Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session X23: Spin-orbit Coupling and Topological Phenomena in Oxide HeterostructuresFocus
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Sponsoring Units: GMAG DMP DCOMP Chair: Di Yi, Stanford Univ Room: LACC 402B |
Friday, March 9, 2018 8:00AM - 8:12AM |
X23.00001: Control of magnetism in bilayer iridates Choong Hyun Kim We have studied from first principles the magnetic properties of the bilayer perovskite iridates, A3Ir2O7. In Sr3Ir2O7, an easy c-axis collinear antiferromagnetic structure have been reported, a significant constrast to single layer Sr2IrO4 with in-plane canted moments. This behavior is understood by competition among intra- and interlayer bond-directional pseudodipolar interactions. Here we suggest two ways to control magnetic structure of the system from our first-principles calculations.. At first, we show that these two energy scales are controllable via epitaxial strain to drive spin-flop transition. Secondly, we suggest Ca-doping of Sr3Ir2O7 could derive hybrid improper ferroelectricity to show strong magnetoelectric coupling effect. |
Friday, March 9, 2018 8:12AM - 8:24AM |
X23.00002: Capturing the hidden symmetry in layered iridate heterostructures Lin Hao, Derek Meyers, Hidemaro Suwa, Junyi Yang, Clayton Frederick, Tamene Dasa, Gilberto Fabbris, Lukas Horak, Dominik Kriegner, Yongseong Choi, Jong Woo Kim, Daniel Haskel, Philip Ryan, Haixuan Xu, Cristian Batista, Mark Dean, Jian Liu Layered iridates hosting square lattices have recently gained plenty of interests with potential of unconventional superconductivity leading the charge. The large spin-orbit coupling of the Ir pseudospin half state, on the other hand, may enable a hidden SU(2) symmetry much stronger than that in cuprates. Probing and unveiling this symmetry is however highly challenging and hindered in the bulk material, because of the non-trivial interlayer coupling. Through accurately tailoring the magnetic structure, we exploited this symmetry in heterostructures as composed of perovskite SrIrO3 and SrTiO3. Upon approaching the 2D limit, the hidden symmetry triggers large magnetic fluctuations and enables an unprecedented strong coupling between the antiferromagnetic order and external magnetic field. The capability of materializing the hidden symmetry in artificial structures provides a fruitful playground for pursuing novel phenomena beyond the cuprate physics. |
Friday, March 9, 2018 8:24AM - 8:36AM |
X23.00003: Magnetic characterizations of the epitaxial Pr2Ir2O7 thin films via solid state epitaxy Lu Guo, Neil Campbell, Tianxiang Nan, Xianglin Ke, Mark Rzchowski, Chang-Beom Eom Pyrochlore iridates (RE2Ir2O7, RE=Rare Earth) exhibit various novel electronic and magnetic properties resulting from the interplay of electronic correlation, spin-orbit coupling and geometrically frustrated spin configurations. Bulk Pr2Ir2O7 single crystals show metallic spin liquid behavior1, quantum criticality2 and a large anomalous Hall Effect without long range magnetic ordering3. Here we present the synthesis and characterization results of high-quality epitaxial Pr2Ir2O7 thin films stabilized via the solid state epitaxy method. Hall measurements reveal a low field magnetic phase transition at 30K, which is much higher than the bulk transition temperature of 1.5K4. Further magnetic effects have been observed in the Hall conductivity under higher magnetic field. We reveal the possible mechanism behind these novel magnetic behaviors by comparing the anomalous Hall behaviors with the magnetization measurements. |
Friday, March 9, 2018 8:36AM - 8:48AM |
X23.00004: Lattice Energetics, Geometrical Constraints and Correlation-Driven Metal-Insulator Transitions: the case of Ca$_2$RuO$_4$ Qiang Han, Andrew Millis Many materials exhibit metal-insulator transitions that are driven by electron correlation effects but are closely associated with changes in local lattice structure. This paper uses density functional and dynamical mean field theory methods to construct a free energy that elucidates the interplay of electronic and lattice energies in one such material, Ca$_2$RuO$_4$. We find that the change in lattice energies across the metal-insulator transition is comparable to the change in electronic energies. An important consequence is that the imposition of geometrical constraints (for example a lattice parameter fixed by epitaxial growth on a substrate) can change the lattice energetics enough to eliminate the metal-insulator transition entirely. A comparison to recent data is presented, and the generalization of the theory to other transition metal oxides is discussed. |
