Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session F23: Magnetism and Magnetic Coupling at Oxide InterfacesFocus
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Sponsoring Units: GMAG DMP DCOMP Chair: Manuel Bibes, CNRS Paris Room: LACC 402B |
Tuesday, March 6, 2018 11:15AM - 11:27AM |
F23.00001: Thickness Dependent and Interfacial Magnetism in LaMnO3 Thin Films and LaCoO3/LaTiO3 Heterointerfaces Mark Golden, George Araizi-Kanoutas, Jaap Geessinck, Gertjan Koster, Pim Reith, Hans Hilgenkamp Charge transfer within thin LaMnO3 films and between LaTiO3 and LaCoO3 in bi- and trilayer samples grown with PLD has been studied using soft X-ray absorption, dichroism and hard X-ray photoemission. Clear signs of charge transfer are seen in both systems, yielding divalent Mn and Co. Interestigly, in the LMO system, this charge transfer is also present in films of sub-critical LMO thickness, indicating a more complex behaviour than a simple thickness-dependent potential staircase leading to self-doping of the LMO. XMCD measurements of the sub-critical thickness LMO films reveal polarisable Mn spins even for only 2 or 3 unit cells thickness, whereas robust ferromagnetism emerges in thicker films. In the LCO case, in which an LaAlO3 spacer layer is used to eliminate polarity changes at any of the interfaces, charge transfer is driven by O2p band alignment across the interface, and the degree of charge transfer and the accompanying Co magnetic moments are controlable via the thickness of the LCO layer, and the number of LTO interfaces. |
Tuesday, March 6, 2018 11:27AM - 11:39AM |
F23.00002: Abnormal Spin Hall magnetoresistance in ultra-thin Pt/LSMO Yu Bai, Na Lei, Lifeng Yin, Jian Shen Thin film bilayer systems comprised of a normal metal (NM) and a ferromagnetic insulator (FI) revealed a new type of magnetoresistance called spin Hall magnetoresistance (SMR) [1]. Very recently, a negative SMR was reported in antiferromagnetic (AFM) insulator [2]. Competing effects may mix in the system and might complicate the data analysis and interpretation. Here we present a case of ultra-thin Pt/LSMO, in which the magnetic state of the LSMO layers depends sensitively on the thickness. Specifically, 6 unit cell (UC) LSMO is a magnetic insulator with Curie temperature (Tc) of 120 K and 3 UC LSMO is an antiferromagnetic insulating. In the Pt/6UC LSMO system, the magnetoresistance (MR) includes contributions from both the anisotropic magnetoresistance (AMR) and SMR effects below curie temperature. Strikingly, the MR increases with temperature even up to temperatures above Tc, which is distinctly different from the case of Pt/YIG [3]. Anomalous Hall effect was also performed showing consistentency with SMR measurement. In the Pt/3UC LSMO system, the SMR signal also shows similar abnormal temperature dependence. We attribute the underlying mechansim to the competing effects from the coexisting electronic phases in the LSMO system. |
Tuesday, March 6, 2018 11:39AM - 11:51AM |
F23.00003: Probing the antiferromagnetic structure of (111)-oriented heterostructures using angle dependent x-ray absorption spectroscopy Yue Jia, Rajesh Chopdekar, Padraic Shafer, Elke Arenholz, Zhiqi Liu, Michael Biegalski, Yayoi Takamura (111)-oriented perovskite oxide heterostructures offer unique properties such as a buckled honeycomb structure resembling that of graphene with the stacking of highly polar layers. These factors impact the magnetic structure and exchange coupling in epitaxial heterostructures composed of ferromagnetic (FM) and antiferromagnetic (AF) layers. We demonstrate the development of measurement protocols for angle-dependent soft x-ray absorption spectroscopy measurements from (111)-oriented heterostructures which are used to unambiguously determine the orientation of the AF spin axis and how it responds to an applied magnetic field due to exchange coupling interactions with the adjacent FM layer. For the La0.7Sr0.3MnO3/La0.7Sr0.3FeO3 (LSFO) system, the LSFO layers possess two populations of AF order: the majority of AF moments cant out-of-the-plane of the film along low-index crystallographic directions, while a minority of AF moments lie within the (111)-plane. The relative orientation of the AF and FM spins differs for each type of AF domain. These results highlight the complexity of magnetic interactions which depend on factors such as crystalline orientation, sublayer thickness, and applied magnetic field which can be used to tune the AF order in perovskite oxide heterostructures. |
