Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Q32: Focus Session: Magnetic Oxide Thin Films and Heterostructures: Electric Field and Magnetoelectric Effects |
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Sponsoring Units: GMAG DMP Chair: Christian Binek, University of Nebraska - Lincoln Room: 207B |
Wednesday, March 4, 2015 2:30PM - 2:42PM |
Q32.00001: Oxygen vacancy control of a ferroelectric memristor J. Santamaria, Yaohua Liu, S. G. E. te Velthuis, D. Hernandez-Martin, A. Perez Munoz, M. Cabero, G. Sanchez-Santolino, J. Tornos, M. Varela, C. Leon, Z. Sefrioui, S. J. Pennycook The rich phenomenology exhibited by correlated oxide interfaces can be expanded by the control and manipulation of point defects. In particular oxygen vacancies can be induced by electro forming processes at redox active electrodes and, when ionized, can be manipulated by electric fields. We aim at using the strong electric fields building up in ultrathin tunnel barriers to generate and manipulate oxygen vacancies in Ag / BaTiO$_{\mathrm{3}}$/ La$_{\mathrm{0.7}}$Sr$_{\mathrm{0.3}}$MnO$_{\mathrm{3\thinspace }}$(LSMO) micron-size structures defined by optical and electronic lithography techniques. Controlling accumulation of oxygen vacancies in the BTO barrier at either Ag or LSMO interfaces allows modification of the magnetic state of the LSMO at the interface, enabling independent variation of the width or the height of the tunnel barrier. This yields a controlled sign change of the electroresistance upon polarization switching. We show that clockwise and counter-clockwise memristors can be tailored on the same sample yielding multiple resistance states. The memristive response resulting from the controlled switching of polarization and-or oxygen vacancies may open new routes towards new computing architectures. [Preview Abstract] |
Wednesday, March 4, 2015 2:42PM - 2:54PM |
Q32.00002: Memristive Switching and Interfacial Magnetoelectricity in LCMO/PBCO Heterostructure Xiao Shen, Timothy J. Pennycook, David Hernandez Martin, Ana P\'erez, Maria Varela, Yevgeniy S. Puzyrev, Carlos Leon, Zouhair Sefrioui, Jacobo Santamaria, Sokrates T. Pantelides New phenomena emerge at the interfaces of transition metal oxides. Here we report memristive switching in a La$_{0.7}$Ca$_{0.3}$MnO$_{3}$PrBa$_{2}$Cu$_{3}$O$_7$ bilayer structure with an On/Off ratio greater than 10$^3$ that originates from a new type of interfacial magnetoelectricity. Using DFT calculations, we show that at the LCMO/PBCO interface, a ``magnetic dead layer (MDL)''\footnote{W. Luo, S. J. Pennycook, S. T. Pantelides, Phy. Rev. Lett. 101, 247204 (2008)} can be switched on and off by a small displacement of the interfacial Mn atoms generated by an external voltage. Initially, the LCMO is ferromagnetic with no MDL. This is the Low Resistance State (LRS) as majority-spin carriers tunnel through the PBCO. A negative voltage creates an electric field that displaces the interfacial Mn atoms towards the bulk LCMO by a few hundredths of an Angstrom. In such position, the interfacial Mn layer is coupled anti-ferromagnetically to the bulk LCMO, whereby a MDL is present, adding a barrier for the majority-spin carriers to tunnel and thus the bilayer is at High Resistance State (HRS). A positive bias drives the Mn atoms back to their original positions that favor ferromagnetic coupling, thus destroying the MDL and switching the bilayer back to LRS. [Preview Abstract] |
Wednesday, March 4, 2015 2:54PM - 3:06PM |
Q32.00003: Enhanced magneto-ionic switching of interface anisotropy in Pt/Co/GdOx films Aik Jun Tan, Uwe Bauer, Geoffrey Beach Voltage control of magnetic anisotropy is of great interest for reducing the switching energy barrier in spintronic devices. It has recently been shown that electric field-driven oxygen ion migration near the interface of ferromagnet/oxide bilayers can lead to very large changes in magnetic anisotropy [1], but these changes required elevated temperature and a voltage dwell time on the order of minutes. Here, we examine magneto-ionic switching in ultrathin Co/GdOx films with perpendicular anisotropy, in which the the GdOx gate dielectric acts as an oxygen ion conductor. We examine the switching efficiency as a function of GdOx layer thickness and electrode geometry, and show that the voltage, operating temperature, and switching timescale can be significantly reduced by optimizing the GdOx thickness and defect structure. We demonstrate reversible toggling of magnetic properties for \textgreater 50 cycles, and correlate the magnetic switching behavior with changes in the electrical properties of the GdOx. 1. U. Bauer et al., arXiv:1409.1843v1(2014) [Preview Abstract] |
