Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session W13: Focus Session: Manganite Thin Films and Interfaces |
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Chair: Ganesh Panchapakesan, Oak Ridge National Laboratory Room: 007D |
Thursday, March 5, 2015 2:30PM - 3:06PM |
W13.00001: Emergent Magnetic Phenomena at Manganite Interfaces Invited Speaker: Yuri Suzuki Emergent phenomena at transition metal oxide interfaces have been the focus of recent intense study since the discovery of metallicity at the interface of LaAlO$_{3}$ and SrTiO$_{3}$ a decade ago. Emergent magnetic phenomena at transition metal oxide interfaces had been studied even earlier. However there have been surprisingly few systems demonstrating interfacial ferromagnetism especially combined with metallicity. Recently, we have developed a general picture describing the origin of interfacial ferromagnetism in CaMnO$_{3}$ based systems. Density functional theory attributed the interfacial ferromagnetism to a double exchange interaction among interfacial Mn ions (just in the first single unit cell of CaMnO$_{3}$) mediated by conduction electrons from the neighboring itinerant layer. We have demonstrated interfacial ferromagnetism in superlattices composed of the antiferromagnetic insulator CaMnO$_{3}$ and an itinerant metal (CaRuO$_{3}$ or LaNiO$_{3}$). Through polarized neutron spectrometry, x-ray magnetic circular dichroism and bulk magmetometry, we have shown that the ferromagnetism originates from Mn ions in a single unit cell of the CaMnO$_{3}$ at the interfaces as theoretically predicted. The modulation of interfacial ferromagnetic moment as a function of constituent layer thicknesses as well as long-range antiferromagnetic correlations in the CaMnO$_{3}$, observed by neutron diffraction, are indicative of the competing magnetic interactions at play. [Preview Abstract] |
Thursday, March 5, 2015 3:06PM - 3:18PM |
W13.00002: Thickness effect on magnetic and electronic response in phase separated manganite thin films Hyoung Jeen Jeen, Amlan Biswas Thickness variation can be used to observe confinement effects and to control the strain state of thin films. Such confinement and strain state variation often creates unconventional physical properties in thin films of complex oxides such as phase separated (La{\$}\textunderscore \textbraceleft 1-y\textbraceright {\$}Pr{\$}\textunderscore \textbraceleft y\textbraceright {\$}){\$}\textunderscore \textbraceleft 0.67\textbraceright {\$}Ca{\$}\textunderscore \textbraceleft 0.33\textbraceright {\$}MnO{\$}\textunderscore \textbraceleft 3\textbraceright {\$} (LPCMO). Thin films of LPCMO show physical properties different from bulk samples such as, in-plane magnetic anisotropy and strain-driven anisotropic dynamic percolation. In this presentation, we will show the relation between lattice strain and the formation of an antiferromagnetic charge ordered insulating phase and the effect of strain relaxation on the magnetic anisotropy and single domain to multi-domain transition in electronically phase separated LPCMO thin films. We observed an increase in residual resistivity, a reduction of in-plane magnetic anisotropy, and an increase of the domain transition temperature as the thickness of the thin films is increased. [Preview Abstract] |
Thursday, March 5, 2015 3:18PM - 3:30PM |
W13.00003: First-principles study of magnetic, electronic and optical properties of double perovskite Bi$_2$FeMnO$_6$ Towfiq Ahmed, Dzmitry Yarotski, Quanxi Jia, Jian-Xin Zhu We study magnetic, electronic and optical properties of double perovskite Bi2FeMnO6 (BFMO) using density functional theory. In these systems, the exchange interaction between Fe and Mn sites gives rise to a ferrimagnetic ordering, which is captured in our ab initio calculations. Thin film Bi2FeMnO6 (BFMO) are generally grown on substrates such as SrTiO3 and Si. Significant strain has been experimentally observed in BFMO unit cells due to slight lattice mismatch between the thin film and substrate unit cells. In this work, we find that the net magnetic moment in BFMO depends on the ``c/a'' ratio of the unit cell, suggesting the strain dependence of magnetization in such system. We further calculate x-ray magnetic dichroism (XMCD) signals of Fe and Mn ions in BFMO for L2 and L3 edges. By applying the XMCD sum rules, we adopted an alternative approach to estimate the spin and orbital magnetic moment from our DFT calculations. We find qualitative agreement between our calculated values and the experimental measurements based on different techniques.Moreover, we study spin resolved optical conductivity and density of states in BFMO. These calculations give insight into electronic structure near Fermi energy, and dominant electronic excitations in the valence-conduction region of BFMO. [Preview Abstract] |
