Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session D30: Focus Session: Cobaltite and Manganite films |
Hide Abstracts |
Sponsoring Units: DMP GMAG Chair: Chris Leighton, University of Minnesota Room: 334 |
Monday, March 16, 2009 2:30PM - 3:06PM |
D30.00001: Emergent Phenomena in Spatially Confined Manganites Invited Speaker: There is evidence that chemically disordered single crystal manganites exhibit electronic inhomogeneity in which areas with vastly different electronic and magnetic properties form and coexist in phase separated domains ranging in size from a few nanometers to micrometers. This phase separation is of particular interest, as it has been suggested that it is the central feature that leads to colossal magnetoresistance in manganites, the Mott transition in VO2 and may play a part in high-TC superconductivity in cuprates. We will discuss our ongoing efforts to answer fundamental questions about the specific role of PS in complex oxides using a novel spatial confinement technique. Unlike transport measurements on bulk or thin films where the electrons follow only the metallic path of least resistance, spatially confining a phase separated material to the scale of its inherent domains forces electrons to travel through both the metallic and insulating regions that lie along the conduction path. This has led to observations of several new phenomena such as a reemergent metal-insulator transition, ultra-sharp jumps in resistivity at the metal-insulator transition, and the first high resolution observation of stable single domain electronic phase transitions in time. La(5/8-x)Pr(x)Ca(3/8)MnO(3) (LPCMO) is used as a model system due to its large scale electronic phase separation into ferromagnetic metal (FMM) and charge ordered insulator (COI) electronic phase domains and well documented spin-charge-lattice interactions. These properties allow us to isolate domains through conventional wet etch techniques and offer a wide range of tunability through doping and substrate strain. This ability to control key elements of the underlying complexity and observe the resulting changes in the emergent behavior help answer questions about the fundamental physics that rule complex materials. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D30.00002: Electric field induced anisotropic transport properties of phase separated (La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3}$ thin films Hyoung Jeen Jeen, Alessandra Gallastegui, Amlan Biswas The perovskite manganese oxide (La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LPCMO) exhibits electronic phase separation i.e., a ferromagnetic metallic phase and a charge ordered insulating phase coexist in a certain temperature range. It was shown that in such a phase separated state thin films of manganites show a colossal electroresistance (CER) although the mechanism driving this phenomenon is still unknown. We present transport measurements which show that the CER is due to an electric field driven anisotropy in phase separated manganites. LPCMO thin films were grown on NdGaO$_{3}$ (110) substrates using Pulsed Laser Deposition. A cross shaped micro-structure, with 60 by 10 $\mu $m legs, was fabricated using UV photolithography and chemical etching. We observe CER close to the insulator to metal transition temperature (T$_{IM})$ in the longitudinal direction i.e. parallel to the applied electric field. We simultaneously measure the transverse resistance in the other (orthogonal) leg of the microstructure. We observe a clear anisotropy in the conduction of the cross shaped microstructure which could be the origin of CER in manganites. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D30.00003: Colossal electroresistance in phase separated manganite nanobridges G. Singh-Bhalla, A. Biswas, A. F. Hebard We have examined the electric field effect on the nanometer scale in the manganite (La,Pr,Ca)MnO{\$}{\_}{\{}3{\}}{\$}which is well known for its micrometer scale phase separation (PS) into coexisting metallic and insulating regions. When thin films of this material are patterned into micrometer and nanometer wide bridges, alternating insulating and metallic regions may form along the length of the bridge within the PS temperature range. At the onset of PS, nanoscale ferromagnetic islands appear within the insulating antiferromagnetic background. Within this temperature range, transport properties along the length of the bridge are reminiscent of transport across metallic islands in the Coulomb blockade regime. Applications of a magnetic field induce unusual bifurcations in the current-voltage characteristics implying either a change in resistance arising from spin canting or a change in the ferromagnetic phase fraction. Next, within the micrometer scale PS temperature regime, current-voltage measurements reveal colossal, step like drops in resistance with increasing current. We will discuss our results both in the context of previously considered models for manganite electroresistance and new interpretations with a focus on the microscopic details of the metallic and insulating regions. