Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session A32: Focus Session: Magnetic Oxide Films. Surface and Interface Effects |
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Sponsoring Units: GMAG DMP Chair: J. D. Burton, University of Nebraska, Lincoln Room: 207B |
Monday, March 2, 2015 8:00AM - 8:12AM |
A32.00001: New chemical and magnetic structure at the domain walls of an epitaxial oxide Saeedeh Farokhipoor, C. Magen, S. Venkatesan, J. Iniguez, C.J.M Daumont, D. Rubi, E. Snoeck, M. Mostovoy, C. de Graaf, A. Muller, M. Doblinger, C. Scheu, B. Noheda Domain walls (DWs) in multiferroic thin films are nanoscale regions presenting different properties compared to the adjacent domains. This distinct behavior originates from the broken crystal symmetry and intense strain gradients around the walls. Therefore, engineering and controlling the properties of DWs in different types of functional materials, in particular in complex oxides, can become a promising path to design and tailor novel nano-electronic and spintronic devices. In TbMnO$_{\mathrm{3}}$, an antiferromagnetic orthorhombic perovskite in bulk form, ferroelastic DWs can also be achieved in a very controlled way, with densities that increase inversely proportional to the film thickness, such that for the thinnest films, the volume fraction of DWs can become up to 25{\%} of the total film volume. These DWs, display a net magnetic moment that originates in a unique chemical environment: a novel Mn coordination has been locally synthesized due to the local stress present at the DWs. We believe that this method can be applied more generally to obtain embedded 2D ferromagnetic sheets of interest in electronics and spintronics. S. Farokhipoor, et al., Nature (20.Nov.2014). [Preview Abstract] |
Monday, March 2, 2015 8:12AM - 8:24AM |
A32.00002: The stoichiometry and Surface termination of La2/3Sr1/3MnO3 film on SrTiO3 (001) substrate * Lina Chen, Zhen Wang, Gaomin Wang, Hangwen Guo, Mohammad Saghayezhian, E.W. Plummer, Jiandi Zhang, Jing Tao, Lijun Wu, Yimei Zhu It has been observed that films of La2/3S1/3MnO3 (LSMO) are insulating when the films are below a certain critical thickness. The nature of such thickness-driven metal-to-insulator transition is still under debate: is this an intrinsic (dimensional confinement) or an extrinsic (oxygen vacancies or change of stoichiometry) effect. We have investigated the thickness-dependent composition variation in the LSMO films on SrTiO3 (001) by Low Energy Electron Diffraction (LEED), Angle-resolved XPS (ARXPS), and scanning transmission electron microscopy (STEM). Both ARXPS and STEM results show that the LSMO films prefer to be terminated with (La/Sr)-O layer and exists a thickness-dependence of Sr vs La concentration ratio in the film as well as the Sr surface segregation, incorporated with the thickness-dependence of the out-of-plane buckling in the MnO2 basal plane. These changes should have profound effect on the electronic and magnetic property, including the emergent insulating behavior in ultrathin films. * Supported by U.S. DOE under Grant No. DOE DE-SC0002136 [Preview Abstract] |
Monday, March 2, 2015 8:24AM - 8:36AM |
A32.00003: Exchange Bias and Unusual Initial Magnetization in Nanocrystalline Spinel Ferrite Thin Films Urusa Alaan, Sreenivasulu Gollapudi, Kin Man Yu, Padraic Shafer, Elke Arenholz, Gopalan Srinivasan, Yuri Suzuki We report on unconventional magnetic behavior in nanocrystalline (Mn,Zn,Fe)$_{3}$O$_{4}$ (MZFO) thin films grown at room temperature. Structural studies show no secondary phases, yet these films are exchange biased, with magnetic hysteresis loops shifted by as much as $\sim$ 200 Oe at 10 K after field-cooling. The samples can be ``trained'' so that successive magnetization loops exhibit reduced exchange bias. Shifts of the hysteresis loops exist even after cooling in zero field, indicating that the MZFO is not externally biased. We attribute the exchange bias to disordered, grain-boundary-like regions that bias more ordered MZFO. Annealing