Wednesday, March 4, 2015 3:06PM - 3:42PM |
Q32.00004: Dual field effects in spinel ferrite field effect devices: electrostatic carrier doping and redox reactions Invited Speaker: Hidekazu Tanaka Spinel ferrite is a good candidate as a tunable magnetic semiconductor with high T$_{\mathrm{C}}$. Here, we report the gate-induced conductance modulation of (Fe$_{\mathrm{3-x}}$Zn$_{\mathrm{x}})$O$_{\mathrm{4}}$ solid solution to demonstrate the dual contributions of volatile and non-volatile field effects arising from electronic carrier doping and redox reactions using field effect device structure with a ferroelectric Pb(Zr,Ti)O$_{\mathrm{3}}$ and an ionic liquid DEME-TFSI. In the Pb(Zr,Ti)O$_{\mathrm{3}}$/(Fe$_{\mathrm{2.5}}$Zn$_{\mathrm{0.5}})$O$_{\mathrm{4}}$ FET, the gate voltage dependence of channel conductance on the (Fe,Zn)$_{\mathrm{3}}$O$_{\mathrm{4}}$ layer shows the typical hysteresis behavior reflecting the ferroelectric polarization, indicating the static carrier modulation [1] . In contrast, in the DEME-TFSI/(Fe$_{\mathrm{2.5}}$Zn$_{\mathrm{0.5}})$O$_{\mathrm{4}}$ FET, a large hysteresis observed in the drain current vs gate voltage characteristics is not accounted for solely by electrostatic doping, strongly suggesting the presence of chemical reactions[2]. In more details, the characteristic hysteresis virtually disappears for the heavily Zn substituted system,(Fe$_{\mathrm{2.2}}$Zn$_{\mathrm{0.8}})$O$_{\mathrm{4}}$ with less carrier concentration [3]. These observations revealed the coexistence of two types of field effects in the Fe$_{\mathrm{3-x}}$Zn$_{\mathrm{x}}$O$_{\mathrm{4}}$ devices, and the tuning of field-effect characteristics via composition engineering should be extremely useful for fabricating high-performance oxide field-effect devices. References; [1] Appl. Phys. Lett. 98 (2011) 102506, [2] Adv. Mater. Interfaces 1 (2014) 1300108, [3] Sci. Rep. 4 (2014) 5818. [Preview Abstract] |
Wednesday, March 4, 2015 3:42PM - 3:54PM |
Q32.00005: Tuning magnetic and electronic properties of (La$_{1-x}$Pr$_{x})_{0.67}$Ca$_{0.33}$MnO$_{3}$ thin films by composition spread deposition and electrolyte gating Xiaohang Zhang, Y.G. Liang, S. Fackler, J.-M. Shin, Ichiro Takeuchi, A.T. N'Diaye, E. Arenholz The magnetic and electronic properties of mixed-valence manganites are known to be sensitive not only to chemical doping but also to oxygen concentration. However, it is difficult to consistently attain exactly the same set of deposition conditions and treatment history for samples that are fabricated individually. In order to perform systematic studies on each of the two effects, we fabricated epitaxial (La$_{1-x}$Pr$_{x})_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LPCMO) composition spread thin films with the Pr concentration changing continuously across 1 cm to ensure that all the sample segments of interest experience the same processing history. X-ray magnetic circular dichroism (XMCD) and electronic transport measurements indicated that the Curie temperatures and the metal-insulator transition temperatures change continuously from $\sim$ 260 K to $\sim$ 120 K as the Pr concentration is varied from 0 to 0.33. Systematic comparison between experimental data obtained on as-grown and post-annealed samples reveals the role of oxygen in the observed magnetic and electronic transitions. Moreover, changes in the magnetic and electronic properties of LPCMO films under electrolyte-gating have also been observed. A proposed mechanism to explain the effect will be discussed. [Preview Abstract] |
Wednesday, March 4, 2015 3:54PM - 4:06PM |
Q32.00006: Aspects of bulk and surface magnetism of magnetoelectric Fe$_2$TeO$_6$ Sai Mu, Kirill Belashchenko Magnetoelectric antiferromagnets can be used to implement voltage-controlled magnetism, but materials design for above room-temperature operation is a challenge. Here we focus on the trirutile Fe$_2$TeO$_6$ magnetoelectric and use first-principles calculations to develop several strategies for increasing its N\'eel temperature $T_N$ above the bulk 210 K value. We find that substitution of larger ions like Zr or Hf for Te increases $T_N$ by increasing the superexchange angles. The compensating O vacancies tend to form bound complexes with such dopants, preserving the electronic band gap. Substitution of N for O is favorable due to the decreased charge-transfer gap. $T_N$ is also increased by compressive [001] epitaxial strain. To help interpret the XMCD signal observed from the (110) surface of Fe$_2$TeO$_6$,\footnote{Wang \emph{et al.}, J. Phys: Condens. Matter 26, 055012 (2014)} we compare the energies of several terminations of this surface and find the known TiO$_2$-like termination is the most stable. The perpendicular magnetic moment at this surface, which appears through spin canting due to spin-orbit coupling, is found to be only 0.015 $\mu_B$ per surface Fe. The XMCD signal likely originates from the lowered symmetry of the combined surface and X-ray beam configuration. [Preview Abstract] |
