Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session B30: Focus Session: Manganite Superlattices |
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Sponsoring Units: GMAG DMP Chair: Satoshi Okamoto, Oak Ridge National Laboratory Room: 334 |
Monday, March 16, 2009 11:15AM - 11:51AM |
B30.00001: Emergent properties of digital superlattices of LaMnO$_{3}$/SrMnO$_{3}$ Invited Speaker: LaMnO$_{3}$ and SrMnO$_{3}$, both antiferromagnetic insulators, are end members of the La$_{1-x}$Sr$_{x}$MnO$_{3}$ phase diagram, which includes a highly spin-polarized ferromagnetic metal and a variety of orbital-ordered antiferromagnets. Interfaces between LaMnO$_{3}$ and SrMnO$_{3}$ provide a unique environment where the spin, charge and orbital degrees of freedom of each of the constituents may `reconstruct', giving rise to collective states at interfaces that are qualitatively distinct from those in either LaMnO$_{3}$ or SrMnO$_{3}$. We have synthesized superlattices of (LaMnO$_{3})_{p}$/(SrMnO$_{3})_{q}$ , where $x=q/(p+q),$ using ozone-assisted molecular beam epitaxy. Here, $p $ and $q $represent integer layers of the constituents. These superlattices can be realized with interfacial roughness/intermixing limited to a region less than one unit-cell in extent. We will explore the properties of these `digital manganites' for a range of $p/q$, including enhanced ordering temperatures compared to randomly alloyed samples, and provide experimental evidence for the interfacial reconstruction that is responsible for their emergent properties. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B30.00002: Enhanced Antiferromagnetic Ordering Temperature in Metallic LaMnO$_{3}$/SrMnO$_{3}$ Superlattices Tiffany Santos, Steven May, Anand Bhattacharya, J. Lee Robertson The perovskite manganite La$_{1-x}$Sr$_{x}$MnO$_{3}$ has a rich magnetic phase diagram, exhibiting ferromagnetism (F) for La-rich compositions and antiferromagnetism (AF) for those that are Sr-rich. Our study focuses on the x=0.5 doping region containing the F-AF phase transition, particularly the role of strain and cation-site disorder in nucleating the F or AF state. Using ozone-assisted molecular beam epitaxy, we have prepared fully-epitaxial superlattices of LaMnO$_{3}$ and SrMnO$_{3}$ on SrTiO$_{3}$ substrates, along with random alloy films of La$_{1-x}$Sr$_{x}$MnO$_{3}$ with equivalent composition. In our digital synthesis method, whereby we interleave single unit-cell layers of undoped LaMnO$_{3}$ and SrMnO$_{3}$, we have eliminated disorder at the La/Sr cation site. Our structural characterization shows atomic layer precision in the synthesis of these superlattices. The structural, magnetic and transport properties of the superlattices are compared with those of the random alloys. A-type AF order (F alignment in-plane, AF alignment of adjacent planes) is verified by neutron diffraction, also revealing an enhanced N\'{e}el temperature with no F phase at higher temperature, in contrast to bulk. These AF thin films display metal-like behavior, opening the possibility of using the discrete layers of opposite spins for coherent spin transport. Supported by DOE, Office of Basic Energy Sciences. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B30.00003: Theory of Manganite Superlattices Andrew Millis, Chungwei Lin A comprehensive theoretical treatment of (001) $(LaMnO_3)_n(SrMnO_3)_m$ manganite superlattices is presented. The charge distribution, conductivity, and propagation through the superlattice of orbital and magnetic order are determined using dynamical mean field calculations in the superlattice geometry. General rules for predicting the behavior of manganite superlattices are outlined. Comparison is made to existing data and inconsistencies between theory and experiment are identified and discussed. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B30.00004: Control of Magnetism via Layer Thickness Modification in the LaMnO$_3$/SrMnO$_3$ Digital Superlattices and the Prediction of a Spin-Polarized 2DEG Birabar Nanda, Sashi Satpathy We study the effect of layer thickness on the magnetic properties in the (LMO)$_{2n}$/(SMO)$_{n}$ superlattices using density-functional calculations. The change in the magnetic properties is shown to be controlled by the leakage of the Mn-e$_g$ electrons from the LMO side to the SMO side. For n = 1 superlattice, the weak potential barrier allows the Mn-e$_g$ electrons to spread across the entire superlattice, so that a uniform ferromagnetic behavior is obtained through carrier mediated Zener double exchange. For larger n, the strong potential barrier restricts the e$_g$ electron transfer to few layers adjacent to the interface, thus leaving the magnetism unchanged and bulk like away from the interface, while modifying the magnetism in the interfacial region. Finally, taking the example of a delta doped superlattice, (SMO)/(LMO)$_1$/(SMO), we predict the formation of a spin-polarized two dimensional electron gas. The 2DEG, generated due to the confinement of the La (d) electrons in the direction normal to the interface, mediates a ferromagnetic alignment of the Mn-t$_{2g}$ spins via double exchange which in turn spin polarizes the 2DEG. \\ $\dagger$ Work supported by the US Department of Energy \\ 1. B. R. K. Nanda and S. Satpathy, arXiv:0810.2126; B. R. K. Nanda and S. Satpathy, Phys. Rev. Lett. {\bf 101}, 127201 (2008) [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B30.00005: The onset of metallic behavior in strained (LaNiO$_3$)$_n$/(SrMnO$_3$)$_2$ superlattices Steven May, Tiffany Santos, Anand Bhattacharya Motivated by predictions of collective ordering phenomena in LaNiO$_3$, we have grown strained (LaNiO$_3$)$_n$/(SrMnO$_3$)$_2$ superlattices on SrTiO$_3$ using ozone-assisted molecular beam epitaxy. The superlattices exhibit excellent crystallinity and interfacial roughness of less than one unit cell. The samples undergo a metal-insulator transition as $n$ is reduced from 4 to 2. Both $n$ = 1 and 2 samples are insulating, however, they exhibit different transport behavior. The $n$ = 1 sample acts as a gapped insulator, while the addition of a second LNO layer ($n$ = 2) leads to hopping transport through non-gapped conduction channels. These results will be compared to (LaMnO$_3$)/(SrMnO$_3$) superlattices to highlight how interfacial charge transfer differs in the nickelate/manganite superlattices from their all-manganite counterparts. Supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences under contract DE-AC02-06CH11357. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B30.00006: Electronic and crystal-field effects in the fine structure of electron energy-loss spectra of La$_x$Ca$_{1-x}$MnO$_3$ Weidong Luo, Maria Varela, Jing Tao, Stephen J. Pennycook, Sokrates T. Pantelides The fine structure of oxygen K-edge electron energy-loss spectra (EELS) of transition-metal oxides is known to correlate with nominal oxidation states (NOS) that are often interpreted as charge states. We report the results of a systematic study of O K-edge EELS fine structures in La$_x$Ca$_{1-x}$MnO$_3$ and their evolution as functions of doping $x$. The calculated spectra, specifically the pre-peak intensities and peak separations, as functions of $x$ are in excellent agreement with experimental data. The calculations show that the variation of the pre-peak's intensity with doping is controlled by the orbital occupancy of the majority-spin Mn 3$d$ states while its width is controlled by crystal-field splitting. The energy separation between the pre-peak and the main peak also has a correlation with the doping parameter $x$ and the NOS. The results confirm that the NOS extracted from EELS correlates with orbital occupancies but does not probe physical charges of the Mn sites. This research was sponsored in part by the DOE Office of Basic Energy Sciences, Division of Materials Sciences and Engineering and by the McMinn Endowment at Vanderbilt University. Computations were performed at the National Energy Research Scientific Computing Center. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B30.00007: Local Electronic Structure at Oxide-Oxide Interfaces Probed by Atomic Resolution Electron Energy Loss Spectroscopy Amish Shah, Q. M. Ramasse, S.J. May, J.G. Wen, J.N. Eckstein, A. Bhattacharya, J.M. Zuo We report an atomic resolution study of the electronic structure of 12 x 4 LaMnO$_{3}$-SrMnO$_{3}$ and 2 x 2 LaMnO$_{3}$-SrTiO$_{3}$ superlattices and their interfaces grown on SrTiO$_{3}$ by EELS. We correlated the interfacial electronic structure with the interfacial atomic structure using atomic resolution Z-contrast STEM using an electron probes of $<$0.1 nm. The oxide superlattices were synthesized using molecular beam epitaxy. We measured the site-specific unoccupied states of oxygen atoms and transition metals. In the LMO-SMO system we found extra states (holes) near the Fermi level and their dependence on abruptness of interface. In LMO-STO, we will present evidence of site-dependent electronic structure of oxygen and the Mn valence based on the L-edge ratios. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B30.00008: Electronic properties of manganite / titanate superlattices Maria Varela, H. Christen, H.N. Lee, L. Petit, T. Schulthess, S. Pennycook, J. Garcia-Barriocanal, A. Rivera, F.Y. Bruno, Z. Sefrioui, C. Leon, J. Santamaria Here we report on the study of LaMnO$_{3}$/SrTiO$_{3}$ interfaces. While LMO in bulk is an antiferromagnetic Mott insulator and STO is a band insulator, LMO/STO superlattices exhibit ferromagnetism and in some cases metallicity, both of which can be tuned by changing the layer thicknesses. We will compare the structure, chemistry and electronic properties of LMO/STO interfaces in high quality superlattices grown by pulsed laser deposition and high O$_{2}$ pressure sputtering. The distribution of defects and electronic properties will be studied through aberration corrected electron microscopy and electron energy loss spectroscopy. PLD superlattices show two alternating interface terminations, LaO-TiO$_{2}$ and SrO-MnO$_{2}$, which cause an asymmetry in the LMO layer electronic properties. Superlattices grown by sputtering only show one termination, LaO-TiO$_{2}$, giving an overall electron doping to the system. The role of interfacial charge transfer or localization, and any changes in electronic properties due to structural relaxations induced by epitaxial strain will be examined. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B30.00009: Magnetic and structural phase transitions in epitaxial thin films of Manganites Valeria Lauter, Hailemariam Ambaye, Steven Nagler, Hans Christen, Mike Biegalski Understanding the magnetic properties of complex materials near interfaces is important for the development of functional nanostructures and devices. Epitaxial LaMnO$_{3}$ films were grown on SrTiO$_{3}$ substrates. Recent work on such thin-film samples has shown that ``interface doping'' can induce magnetism at interfaces. Our work on LaMnO$_{3}$/SrTiO$_{3}$ interfaces has shown that the nature of the interface determines its magnetic structure - with the MnO$_{2}$-SrO interface showing a different magnetization than the LaO-TiO$_{2}$ interface. To investigate interfacial structures, we used polarized neutron reflectometry with off-specular scattering. Our results give evidence of reversible temperature- and field- dependent structural changes in LaMnO$_{3}$ film which undergo a phase transition. We determined that a structural phase transition in SrTiO$_{3}$ and the misfit strain trigger appearance of twins to reduce stresses and to adjust lattice mismatch between the film and the substrate. We show that a laterally correlated superstructure appear due to interaction of structural modifications with the magnetization the film [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B30.00010: Ferromagnetic Spin State of Manganite/SrTiO$_{3}$ Interfaces in (110) Orientation Xinfei Liu, Jianxing Ma, Tao Lin, G. Y. Gao, W. B. Wu, X. G. Li, Jing Shi The interface spin state of a ferromagnet (FM) can deviate significantly from its bulk spin state and this effect could be strongly orientation-dependent especially in manganites. We have successfully fabricated high-quality (110)-oriented [La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (t) /SrTiO$_{3}$ (3ML)]$_{n}$ superlattices (t ranging from 3 to 15 ML), which are characterized by the atomic force microscopy, high-resolution transmission electron microscopy, x-ray diffraction and magnetometry. Compared to (100)-oriented counterparts, LSMO in (110)-oriented superlattices has a thinner deadlayer at LSMO/STO interface. From the thickness (t) dependence of the superlattice magnetic moment, we extract the interface contribution and find that the interface moment is close to that of the bulk, suggesting that the (110)-oriented interface adopts the FM spin ground state. This differs significantly from the spin canting state at (100)-oriented LSMO/STO interface that was previously reported by other groups. Our results indicate that the magnetism of manganite interface may be manipulated by taking advantage of the orientation-dependent nature of the exchange interactions. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B30.00011: Enhanced Low Field Magnetoresistance in La$_{0.67}$Ca$_{0.33}$MnO$_{3}$/La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ superlattices P.V. Wadekar, Q.Y. Chen, O. Lozano, P.V. Chinta, W.K. Chu, D. Wijesundera, C.S. Lin, P.H. Tseng, Y.T. Lin, L.W. Tu, C.P. Lin, H. Chou, C.C. Kuo, N.J. Ho, H.W. Seo We have grown [1 nm/ 1nm]$_{n }$ superlattices of manganite (n = 20, 30,40) in which La$_{1-x}$Ca$_{x}$MnO$_{3}$(x=0.33) serves as the ferromagnetic layer while La$_{1-x}$Ca$_{x}$MnO$_{3}$(x=0.5) serves as the spacer layer on LaAlO$_{3}$ substrates by magnetron sputtering. The samples were characterized by XRD, Magneto-transport measurements, Rutherford backscattering spectroscopy, and atomic force microscopy. Enhanced longitudinal magnetoresistance (MR) under an applied field B, defined as MR (B) = $\rho $(B)/$\rho $(0) -- 1, was as much as -49{\%} at B=0.5 Tesla and T=90 K. The causes for this enhancement not seen at low field in other single-layered films of x=0.33 and the correlation of oxygen annealing with the MR effects for the superlattices will be discussed. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B30.00012: Strain-controlled electronic properties and magnetorelaxor behaviors in electron-doped CaMnO$_{3}$ thin films and superlattices P.-H. Xiang, H. Yamada, A. Sawa, H. Akoh We present a systematic study on electronic properties of the \textit{electron-doped} manganite Ca$_{1-x}$Ce$_{x}$MnO$_{3}$ (CCMO,$_{ }$0 $\le x\le $ 0.08) single-layer films and superlattices composed of alternating stacks of non-doped CaMnO$_{3}$ (CMO) and CCMO($x$ = 0.08) layers. The transport properties of the CCMO films are found to be very sensitive to the epitaxial strain. Metallic characteristic observed in the CCMO(0.04 $\le x\le $ 0.06) bulk polycrystal can be realized only in the practically strain-free CCMO epitaxial films on the NdAlO$_{3}$ (NAO) substrate. A large magnetoresistance accompanied with magnetorelaxor-like behavior is observed in the CCMO($x$ = 0.06) film, which can be explained in terms of the phase separation and the irreversible growth of metallic domain in antiferromagnetic insulating matrix. The metallic property is also realized in the superlattices, indicating a charge transfer at the interfaces between CMO and CCMO($x$ = 0.08) layers. When the CCMO($x$ = 0.08) layer in the superlattice is thicker than 8 unit cells, the superlattice exhibits magnetorelaxor-like phenomenon This can be attributed to a phase competition between different antiferromagnetic orderings at the interfaces, resulting in the phase separation [Preview Abstract] |
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