Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session V23: Oxide Thin Film MagnetoelectricsFocus
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Sponsoring Units: GMAG DMP DCOMP Chair: Julia Mundy Room: LACC 402B |
Thursday, March 8, 2018 2:30PM - 3:06PM |
V23.00001: Magnetoelectric coupling through the electric-field controlled ionic evolution Invited Speaker: Pu Yu Electric-field control of phase transformation with ion transfer is of great interests in materials science with enormous practical applications. Due to the strong electron-ion interaction, the ionic evolution would naturally have dramatic influence ont the material magnetic properties. In this talk, I will propose two strategies/pathways to achieve the magnetoelectric coupling through the electric-field controlled ionic evolution. Firstly, I will present a reversible and nonvolatile electric-field control of oxygen and hydrogen ion evolutions within the model system of brownmillerite SrCoO2.5 by ionic liquid gating. Due to the selectively controllable ionic evolutions, we achieved a tri-state phase transformations among SrCoO2.5 and its counterpart of perovskite SrCoO3-δ and a hitherto-unexplored HSrCoO2.5 phase. Because of the extremely distinct magnetic, electrical and optical properties among these phases, this result forms solid foundations for the conceptually new tri-state magnetoelectric and electrochromic effects. Next, using Co/SrCoO2.5 heterostructure as model system, I will introduce a room temperature electric-field control of magnetic state in the Co layer accompanied by the bipolar resistance switch. In this case, the electric field controlled oxygen evolution leads to the oxygen ion accumulation (gating) at the interface, in the same manner as the conventional charge-gating device. As the consequence, the interfacial oxygen contents modulate the magnetic interaction within the Co surface layer and eventually results in the intriguing magnetoelectric coupling. We envision that the ionic evolution brings in a new tuning knob to manipulate the coupling and correlation between charge, spin, orbital and lattice degrees of freedom and paves a new playground for the discovery of novel materials and rich functionalities. |
Thursday, March 8, 2018 3:06PM - 3:18PM |
V23.00002: Electric-Field-Induced Ferromagnetism via Cluster Percolation in Ion-Gel-Gated La1-xSrxCoO3-δ Films Jeffery Walter, Biqiong Yu, Guichuan Yu, Alexander Grutter, Brian Kirby, Julie Borchers, Zhan Zhang, Hua Zhou, Timothy Charlton, Haile Ambaye, Michael Fitzsimmons, Peter Orth, Rafael Fernandes, Boris Shklovskii, Turan Birol, Martin Greven, Chris Leighton Electrolyte gating techniques have proven remarkably effective in the study of charge density effects in a variety of conductors. The ability to controllably induce surface charge densities > 1014 cm-2 has led to their extensive use in the study of superconductivity, insulator-metal transitions, and magnetism. In this work we apply electrolyte gating to epitaxial films of La1-xSrxCoO3-δ (LSCO), seeking gate-control of ferromagnetism. Our previous work provided direct evidence of oxygen vacancy creation at positive bias, vs. electrostatic hole accumulation at negative bias, using transport as well as in operando X-ray synchrotron and polarized neutron reflectometry (PNR) measurements [1]. Here, we present detailed gate-control over resistivity, magnetoresistance, magnetization, and Curie temperature under hole accumulation, including gate-induced ferromagnetism verified by PNR. These are discussed in terms of recently developed theory for electrolyte-gate-induced percolation [2]. |
Thursday, March 8, 2018 3:18PM - 3:30PM |
V23.00003: Study of magnetic phase transition in LaCoO3 and La0.7Sr0.3CoO3 thin films grown by pulsed laser deposition Toyanath Joshi, David Belanger, David Lederman The rare-earth cobaltite, LaCoO3 has attracted researchers’ interest because of its intriguing electronic and magnetic properties. In bulk LaCoO3, Co3+ ions have the low spin configuration (S=0) at low temperatures. Thin films of LaCoO3, however, exhibit a ferromagnetic ground state below ~85 K. There are several efforts to explain underlying physics in terms of multiple spin-state transitions, Jahn-Teller distortions, the Co-O-Co bond angle or the rotation of the CoO6 octrahedra, oxygen vacancies, and strain induced by doping with Sr or the substrate-film lattice mismatch. A complete understanding of the mechanism, however, has not been achieved. In this presentation, we discuss the phase transition mechanism of LaCoO3 and La0.3Sr0.7CoO3 thin films less than 30 nm thick grown by pulsed laser deposition under compressive and tensile strain using SrTiO3 and LaAlO3 substrates. We demonstrate that strain plays an important role in the transition and discuss mechanisms related to substrate-induced strain and the formation of structural domains. We will also discuss the roles of effective dimensionality in the thinnest films and anisotropic exchange interactions. |
Thursday, March 8, 2018 3:30PM - 3:42PM |