Thursday, March 5, 2015 3:30PM - 3:42PM |
W13.00004: Pressure-induced modification of colossal magnetoresistive magnanites Petro Maksymovych, S. Kelly, R. Vasudevan, E. Eliseev, A. Morozovska, M.D. Biegalski, J.F. Mitchell, H. Zheng, J. Aarts, S.V. Kalinin Nanoscale chemical control of oxides using confined fields, conceptually similar to electrical switching of ferroelectrics, is not obvious. We investigated CMR manganites using UHV force microscopy and tunneling microscopy. Scanning the surface with a metal tip was found to create a strongly insulating state, at least a few nm deep, even at zero applied bias. The state could be due to charge order, polaron disorder, chemical disorder or a combination thereof. Based on concomitant changes of surface potential we propose that contact-pressure modifies electrochemical potential of oxygen vacancies via the Vegard effect, causing vacancy motion and changes of electronic properties. Given broad similarities in defect chemistry, mechanical control of oxides may be universal. (MD) supported by Center for Nanophase Materials Sciences, a DOE Office of Science User Facility. (PM, SVK, RV, JFM, HZ) supported by U.S. DOE Office of Basic Energy Sciences, Division of Materials Sciences and Engineering. [1] Nanotechnology 25 (2014) 475302. [Preview Abstract] |
Thursday, March 5, 2015 3:42PM - 3:54PM |
W13.00005: Ferroelectric control of spin injection in La0.7Sr0.3MnO3/BaTiO3/La0.5Ca0.5MnO3/La0.7Sr0.3MnO3 multiferroic tunnel junctions with a bilayer barrier Yuewei Yin, L.D. Miao, R.Z. Du, Q. Li Using a ferroelectric (FE) barrier with ferromagnetic electrodes has become a promising method for controlling spin injection by purely electrical means, which is an important challenge in spintronics. Recently, we have designed a La0.7Sr0.3MnO3(LSMO) /BaTiO3(BTO) /La0.5Ca0.5MnO3(LCMO) /LSMO tunnel junctions in which the reversal of FE polarization of BTO will magnetoelectrically lead to a FM metallic - antiferromagnetic insulating phase transition in LCMO and result in an enhanced tunneling electroresistance (TER).[1] Using the bilayer barrier, we observed that the spin injection can be controlled by barrier polarization reversal as shown in the change of tunneling magnetoresistance (TMR). The temperature evolution of tunnel electromagnetoresistance (TEMR) (percentage ratio between the TMR values for the two polarization states), which is directly proportional to the change of tunnel-current spin polarization, was studied and larger TEMR was obtained with increasing temperature. Meanwhile, TEMR increases with TER effect for samples with different LCMO insertion thicknesses, suggesting a controllable strong electric control of tunnel-current spin polarization using a designed structure with proper interfaces. [1] Y. W. Yin et al, Nat. Mater. 12, 397 (2013) [Preview Abstract] |
Thursday, March 5, 2015 3:54PM - 4:06PM |
W13.00006: Influence of Oxygen Pressure during Deposition on Atomic-Scale Surface Features of La$_{5/8}$Ca$_{3/8}$MnO$_{3}$ Films A. Tselev, R.K. Vasudevan, A. Gianfrancesco, L. Qiao, P. Ganesh, T.L. Meyer, H.-N. Lee, M.D. Biegalski, A.P. Baddorf, S.V. Kalinin We have used \textit{in situ} scanning tunneling microscopy to visualize atomic-scale surface structure of a mixed-valence manganite La$_{5/8}$Ca$_{3/8}$MnO$_{3}$ films grown by pulsed laser deposition. Surface termination and chemical composition were identified \textit{in situ} with angle-resolved x-ray photoelectron spectroscopy. We find a strong effect of the background oxygen pressure during deposition on structural and chemical features of the film surface. Deposition at 50 mTorr leads to mixed-terminated films with atomic-scale structurally imperfect B-site (MnO$_{2})$ termination. A small reduction of the pressure from 50 mTorr to 20 mTorr results in a dramatic change of the atomic-scale surface structure. The surface is dominated by nearly perfectly ordered B-site termination. However, this was accompanied by surface roughening at a mesoscopic length scale with formation of mound-like structures. These results can be interpreted as a strong influence of oxygen on the adatom mobility during growth. The effect of the oxygen pressure on dopant surface segregation is also noticeable: Ca surface segregation is reduced with decrease of the oxygen pressure. [Preview Abstract] |