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D30.00004: Phase modification of La$_{1/4}$Pr$_{3/8}$Ca$_{3/8}$MnO$_{3}$ thin films by light, magnetic field and applied stress Justin Olamit, Mikhail Zhernenkov, Mike Fitzsimmons, Haile Ambaye, Valeria Lauter, Hyoung Jeen Jeen, Amlan Biswas Complex oxide materials exhibit a wide variety of fascinating electromagnetic properties related to the coexistence of multiple electronic and magnetic phases.[1,2] The temperature-magnetic field phase diagram of La$_{0.27}$Pr$_{0.40}$Ca$_{0.33}$MnO$_{3}$ (LPCMO) is intriguing; a ferromagnetic metallic (FMM) phase, charge ordered insulating (COI) phase and conditions where both phases coexist are accessible with changes in temperature and magnetic field.[3] We have performed neutron reflectometry to understand the emergence of the FMM phase in a COI matrix of a LPCMO thin film as functions of temperature, irradiation with light, magnetic field and stress. Specular reflectivity reveals the emergence of ferromagnetism below the phase transition temperature. Diffuse scattering shows that the length scale of FM domains is 1-2 microns. [1] Ch. Renner et al., Nature \textbf{416}, 518 (2002). [2] L. Zhang et al., Science \textbf{298}, 805 (2002). [3] T. Dhakal et al., Phys. Rev. B \textbf{75}, 092404 (2007). [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D30.00005: Real-Space Imaging of Electronic Phase Separation in a Mn-Doped Bilayered Ruthenate Tae-Hwan Kim, M. Angst, R. Jin, X.G. Zhang, J.F. Wendelken, A.P. Li, B. Hu, E.W. Plummer Transition-metal oxides with multiple nearly degenerate states show very complicated phase diagrams. Small perturbations can often dramatically change their functionalities. It is believed that electronic phase separations (PS) play an important role in the exotic functionality. Direct experimental observation of PS has thus become crucial to understanding underlying mechanisms of the striking functionalities. We have studied the PS and the evolutions of phase domains with temperature near the Mott transition in a Mn-doped bilayered ruthenate Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7}$. Our experimental approach combines electron microscopy, scanning tunneling microscopy, and electron transport spectroscopy, which provide unprecedented capabilities of imaging PS and interrogating individual microscopic domains in situ. A quantitative correlation has been determined between the macroscopic metal-insulator transition and the microscopic phase domain percolation in Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7}$. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D30.00006: Mapping the phase boundaries in thin-film manganites using scale-invariant dielectric response P. Mickel, G. Singh-Bhalla, S. Tongay, A. Biswas, A. F. Hebard Magnetocapacitance techniques[1] have been used in a study of (La$_{1-y}$Pr$_{y})_{1-x}$Ca$_{x}$MnO$_{3}$ (LPCMO) thin films to determine the range of phase space, described by frequency ($\omega )$, temperature ($T)$ and field ($H)$, over which a dielectric response of the form, C"($\omega $,T,H) = [C'($\omega $,T,H) - C$_{\infty }$]$^{\gamma }$ , is found to hold. This power-law scaling collapse (PLSC) of the complex capacitance (C', C"), expressed in a Cole-Cole formulation, differs from the well-known ``universal'' dielectric response (UDR) [2], where the exponent $\gamma $ = 1. The influence of film thickness and stoichiometry on the extent of the PLSC region is investigated with the implementation of a new phase-space mapping technique. The mappings clearly illustrate the onset of phase competition in LPCMO, delineating boundaries which correspond to capacitive minima at low temperatures, where the first-order insulator-metal transition occurs, and to the second-order paramagnetic-insulator/charge-ordered-insulator transition at higher temperatures, where a resistive transport signature exists in bulk but not in thin films. Modeling with distributions of UDR elements corresponding to the different manganite phases gives a good qualitative account of the observed behavior, and can lead to the determination of individual phase fractions [1] R. Rairigh, Nature Physics 3, 551 - 555 (2007) [2] Jonscher - J. Phys. D: Appl. Phys. 32* *(1999) [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D30.00007: Direct measurement of the