V23.00004: Effect of non-uniform electric field in microstructured (La1-yPry)1-xCaxMnO3 thin films AMBIKA SHAKYA, Ashkan Paykar, Amlan Biswas Thin films of the hole-doped manganite (La1-yPry)1-xCaxMnO3 (LPCMO) grown on NdGaO3 (NGO) substrates show effects such as phase coexistence and colossal electroresistance (CER). Using lift-off photolithography we deposited micrometer scale gold contacts on LPCMO thin films. The geometry of these gold contacts allows us to apply non-uniform electric field across the sample. Two-probe voltage source measurements using these contacts showed overall reduction in sample resistance with increasing applied voltages between 5 V and 50 V. This electroresistance behavior is due to the realignment of the fluid-like ferromagnetic metallic (FMM) regions embedded in a charge- ordered insulating (COI) background and has been observed before with uniform electric fields. However, unlike in the case of uniform field, the non-uniform field leads to a non-monotonic change in the insulator to metal transition temperature as a function of applied voltage. This behavior is due to the dielectrophoresis of the FMM regions. We will also compare the behavior of electroresistance and the insulator to metal transition temperature under non- uniform electric field for various sizes of microstructures on LPCMO thin films. |
Thursday, March 8, 2018 3:42PM - 3:54PM |
V23.00005: Voltage Control of Magnetization in a Chromia Based Thin Film Heterostructure Will Echtenkamp, Michael Street, Ather Mahmood, Christian Binek Controlling magnetism by electrical means is a key challenge in the field of spintronics. Utilizing magnetoelectric Cr2O3 (chromia) based heterostructures, electric control of ferromagnetic exchange bias has been achieved near room temperature. In this study the electrically-controlled magnetization is directly investigated in thin film chromia using MBE grown palladium as a sensing layer to detect the boundary magnetization intrinsic to all magnetoelectric antiferromagnets. Prototype hall bar devices are fabricated using lithography techniques. Using a process of magnetoelectric annealing, voltage control of magnetization is demonstrated in exceptionally thin films with significant implications for scalability of ultra-low power memory and logical devices. |
Thursday, March 8, 2018 3:54PM - 4:06PM |
V23.00006: Electrode optimization for improved magnetoelectric switching behavior in ultra-thin Cr2O3 films Ather Mahmood, Michael Street, Will Echtenkamp, Christian Binek Voltage-controlled switching of magnetization is manifested through exchange bias (EB) and promises non-volatile spintronic memory and logic devices. The influence of seed layer properties on the morphology and dielectric properties of thin films of the magnetoelectric antiferromagnet chromia (α-Cr2O3) is investigated. The films were grown on metallic electrodes Pd (111) and Pt (111) or on metallic oxide film V2O3. Correlation between nanoscale structure and electrical properties of chromia, was probed by conductive Atomic-Force-Microscopy (C-AFM). Whereas films grown on elemental metal substrates showed either leakage pathways (Pd) or partially mitigated leakage (Pt), a remarkable suppression of leakage and surface defects was found in films formed on V2O3. X-ray analyses attribute these differences to the presence of in-plane rotational domains in the films grown on Pd substrates, a feature that is reduced in films grown on Pt and absent in films grown on V2O3. This growth strategy demonstrates a vastly improved magnetoelectric switching characteristics in 20 nm thin Cr2O3 films. |
Thursday, March 8, 2018 4:06PM - 4:18PM |
V23.00007: Tuning Néel temperature and anisotropy of magnetoelectric Cr2O3 for enhanced performance in voltage-controlled spintronic devices Michael Street, Will Echtenkamp, Takashi Komesu, Shi Cao, Peter Dowben, Christian Binek Electric control of magnetization through the magnetoelectric effect has received considerable attention as a promising route for next-generation low-energy magnetic recording devices. This work is an effort to realize such spintronic devices by voltage-control of the magnetoelectric α-Cr2O3. The electrically switchable boundary magnetization of Cr2O3 can be used to voltage-control the magnetic states of an adjacent ferromagnet. For this technique to be implemented into a spintronic device, the Néel temperature of Cr2O3 must be increased above the bulk value of TN=307K. Previously, boron doped Cr2O3 thin films were fabricated via PLD showing boundary magnetization at elevated temperatures via magnetometry and spin polarized inverse photoemission spectroscopy (SPIPES). In this work, we investigate the influence of boron doping of Cr2O3 on the temperature dependence of exchange bias and the consequences it has on the blocking temperature of these exchange-coupled thin film systems. |
Thursday, March 8, 2018 4:18PM - 4:30PM |
V23.00008: First-principles study of the graphene/Cr2O3 (0001) interface Hiroyuki Takenaka, Evgeny Tsymbal The magnetoelectric properties of Cr2O3 in the corundum structure (chromia) has been drawn special attention due to possible implications for the next generation of spintronics devices. Due to the presence of surface magnetization, intrinsically coupled to the antiferromagnetic order parameter and robust with respect to roughness, the magnetoelectric switching of chromia may be used to electrically control the spin-dependent transport properties of an adjacent two-dimensional (2D) channel material to design a spin transistor. Graphene is the obvious choice for the 2D channel conductor, due to its unique electronic structure and excellent match of its lattice structure to the lattice structure of the (0001) surface of chromia. In this work, we use a first-principles approach to explore the atomic, electronic, and magnetic properties of the graphene/Cr2O3 (0001) interface. Considering different atomic configurations of the interface, we explore the induced magnetism in graphene, driven by proximity of the chromia surface, and a possibility of its modulation by the chromia surface magnetization. We discuss an impact this induced magnetism can make on the spin-dependent transport in the 2D graphene channel and the potential for using the interface structure as a spin transistor. |