Thursday, March 5, 2015 4:06PM - 4:18PM |
W13.00007: Trends in (LaMnO$_3$)$_n$/(SrTiO$_3$)$_m$ superlattices with varying layer thicknesses Jilili Jiwuer, Fabrizio Cossu, Udo Schwingenschl\"ogl We investigate the thickness dependence of the structural, electronic, and magnetic properties of (LaMnO$_3$)$_n$/(SrTiO$_3$)$_m$ ($n$, $m$ = 2, 4, 6, 8) superlattices using density functional theory. The structure relaxation turns out to be highly sensitive to the onsite Coulomb interaction. In contrast to bulk SrTiO$_3$, strongly distorted O octahedra are observed in the SrTiO$_3$ layers with a systematic off centering of the Ti atoms. The systems favour ferromagnetic spin ordering rather than the antiferromagnetic spin ordering of bulk LaMnO$_3$ and all show half-metallicity, while a systematic reduction of the minority spin band gaps as a function of the LaMnO$_3$ and SrTiO$_3$ layer thicknesses originates from modifications of the Ti $d_{xy}$ states. [Preview Abstract] |
Thursday, March 5, 2015 4:18PM - 4:30PM |
W13.00008: Variational calculations for spin canting at ferromagnetic/antiferromagnetic interfaces G. Richard, J.-X. Zhu, A.V. Balatsky, J.T. Haraldsen Understanding the complex interaction between materials is critical for the development of spintronic and electronic devices in the technology industry. In this report, we examine the canting of local moments throughout a ferromagnetic/antiferromagnetic heterostructure, where a combination of interlayer mixing and orbital reconstruction can be described as a local exchange field at the interface. Using a variational method and semi-classical approach, we examine the canting of spins throughout the full multilayer heterostructure. We approximate the interlayer interactions as an effective field throughout the interface and apply a standard spin Hamiltonian with spin anisotropy for the intralayer interactions of the ferromagnetic and antiferromagnetic layers. Overall, we show that observed finite magnetization and rotation of the local moment observed in LSMO/BFO is due to the interface interactions. Furthermore, we predict a size limit for this effect in the antiferromagnetic (BFO) layer. [Preview Abstract] |
Thursday, March 5, 2015 4:30PM - 4:42PM |
W13.00009: Phase separation in strained ultrathin La$_{0.67}$Sr$_{0.33}$MnO$_{3}$/SrTiO$_{3}$(100) films In Hae Kwak, Sarah Toth, Amlan Biswas Atomically smooth and ultrathin La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO) films were grown on TiO$_{2}$ terminated SrTiO$_{3}$(100) (STO) substrates using pulsed laser deposition. The lattice mismatch between LSMO and STO generated uniform tensile strain on the LSMO film, and it significantly changed the electrical and magnetic properties of LSMO/STO thin films. Compared to the fully spin polarized ferromagnetic metallic state of bulk LSMO below a Curie temperature near 350 K, we observed a decrease in Curie temperature and the insulator to metal transition temperature as the thickness of LSMO was reduced down to 5 unit cells. Furthermore, the reduction of the saturation magnetization as the film thickness is reduced indicates phase separation in ultrathin LSMO/STO films. Therefore, it is possible to obtain a phase separated state close to room temperature in uniformly strained LSMO thin films. [Preview Abstract] |
Thursday, March 5, 2015 4:42PM - 4:54PM |
W13.00010: Magnetic coercive field changes in microstructured (La$_{\mathrm{1-y}}$Pr$_{\mathrm{y}})_{\mathrm{1-x}}$Ca$_{\mathrm{x}}$MnO$_{3}$ thin films Daniel Grant, Michael Ryan, Amlan Biswas The hole-doped manganite (La$_{\mathrm{1-y}}$Pr$_{\mathrm{y}})_{\mathrm{1-x}}$Ca$_{\mathrm{x}}$MnO$_{3}$ (LPCMO) shows effects such as phase coexistence and colossal magnetoresistance. Since the phase coexistence occurs at length scales of up to 10 micrometers, it is relatively straightforward to reduce the sample size to the scale of phase separation. We will present magnetization data that show a change in the magnetic coercive fields of LPCMO thin films by a factor of about 2 when the sample size is reduced to 100 micrometers using a photolithography process. The amount of the increase of the coercive field increases with film thickness. We will discuss the increased coercive field in the context of the competition between shape and stress magnetic anisotropies. We will also describe the role of dimensionality in determining the coercive field behavior. This process can be used to control the phase separation and the magnetic hardness of manganites. [Preview Abstract] |