Friday, March 9, 2018 8:48AM - 9:00AM |
X23.00005: Topological and interfacial phenomena in oxide heterostructures Fengyuan Yang, Adam Ahmed, Keng-Yuan Meng, Bryan Esser, David McComb Oxide materials show a plethora of electronic and magnetic phases from strong electron-electron interactions, in particular, the presence of magnetic skyrmions in oxide heterostructures. In this talk, I will present recent data on topological and interfacial phenomena in oxide heterostructures SrRuO3/SrIrO3, where the SrRuO3 film is ferromagnetic while the SrIrO3 provides a large interfacial Dzyaloshinskii-Moriya interaction (DMI). The interfacial DMI is a key proponent in stabilizing topological spin textures such as skyrmions. Our samples were grown be dc sputtering on SrTiO3 (100) substrates. The films were characterized with AFM and XRD to confirm excellent crystalline quality in conjunction with atomic scale roughness. Hall measurements show features not seen after accounting for the ordinary and anomalous Hall contributions. Specifically, we observed a pronounced topological Hall signal at temperatures below 60 K and at high magnetic fields of 1-3 T. Additionally, SQUID magnetometry measurements reveal a magnetic feature near zero field not captured by the resistivity measurements, indicating this magnetic phase to be insulating in nature. We conclude by showing recent measurements that demonstrate the tunability of these topological properties. |
Friday, March 9, 2018 9:00AM - 9:12AM |
X23.00006: Hot-electron dynamics in oxide heterostructures Brian Kim, Yasuyuki Hikita, Di Lu, Adrian Swartz, Hyeok Yoon, Harold Hwang The behavior of hot electrons has potential to provide useful insight in the dynamics of strongly correlated electron systems, but it is inherently challenging as an experimental probe due to rapid relaxation (typically on the order of femtoseconds) within very short length scales (typically few nanometers). The hot-electron transistor (HET) provides a unique measurement geometry for this purpose, where the (correlated) metal base can be studied by vertically setting the transport length scale comparable to the hot-electron mean free path. Here we use the HET to probe the evolution of vertical hot-electron transport across oxide heterostructures with a metallic SrRuO3 (SRO) base, well-known for ‘badly metallic’ behavior as a consequence of intermediate electron-correlations. We observe an abrupt increase of the hot-electron mean free path as a function of decreasing temperature at the ferromagnetic transition. Furthermore, we present results on the anisotropic scattering of hot-electrons as a function of the magnetization orientation deep in the ferromagnetic state. |
Friday, March 9, 2018 9:12AM - 9:24AM |
X23.00007: Confinement-driven electronic, magnetic and topological phases in corundum-derived oxide honeycomb superlattices Okan Koeksal, Rossitza Pentcheva Based on density functional theory calculations with a Hubbard U parameter, we explore electronic, magnetic and possibly topologically non-trivial phases in X2O3 honeycomb layers confined in the corundum structure Al2O3 (0001). For X=3d cation the ground state is in most cases a trivial antiferromagnetic Mott insulator. However, with imposed symmetry of the two sublattices the ferromagnetic phases of Ti, Mn, Co and Ni exhibit a characteristic set of four bands associated with the single occupation of eg’ (Ti) and eg (Mn, Co, Ni). The Dirac point can be tuned to the Fermi level by strain and a substantial anomalous Hall conductivity arises when spin-orbit coupling (SOC) is switched on. Moreover, a particularly strong effect of SOC is identified for X=Ti at aAl2O3 = 4.81Å with an unusually high orbital moment of -0.88 μB nearly compensating the spin moment of 1.01 μB. Thus, this system emerges as a possible realization of Haldane’s model of spinless fermions. We further extend our work to the 4d and 5d series and identify cases of high orbital moment (Os) or candidates for Chern insulators (CI), i.e. X=Tc and Pt with C=-2 and -1, as a function of the Coulomb repulsion strength. |
Friday, March 9, 2018 9:24AM - 9:36AM |
