Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session P09: Magnetism in Thin Film OxidesFocus
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Sponsoring Units: DMP GMAG Chair: Xifan Wu, Temple Univ Room: LACC 301A |
Wednesday, March 7, 2018 2:30PM - 2:42PM |
P09.00001: A polarity-driven instability to itinerant surface ferromagnetism in the delafossite oxide PdCoO2 Federico Mazzola, Veronika Sunko, Seunghyun Khim, Helge Rosner, Pallavi Kushwaha, Oliver Clark, Lewis Bawden, Igor Marković, Timur Kim, Moritz Hoesch, Andrew Mackenzie, Philip King The ability to manipulate the surface and interface properties of correlated electron systems underpins the burgeoning field of designer quantum matter. In this context, electronic reconstructions arising from polar interfaces have been widely discussed as a way to drastically modify the interface electronic properties of complex oxide heterostructures [1]. Here, we show that an electronic reconstruction occurs at the polar surfaces of delafossite oxides, an emerging class of oxide materials [2]. Using angle-resolved photoemission, we demonstrate how this leads to a Stoner-instability towards itinerant ferromagnetism at the Pd-terminated surface of the non-magnetic PdCoO2 [3], thereby stabilising a magnetic environment which does not exist in the bulk. We also show that this occurs for the Pd-terminated surface of PdCrO2, an antiferromagnet in the bulk, thus leading to a ferromagnetic-antiferromagnetic heterostructure at its surface. For the PdCoO2 states, we also uncover a strong coupling of the surface electrons with magnons. Our work paves the way for the creation, control and manipulation of magnetic states in oxide surfaces and heterostructures. [1] Hwang et al. Nature Mater. 11, 103 (2012) [2] Mackenzie, Rep. Prog. Phys. 80, 32501 (2017) [3] Mazzola et al., arXiv1710.05392 |
Wednesday, March 7, 2018 2:42PM - 2:54PM |
P09.00002: Cubic CuO: a Prototype to Study Magnetism in High-temperature Superconductors Xucun Ma As an exceptional member of the transition metal monoxides with a rocksalt structure, CuO exhibits a monoclinic structure and lowered Néel temperature. Since the discovery of high temperature superconductivity in the copper oxide compounds, a higher symmetry structure of CuO is highly anticipated with the highest Néel temperature among the transition metal oxides, which would be of very importance to understand the copper oxide superconductors and other correlated materials. Recently, we have grown the CuO films in a combined MBE and STM system. Both scanning transmission electron microscopy and X-ray diffraction measurements confirm the cubic rocksalt structure for CuO films. Further magnetic measurements will be performed to determine if the magnetic interactions in cubic CuO are comparable with those of high-temperature superconducting oxides. |
Wednesday, March 7, 2018 2:54PM - 3:06PM |
P09.00003: Stabilizing Ferromagnetism in Low-Dimensional Rare-Earth Manganites Sanaz Koohfar, Elke Arenholz, Divine Kumah The confinement of magnetism in atomically thin oxide layers has important implications for understanding and manipulating magnetic interactions in two-dimensional systems and for the realization of novel spintronic device architectures. In the rare-earth manganites which exhibit colossal magnetoresitive effects in bulk, a thickness-dependent suppression of magnetism has been observed for films with thicknesses on the order of 5-10 unit cells. By using synchrotron-based surface x-ray diffraction to image the atomic structures of ultra-thin manganite films, we identify polar structural relaxations at the manganite film interfaces which are correlated with the suppressed magnetism. We achieve robust ferromagnetism in unit cell-thick manganite films by using La1-xSrxCrO3 spacers to effectively suppress the polar discontinuity at the manganite interfaces. These results demonstrate the control of atomic-scale structural distortions to engineer the electronic, orbital and magnetic properties of complex oxide systems. |
Wednesday, March 7, 2018 3:06PM - 3:42PM |
P09.00004: Tailoring interface-induced emergent phases in magnetic complex oxides with atomic precision Invited Speaker: Hangwen Guo Developments in synthesis and characterizing artificially structured materials have greatly advanced the possibility to explore new quantum states of matter at heterointerfaces. Atomically resolved electron microscopy and spectroscopy has been shown to hold the key to identify the structural, compositional and electronic behavior across the interfaces with atomic precision. This is especially crucial for complex oxides system since subtle variation in order parameters will give rise to totally unexpected phenomena. In this talk, we show that via the combination of high quality growth, electron microscopy, magnetic/transport characterization and theory, one can make self-assembled and artificially designed interface-induced phases that are inaccessible in bulk counterparts. Examples include interface-induced multiferroism with reemergent magnetic & electronic characters [1]; interface-driven unusual magnetic response in self-assembled magnetic nanostructures etc. Through these examples, we will illustrate that structural /composition evolution and correlation at interface are the most essential ingredients and serve as the foundation on the discovery of these emergent phenomena. We will also suggest a few recipes on designing low dimensional materials with amendable functionalities. [1] Guo et al, Proc. Natl. Acad. Sci. U.S.A. 114, E5062 (2017) |