experiments that improved sample crystallinity decreased the exchange bias. Higher annealing temperatures resulted in reduced coercivities, higher magnetizations, and even the elimination of the exchange bias. Annealing also removed an unusual crossover of the initial magnetization curve outside of the saturated magnetization loop. This behavior has been seen in so-called ``mictomagnetic'' alloys. Using x-ray magnetic circular dichroism measurements, we have shown that cation disorder was reduced with annealing, and correlated the atypical initial magnetization with the degree of disorder. [Preview Abstract] |
Monday, March 2, 2015 8:36AM - 9:12AM |
A32.00004: Role of electrochemical phenomena at magnetic oxide interfaces Invited Speaker: Albina Borisevich |
Monday, March 2, 2015 9:12AM - 9:24AM |
A32.00005: New features in electronic transport across the ferromagnetic transition in SrRuO$_{3}$ /Nb:SrTiO$_{3}$ devices Saurabh Roy SrRuO$_{3}$(SRO), a moderately correlated material system, exhibits unique structural and magnetoelectric properties at interfaces with other correlated oxides. Here we report on new features in electronic transport across a functional interface between SRO and Nb:SrTiO$_{3}$, a n-type semiconductor. We map the potential landscape across such an interface using a nanoscale transport probe and find it to be strongly influenced by differences in local substrate termination. A difference in Schottky barrier height of 0.19 eV for SRO grown on local TiO$_{2}$ or SrO substrate terminations is found. This difference is attributed to different metal-oxygen displacements of the first unit cell of SRO at the different terminations; further supported by High-Resolution Transmission-Electron-Microscopy studies and Density Functional Theory calculations. This strong correlation of structure with electronic transport at the interface is reflected in a concomitant decrease of hot electron attenuation length from 1.6 u.c. to 0.88 u.c. at -2.1 V with the onset of the ferromagnetic state. This is attributed to the increased buckling of Ru-O-Ru bonds in ferromagnetic SRO, highlighting the role of strong correlations at such interfaces. [Preview Abstract] |
Monday, March 2, 2015 9:24AM - 9:36AM |
A32.00006: Controlling the dimensionality of the octahedra network in SrRuO3/SrTiO3 superlattice Mingqiang Gu, Qiyun Xie, Guoping Zhang, Xiaoshan Wu Two dimensional (2D) systems in perovskites have been widely investigated by designing superlattices. We propose a way to control the dimensionality of the octahedra network in perovskite superlattices by selecting different substrate orientation and superlattice periods. Lower dimensionality like one-dimension (1D) and zero-dimension (0D) can be achieved. Taking SrRuO3/SrTiO3 as an example, we demonstrate that the 1D structure is in a 1D Ising state, which is paramagnetic, while the 0D structure is ferromagnetic insulator with fully saturated magnetic moment on the Ru sites. New phenomena in the magnetic and electronic properties are observed, including large strain response, half-metallicity, and orbital-selective quantum confinement effects. [Preview Abstract] |
Monday, March 2, 2015 9:36AM - 9:48AM |
A32.00007: The surface study of ReFeO$_{3}$ (Re$=$Lu, Yb) thin films by X-ray Photoemission Spectroscopy and Density Function Calculation Shi Cao, Tula Paudel, Kishan Sinha, Xuanyuan Jiang, Wenbin Wang, Evgeny Tsymbal, Xiaoshan Xu, Peter Dowben The rare-earth ferrites, ReFeO3, may have a large magneto-electric response, with high surface/interface polarization, thus the surfaces are of considerable interest. We have characterized the surfaces of hexagonal ReFeO3 (Re$=$ Lu, Yb) and orthorhombic LuFeO3 thin films by angle resolved X-ray photoemission spectroscopy (ARXPS) and compared with density function theory (DFT). The surfaces will terminate in either Fe-O or Re-O depending on whether in the hexagonal or orthorhombic phase of the rare earth ferrite, but consistent with the expectations of DFT. The orthorhombic or hexagonal phases of these rare earth ferrites have the Fe in different crystal fields, which in turn affects the of Fe-O ligands. These changes in electronegativity are experimentally evident as differences in the Fe 2p core level photoemission satellite features. Surface preparation also effects surface termination and will be discussed. [Preview Abstract] |
