2017).


CK-3:IL02  Charge Transfer Effects in Rare Earth Nickelates
J. VARIGNON¹, M.N. GRISOLIA¹, J. INIGUEZ², A. BARTHELEMY¹, M. BIBES¹, ¹Unité Mixte de Physique, CNRS, Thales, Université Paris Sud, Université Paris-Saclay, Palaiseau, France; ²Materials Research and Technology Department, Luxembourg Institue of Science and Technology (LIST), Esch/Alzette, Luxemburg

Transition metal oxide perovskites display a wide range of properties that take their origin from the interplay between lattice, electronic and magnetic degrees of freedom. A special emphasis has been devoted to 3d elements allowing for strong electronic correlations and ionicity or covalency that in turn have a dramatic influence on the properties. This is the case for instance of rare-earth nickelates R3+Ni3+O3 (R-Lu-Pr,Y) that feature a metal-insulator phase transition at TMI due to possible charge disproportionation effects (2Ni3+→Ni(3+δ)+ + Ni(3-δ)+) and a breathing of the oxygen cage octahedra. In their ground state, they also exhibit complex antiferromagnetic orders and strong covalent effects. Nevertheless, their electronic structure is still under debate. On the basis of first-principles simulations, we show that their ground state electronic structure is based on charge disproportionation effects and electron transfers from surrounding O to Ni leaving the impression of equivalent Ni sites in the whole structure. Finally, we discuss the implication of the level of covalency on the ground state properties, revealing a new pathway to control electronic and magnetic phases in perovskites.
Work supported by the ERC consolidator grant MINT (Contract 615759)


CK-3:IL03  Emergent Quantum Phases in Relativistic Magnetic Oxides
C. FRANCHINI, University of Vienna, Vienna, Austria

The novel electronic phases found in 5d iridates driven by the interplay of spin-orbit coupling and electronic correlation has prompted intense efforts to identify and understand exotic quantum states in relativistic solids with great scientific and technological potential. In this talk, we shall present results on magnetic 4d and 5d oxides in which spin-orbit coupling and its interaction with the lattice and with electronic correlation plays a pivotal role to establish different physical behaviors including: metal-to-insulator transitions (relativistic-Mott, Lifshitz, doping/phonon-induced), anisotropic magnetic interactions (Dzyaloshinskii-Moriya, Kitaev, spin-compass), and (topological) Dirac/Weyl semimetal states. The results are primarly based on density functional theory, extended Heisenberg Hamiltonians and the constrained random phase approximation.


CK-3:IL04  High Pressure Synthesis of Oxides and Mixed-anion Oxides with Novel Magnetic and Transport Properties
HIROSHI KAGEYAMA, HIRASHI TAKATSU, Kyoto University, Kyoto, Japan

High pressure reaction enables us to obtain a fully occupied tetragonal tungsten bronze (TTB), K3W5O15 (K0.6WO3) [1]. The terminal phase shows an unusual transport property featured by slightly positive temperature dependence in resistivity and a large Wilson ratio of R_W=3.2. Such anomalous metallic behavior arises possibly from the low dimensional electronic structure with a van Hove singularity at the Fermi level and/or from the enhanced magnetic fluctuations by geometrical frustration of the W-sublattice. The ‘asymmetric’ nature of the TTB KxWO3–K0.6–yBayWO3 phase diagram implies that superconductivity for x≤0.45 originates from the lattice instability due to K-deficiency. This study opens a versatile opportunity to extend the solubility limit in tungsten bronze oxides. In my contribution, high-pressure stabilized mixed-anion compounds (oxyhydrides and oxynitrides), such as SrCrO2H, LaMnO3.3H0.7 and MnTaO2N will also be shown [2-5].
[1] Ikeuchi et al., Angew. Chem. Int. Ed. 56, 5770 (2017). [2] Tassel et al., Angew. Chem. Int. Ed. 54, 516 (2015). [3] Tassel et al., Angew. Chem. Int. Ed. 55, 9667 (2016). [4] Tassel et al., Angew. Chem. Int. Ed. 53, 10377 (2014). [5] Kuno et al., J. Am. Chem. Soc. 138, 15950 (2016).

