Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session B40: Complex Oxide Films and Heterostructures IFocus
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Sponsoring Units: GMAG DMP DCOMP Chair: George Sterbinsky, Argonne National Laboratory Room: BCEC 208 |
Monday, March 4, 2019 11:15AM - 11:51AM |
B40.00001: Evidence of antiferromagnetism as the driver of the metal-insulator transition in vanadium sesquioxide (V2O3) Invited Speaker: Juan Trastoy Metal-insulator transitions (MITs) in strongly-correlated materials result from the interplay of many degrees of freedom, such as electronic, magnetic or structural. Untangling these contributions has remained a longstanding problem in condensed matter physics. In this work, we have investigated V2O3, an archetypal strongly-correlated material with a MIT where electronic, structural and magnetic phase transitions occur simultaneously. By performing magneto-resistance (MR) measurements across the MIT we acted on the magnetic degree of freedom and revealed an anomalous behavior: the MR crosses over to negative values and seemly diverges as the MIT takes place. To gain physical insight, we study the antiferromagnetic MIT in a Hubbard model in the presence of a magnetic field by high precision dynamical mean-field theory calculations. We find the model results accurately reproduce the unusual experimental behavior. Furthermore, they reveal a simple mechanism where the magnetic field impedes the antiferromagnetic ordering of one sublattice, thus preventing the opening of the gap. Our study provides strong evidence that the origin of the MIT in V2O3 is the opening of an antiferromagnetic gap. |
Monday, March 4, 2019 11:51AM - 12:03PM |
B40.00002: Large linear magnetoresistance in ultraclean SrVO3 Matthew Brahlek, Jong Mok Ok, Elizabeth Skoropata, Ho Nyung Lee, jason lapano, Roman Engel-Herbert, patrick Irvin, jeremy levy Observations of linear magnetoresistance has a near 100-year history with origins ranging from effects of open Fermi surface in polycrystalline metals, pushing materials like bismuth to the quantum limit, to large scale inhomogeneity/defect clusters in silver chalcogenides—such diversity has long given rise to controversy. This talk will focus on the surprising observation of a large non-saturating linear magnetoresistance that appears in ultraclean, molecular beam epitaxy-grown thin films of the correlated metal SrVO3. The unusual nature of this observation is that the linear magnetoresistance appears only when the samples are in the ultraclean limit and sets in far below the quantum limit; this strongly implies that this is an intrinsic feature of the electronic structure. As such both intrinsic effects, coming from the Fermi surface geometry and electron-electron correlations, and extrinsic effects, particularly defect structures common to perovskites, will be discussed. |
Monday, March 4, 2019 12:03PM - 12:15PM |
B40.00003: Spin colossal magnetoresistance in an antiferromagnetic insulator Dazhi Hou, Zhiyong Qiu, Joseph Barker, Kei Yamamoto, Olena Gomonay, Eiji Saitoh Colossal magnetoresistance (CMR) refers to a large change in electrical conductivity induced by a magnetic field in the vicinity of a metal–insulator transition and has inspired extensive studies for decades. Here we demonstrate an analogous spin effect near the Néel temperature, TN = 296 K, of the antiferromagnetic insulator Cr2O3. Using a yttrium iron garnet YIG/Cr2O3/Pt trilayer, we injected a spin current from the YIG into the Cr2O3 layer and collected, via the inverse spin Hall effect, the spin signal transmitted into the heavy metal Pt. We observed a two orders of magnitude difference in the transmitted spin current within 14 K of the Néel temperature. This transition between spin conducting and non-conducting states was also modulated by a magnetic field in isothermal conditions. This effect, which we term spin colossal magnetoresistance (SCMR), has the potential to simplify the design of fundamental spintronics components, for instance, by enabling the realization of spin-current switches or spin-current-based memories. |
Monday, March 4, 2019 12:15PM - 12:27PM |
B40.00004: Observation of non-Fermi liquid behaviors on high-mobility LaNiO3 Changjiang Liu, Terence Bretz-Sullivan, Friederike Wrobel, Gensheng Wang, Deshun Hong, JOHN E. PEARSON, J Samuel Jiang, Jianjie Zhang, Clarence Chang, Alexey Suslov, Michael Norman, Anand Bhattacharya LaNiO3 (LNO) is a special member of the rare-earth perovskite nickelate family, as it does not show metal-insulator or magnetic transitions at low temperatures. Whether or not LNO behaves like a Fermi liquid in the low temperature regime with quadratic temperature dependence of resistivity remains an open question. Recent measurements of the electronic structure and theoretical calculations support the presence of structural and magnetic instabilities in LNO. However, the impact of these instabilities has yet to be clarified in transport measurements, partly because preparing high-quality LNO samples had been challenging due to a propensity towards oxygen vacancy formation. Here, we report on the observation of non-Fermi liquid transport behaviors at sub-Kelvin temperatures in high-mobility LNO thin films grown by ozone-assisted molecular beam epitaxy. The resistivity of these samples can be lower than 4 μΩ cm at T = 2 K, with residual resistivity ratios (RRR) [ρ (300 K) / ρ (2 K)] > 24. |
