Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session N54: Emergent Properties of Complex Oxides Bulk, Thin Films, and Heterostructures IVFocus
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Sponsoring Units: GMAG Chair: Mark Dean, Brookhaven National Laboratory Room: Room 306 |
Wednesday, March 8, 2023 11:30AM - 12:06PM |
N54.00001: Emergent spin-orbital states and magnetic regimes in the second-generation Kitaev magnets Invited Speaker: Fazel Tafti Honeycomb iridate materials have been intensely studied in the past two decades due to the potential realization of a Kitaev spin-liquid phase. The basic ingredients of the Kitaev model including a honeycomb lattice, spin ½ ions, and bond-directional Ising-like interactions are present in materials such as α-Li2IrO3 and Na2IrO3. However, non-Kitaev interactions such as the Heisenberg and off-diagonal exchange are also present in real materials. These competing interactions create a complex phase diagram with non-collinear magnetic orders as well as the spin-liquid phase. In this talk, I will explain how topochemical methods can be used to tune the competing interactions and access different regimes in the phase diagram of Kitaev magnets. In a typical topochemical reaction, alkali ions are replaced by monovalent transition metal ions. As a result, the bond angles across the super-exchange paths change, providing a mechanism for tuning the relative strength of different exchange interactions and a potential route to discovering the quantum spin-liquid phase. I will present several materials synthesized via the topochemical reactions including Cu2IrO3 which exhibits a competition between static and dynamic magnetism [1,2,3], Ag3LiIr2O6 which exhibits thermodynamic evidence of proximity to the Kitaev spin-liquid phase [4,5,6,7], and Ag3LiRh2O6 that falls in a dramatically different region of the phase diagram away from the spin-liquid phase [8]. |
Wednesday, March 8, 2023 12:06PM - 12:18PM |
N54.00002: Hidden order in the osmates: Microscopic theory and phenomenology Arun Paramekanti, Sreekar Voleti Phases associated with multipolar broken symmetries are often termed 'hidden orders' since they elude conventional experimental probes. Understanding the origin of the multipolar degrees of freedom, and their exotic ordering has been a subject of intense interest over the past decade. While much of the initial work on the subject was done in the context of f-electron heavy fermion systems, recent experiments have put the spotlight on d-orbital systems as a rich playground for exploring these multipolar orders. In particular, heavy d-orbital spin-orbit coupled Mott insulators provide a very natural way to stabilize ground states with multipolar moments. In this talk, I will review the advancements in our theoretical understanding of a class of osmates, and various proposals for their multipolar ordering. I will present ideas both from a phenomenological point of view, inspired by experimental evidence, as well as from microscopic calculations, which have given us insight into not only the rich phases that can be exhibited in these compounds, but also into limitations of existing perturbative techniques used to study "standard" Mott insulators. I will conclude with our understanding of recent NMR experiments that have provided a further peek into these complex orders. |
Wednesday, March 8, 2023 12:18PM - 12:30PM |
N54.00003: Antiferromagnetic structure refinement in epitaxially-strained RuO2 by resonant elastic x-ray scattering Benjamin Z Gregory, Neha Wadehra, Joerg Strempfer, Anita Verma, Jacob Ruf, Hari P Nair, Nathaniel J Schreiber, Darrell G Schlom, Kyle M Shen, Andrej Singer The simple, rutile oxide, RuO2, is the parent material of the perovskite ruthenates and a source of current intrigue following the surprising discoveries of antiferromagnetism [1, 2], and strain-induced superconductivity [3] in this material. The details of the magnetic structure and its relationship to the novel superconducting phase, however, remain open questions. Recent magnetic resonant scattering results from the highly-strained, superconducting films deviate from those in bulk RuO2 [4], and simulations of resonant diffraction from several antiferromagnetic motifs have led some researchers to question the evidence for collinear antiferromagnetism in RuO2 [5]. These critics instead suggest a “chiral signature,” measured with circularly-polarized, resonant x-rays, as an unambiguous indicator of long-range magnetic order. Here we present the results of such an experiment, exploiting both circular and linear polarization analysis, to conclusively determine the magnetic structure of RuO2. We report findings from a set of superconducting thin films grown by molecular beam epitaxy, ranging from relaxed and bulk-like to ultrathin and coherently strained. |