 
Session CK-4 - Interplay Between Spin, Charge and Lattice Degrees of Freedom

CK-4:IL01  Unconventional Metals from Doped Spin-orbit Assisted Mott States
S.D. WILSON, University of California at Santa Barbara, Santa Barbara, CA, USA

A variety of new electronic states are predicted to arise in the presence of strong spin-orbit coupling and appreciable Coulomb interactions. Depending on the crystal lattice type and the relative balance of these energy scales, states ranging from new forms of quantum spin liquids to correlated topological phases to high temperature superconductivity are predicted to arise in materials at this frontier. The spin-orbit assisted (or J_eff=1/2 ) Mott state is one such example where spin-orbit coupling, strong crystal field splitting, and residual on-site Coulomb interactions act together to stabilize an unexpected Mott insulator. Here I will present some of our group’s recent work exploring how this spin-orbit Mott state melts into nearby unconventional metallic states upon carrier substitution. Competing electronic states discovered close to the spin-orbit Mott phase will be a particular focus and their implications for more exotic phases at higher doping concentrations will be discussed.


CK-4:L02  Magnon Study in (Y, Lu)MnO3 System using Raman Spectroscopy
SEUNG KIM¹, JIYEON NAM¹, THI HUYEN NGUYEN¹, IN-SANG YANG¹, XUEYUN WANG², SANG-WOOK CHEONG², ¹Department of Physics, Ewha Womans University, Seoul, South Korea; ²Rutgers Center for Emergent Materials and Department of Physics and Astronomy, Rutgers University, Piscataway, NJ, USA

We have investigated spin waves (magnons) in the hexagonal RMnO3 (R= Y, Lu) manganite systems using Raman spectroscopy. Below the Neel temperature (TN), we have found the prominent magnon scattering peaks as in other hexagonal RMnO3.[1-2] We observed the magnon peaks as the Mn3+ ions are replaced by non-magnetic/magnetic ions, as the temperature varies from 15 K to 300 K. The magnon peaks are found in different range (775~960 cm-1 vs 810~1000 cm-1) depending on the substituent element being non-magnetic Al3+ ions or magnetic Fe3+ ions, respectively. From these values, we could deduce the spin exchange interaction intergral values J1 and J2 of these samples below TN using our simple magnon model. Moreover we could infer the TN values, which the magnetization cannot easily observe. Magnons observed by optical method (Raman spectroscopy) is a good entity which enables easy investigation of the magnetic properties related with the Mn3+ ions of these material.
1. N.T.M. Hien, X.B. Chen, L.H. Hoang, D. Lee, S.-Y. Jang, T.W. Noh, and I.S. Yang, J. Raman Spectrosc. 41, 983 (2010). 2. X.B. Chen, P.C. Guo, N.T. Huyen, S. Kim, I.S. Yang, X. Wang, and S.-W. Cheong, Appl. Phys. Lett. 110, 122405 (2017).

 
Session CK-5 -  Multiferroic and Magnetoelectric Compounds

CK-5:IL01  Marrying Ferroelectricity and Metallicity: "It's Complicated"
V. FIORENTINI, A. FILIPPETTI, Cagliari University, Monserrato (CA), Italy; F. RICCI, Louvain University, Belgium; A. URRU, P. DELUGAS, SISSA, Trieste, Italy; J. INIGUEZ, H.J. ZHAO, LIST, Luxembourg; E. CANADELL, ICMAB, Spain; L. BELLAICHE, University of Arkansas, USA