Wednesday, March 7, 2018 3:42PM - 3:54PM |
P09.00005: Interface-Induced Emergent Properties in Self-Assembled La1-xSrxMnO3/La1-ySryMnO3 Heterostructures Summayya Kouser, Saurabh Ghosh, Mohammad Saghayezhian, Hangwen Guo, E Plummer, Sokrates Pantelides The strong correlation of charge, lattice, spin and orbital degrees of freedom in manganites provides a rich phase diagram for La1-xSrxMnO3 (LSMO) that ranges from canted antiferromagnetism (AFM) to ferromagnetism (FM) to paramagnetism (PM). Here we report the growth of self-assembled (La0.67Sr0.33MnO3/La0.73Sr0.27MnO3/SrTiO3) thin-film heterostructures that show unusual magnetic properties like exchange bias, spontaneous magnetic reversal and inverted hysteresis, with substantial suppression of octahedral tilts in La-poor region. Using first-principles calculations, we provide a theoretical understanding of the correlation of structural and magnetic properties observed in these self-assembled heterostructures and complement the experiments. We address the following three questions: (I) How does change of x in La1-xSrxMnO3 affect magnetism? (II) What is the origin of the observed exchange bias? and (III) How do we explain the suppression of octahedral tilts in La0.67Sr0.33MnO3 identified as critical for the novel magnetic properties?. |
Wednesday, March 7, 2018 3:54PM - 4:06PM |
P09.00006: Oxygen-isotope effects in the electrical and magnetic properties of epitaxial thin films of La1-xCaxMnO3 Bo Truong, Oscar Bernal, Guo-meng Zhao Oxygen-isotope effects in the electrical and magnetic properties of epitaxial thin films of La1-xCaxMnO3 (x = 0.2, 0.3, and 0.4) have been studied. The films were grown on [100] LaAlO3 single crystal substrates by pulsed laser deposition using KrF excimer. We find that the oxygen-isotope shift of the Curie temperature decreases significantly with increasing hole concentration. The activation energies of the Hall coefficient and resistivity are slightly different and both depend on the oxygen-isotope mass. The field-dependent data of the Hall Effect and electrical conductivity for the oxygen-isotope-exchanged films clearly demonstrate that the colossal magnetoresistance effect and the oxygen-isotope effects in doped manganites cannot simply be explained by the double-exchange model along with strong lattice polaronic effects. The results could be well explained if the spins of polarons would be quenched by formation of clusters of polaronic pairs. |
Wednesday, March 7, 2018 4:06PM - 4:18PM |
P09.00007: Magnetism in iridate heterostructures leveraged by structural distortions Derek Meyers, Yue Cao, Gilberto Fabbris, Neil Robinson, Lin Hao, Clayton Frederick, Nathan Traynor, Junyi Yang, Jiaqi Lin, Mary Upton, Diego Casa, Jong Woo Kim, Thomas Gog, Jenia Karapetrova, Yongseong Choi, Daniel Haskel, Philip Ryan, Lukas Horak, Xuerong Liu, Jian Liu, Mark Dean Heterostructuring of complex oxides into superlattices is an emerging method that provides new avenues to stabilize ground states unavailable to bulk synthesis. Artificial analogues to the Ruddlesen-Popper series iridates, composed of metallic SrIrO3 interspaced with band insulating SrTiO3, are shown to conserve the bulk-like spin-flop transition through resonant elastic scattering. However, measurements of the magnetic excitation spectrum with resonant inelastic scattering evidence a strong alteration in the stability of the magnetic ground state, driven by structural distortions not present in the bulk materials. These results point to heterostructuring as a powerful method for modulating the magnetic ground state of complex oxides hosting strong spin-orbit coupling. |
Wednesday, March 7, 2018 4:18PM - 4:30PM |
P09.00008: Interface induced magnetic polar metals by design Saurabh Ghosh, Hangwen Guo, E Plummer, Sokrates Pantelides Polar metals with ferroelectric-like (FEL) displacements offer the promise of designing ferroelectrics with tunable band gaps [1,2]. One efficient way to induce FEL displacements in a centrosymmetric phase is by interface-induced coupling [3]. Here, using first-principles density functional theory (DFT) and DFT+U (static d-d Coulomb interaction) we investigate (BaTiO$_3$)$_6$/(SrRuO$_3$)$_n$/(BaTiO$_3$)$_7$ heterostrcutures (6/n/7), with n=1, 2 and 3. DFT calculations predict that a FEL phase coupled with in-plane rotation of RuO$_6$ octahedral ($a^0a^0c^+$) can be induced in centrosymmetric SrRuO$_3$ unit when BaTiO$_3$ is in a ferroelectric phase. Moreover, a spin crossover from low spin (2$\mu_B$, Bulk SrRuO$_3$) to high spin (4$\mu_B$, 6/3/7) is found driven by the FEL distortion, which is a rare phenomenon in case of magnetic polar metals. Experimental investigation has confirmed the induced polar distortion and the spin crossover. Finally, we discuss the design principles for spin crossover magnetic polar metals. [1] W. Cochran, Phys. Rev. Lett., 3, 412 (1959), [2] S. Ghosh et. al. Phys. Rev. Lett. 119, 177603 (2017), [3] H. Guo, PANS, 114, E5062-E5069 (2017). |