Monday, March 4, 2019 12:27PM - 12:39PM |
B40.00005: Direct Imaging Revealing Halved Ferromagnetism in Tensile-Strained LaCoO3 Thin Films Xiaofang Zhai, Qiyuan Feng, Dechao Meng, Qingyou Lu, Yalin Lu The enigma of the emergent ferromagnetic state in tensile-strained LaCoO3 thin films remains to be explored since there is a longstanding controversy between the theory and experiments. It is that all experiments can only find at most half of the magnetic moment predicted by the theory. Here we show the experimental evidence of the rare halved occupation of the ferromagnetic state in tensile-strained LaCoO3 thin films by the direct magnetic imaging technique using a low-temperature magnetic force microscope. The film was grown by the atomic layer-by-layer laser molecular beam epitaxy and exhibits the coherently strained lattice structure and the stoichiometric composition. The direct magnetic imaging revealed that percolated ferromagnetic regions with typical sizes between 100 nm and 200 nm occupy about 50% of the entire film, down to the lowest achievable temperature of 4.5 K and up to the largest magnetic field of 13.4 T. Our study demonstrated the halved occupation of the ferromagnetic phase in the LaCoO3 ultrathin films as the real microscopic state of the emergent ferromagnetism. |
Monday, March 4, 2019 12:39PM - 12:51PM |
B40.00006: Interplay between Strain-Stabilized Ferromagnetism and Charge Transport in Epitaxial LaCoO3-δ Thin Films VIPUL CHATURVEDI, Jeffery J Walter, Arpita Paul, Jong S Jeong, Alexander Grutter, Brian Kirby, Julie Borchers, K. Andre Mkhoyan, Turan Birol, Chris Leighton The presence of ferromagnetic (FM) order in epitaxial LaCoO3 (LCO) thin films, in stark contrast to bulk, has gathered substantial recent attention. Due to the insulating nature of this strain-stabilized FM, however, little attention has been paid to electronic transport. Here, we present a systematic study of epitaxial LCO films (8-22 nm thick) grown under both tensile (on SrTiO3 and LSAT) and compressive strain (on LaAlO3 and SrLaAlO4), emphasizing not only magnetism but also transport. Tensile films exhibit ferromagnetism with saturation magnetization 1-1.5 μB/Co and Curie temperature ~80 K, consistent with prior work. Polarized neutron reflectometry has also been performed, revealing a uniform magnetization depth profile with thickness. Most notably, transport, which is semiconducting in all cases, reveals an unanticipated correlation between strain-state and majority carrier type, supported by density functional theory calculations suggesting significant changes in conduction band structure under strain. This provides important clues to the origin of FM in this material. |
Monday, March 4, 2019 12:51PM - 1:03PM |
B40.00007: Size bistability in magnetoelectric cycloids Marc Allen, Ian Aupiais, Maximilien Cazayous, Rogério de Sousa Most multiferroic materials with coexisting ferroelectric and magnetic order exhibit cycloidal antiferromagnetism with wavelength much larger than lattice spacing. The prototypical example is bismuth ferrite (BiFeO3 or BFO), a room-temperature multiferroic considered for a number of technological applications, including magnetic memories with electric-write capability. While most applications requires small sizes such as nanoparticles, little is known about the state of these materials when their sizes are comparable to the cycloid wavelength. This work describes a theory of cycloid magnetism in nanoparticles. It is argued that magnetic anisotropy close to the surface has a huge impact on the ground state cycloidal wavevector, leading to several observable consequences. For certain sizes the cycloidal wavevector is bistable, an effect that may be exploited in the design of novel memory devices. |
Monday, March 4, 2019 1:03PM - 1:15PM |
B40.00008: Charge Doping and Quantum Monte Carlo of NiO Friederike Wrobel, Changhee Sohn, Hyeondeok Shin, George Sterbinsky, Anouar Benali, Panchapakesan Ganesh, Olle Heinonen, Ho Nyung Lee, Anand Bhattacharya NiO is a Mott insulator that orders antiferromagnetically below 520 K1. The transition temperature drops with Li-doping and becomes first ferrimagnetic and later antiferromagnetic with a Néel temperature of 9 K2. Despite intense study of NiO and other Mott insulators, the evolution of correlations with charge doping, which give rise to spectacular phenomena (including superconductivity in cuprates), is poorly understood to date. Here, we combine atomic layer-by-layer molecular beam epitaxy growth of charge doped NiO with density functional theory (DFT) and Quantum Monte Carlo (QMC) simulations. We analyze the band gap with spectroscopic ellipsometry as a function of hole (K) and electron (In) doping, and compare the results with DFT and QMC. Moreover, we measure the distortion of the lattice caused by the large K with extended x-ray absorption fine structure and find a distortion much larger than predicted by DFT. |