X23.00008: Anomalous Hall effect in perovskite cobalt oxide thin films Yuki Ohuchi, Jobu Matsuno, Yusuke Kozuka, Masaki Uchida, Masashi Kawasaki Magnetic oxide thin films have attracted increasing attention as the spintronic materials such as for magnetic tunnel junctions or for spin injection. SrCoO3 is one of the unique ferromagnetic perovskite oxides in its high Curie temperature (Bulk: 305 K) and tunable magnetic properties through topotactic oxygen deinsertion or La substitution. Despite these intriguing characteristics, the basic magneto-transport properties have remained to be clarified in this material. Here, we investigate anomalous Hall effect (AHE) in epitaxially grown SrCoO3 single crystalline thin films and La-doped ones. Their temperature dependences of AHE are examined as well as those of magnetization. We observe the characteristic behavior of intrinsic AHE as demonstrated in SrRuO3; the sign of AHE is inverted depending on magnetization and/or La substitution. The result suggests that AHE in SrCoO3 is subject to the berry phase in momentum space determined by the band structure, leading to a candidate for detecting other spin-orbit related phenomena. |
Friday, March 9, 2018 9:36AM - 9:48AM |
X23.00009: Thickness Dependent Topological Hall Effect at SrIrO3/SrRuO3 Interface Keng-Yuan Meng, Adam Ahmed, Bryan Esser, David McComb, Fengyuan Yang The Dzyaloshinskii-Moriya interaction (DMI) has recently been found to enable chiral spin structures such as skyrmions, which have potential for next-generation magnetic memory and logic devices. Oxide materials are a desired platform to study novel physics owing to their stability under ambient conditions and tunability through strain. To this end, we study an oxide bilayer system, SrIrO3/SrRuO3, in order to stabilize skyrmions via interfacial DMI. In this system, SrRuO3 is a ferromagnet while nonmagnetic SrIrO3 provides strong spin-orbit coupling. We report the synthesis of single crystal SIO/SRO bilayers by off-axis magnetron sputtering, and our epitaxial films were shown to have exceptional crystalline quality by XRD, AFM, and cross section TEM. Our Hall effect measurements reveal a high field topological Hall peak (~1-3 T) below 60 K that cannot be solely accounted for from anomalous Hall signal. In addition, zero field topological Hall signals are consistently seen in a 40 K temperature window below the Curie temperature—independent of thickness. To elucidate these features, we study single layer SRO films which also show zero field topological Hall signals. |
Friday, March 9, 2018 9:48AM - 10:00AM |
X23.00010: Magnetism of Ferromagnetic Manganite/Iridate Interfaces Suzanne te Velthuis, Stephan Rosenkranz, David Keavney, John Freeland, Yongseong Choi, Joerg Strempfer, Daniel Haskel, Xiao Wang, Brian Kirby, Timothy Charlton, Javier Tornos, Jacobo Santamaria The relationship between spin-orbit coupling (SOC), emergent topological states, and spin Hall effects, which are highly relevant for spintronics, has driven the interest in materials with strong SOC in recent years. Likewise, complex oxide heterostructures have garnished much interest due to emergent interfacial phenomena resulting from the competition between charge, spin, and lattice order parameters, and the ability to tune this competition. The question arises as to whether SOC can result in modified interfacial magnetic states. To explore this possibility we have investigated ferromagnetic La0.7Sr0.3MnO3/SrIrO3 bilayers deposited on a SrTiO3 substrate. First of all we find that depending on the growth order, the coercive field of the magnetization loop differs significantly at low temperatures. Similar to previous reports, a net moment on Ir has been recorded with element specific XMCD experiments, which is aligned antiparallel to the Mn moments within the manganite. Interestingly, we find that the Ir moment does not follow the same temperature dependence as the magnetization of the manganite, but drops off almost linearly with increasing temperature. Possible origins of this result will be discussed. |
Friday, March 9, 2018 10:00AM - 10:12AM |
X23.00011: Visualization of topological Hall effect in ultra-thin SrRuO3 thin films Weida Wu, Wenbo Wang, Zhaoliang Liao, Jun Wang The topological Hall effect (THE), which originates from the non-coplanar spin textures in real space, has attracted much attention over years. Recently, an interface-driven topological Hall effect has been realized in SrRuO3-SrIrO3 bilayer systems[1]. By combining a ferromagnetic SrRuO3 layer and a paramagnetic SrIrO3 layer with strong spin-orbital coupling, large DM interaction was introduced on the interface, which presumably results in a Neel-type magnetic skyrmion phase with substantial THE. In this talk, we will report the observation of THE in the SrRuO3 ultrathin films without SrIrO3 layer using magnetic force microscopy (MFM) with in-situ transport. Possible mechanisms of the THE in these SrRuO3 ultrathin films will be presented. |
Friday, March 9, 2018 10:12AM - 10:24AM |