The idea of symmetry-allowed polar distortions in metals, or ‘ferroelectric metals’, has been around for over 50 years: can a material be simultaneously metallic and ferroelectric — i.e. have a switchable intrinsic electric polarization) ? Subordinately, can at least symmetry-polar metals exist, and what are they good for? Here we explore two of the many routes to approach this problem. One: do polar distortions in bona-fide ferroelectrics survive when free charge is added by doping ? We answer ‘yes, in most cases’, analyzing the free-charge screening mechanisms involved in various classes of ferroelectrics. Two: are there any (perhaps marginal, or sub-3D) metals than can legitimately be labeled ferroelectrics? Here too we answer yes, studying a family of metallic layered perovskites with very special Fermi surfaces, and finding one that is ferroelectric (Bi5Ti5O17), and one that is multiferroic (Bi5Mn5O17).


CK-5:L03  Nanopillars with E-field Accessible Multi-state (N≥4) Magnetization with Giant Magnetization Changes in Self-assembled BiFeO3-CoFe2O4/Pb(Mg1/3Nb2/3)-38at%PbTiO3 Heterostructures
XIAO TANG, JIEFANG LI, D. VIEHLAND, Dept. of Materials Science and Engineering, Virginia Tech, Blacksburg, VA, USA

We have deposited self-assembled BiFeO3-CoFe2O4 (BFO-CFO) thin films on (100) Pb(Mg1/3Nb2/3)0.62Ti0.38O3 (PMN-38PT) single crystal substrates. These heterostructures were used for the study of real-time changes in the magnetization with applied DC electric field (E_DC). With increasing E_DC, a giant magnetization change was observed along the out-of-plane (easy) axis. The induced magnetization changes of the CFO nanopillars in the BFO/CFO layer were about ∆M=93%. A giant converse magnetoelectric (CME) coefficient of 1.3×〖10〗^(-7)s/m was estimated from the data. By changing E_DC, we found multiple (N≥4) unique possible values of a stable magnetization with memory on removal of the field.


CK-5:L05  Origin of High Magnetoelectric Coupling in Multiferroic BiFeO3-BaTiO3 Superlattices
M. LORENZ¹, V. LAZENKA², C. PATZIG³, S. SELLE³, D. HIRSCH⁴, T. HÖCHE³, K. TEMST², M. GRUNDMANN¹, ¹Universität Leipzig, Felix-Bloch-Institut für Festkörperphysik, Semiconductor Physics Group, Leipzig, Germany; ²KU Leuven, Instituut voor Kern- en Stralingsfysica, Leuven, Belgium; ³Fraunhofer-Institut für Mikrostruktur von Werkstoffen und Systemen, Center for Applied Microstructure Diagnostics, Halle, Germany; ⁴Leibniz-Institut für Oberflächenmodifizierung e.V., Physikalische Abteilung, Leipzig, Germany

The understanding of magnetoelectric coupling in multiferroic oxide heterostructures is still a challenge. Little is known to date concerning the impact of the structural and chemical interface structure and unwanted impurities that may be buried within short-period multiferroic BiFeO3-BaTiO3 superlattices during growth. Here, we demonstrate how epitaxial coherence [1], trace impurities and elemental concentration gradients [2] contribute to high magnetoelectric voltage coefficients in thin film superlattices built of 15 double layers of BiFeO3-BaTiO3. Interestingly, the highest ME voltage coefficient of 55 Vcm-1Oe-1 at 300 K was measured for a coherently grown superlattice with a few atomic percent of Ba and Ti that diffused into the nominally 5 nm thin BiFeO3 layers, according to analytical transmission electron microscopy [2]. In addition, clear enhancements of the cation signals were observed in depth profiles by secondary ion mass spectrometry at the interfaces of BaTiO3 and BiFeO3 [2] The challenge is to provide cleaner materials and processes, as well as a well defined control of the structural and chemical interface structure [1, 2].
[1] M. Lorenz et al. Advanced Materials Interfaces 3, 1500822 (2016). [2] M. Lorenz et al. ACS Appl. Mater. Interfaces 9, 18956 (2017).