Wednesday, March 8, 2023 12:30PM - 12:42PM |
N54.00004: Electronic structure of itinerant antiferromagnet RuO2 under strain Quoc Dai HO, Quang D To, Ruiqi Hu, Garnett W Bryant, Anderson Janotti Rutile RuO2 has long been considered as an ordinary metallic paramagnet in its ground state. It is, however, recently proved by neutron diffraction experiment to be an itinerant antiferromagnet with Néel temperature above 300 K. More interestingly, RuO2 is shown to have momentum-dependent spin polarization due to its unique symmetry in real space, leading to non-spin degenerate Fermi surface. The spin splitting effect in AFM RuO2 is regarded as an anomalous phenomenon since the AFM state is collinear and non-frustrated with a centrosymmetric center and without spin-orbit coupling. Given this unique magnetic structure, RuO2 proved to host interesting properties such as giant tunneling magnetoresistance, anomalous Hall effect, time-reversal-odd spin Hall conductivity amongst others. In addition, epitaxial and uniaxial strain were applied to thin film RuO2, giving rise to fascinating observations ranging from superconductivity below 2 K to enhanced oxygen evolution reaction performance. In this study, using the state-of-the-art first principles calculations we explore the electronic structure of RuO2 under various strain conditions. We address the interplay between spin, orbital, and lattice degrees of freedom as well as the strained-induced effects on its band structure. Initial results show that applying strain modifies the RuO2 electronic structure, i.e., altering the Fermi surface topology, establishing the controlled strain-manipulation of the electronic transport properties of RuO2. |
Wednesday, March 8, 2023 12:42PM - 12:54PM |
N54.00005: In-depth structural and magnetic study of Ni/NiO epitaxial thin films on single crystalline sapphire grown using pulsed laser deposition Md Shaihan Bin Iqbal The crystallographic orientation plays a significant role in magnetic heterostructure devices, especially in the arena of spintronic devices. In terms of magnetic property, domain orientation is solely dependent on the structural homogeneity. This research work deals with the development of epitaxial thin films of Ni/NiO on single-crystalline sapphire (0006) substrate. PLD was used at 650°C in constant oxygen pressure of 0.01 mbar to deposit films. Phase mixture of Ni/NiO in the thin films was achieved via reduction annealing. XRD data shows preferential growth of NiO on sapphire along the (111) plane (peak at 37.25°), sharp peak of Ni at 44.4° indicates a handful amount of NiO convert to Ni phase. Crystallographic orientation of NiO was analyzed by phi scan and pole figure technique. TEM-EDS images show the surface homogeneity with presence of both Ni and NiO on the film. Magnetic force microscopy and ferromagnetic resonance indicated the ferromagnetic nature of the film. Temperature and field dependent magnetization data collected using SQUID magnetometer show ferromagnetism and antiferromagnetism in Ni and NiO films, respectively. Spin canting has been observed in phase mixture, holding a potential wide range magnetic exchange coupled phenomena. |
Wednesday, March 8, 2023 12:54PM - 1:06PM |
N54.00006: Origin of the static magnetic order in uniaxially strained Sr2RuO4 Bongjae Kim, Sergii Khmelevskyi, Cesare Franchini, Igor I Mazin Combining first principle density functional calculations and Moriya's self-consistent renormalization theory, we explain the recently reported counterintuitive appearance of an ordered magnetic state in uniaxially strained Sr2RuO4 beyond the Lifshits transition. We show that strain weakens the quantum spin fluctuations, which destroy the static order, more strongly than the tendency to magnetism. A different rate of decrease of the spin fluctuations vs. magnetic stabilization energy promotes the onset of a static magnetic order beyond a critical strain. |
Wednesday, March 8, 2023 1:06PM - 1:18PM |
N54.00007: Manipulating spin-waves by strain in BiFeO3 thin films Taehun Kim, Jiemin Li, Yanhong Gu, Yuwei Liu, Zhihai Zhu, Je-Geun Park, Sang-Wook Cheong, Chang-Yang Kuo, Yuefeng Nie, Mark P Dean, Jonathan Pelliciari, Valentina Bisogni Since the first realization of BiFeO3 film 20 years ago [1], this material form has been extensively studied as it is promising for spintronic and magnonic applications. Due to the strong magneto-electric and magneto-elastic couplings of BiFeO3, the magnetic properties of thin films (e.g., the magnetic ground states, the spin-wave energies at the zone center) can be effectively manipulated by strain [2-3]. However, the lack of suitable techniques sensitive to the spin-wave dispersion in thin films prevented so far to understand the relation between the magnetic exchange interaction and the strain. |