Monday, March 2, 2015 9:48AM - 10:00AM |
A32.00008: LaAlO$_3$ thickness window for electronically controlled magnetism in LaAlO$_3$/SrTiO$_3$ heterostructures Feng Bi, Mengchen Huang, Hyungwoo Lee, Sangwoo Ryu, Patrick Irvin, Chang-Beom Eom, Jeremy Levy Complex oxide heterostructures, especially LaAlO$_3$/SrTiO$_3$ (LAO/STO) exhibit emergent conductivity, superconductivity, and magnetism. A recent investigation\footnote{F. Bi, \textit{et. al}, Nature Communications \textbf{5}, 5019 (2014).} demonstrates that in-plane magnetism at the LAO/STO interface can be controlled electronically at room temperature. Here we employ the magnetic force microscopy to investigate the emergence of magnetism as the LAO thickness is varied between 4 u.c. and 40 u.c. Magnetism is observed only within a certain thickness window 8 u.c.-25 u.c; outside of this window the magnetic signal becomes negligible. Simultaneous capacitance measurements for different LAO thickness show that the devices with negligible magnetic signatures all have capacitance values far below the geometric capacitance. The existence of the observed window is attributed to the existence of direct or Zener tunneling, which prevents the insulating phase from being reached. [Preview Abstract] |
Monday, March 2, 2015 10:00AM - 10:12AM |
A32.00009: ABSTRACT WITHDRAWN |
Monday, March 2, 2015 10:12AM - 10:24AM |
A32.00010: Spectral and magnetic properties of hematite Fe2O3(001) surface: results from DFT+DMFT Alamgir Kabir, Volodymyr Turkowski, Talat S. Rahman It has been demonstrated that strong correlation effects may significantly modify the spectrum of a system, in particular leading to an increase of the bandgap and to a change in the orbital occupancies, which affects the magnetic properties of the system. With this in mind, we have examined the spectral and magnetic properties of the hematite Fe2O3 film system with (001) surface orientation by using the combined density functional theory (DFT) and dynamical mean-field theory (DMFT) approach. We pay special attention to the surface geometry and electronic structure, magnetization and magnetic anisotropy (MA) of the system by performing calculations at different values of the parameters for the local Coulomb repulsion and exchange energy. To calculate the MA of the system, we propose and apply a combined Bruno model [1] within DMFT, and demonstrate that under-coordinated surface Fe atoms contribute significantly to the MA of the film. We also compare our results with the DFT+U solution [2] and show that the dynamical effects taken into account by the DMFT significantly affect system properties, notably leading to a decrease of the atomic magnetic moments. [1] P. Bruno, Phys. Rev. B 39, 865 (1989). [2] A. Kiejna and T. Pabisiak, J. Phys.: Cond. Mat. 24, 095003 (2012). [Preview Abstract] |
Monday, March 2, 2015 10:24AM - 10:36AM |
A32.00011: Tunable Positive/Negative Exchange Bias in Gd$_{x}$Fe$_{1-x}$/NiCoO Thin Films Dustin Gilbert, Justin Olamit, Brian J. Kirby, Randy K. Dumas, Elke Arenholz, Kai Liu The exchange bias phenomenon in ferromagnet/antiferromagnet has been a long-standing problem due to its intriguing mechanisms and critical applications in spin-valve devices. Generally, exchange biased systems which are cooled below the AF N\'{e}el temperature in a positive magnetic field become biased to negative fields, referred to as negative exchange bias. Here, we investigate thin films of ferrimagnet/antiferromagnet Gd$_{x}$Fe$_{1-x}$/NiCoO, with 0.47$\le $x$\le $0.63. A multi-stepped hysteresis loop is observed, with a primary loop (phase one, low anisotropy) and a pair of sub-loops (phase two, higher anisotropy). The exchange bias can be tuned by the cooling