Thursday, March 5, 2015 4:54PM - 5:06PM |
W13.00011: Effect of A-site ordering on the magnetoelectric properties in (111)-oriented LaMnO$_{3}$/SrMnO$_{3}$ superlattices Minhui Hu, Ruinan Song, Jiandong Guo It is expected that the chemical order that occurs over the crystallographic A-sites might strongly influence the distribution of the charges, magnetic and transport properties of the perovskite magnates. In this work, we focus on growing LaMnO$_{3}$/SrMnO$_{3}$ superlattices on (111)-oriented by PLD. The superlattices were characterized by magnetic as well as electronic transport measurements, and compared with the La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ thin film having the same components in which the A-site dopants are randomly distributed. The superlattices had different properties from the thin film: higher Curie temperature of FM and metal-insulator transitions, larger magnetization, and lower resistivity. This will allow for an understanding of the dependence of the intrinsic properties with respect to the long-range ordering of dopants. These differences can be explained by Mn$^{3+}$/Mn$^{4+}$ double exchange separation arising from the artificially induced the order of the A-site cations in the superlattices. Our proposed superlattices will allow for understanding of the fundamental magnetic and electronic interactions arising from dopant ordering in transition metal oxides. [Preview Abstract] |
Thursday, March 5, 2015 5:06PM - 5:18PM |
W13.00012: Large Relaxation (Polar distortion) of SrTiO$_{3}$ interfaced with La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ Zhen Wang, Hangwen Guo, Lina Chen, Mohammad Saghayezhian, E.W. Plummer, Jiandi Zhang, Jing Tao, Lijun Wu, Huolin Xin, Yimei Zhu The physics of thickness-induced metal-insulating transition in metallic oxide thin films is very interesting. The question is the behavior intrinsic or extrinsic. We explore the origin of such transition by manipulating the thickness of La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ (LSMO) thin film grown on SrTiO$_{3}$ (STO) substrate and detailed property measurements and structural characterization. We observed an unexpected structural relaxation in STO when interfaced with La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ (LSMO) by using scanning transmission electron microscopy (STEM). A large out-of-plane polar distortion of STO extends up to 8 $\sim$ 10 unit cell (u.c.) across the interface with the 4 u.c. insulating LSMO film, while only very moderate relaxation were found with thicker and metallic LSMO films. The electron energy loss spectrum (EELS) studies reveal that the charge transfer across the interface is similar in the both films. The nature of such an overlayer-dependent structural relaxation will be discussed: Is this thickness dependent relaxation of the STO an inherent property of metallic/insulating properties of the ultrathin film? [Preview Abstract] |
Thursday, March 5, 2015 5:18PM - 5:30PM |
W13.00013: Interfacial intermixing in $\delta$-doped oxide superlattices Valentino R. Cooper, Houlong L. Zhuang, P. Ganesh, Haixuan Xu, P. R. C. Kent First principles studies of the interfaces between dissimilar insulating oxides have been fundamental in understanding both the origin of emergent interfacial phenomena and ways to control conduction pathways and charge carrier densities. However, these calculations often assume sharp interfaces; neglecting the effects of interfacial cation intermixing. Using density functional theory, we examine the effect of $A$-site interfacial intermixing on the stability and electronic structure of oxide $\delta$-doped heterointerfaces. We find that the dominant effect of interfacial intermixing is the reduction in carrier densities of the 2DEG states at the interface. Our previous work suggest that this reduction in carrier density (i.e. fractional $\delta$-doping) may lead to enhancements in electron mobilities. These results offer a plausible explanation for the deviations in carrier mobilities and densities measured in different experimental samples. Furthermore, our calculations show that intermixing above 1/4 concentration is unstable relative to a clean interface; thus having implications for large scale production, where experimental growth techniques, such as chemical vapor deposition, may be a viable alternative. [Preview Abstract] |
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