Session CK-6 - Coexistence of Superconductivity and Magnetism

CK-6:IL01  Superconducting Properties of Ultrathin FeSe Films on Oxide Substrates
TSUTOMU NOJIMA, J. SHIOGAI, T. MIYAKAWA, Y. ITO, T. HARADA, A. TSUKAZAKI, Institute for Materials Research, Tohoku University, Sendai, Japan

Among variety of iron-based superconductors, the ultrathin FeSe films on SrTiO3 have been uniquely evolved into a high transition temperature Tc superconductor. Although the microscopic mechanism is still under debate, the interfacial effects are the center of attraction. Recently, we developed an electrochemical etching technique as a new access to the thin limit by using an electric-double-layer transistor configuration, which enables the investigation of superconductivity as a function of thickness, substrate material, and electric field [1]. In this presentation, we will introduce our recent progress in the studies on FeSe ultrathin films grown on SrTiO3, MgO and KTaO3 [2]. For all the substrates, Tc is enhanced above 40 K with thinning below the substrate dependent critical thickness. We discuss that the high-Tc superconductivity does not primarily originate from a specific interface combination but from the electron filling at specific electronic band by the electrostatic doping and/or the charge transfer from the substrate. We will also demonstrate the robustness of high-Tc phase based on the enormously large critical magnetic field and critical current density.
[1] J. Shiogai et al., Nat. Phys. 12 (2016) 42. [2] J. Shiogai et al., Phys. Rev. B 95 (2017) 115101.


CK-6:IL02  Novel Proximity Phenomena at High Tc Superconductor Interfaces
D. SANCHEZ-MANZANO¹, M. ROCCI¹, F.A. CUELLAR¹, M. VARELA¹, Z. SEFRIOUI¹, C. LEON¹, J. TRASTOY², X. PALERMO², V. ROUCO², J. VILLEGAS², M. GARCIA HERNANDEZ³, Q. WANG⁴, Y.H. LIU⁴, S.G.E. TE VELTHUIS⁴, M.R. FITZSIMMONS⁵, J. SANTAMARIA¹, ¹GFMC, Depto. Física de Materiales, Universidad Complutense de Madrid, Madrid, Spain; ²Unité Mixte de Physique CNRS/Thales, Campus de Polytechnique, Palaiseau and Université Paris-Sud, Orsay, France; ³Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), Cantoblanco. Madrid, Spain; ⁴Materials Science Division, Argonne National Laboratory, Argonne, IL, USA; ⁵Oak Ridge National Laboratory, Oak Ridge,TN, USA

The emergent electronic states nucleating at the interfaces between correlated oxides are being the subject of great interest. They give rise to unexpected physical properties and there is a strong drive to utilize them in new device concepts for a future oxide electronics. The perovskite structure common to many correlated oxides allows for the combination of very different, even antagonistic, ground states. This is the case of interfaces combining ferromagnetic manganites and superconducting cuprates. In this talk I will examine several interface problems in oxide heterostructures with special focus on the possibility of creating novel interfacial magnetic or superconducting states at the interface between a cuprate and a manganite. Induced magnetism at the cuprate interface may result from the superexchange path resulting from the bonding reconstruction at the interface. On the other hand, triplet superconducting correlations resulting from magnetization inhomogeneities are generated at these interfaces. Triplet pairing amplitude may propagate deep into the highly spin polarized ferromagnet giving rise to non conventional proximity phenomena between a half metal and a superconductor, yielding an interesting form of spin polarized supercurrents of interest for spintronics.