Monday, March 4, 2019 1:15PM - 1:27PM |
B40.00009: Magnon spin transport through antiferromagnetic NiO Geert Hoogeboom, Bart Van Wees Thus far, antiferromagnets (AFMs) played a passive role of pinning in giant magnetoresistance. AFMs hold promise of ultrafast and lossless AFM transport devices [1] when one is able to simultaneously generate and detect spin currents, while having control over the magnetic moment directions. AFMs have no stray fields so we made use of either the magnetic easy-plane character[2] or an attached ferromagnet to manipulate its magnetic moments. |
Monday, March 4, 2019 1:27PM - 1:39PM |
B40.00010: The cation distribution in piezoelectric ferromagnetic GaFeO3(010) thin films Jaeyoung Kim, Dong-Hwan Kim GaFeO3 is well known piezoelectric ferrimagnet where electron and spin degree of freedoms are coupled. There are four different cationic sites and the disorder in the occupation of them makes it ferrimagnet. Therefore, it is very important to study the site disorder in a specific specimen so as to understand a wide variety of ferromagnetic properties found in different GaFeO3’s. Usually the site occupation of cations can be studied with neutron diffraction and Mösbauer spectroscopy. But, mostly they are not available for thin films. |
Monday, March 4, 2019 1:39PM - 1:51PM |
B40.00011: Spin Transport across Antiferromagnetic Cr2O3 Films Grown on Y3Fe5O12 Yawen Liu, Wei Yuan, Qiming Shao, Kang Wang, Jing Shi Spin transport in heterostructures containg antiferromagnetic materials has attracted a lot of attention due to the enhanced transmission of the pure spin current and the interesting orientation dependence. Compared with NiO, Cr2O3 is a better-defined antiferromagnet for its uniaxial anisotropy. Here we report a spin transport study of YIG/Cr2O3/Pt heterostructures, in which both YIG and Cr2O3 are grown by PLD. RHEED patterns indicate that Cr2O3 is textured polycrystalline on single crystal YIG with preferred c-axis along the easy axis of the YIG. We perform both magnetoresistance(MR) and spin Seebeck effect (SSE) measurements. When the in-plane magnetic field is applied along or perpendicular to the c-axis of textured Cr2O3, the high-field MR responses in both directions resemble those of the YIG/Pt. When the field is along the c-axis of textured Cr2O3, MR shows an additional peak feature ~1T at 5 K, and the peak position moves to higher fields at higher temperatures, indicative of the spin-flop transition of Cr2O3. The SSE signal shows a sign reversal at low temperature, which does not exist in YIG/Pt and therefore is also attributed to the Cr2O3. |
Monday, March 4, 2019 1:51PM - 2:03PM |
B40.00012: Hexagonal YbFeO3 thin-film heterostructures grown by pulsed laser deposition Yu Yun, Yuewei Yin, Xuanyuan Jiang, Xiaoshan Xu
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Monday, March 4, 2019 2:03PM - 2:15PM |
B40.00013: The role of disorder and defects in the ferromagnetic resonance of spinel ferrite thin films Jacob Wisser, Satoru Emori, Lauren Riddiford, Peng Li, Krishnamurthy Mahalingham, Brittany T. Urwin, Brandon Howe, Alexander Grutter, Brian Kirby, Yuri Suzuki Spin current generation in heterostructures requires low magnetic damping spin-current sources. To this end, we seek to minimize disorder and defects in spin-current source materials and improve interface quality. In this work, we find that epitaxial spinel ferrite MgAl0.5Fe1.5O4 (MAFO) films exhibit low damping and coercivity in the 10-15nm thickness range with a Gilbert damping parameter α≈0.001 and coercive field Hc<5 Oe. Transmission electron microscopy results indicate these films grow coherently strained on MgAl2O4 substrates with minimal defects. Gilbert damping rapidly increases outside of this thickness range, with α≈0.004 for 5nm films and α≈0.03 for 40nm films. Dislocations form at the film/substrate interface in 40nm films, indicating that the film relaxes, causing increased damping and coercivity (Hc>50 Oe). A thickness-dependent magnetization study of MAFO films from 1-45nm thick indicates the presence of a 1.6nm magnetically-dead layer, less than half that found in yttrium iron garnet films. This layer is confirmed to exist at the film/substrate interface by depth profiling via polarized beam neutron reflectivity. The nature of this dead layer is crucial to device integration, and we will discuss interfacing this oxide with platinum and copper/platinum overlayers. |
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