Wednesday, March 7, 2018 4:30PM - 4:42PM |
P09.00009: Electronic reconstruction and spontaneous magnetic reversal driven by interface dipole layer Mohammad Saghayezhian, Zhen Wang, Hangwen Guo, Rongying Jin, Yimei Zhu, Jiandi Zhang, E Plummer Between two insulators, the electric dipole formed at the polar-nonpolar interface, triggers charge transfer that results in emergent phenomena. However, at metal-insulator junctions, a polar-nonpolar interface behaves very differently. We show that in this case, itinerant electronic screening becomes indispensable. The caveat to this premise is that, within the Thomas-Fermi screening length, the charge transfer is non-negligible. To illustrate this effect, we performed a detailed study of La0.67Sr0.33MnO3/SrTiO3 (001). Electronic reconstruction and subtle lattice distortion at the interface creates a strong antiferromagnetic (AFM) exchange coupling, producing an exchange-spring type interaction. Surprisingly, the AFM coupled layers at the interface prompt a spontaneous magnetic reversal and an inverted hysteresis persisting above room temperature. Our results demonstrate that an interface dipole leads to magnetic switching in a monolithic thin film, showing a new design pathway that activates the interface, on the length scale of Thomas-Fermi screening, a single unit cell. |
Wednesday, March 7, 2018 4:42PM - 4:54PM |
P09.00010: Prediction of spin-polarized two-dimensional t2g electron gas at oxygen-deficient SrTiO3/EuO interface Lingyuan Gao, Alexander Demkov By performing first-principles calculations, we predict the existence of a spin-polarized two-dimensional electron gas (2DEG) at the interface of a ferromagnetic insulator EuO and oxygen-deficient SrTiO3 (STO). The carriers are generated by the oxygen vacancies in STO near the interface and have predominantly Ti-t2g orbital character. At the interface, the split off dxy-derived conduction band of STO is fully spin-polarized as is the in-gap vacancy-related deep state, found below the conduction band edge. The electron in the gap state is aligned ferromagnetically with EuO. Combined with the calculated electronic structure, we use the conventional 2D Boltzmann transport theory to explain the observed positive linear magnetoresistance (LMR) in this system. We attribute the LMR to the proximity-induced-Zeeman effect. We develop a three-orbital model to explain how the DFT result depends on the strength of the on-site Coulomb repulsion U. The calculations suggest a possible mechanism for generating spin-polarized 2DEG for future spintronic applications. |
Wednesday, March 7, 2018 4:54PM - 5:06PM |
P09.00011: Structural Control of Magnetism in SrRuO3/SrTiO3 Artificial Superlattices Seunggyo Jeong, Sungmin Woo, Jiwoong Kim, Youngmin Kim, Sungkyun Park, HuYoung Jeong, Woo Seok Choi Artificial superlattices composed of perovskite oxides are attractive model systems for studying low dimensional magnetism via structural periodicity control. Novel magnetic behaviors such as suppression of magnetism at the 2D limit, control of magnetic anisotropy, and modified hysteresis have been observed in oxide superlattices. Consideration on the periodic lattice, epitaxial strain, electronic structure, orbital arrangements, and their couplings is required for the fundamental understanding of such novel magnetic behaviors in the superlattices. In this presentation, we introduce a close relationship between the crystal structure and magnetic orderings in SrRuO3/SrTiO3 superlattices fabricated by pulsed laser epitaxy. In particular, the thickness of the SrTiO3 (STO, a paramagnetic insulator) layers could be controlled to induce an orthorhombic-to-tetragonal structural phase transition in the thin SrRuO3 (SRO, ferromagnetic metal) layers. The structural phase transition influences the magnetic properties of the SRO layer significantly. Furthermore, the long-range coupling of ferromagnetic SRO layers across the nonmagnetic STO layers will be discussed in the course of understanding the magnetic ground state of the oxide superlattices. |
Wednesday, March 7, 2018 5:06PM - 5:18PM |
P09.00012: Evolution of ferromagnetism in two-dimensional electron gas of LaTiO3/SrTiO3 Fangdi Wen, Yanwei Cao, Xiaoran Liu, Banabir Pal, Srimanta Middey, Mikhail Kareev, Jak Chakhalian Despite almost one-decade-long research effort, the microscopic origin of ferromagnetism in LaAlO3/SrTiO3 is still an open question. Here we report the antiferromagnetic affected two-dimensional electron gas (2DEG) on the interface between antiferromagnetic Mott insulator LaAlO3 and non-magnetic band insulator SrTiO3. The clear two-dimensional reflection high-energy electron diffraction (RHEED) pattern and X-ray photoelectron spectroscopy confirm the crystal structure and chemical composition, while the negative correlation between temperature and resistivity in low temperature shows the existence of magnetic islands. The observation of clear hysteresis in magnetotransport at low magnetic fields (~ 2 T) shows the effect of canted magnetic islands in the hole-rich LaAlO3 near the interface. Our result provides another path for designing all-oxide structures and is relevant to spintronics applications. |
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