CK-6:IL03  Helical Magnetic Order and Pressure Induced Super-conductivity in Binary Pnictides CrAs and MnP
R. KHASANOV, A. AMATO, P.K. BISWAS, P. BONFA', I. EREMIN, Z. GUGUCHIA, H. LUETKENS, E. MORENZONI, R. DE RENZI, CH. RÜEGG, A.S. SEFAT, M.A. SUSNER, N.D. ZHIGADLO, Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institut, Villigen, Switzerland

The recent discovery of pressure induced superconductivity in the binary helimagnets CrAs and MnP has attracted much attention. How superconductivity emerges from the magnetic state and what is the mechanism of the superconducting pairing are two important issues which need to be resolved. In the current talk I will present the results of the muon-spin rotation relaxation under pressure experiments on CrAs and MnP.
CrAs.  In this compound the magnetism was found to remain bulk up to p ~ 0.35 GPa while its volume fraction gradually decreases with increasing pressure until it vanishes at p ~0.7 GPa. At 0.35 GPa superconductivity abruptly appears with its maximum T_c ~ 1.2 K which decreases upon increasing the pressure. In the intermediate pressure region (0.35 < p < 0.7 GPa) the superconducting and the magnetic volume fractions are spatially phase separated and compete for phase volume. A scaling of \rho_s with T^3.2 as well as the phase separation between magnetism and superconductivity point to a conventional mechanism of the Cooper-pairing in CrAs [1].
MnP. In MnP a ferromagnetic state as well as two incommensurate helical states (with propagation vectors Q aligned along the crystallographic c- and b-directions, respectively) were identified. This states transform into each other through first order phase transitions as a function of pressure and temperature. The analysis of the high-pressure muon-spin rotation data reveals that the new magnetic order appearing for pressures exceeding 1.5 GPa can not be described by keeping the propagation vector Q||c. Even the extreme case -- decoupling the double-helical structure into four individual helices -- remains inconsistent with the experiment. Overall, the high-pressure magnetic phase which is a precursor of superconductivity in MnP is an incommensurate helical state with Q||b [2,3].
References: 1. R. Khasanov et al., Scientific Reports 5, 13788 (2015). 2. R. Khasanov et al.,  Phys. Rev. B, 93, 180509 (2016). 3. R. Khasanov et al., Journal of Physics: Condensed Matter, 29, 164003 (2017).


Session CK-8 -  Novel Synthesis and Characterization Techniques

CK-8:IL02  Atomic-scale Characterization of Charge Distributions at Oxide Interfaces and Néel-Type Domain Walls Using Scanning Transmission Electron Microscopy
MING-WEN CHU, National Taiwan University, Taipei, Taiwan

The two-dimensional electron gas (2DEG) at the heterointerface between insulating LaAlO3 (LAO) and SrTiO3 (STO) boasts oxide electronics and rejuvenates the interest in charged domain walls (DWs, i.e., 2D homointerfaces). The origin for the metallic LAO/STO is, however, unsettled and the physics of charged DWs is under development. Using scanning transmission electron microscopy, we unveiled that the metallic LAO/STO is accompanied with head-to-head (HH) ferroelectric(FE)-like polarizations across the interface due to strain-rejuvenated FE-like instabilities in the materials. The divergent depolarization fields of the HH polarizations cast the interface into an electron reservoir, leading to the formation of screening 2DEG. We then tackled the coexisting HH and tail-to-tail (TT) DWs in FE (Ca,Sr)3Ti2O7 (CSTO), with the respective DWs demanding for screening charges with an opposite sign while only electrons available in n-type titanates. Using group-theoretical analysis, we established the existence of a hidden antipolar order parameter, turning the order-parameter spaces to be multicomponent with the joint formation of Néel-type twinlike antipolar domain walls between the HH and TT polar domains. This distinct domain topology lifts the polar divergences between the domains.