field to be either positive or negative. As the cooling field is varied, the bias of phase one moves opposite to that of phase two. Element-specific hysteresis loops were measured by x-ray magnetic circular dichroism (XMCD), identifying phase one as GdFe, while phase two is associated with rotatable Co and Ni. Polarized neutron reflectometry identified the CoNi as a 3nm interfacial layer, which is likely the cause for the tunable exchange bias. We attribute this CoNi layer to an interfacial redox reaction between the Gd and NiCoO. This work has been supported by the NSF (DMR-1008791 and ECCS-1232275). [Preview Abstract] |
Monday, March 2, 2015 10:36AM - 10:48AM |
A32.00012: Surface reconstruction in (LaFeO3)m/(SrFeO3)n superlattices Albina Borisevich, Rohan Mishra, Young-Min Kim, Yuyuang Zhang, Seohyoung Chang, Seong Keun Kim, Anand Bhattacharya, Sokrates Pantelides We use scanning transmission electron microscopy and electron energy loss spectroscopy (EELS) to study the surface of a series of (LaFeO$_{\mathrm{3}})_{m}$/(SrFeO$_{\mathrm{3}})_{n}$ films grown on SrTiO$_{\mathrm{3}}$ substrates using molecular beam epitaxy. From EELS, we observe dramatic changes in the Fe L$_{\mathrm{2,3}}$ edges, including shifts in the peak positions and changes in their crystal-field splitting, on moving from the bulk to the surface. Moreover, we observe these changes to extend several layers into the films rather than being restricted to the topmost layer. For example in a (LaFeO$_{\mathrm{3}})_{\mathrm{8}}$/(SrFeO$_{\mathrm{3}})_{\mathrm{1}}$ superlattice, we observe that while the Fe oxidation state remains unchanged, a reversal in the intensity of the crystal-field split $t_{\mathrm{2g}}$ and $e_{\mathrm{g}}$ peaks occurs over a length of 5 unit cells suggesting a shift from octahedral coordination in the bulk to tetrahedral coordination at the surface. Simultaneously acquired annular bright field and dark field images allow us to map the associated changes in their structure, such as cation displacements and changes in oxygen columns. We combine these results with density functional theory calculations to give a complete picture of surface reconstruction in the studied films including changes in the electronic, magnetic properties and its effect on the adsorption of CO$_{\mathrm{2}}$ and H$_{\mathrm{2}}$O molecules that could point to routes to tune surface properties for water splitting and other applications. [Preview Abstract] |
Monday, March 2, 2015 10:48AM - 11:00AM |
A32.00013: Magnetic anomalies in self-assembled SrRuO3 -CoFe2O4 nanostructures studied by Raman spectroscopy Yi-Chun Chen, Yen-Chin Huang, Chia-Hsien Chien, Heng-Jui Liu, Ying-Hao Chu Self-assembled nanostructures with high interface-to-volume ratio usually possess interesting physical properties through the coupling between neighboring materials. In complex-oxide nanocomposites, the interplay of spin, charge, orbital, and lattice degrees of freedom especially provides various functionalities. Our recent study had shown photo-induced magnetization switching in a self-assembled system, CoFe2O4 (CFO)- SrRuO3(SRO), where the CFO nanopillars were embedded in the SRO matrix. Moreover, this system also has significant magnetoresistance behaviors. In this study, we used Raman spectroscopy to investigate the magnetic coupling mechanisms in CFO-SRO nanostructures. Compared to the pure CFO films, the CFO nano-pillars under out-of-plane compressive strain show a slightly increase of A1g(Co)/A1g(Fe) intensity ratio, which corresponds to a migration of Co ions from O-site (oxygen octahedron) to T-site (oxygen tetrahedron). This behavior can be further tuned by external stimulus, such as magnetic fields and temperatures. A strong increase of A1g(Co)/A1g(Fe) ratio together with a discontinuous A1g frequency shift occur at the SRO magnetic transition temperature. This result indicated that the spin-orbital interaction in CFO can be modulated by the SRO magnetic orderings. [Preview Abstract] |
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