Wednesday, March 8, 2023 1:18PM - 1:30PM |
N54.00008: Evidence for Long Range Magnetic Ordering in Perovskite Calcium Iridate Thin Films Emily R Lindgren, Xin Yu Zheng, Sanyum Channa, Mingde Jiang, Young S Lee, Yuri Suzuki The 5d transition metal oxides are promising materials for studying the interplay between electron-electron interactions and strong-spin orbit coupling, which can lead to topological states and other emergent phenomena. Within this class of materials, the semimetal CaIrO3 (CIO) is of particular interest because it has strong spin-orbit coupling, is predicted to have Dirac electrons, and has exhibited mobility values up to 60,000 cm2/Vs in the bulk. We have synthesized high quality CIO films 2-40nm thick on (001)-oriented LSAT substrates and report on their electronic and magnetic properties. Our CIO films exhibit semi-metallic resistivity and electron-dominated conduction at room temperature, but the majority carrier switches to hole-type at approximately 190K, indicated by a sign change of the Hall coefficient. Hole-dominated conduction persists down to 2K, but at low temperatures the Hall resistance becomes non-linear as a function of field, thus suggesting an additional contribution. Using SQUID magnetometry, we observe a hysteretic magnetization as a function of applied field that is consistent with an anomalous Hall effect and may be attributed to ferromagnetic or canted antiferromagnetic order. Additionally, angular magnetoresistance (MR) measurements indicate an anisotropy in MR when the magnetic field is rotated through the plane of the current, in plane with the current, and about the axis of the current. This anisotropic MR suggests the presence of long range magnetic ordering in our CaIrO3 thin films. |
Wednesday, March 8, 2023 1:30PM - 1:42PM |
N54.00009: Strain tunable emergent magnetic state in Sr2IrO4 Shashi K Pandey, Dongliang Gong, Han Zhang, Jian Liu, Haidong Zhou, Philip J Ryan, Jong Woo Kim, Zhaoyu Liu, Jiun-Haw Chu, Andrew F May Iridates are one of the extensively studied transitional metal oxides because of the unique combination of the electron-electron and spin-orbit interaction. Sr2IrO4 is a notable example, which is a quasi-two-dimensional Jeff =1/2 canted antiferromagnetic (AF) Mott insulator with a layered structure that is remarkably similar to the parent phase of weakly spin-orbit-coupled high-Tc cuprates. However, due to the built-in spin-orbit entanglement, the Jeff = 1/2 moments can form significant inter-site quadrupoles in contrast to the S=1/2 moments of Cu ions. The resulting magnetoelastic coupling leads to spontaneous tetragonal symmetry breaking by the AF order in the B1g channel. In the experiment, we compared the elasto-responses of the AF order to in-situ B1g and B2g strains representing two orthogonal symmetry configurations. While the B1g strain efficiently detwins the spontaneous AF domains, new states that break the translational symmetry along the c-axis emerge with the B2g strain. Our model analysis shows that such an emergent state is driven by an unusual quartic interaction of B2g symmetry, competing with the intrinsic B1g anisotropy, and can be in situ tuned by the applied strain. |
Wednesday, March 8, 2023 1:42PM - 1:54PM |
N54.00010: Dimensional tuning of the metal-to-insulator transition in epitaxially strained NdNiO3 films Christopher T Parzyck, Vivek Anil, Berit H Goodge, Ludi Miao, Shuyuan Zhang, Lena F Kourkoutis, Darrell G Schlom, Kyle M Shen The rare earth nickelate perovskites, RENiO3 RE=La,Pr,Nd,…, exhibit a variety of intertwined phase transitions including a structural rearrangement, charge disproportionation, antiferromagnetic order, and a metal-to-insulator (MIT) transition. Epitaxial thin films have offered a fruitful platform to engineer these phases using doping, epitaxial strain, heterostructuring, and confinement. For example, it has been previously established that in ultrathin films of LaNiO3 under compressive strain the metallic ground state is suppressed in favor of an insulating one as the film thickness is reduced. In this talk we illustrate an opposite effect where the MIT in highly tensily strained NdNiO3/SrTiO3 (100) films can be continuously suppressed by decreasing the film thickness. At a critical thickness we observe a complete suppression of the insulating ground state in favor of a metallic one. We discuss the investigation of this crossover using a variety of measurement techniques as well as the connection between the suppression of the MIT and a simultaneously observed change in the octahedral rotation pattern of the films. |
Wednesday, March 8, 2023 1:54PM - 2:06PM |