CK-8:IL04  New Directions in Materials Characterization with Hard X-ray Photoemission
J.-P. RUEFF, Synchrotron SOLEIL, Gif sur Yvette, and LCPMR, CNRS - Sorbonne Université, France; C.S. FADLEY, Department of Physics, University of California Davis and Materials Sciences Division, Lawrence Berkeley National Laboratory, Davis, CA, USA

X-ray photoemission (XPS) has traditionally been carried out with photon energies up to about 1.5 keV. However, over the last 15 years or so, photoemission with hard x-ray photon energies from ca. 2-12 keV (HAXPES), has by now become available from both synchrotron and laboratory sources, thus opening up exciting new possibilities, especially connected with the greater probing depths that permit studying more bulklike properties in complex materials, as well as buried interfaces in thin films and heterostructures. Overviews of this new technique appear elsewhere [1,2].  We will discuss several illustrative applications of HAXPES in oxide materials, heavy fermions compounds and devices studied in operando, exploring the capabilities of HAXPES especially on the GALAXIES beamline at SOLEIL Synchrotron [3]. These energies also permit using x-ray standing-wave (SW) excitation to provide much improved depth resolution, which will be illustrated in oxide heterostructures.
[1] Hard X-ray Photoelectron Spectroscopy, J.C. Woicik, Editor, Springer Series in Surface Sciences, Vol. 59, (2016), including “Hard X-ray Photoemission: An Overview and Future Perspective”, C.S. Fadley, op. cit. pp. 1-34. [2] Slide and abstract archives of the 7th International Conference on Hard X-Ray Photoelectron Spectroscopy: https://sites.google.com/a/lbl.gov/haxpes2017/. [3] J.-P. Rueff et al., The GALAXIES Beamline at SOLEIL Synchrotron: Inelastic X-ray Scattering and Photoelectron Spectroscopy in the Hard X-ray Range, J. Synchrotron Rad. 22 (2015), 175.

 
Session CK-9 -  Applications in Electronics and Energy

CK-9:IL01  Energy Efficiency in Electrocaloric Heat Exchangers
E. DEFAY, R. FAYE, S. NICOLAU, D. SETTE, H. STROZYK, Luxembourg Institute of Science and Technology, Belvaux, Luxembourg

Electrocaloric (EC) materials experience a variation of temperature when an external electric field is applied but also removed. Heat exchangers stand therefore for a natural application of this EC effect. Although other caloric effects such as magneto- or elasto-caloric are more advanced in terms of devices, the EC effect exhibits a built-in advantage: the triggering field is the electric field. It means that one can think of recycling the electrical charges used to induce the EC effect. Besides this extrinsic means, some oxides such as antiferroelectrics or relaxors present an intrinsic advantage thanks to their slim polarisation-electric field loops, which can make a crucial difference when it comes to assessing the overall energy efficiency of heat exchangers. In this presentation, I will review the best EC materials regarding efficiency but also give some applied examples developed in our lab showing the interest of recovering energy for EC devices.


CK-9:IL03  Purely Antiferromagnetic Magnetoelectric Random Access Memory (AF-MERAM)
T. KOSUB, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany

A novel magnetic solid-state memory type, the AF-MERAM [1], is presented. It unites the benefits of electric field effect writing [2] with those of antiferromagnets [3]. It offers a remarkable 50 fold reduction of the writing threshold compared to state-of-the-art ferromagnet-based counterparts [1], is robust against magnetic disturbances and exhibits no ferromagnetic hysteresis losses. We demonstrate reliable isothermal switching via gate voltage pulses and all-electric readout at room temperature. While omitting the ferromagnet from conventional MERAM enables the large improvement in the writing threshold, it also eliminates all possibilities for conventional magnetoresistive read out, such as the AMR/GMR/TMR effects. Thus, a key aspect of the AF-MERAM functionality is its new all-electric read out of the pure Hall resistance [4]. This method is both ultra-sensitive to tiny net magnetization in metallic antiferromagnets and to boundary magnetism between nonmagnetic metals and magnetic insulators, suggesting its applicability to emerging antiferromagnetic/insulator spintronics.
1. T. Kosub et al., Nat. Commun. 7, 13985 (2017) 2. F. Matsukura et al., Nat. Nano. 10, 209 (2015) 3. T. Jungwirth et al., Nat. Nano. 11, 231 (2016) 4. T. Kosub et al., Phys. Rev. Lett. 115, 097201 (2015)