Thursday, March 8, 2018 4:30PM - 4:42PM |
V23.00009: Nonlocal spin transport in antiferromagnet Cr2O3 thin film Wei Yuan, Qiong Zhu, Tang Su, Yunyan Yao, Yang Ma, Xi Lin, Jing Shi, Ryuichi Shindou, Xincheng Xie, Wei Han Cr2O3 is an interesting insulating antiferromagnetic material, which has been widely investigated recently, including in the electric field control of the exchange bias and spin Seebeck effect. In our study, high quality Cr2O3 films are epitaxially grown on the substrates of (0001) Al2O3 using high vacuum pulsed laser deposition. The layer by layer growth is monitored by the in-situ reflection high energy electron reflection oscillations, and atomic force microscope result shows the surface is atomically flat. The spin transport on the thin films is probed by using the nonlocal spin transport geometry with two parallel electrodes via the standard low frequency lock-in technique. Temperature and magnetic field dependences of the nonlocal spin signal are systematically studied. Interestingly, the nonlocal signal can be observed even at a spacing distance of 20 μm, demonstrating the potential of insulating antiferromagnet for the long distance spin transport. |
Thursday, March 8, 2018 4:42PM - 4:54PM |
V23.00010: Designing switchable room-temperature multiferroics via the discovery of a novel magneto-electric coupling Feng Jun Sheng, Ke Xu, laurent bellaiche, Hongjun Xiang Magnetoelectric (ME) coupling is the key ingredient for realizing the cross control of magnetism and ferroelectricity in multiferroics. However, multiferroics are not only rare in nature, especially at room temperature, but also the overwhelming majority of known multiferroics does not exhibit an highly-desired switching of magnetization when the polarization is reversed by an electric field. Here, we report group theory analysis and ab-initio calculations demonstrating, and revealing the origin of the existence of a novel form of ME coupling term in a special class of materials that does allow such switching. This term naturally explains the previously observed electric-field control of magnetism in the first known multiferroics, i.e., the Ni-X boracite family. It is also presently used to design a switchable room-temperature multiferroic having a rather large ferroelectric polarization, strong spontaneous magnetization, and strong ME coupling. |
Thursday, March 8, 2018 4:54PM - 5:06PM |
V23.00011: Magnetoelectric Couplings at Fe/BiFeO3(001) Heterointerfaces Studied by First-Principles Calculations Kazuhiro Fujita, Yoshihiro Gohda
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Thursday, March 8, 2018 5:06PM - 5:18PM |
V23.00012: Spatially-resolved electric-field manipulation of magnetism in multiferroic heterostructures Yonggang Zhao Electric-field control of magnetism is essential for exploiting new concepts for dense, fast, and non-volatile random access memory with reduced energy consumption. We studied electric-field control of magnetism in CoFeB/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 multiferroic heterostructure at the mesoscale using spatially-resolved techniques including scanning Kerr microscopy and scanning electron microscopy with polarization analysis with in situ electric fields. Different types of magnetic responses were revealed, and analysis suggests that they are related to different types of ferroelectric domain-switching, which provides a path for designing magnetoelectric devices through domain engineering. Our work demonstrates the interesting new physics and potential applications of electric-field control of magnetism in the multiferroic heterostructures. |
Thursday, March 8, 2018 5:18PM - 5:30PM |
V23.00013: Giant Voltage-Controlled Magnetic Anisotropy in Strained Ir/FeCo/MgO Heterostructures Sohee Kwon, Xiang Li, Pedram Khalili Amiri, Kang Wang, Nicholas Kioussis Contrary to current-controlled magnetic random access memory devices utilizing spin transfer torque (STT) or spin-orbit torques (SOT), voltage-induced magnetization switching can lead to a new paradigm enabling ultralow-power and high density instant-on nonvolatile magnetoelectric random access memory (MeRAM) devices. Employing ab initio electronic structure calculations we have investigated the effect of epitaxial strain on the voltage-controlled magnetic anisotropy (VCMA) behavior in Ir/FeCo/MgO heterostructures. We find that the magnetic anisotropy energy decreases linearly with electric field where the giant VCMA efficiency is in the range of -1742 to -1,000 fJ/(Vm) in agreement with recent experiments. Furthermore, we predict an electric-field-induced spin-reorientation which depends on epitaxial strain. These findings open interesting prospects for exploiting strain engineering and the appropriate heavy metal cap to harvest higher efficiency VCMA for the next-generation MeRAM devices. |
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