Tuesday, March 6, 2018 11:51AM - 12:03PM |
F23.00004: Substrate effects on magnetic properties of LSMO thin films. Navid Mottaghi, Robbyn Trappen, Shalini Kumari, Chih-Yeh Huang, Guerau Cabrera, Saeed Yousefi Sarraf, Allison Haertter, Mohindar Seehra, Mikel Holcomb Temperature and magnetic field dependence of magnetization in LSMO films of 7.6 and 13.2 nm thicknesses grown on SrTiO3 (100) and LaAlO3 (100) substrates, respectively, were investigated by vibrating sample magnetometer. Coercive field (HC), exchange bias (Hε), and saturation magnetization (MS) were measured from zero-field-cooled hysteresis loops at 5 K. HC and Hε are determined at 5 K using the relation, HC = (HC1 + HC2)/2 and Hε = (HC1 - HC2)/2, where HC2 and HC2 are the two magnetic fields where magnetization changes sign in hysteresis loops. Results show that MS values are 476, 210 emu/cm3 and Hc values are 142, 655 Oe for LSMO film grown on SrTiO3 and LaAlO3 respectively. The sample grown on LaAlO3 has Hε ∼ -98 Oe which suggests the possible presence of both ferromagnetic and antiferromagnetic phases while the LSMO film on SrTiO3 does not have an exchange bias. Further studies on understanding the effect of the substrate on magnetic properties of LSMO are in process. |
Tuesday, March 6, 2018 12:03PM - 12:15PM |
F23.00005: Cation Orderings of Strained CoFe2O4 in Self-Assembled Heterostructures Yi-Chun Chen, Yen-Chin Huang, Yi-De Liou, Heng-Jui Liu, Ying-Hao Chu Complex oxide heteroepitaxy has attracted lots of interests due to their unique physical properties through couplings of spin, charge, orbital and lattice orderings between two neighboring materials. In order to reveal the strain effects caused by heterointerfaces on the spinel ferrite CoFe2O4 (CFO), we studied the crystal symmetry of the epitaxial CFO film and the CoFe2O4-SrRuO3 (CFO-SRO) vertically aligned heterostructure. XRD patterns show a strain-relaxed state in the CFO film, while the interfacial strain results in distorted CFO structures in CFO-SRO film. In addition, the angle-resolved polarized Raman spectroscopy indicates there are different short-range octahedral-site orderings between the CFO film and the heterostructure. Based on Raman polarization selection rules and first principle predictions, the relaxed CFO (c/a=1) is confirmed with the Imma symmetry, while the strained CFO (c/a>1) in heterostructure film is with the P4122 symmetry. This study indicates the interfacial strain plays an important role to manipulate the cation distribution and magnetic properties in spinel ferrite. |
Tuesday, March 6, 2018 12:15PM - 12:51PM |
F23.00006: Magnetism and electronic phase transitions in isovalent manganite and ferrate superlattices Invited Speaker: Steven May The manganites and ferrates exhibit a host of collectively ordered states making them exciting candidates in which to identify new strategies for controlling local magnetism, orbital physics and metal-insulator transitions in oxide heterostructures. In this talk, I will demonstrate how magnetism can be enhanced at the sub-nm length scale through structural “delta doping” of octahedral rotations. By inserting two unit cells of La0.7Sr0.3MnO3 within isovalent 20 unit cell La0.7Ca0.3MnO3 layers, the magnetic exchange interactions are enhanced within spatially confined regions of suppressed octahedral rotations.[1] I will also discuss recent work on strained CaFeO3 films and isovalent CaFeO3/SrFeO3 superlattices. Through a combination of X-ray spectroscopy and density functional theory, element-specific changes to the electronic structure induced by the metal-insulator transition in CaFeO3 films are elucidated, revealing the central role of ligand holes in stabilizing the insulating state and a subtle electronic transfer from oxygen to iron states on cooling through the phase transition.[2] Efforts to understand how helical magnetism, electronic transport, and orbital polarization are altered through interfacial formation in CaFeO3/SrFeO3 superlattices will be presented. |