X23.00012: Unravelling decoupled ferrimagnetism and Skyrmion in Manganite-Iridate perovskites Z. S. Lim, Ariando Ariando Exploring exotic magnetism due to charge transfer in perovskites with 3d and 5d hetero-species B-site ions has remained an intriguing topic with plenty of unexplained features. In this study, Ir- and Mn-based alloy and superlattice were compared and explained in terms of their in-plane and perpendicular magnetic anisotropy supported by evidence of orbital occupancy, decoupling between Mn(4-δ)+ and Ir(4+δ)+ contributions to ferromagnetism and the discovery of high Curie Temperature contributed by Ir(4+δ)+ moment. Topological Hall Effect is also discovered in the superlattice case at a narrow temperature range when the long range order of Mn(4-δ)+ moment is about to vanish, indicating magnetization switching by formation of Skyrmions. Comprehending the interplay of complex Physics involved may facilitate future design of memory devices. |
Friday, March 9, 2018 10:24AM - 10:36AM |
X23.00013: Strain control of electric and magnetic properties in the iridate heterostructure Junyi Yang, Lin Hao, Jian Liu, Derek Meyers, Mark Dean, Clayton Frederick, Lukas Horak, Dominik Kriegner, Yongseong Choi, Jong Woo Kim, Daniel Haskel, Tamene Dasa, Haixuan Xu, Philip Ryan The promising quantum phenomena in Ruddlesden-Popper series iridates (Srn+1IrnO3n+1) have made them a playground to study exotic electronic and magnetic properties. The competition between crystal field splitting, Coulomb repulsion, and spin-orbit coupling in comparable energy scales allows minimal structural and electronic perturbations to drastically alter their ground states. While the bulk Srn+1IrnO3n+1 crystals offer limited structural flexibility, the choice of the artificially stacked [(SrIrO3)n , (SrTiO3)] superlattices can elevate structural tuning of the competing interactions to a higher degree. In our recent work, we have synthesized a series of [(SrIrO3)n , (SrTiO3)] superlattices with various lattice mismatch. Results from structural characterization, transport measurements and magnetic moment measurement indicate strong structural modulability of macroscopic electronic and magnetic properties in the iridates. |
Friday, March 9, 2018 10:36AM - 10:48AM |
X23.00014: Strain induced perpendicular magnetic anisotropy in epitaxial europium iron garnet thin films Víctor H. Ortiz, Mohammed Aldosary, Junxue Li, Yawen Liu, Pathikumar Sellappan, Chad Warren, Javier Garay, Jing Shi Epitaxial growth of rare earth iron garnet (REIG) films offers a unique opportunity to control the magnetic properties such as the magnetic anisotropy and Gilbert damping [1,2]. To achieve desired perpendicular magnetic anisotropy (PMA), for example, different REIG require suitable substrates to deliver either tensile or compressive strain due to different magnetostriction coefficients. In this work, we present an experimental study of the magnetic anisotropy in europium iron garnet (EuIG) thin films grown by pulsed laser deposition. Atomically flat EuIG films of varying thickness (10nm – 100nm) were deposited on (100)-oriented gadolinium gallium garnet substrates. All films show compressive strain as characterized by X-ray diffraction. The magnetometry and angular dependent ferromagnetic resonance results indicate correlation between PMA and strain on the EuIG lattice. PMA was corroborated by spin pumping and anomalous Hall effect experiments on Pt devices fabricated on the EuIG thin films. |
Friday, March 9, 2018 10:48AM - 11:00AM |
X23.00015: Observation of room temperature polar bubbles/ skyrmions in oxide heterostructures SUJIT DAS, Y. L Tang, S.-L. Hsu, A. Ghosh, M. R. McCarter, Z. Hong, Y. Dong, L.-Q. Chen, D. D. Fong, J. Junquera, L.W. Martin, R. Ramesh The complex non-collinear and non-coplanar topological textures in complex oxides have recently attracted of their various exotic phenomena (such as emergent structural chirality). Among them the recent discovery of polarization vortices with toroidal order in superlattices of (PbTiO3)n/(SrTiO3)n [1]. Topologically protected nanometric polar vortices, skyrmions are being are rarely investigated in complex heterostructures. Here, we describe that for a room-temperature polar bubbles/skyrmions with 6-7 nm size in PbTiO3/SrTiO3 heterostructures via real space imaging transmission electron microscopy, phase field simulations and ab-initio calculations. Phase-field and ab-initio studies have revealed the presence of this new polarization topology of ferroelectric bubbles/skyrmions that arise from a combination of epitaxial strain and layer thickness. This finding of nanometre-scale polar topology in a thin film may be useful as a fertile ground for promising applications. |
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