Wednesday, March 4, 2015 4:06PM - 4:18PM |
Q32.00007: Designing asymmetric multiferroics with strong magnetoelectric coupling Xuezeng Lu, Hongjun Xiang, James Rondinelli Multiferroics offer exciting opportunities for electric-field control of magnetism. Single-phase multiferroics suitable for such applications at room temperature need much more study. Here, we propose the concept of an alternative type of multiferroics, namely, the ``asymmetric multiferroic.'' In asymmetric multiferroics, two locally stable ferroelectric states are not symmetrically equivalent, leading to different magnetic properties between these two states. Furthermore, we predict from first principles that a Fe-Cr-Mo superlattice with the LiNbO3-type structure is such an asymmetric multiferroic. The strong ferrimagnetism, high ferroelectric polarization, and significant dependence of the magnetic transition temperature on polarization make this asymmetric multiferroic an ideal candidate for realizing electric-field control of magnetism at room temperature. Our study suggests that the asymmetric multiferroic may provide an alternative playground for voltage control of magnetism and find its applications in spintronics and quantum computing. [Preview Abstract] |
Wednesday, March 4, 2015 4:18PM - 4:54PM |
Q32.00008: Magnetoelectic multiferroic superlattices and interfaces: Designing spintronic materials from first principles Invited Speaker: Zeila Zanolli The research challenges of the near and far future in electronics focus on the quest for new materials and novel device concepts to achieve low energy consumption, increased reliability and high device density. These can be obtained by designing active elements and interconnects whose operating principle is not (only) based on the electron charge but on the spin degree of freedom of the electron. The nanoscopic size of the materials calls for atomistic and parameter free ({\it ab initio}) simulations, which have proven to be crucial in achieving the necessary accuracy and predictive power. Materials which present a coupling between ferroelectricity and magnetism, i.e. magnetoelectric (ME) multiferroics, have been proposed as fundamental building blocks for spintronic devices [1]. However ferroelectricity and magnetism are often exclusive or weakly coupled in bulk. In this talk, we will discuss how superlattices of perovskites can be designed from first principles to achieve strongly coupled ME and, hence, achieve control the weak magnetization via an electric field [2]. Most important, advanced epitaxial techniques allow one to actually grow such magnetoelectric superlattices [3]. Another route to optimize spintronic devices is to exploit the unique electronic and transport properties of Carbon-based nanomaterials [4]. The latter present spin diffusion lengths up to 100 $\mu$m and high electron velocity. However, a large spin diffusion length comes at the price of small Spin Orbit coupling, which limits the possibility of manipulating electrons {\it via} an external applied field. Further, to achieve graphene-based devices one also needs to open its vanishing electronic gap. We use first principle techniques to show that placing graphene on a ME substrate can overcome these limitations by inducing magnetism and opening an electronic band-gap in the hybrid organic-multiferroic material. \\[4pt] [1] S. Fusil, V. Garcia, A. Bart\'eh\'elemy, M. Bibes, Annu. Rev. Mater. Res. {\bf 44}, 91–116 (2014)\\[0pt] [2] Z. Zanolli, J. C. Wojde\l, J. \'{I}\~{n}iguez, and Ph. Ghosez, Phys. Rev. B {\bf 88}, 060102(R) (2013)\\[0pt] [3] G. Rispens, B. Ziegler, Z. Zanolli, J. Iniguez, Ph. Ghosez, P. Paruch, Phys. Rev. B {\bf 90}, 104106 (2014)\\[0pt] [4] S. M.-M. Dubois, Z. Zanolli, X. Declerck, J.-C. Charlier, Eur. Phys. J. B {\bf 72} (2009) 1-24 [Preview Abstract] |
Wednesday, March 4, 2015 4:54PM - 5:06PM |