N54.00011: Evolution of surface collective excitations in NdNiO3 across the metal-insulator transition Shuyuan Zhang, Christopher T Parzyck, Vivek Anil, Neha Wadehra, Darrell G Schlom, Kyle M Shen Rare-earth nickelates exhibit a variety of intriguing phenomena, including a metal-insulator transition, long-range magnetic order, and ferroelectricity. Using a combination of reflection momentum-resolved electron energy loss spectroscopy (EELS), angle-resolved photoemission spectroscopy, and oxide molecular beam epitaxy, we study the collective excitations of NdNiO3 thin films. We observe a multitude of collective excitations, including phonons, plasmons, and d-d excitations in the high-temperature metallic phase. As the insulating antiferromagnetic phase is reached, we observe a strong suppression of the plasmons and novel, dipole-active modes emerge in the EELS spectra. These modes are close in energy to magnon excitations and exhibit highly asymmetric line shapes. We discuss the possible origin of these modes, including the relationship between magnetic excitations and the electric dipole degrees of freedom. |
Wednesday, March 8, 2023 2:06PM - 2:18PM |
N54.00012: Engineering of octahedral rotation and magnetic anisotropy in LaCrO3/LaMnO3 superlattice Xuanyi Zhang, Divine P Kumah As the magnetic anisotropy (MA) has become important for high performance spintronics applications, developing an effective way to control MA in magnetic system has been of great interest. The magnetic properties of perovskite transition-metal oxides are highly tunable with thin film deposition techniques such as molecular beam epitaxy (MBE) and thus provide a fascinating playground to manipulate MA. While the interface effect and strain in the perovskite thin film/superlattice has been widely investigated to tune the MA, open questions remain related to the effect oxygen vacancy on MA. Here, we studied the magnetic anisotropy and oxygen octahedral rotation of MBE grown LaCrO3/LaMnO3(LCO/LMO) superlattices with different oxygen vacancy concentrations. The atomic-scale structure analyzation based on high-resolution synchrotron X-ray diffraction indicates differences in the oxygen octahedral rotation configuration of the LCO/LMO superlattice before and after annealing in oxygen. The antiferromagnetic coupling between Mn and Cr is verified by X-ray magnetic circular dichroism measurements. The angular dependent magnetism of LCO/LMO superlattice is investigated by SQUID measurement and shows a changes in the magnetic anisotropy which are correlated with the oxygen-vacancy induced structural transitions. Our finding leads to a better understanding of microscopic origin of the magnetic anisotropy in LCO/LMO superlattices and demonstrates a pathway to engineer the magnetic anisotropy with oxygen vacancy in transition-metal oxide heterostructures. |
Wednesday, March 8, 2023 2:18PM - 2:30PM |
N54.00013: Emergent Ferromagnetism at LaFeO3/SrTiO3 Interface Induced by Spin-State Transition Menglin Zhu, Joseph A Lanier, Sevim Polat Genlik, Jose Flores, Victor Barbosa, Patrick Woodward, Maryam Ghazisaeidi, Fengyuan Yang, Jinwoo Hwang Creating and manipulating magnetic states through the interface is of particular interest for developing novel spintronics. Despite recent progress, challenges remain in understanding emergent interface magnetism. In magnetic insulators with complex structures, multiple mechanisms can play their roles in modifying interfacial magnetism, and the spatial extent of modifications is often confined to a few unit cells near the interface. Therefore, a comprehensive characterization approach with the highest spatial resolution and precision is required for an unequivocal understanding of the origin of interfacial magnetism. We present a novel conducting ferromagnetic state at the epitaxial LaFeO3/SrTiO3 interface, where the atomistic origin is traced down using a combination of atomic resolution imaging/spectroscopy and first principal calculations. Our result shows that the interfacial ferromagnetism originates from spin state transitions of Fe ions within the first few unit cells near the interface, which is mediated by two factors. First, the oxygen octahedral volume shrinks to maintain oxygen connectivity with the substrate. Further, polar mismatch leads to the accumulation of extra charge on Fe site. Both factors combined result in a checkerboard-type arrangement of low and high-spin Fe ions at the LaFeO3 interface, leading to the observed interfacial ferromagnetism. The result here demonstrates a new paradigm of creating interfacial magnetic states by combining multiple degrees of freedom, and also presents a new routine to understand the atomistic origin of emergent magnetism. |
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