Tuesday, March 6, 2018 12:51PM - 1:03PM |
F23.00007: Magnetic Proximity Effect in Pt/CoFe2O4 Bilayers Walid Amamou, Igor Pinchuk, Amanda Trout, Robert Williams, Nikolas Antolin, Adam Goad, Dante O'Hara, Adam Ahmed, Wolfgang Windl, David McComb, Roland Kawakami We observe the magnetic proximity effect (MPE) in Pt/CoFe2O4 bilayers grown by molecular beam epitaxy (MBE). This is revealed through angle-dependent magnetoresistance measurements at 5 K, which isolate the contributions of induced ferromagnetism (i.e. anisotropic magnetoresistance) and spin Hall effect (i.e. spin Hall magnetoresistance) in the Pt layer. The observation of induced ferromagnetism in Pt via AMR is further supported by density functional theory calculations and various control measurements including insertion of a Cu spacer layer to suppress the induced ferromagnetism. In addition, anomalous Hall effect measurements show an out-of-plane magnetic hysteresis loop of the induced ferromagnetic phase with larger coercivity and larger remanence than the bulk CoFe2O4. By demonstrating MPE in Pt/CoFe2O4, these results establish the spinel ferrite family as a promising material for MPE and spin manipulation via proximity exchange fields. |
Tuesday, March 6, 2018 1:03PM - 1:15PM |
F23.00008: Observation of Vortex/Meron Pairs at Room Temperature in a planar α-Fe2O3/Co Heterostructure. Paolo G. Radaelli, Francis Chmiel, Noah Waterfield-Price, Roger Johnson, Anne Lamirand, Jonathan Schad, Chang-Beom Eom Vortices are among the simplest topological structures. They occur whenever a flow field `whirls' around a one-dimensional core, and are ubiquitous to many branches of physics. In the crystalline state, vortex formation is rare, since it is generally hampered by long-range interactions. Here, we present the discovery of a novel form of crystalline vortices in antiferromagnetic (AFM) hematite (α-Fe2O3) epitaxial films, in which the primary whirling parameter is the non-ferroics staggered magnetisation. Remarkably, ferromagnetic (FM) topological objects with the same winding number of the α-Fe2O3 vortices are imprinted onto an ultra-thin Co ferromagnetic over-layer by interface exchange. Our data suggest that the FM objects are merons (half-skyrmions), and that the vortex/meron pairs are relatively robust well beyond the Co coercive field, but can be manipulated by the application of a larger in-plane magnetic field (Hpar~100mT) , giving rise to large-scale vortex-antivortex annihilation. |
Tuesday, March 6, 2018 1:15PM - 1:27PM |
F23.00009: Theoretical Study of Damping In Perpendicular Anisotropy Materials along Multiple Crystal Orientations Tao Qu, Randall Victora The damping constant α represents the elimination of the magnetic energy and affects the time scale of devices: therefore it is one of the most fundamental features of magnetism. However, there is no integrated model to describe the multiple damping mechanisms, including spin-orbit exchange relaxation, interfacial effects and magnon scattering. Here, we apply Kambersky’s torque correlation technique, within the tight-binding method, to multiple materials with high perpendicular magnetic anisotropy in superlattice and MgO based thin film structures. These materials play a key role in spintronic devices owing to their stability even for nanometer dimensions. The total damping shows a size effect that it is linearly dependent on the magnetic material, and decreases for thicker magnetic layers. This behavior is consistent with experimental data. The interfacial damping is also dependent on the nonmagnetic material and interfacial orientation, for crystals growing along (001), (111), and (011) orientations. We find that the origin of the interfacial damping includes both the distorted electronic states at the interface and the spin-orbit interaction in polarizable materials such as Pt or Pd. Integration of three magnon scattering within this picture will also be discussed. |