Q32.00009: Electric Field Control of the Ferromagnetic CaRuO3/CaMnO3 Interface Alexander Grutter, Brian Kirby, Matthew Gray, Charles Flint, Yuri Suzuki, Julie Borchers Electric field control of magnetism has been recognized as one of the most important goals in nanoscale magnetics research. The most popular routes towards achieving magnetoelectric (ME) coupling have focused on heterostructures incorporating multiferroics or ferroelectrics. Such studies often rely on voltage induced distortion to induce strain in the magnetic film and alter the magnetic properties. However, successful attempts to induce ME coupling without multiferroicity or magnetoelasticity remain relatively rare. The ferromagnetic interface between the antiferromagnetic insulator CaMnO$_3$ and the paramagnetic metal CaRuO$_3$ is a promising candidate for direct magnetization control. This interfacial ferroagnetism is stabilized through the competition between interfacial double exchange and antiferromagnetic superexchange between adjacent Mn$^{4+}$ so that the system is expected to be very sensitive to small changes in interfacial carrier density. Using polarized neutron reflectometry, we have probed the electric field dependence of the interfacial magnetization of CaRuO$_3$/CaMnO$_3$ bilayers deposited on SrTiO$_3$. We find that electric fields of $\pm$8 kV/m are sufficient to switch the interfaces from largely ferromagnetic to completely antiferromagnetic. [Preview Abstract] |
Wednesday, March 4, 2015 5:06PM - 5:18PM |
Q32.00010: Simultaneous Kerr and Faraday investigations of boundary magnetization and order parameter switching in voltage-controllable exchange bias films Junlei Wang, Will Echtenkamp, Mike Street, Christian Binek Magnetoelectric oxides are of great interest for ultra-low power spintronics with memory and logic function. A key property for the realization of electrically switchable state variables is the voltage-controlled boundary magnetization in magnetoelectric antiferromagnets. It allows electric switching of an adjacent exchange coupled ferromagnetic layer in the absence of dissipative currents. Previous surface sensitive measurements of boundary magnetization in thin films of the archetypical magnetoelectric antiferromagnet chromia lacked explicit demonstration of the predicted rigid coupling between the bulk antiferromagnetic order parameter and the boundary magnetization. We designed a magneto-optical setup allowing simultaneous measurement of Kerr and Faraday rotation. Our experiments correlate electric field induced bulk magneto-optical effects (non-reciprocal rotation), including the response on switching of the antiferromagnetic order parameter, with the boundary magnetization. Our results suggest that switching of a ferromagnetic film strongly exchange coupled to a magnetoelectric antiferromagnetic ultra-thin film allows switching of the antiferromagnetic order parameter. We investigate the possibility that this switching phenomenon might induce a voltage pulse via a generalized variation of the inverse linear magnetoelectric effect. [Preview Abstract] |
Wednesday, March 4, 2015 5:18PM - 5:30PM |
Q32.00011: Voltage Controlled Exchange Bias in a Cr$_{2}$O$_{3}$ based heterostructure Will Echtenkamp, Mike Street, Christian Binek Controlling magnetism by electrical means is a key challenge in the field of spintronics, and electric control of exchange bias is one of the most promising routes to address this challenge. Isothermal electric control of exchange bias has been achieved near room temperature using bulk, single crystal, magnetoelectric Cr$_{2}$O$_{3}$, which has a voltage controlled net magnetization at the (0001) surface\footnote{Xi He, et. al, Nat. Mater. 9, 579-585 (2010)}\footnote{W. Echtenkamp, Ch. Binek, Phys. Rev. Lett. 111, 187204 (2013)}. Voltage control of magnetism in a Cr$_{2}$O$_{3}$ thin film system has presented significant challenges. In this study we explore the electric control of exchange bias in an all-thin-film system of decreasing chromia film thickness with significant implications for scalability of ultra-low power memory and logical devices. Cross-sectional HRTEM indicates that grain boundaries in the metallic bottom electrode propagate into the Cr$_{2}$O$_{3}$ thin film with detrimental effects on leakage currents. We address this issue via a three-step growth method for the deposition of epitaxial Pd on sapphire. The resulting microstructure of the films is analyzed by reflection high-energy electron diffraction, tunneling electron microscopy and x-ray diffraction. [Preview Abstract] |
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