Tuesday, March 6, 2018 1:27PM - 1:39PM |
F23.00010: Interface screening effects on semiconducting thin films Simone Altendorf, Andreas Reisner, Brian Tam, Federico Meneghin, Steffen Wirth, Liu Hao Tjeng We report on our study on the influence of interface screening on the properties of semiconducting thin films utilizing EuO as a model system. EuO films of various thicknesses were grown by molecular beam epitaxy and interfaced half with metallic Mg and half with insulating MgO. The magnetic characterization clearly shows that the lowering of the EuO Curie temperature for ultra-thin films is much delayed and less significant for the EuO part covered with the metal compared to the EuO part interfaced with the insulator. Our results strongly suggest that the proximity of a highly polarizable material can counteract finite size effects in semiconducting films, and point out the importance of considering the dielectric properties of each layer when designing a nanoscale heterostructure device. |
Tuesday, March 6, 2018 1:39PM - 1:51PM |
F23.00011: Formation of antiferromagnetic NiO spiral domain and its effect on the magnetic anisotropy in Py/NiO/Co0.5 Ni0.5O/vicinal Ag(001) Qian Li, Mengmeng Yang, Nan Gao, Qiaoyan Dong, Christoph Klewe, Padraic Shafer, Alpha N'Diaye, Elke Arenholz, Chanyong Hwang, Jia Li, Z. Qiu It has been a fundamental issue on how the Mauri’s model describes the effect of an antiferromagnetic (AFM) layer on a ferromagnetic (FM) layer. Although recent result shows that Mauri’s model not necessarily gives raise an exchange bias, it is unclear on how the planar spiral domain in the AFM layer induces a magnetic anisotropy in the FM layer in a FM/AFM system. Here we report an experimental study of Py/NiO/Co0.5Ni0.5O/vicinal Ag(001) system using x-ray linear dichroism (XMLD) and rotating magnetic field magneto-optic Kerr effect (ROTMOKE). The Co0.5Ni0.5O layer has frozen AFM spins which are aligned by atomic steps of the vicinal surface so that they pin the NiO spin orientation at the NiO/Co0.5Ni0.5O interface. In this way, as the top Py layer magnetization is rotated by an external magnetic field, the Py/NiO interfacial coupling rotates the NiO spins at the FM/NiO interface to twist the NiO spins into a planar spiral domain. We find that the NiO AFM spins are almost frozen below ~3nm thickness, gradually develop a planar spiral domain wall in the range of 3-16nm, and complete the domain wall formation thicker than ~16nm. Correspondingly, the frozen, incomplete spiral, and complete spiral NiO domain walls are well revealed in the magnetic anisotropy of the Py layer. |
Tuesday, March 6, 2018 1:51PM - 2:03PM |
F23.00012: Spin reorientation transition of antiferromagnetic NiO/CoO/MgO(001) and its effect on the Py magnetic anisotropy in Py/NiO/CoO/MgO(001) Mengmeng Yang, Qian Li, Alpha N'Diaye, Qiaoyan Dong, Nan Gao, Elke Arenholz, Chanyong Hwang, Yizheng Wu, Z. Qiu The effect of antiferromagnetic (AFM) layer on the magnetic properties of ferromagnetic (FM) layer in FM/AFM systems highly depends on the AFM spin configurations. Comparing to the many available tools for FM measurement, there is limited experimental techniques for the AFM spin measurement, thus limiting the fundamental understanding of why and how the AFM spins induce a magnetic anisotropy in the FM layer. Using x-ray magnetic linear dichroism (XMLD) measurement, we determined the AFM spin orientation in epitaxial NiO/CoO/MgO(001) system at room temperature. We find that the NiO AFM spins undergo a spin reorientation transition (SRT) from out-of-plane to in-plane direction as a function of CoO or NiO thickness. We constructed a complete phase diagram of the NiO spin orientation in the NiO and CoO thickness plane. Then utilizing Magneto-Optic Kerr Effect combined with a rotation magnetic field (ROTMOKE), we investigated the Py magnetic anisotropy in Py/NiO/CoO/MgO(001) system. We find that different NiO spin orientations have very different effects on the induced magnetic anisotropy of the Py film. |
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