low-temperature spin-state transition in epitaxially strained LaCoO$_{3}$ thin films Robert Klie, Guang Yang, Yuan Zhao The perovskite oxide LaCoO$_{3}$ exhibits an anomaly in its magnetic susceptibility at 80 K associated with a thermally excited transition of the Co$^{3+}$-ion spin. We will show that atomic-resolution Z-contrast imaging and electron energy-loss spectroscopy in combination with ab-initio first-principles DFT calculations can be utilized to measure the spin-state transition in LaCoO$_{3}$. In particular, we utilize in-situ cooling experiments in a transmission electron microscope to demonstrate that the O K-edge pre-peak is sensitive to the Co$^{3+}$-ion spin-state. Our experimental results will be compared to first-principles calculations, and we will conclude that the thermally excited spin-state transition occurs from a low to an intermediate spin state, which can be distinguished from the high-spin state. Next, we will examine the effects of bi-axial strain and point defects in LaCoO$_{3}$ thin-films on the Co$^{3+}$-ion spin-state. We will show that a single-crystal pseudo-cubic LaCoO$_{3}$ (001) film can be successfully grown on LaAlO$_{3}$ (001). Moreover, we will show that the epitaxially strained LaCoO$_{3}$ film exhibits a ferro-magnetic transition at low temperature that was not observed in bulk LaCoO$_{3}$. We will discuss the origin of this transition and the possibility of stabilizing different Co$^{3+}$-ion spin-states in LaCoO$_{3}$ using interfacial strain. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D30.00008: First-principles study for low-spin LaCoO$_{3}$ with structurally consistent Hubbard $U$ Han Hsu, Koichiro Umemoto, Matteo Cococcioni, Renata Wentzcovitch We use the local density approximation + Hubbard $U$ (LDA+$U$) method to calculate the structural and electronic properties of low-spin LaCoO$_{3}$. The Hubbard $U$ is obtained by first principles and consistent with each fully-optimized atomic structure at different pressures. With structurally consistent $U$, the fully-optimized atomic structure agrees with experimental data better than the calculations with fixed or vanishing $U$. A discussion on how the Hubbard $U$ affects the electronic and atomic structure of LaCoO$_{3}$ is also given. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D30.00009: Direct observation of local magnetic properties in strain engineered lanthanum cobaltate thin films S. Park, Weida Wu, J. W. Freeland, J. X. Ma, J. Shi Strain engineered thin film devices with emergent properties have significant impacts on both technical application and material science. We studied strain-induced modification of magnetic properties (Co spin state) in epitaxially grown lanthanum cobaltate (LaCoO$_3$) thin films with a variable temperature magnetic force microscopy (VT-MFM). The real space observation confirms long range magnetic ordering on a tensile-strained film and non-magnetic low-spin configuration on a low-strained film at low temperature. Detailed study of local magnetic properties of these films under various external magnetic fields will be discussed. Our results also demonstrate that VT-MFM is a very sensitive tool to detect the nanoscale strain induced magnetic defects. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D30.00010: Understanding the Origin of Ferromagnetism in Strained LaCoO$_3$ Thin Films J.X. Ma, J. Shi, J.W. Freeland Using strain to control the behavior of strongly correlated materials offers new opportunities to control fundamental properties. For the case of magnetism, LaCoO$_3$ offers the ability to use strain through thin film growth to manipulate directly the spin-state of Co in this system. Here we present the results of a detailed polarized x-ray spectroscopy study of LaCoO$_3$ thin films grown on SrTiO$_3$(001) and LaAlO$_3$ (001) substrates. X-ray diffraction from 25 nm thin films confirm the films are fully strained in both cases and, for films under tensile strain, total moment magnetometry shows a clear transition to ferromagnetic state at $\sim$80K. X-ray absorption shows that the films grown from a LaCoO$_3$ target are slightly hole doped due to non-stoichiometry generated during growth (effective doping $\sim$ 0.1 holes per unit cell), which in the bulk is sufficient to destroy the low-spin state. However, even though the films are slightly hole doped, the films under tensile strain show long range ferromagnetic order unlike the bulk system. Since the films are insulating, these results are consistent with a ferromagnetic insulating state arising due to superexchange. Work at UCR is supported by ONR/DMEA under award H94003-08-2-0803. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D30.00011: Structure, Magnetism, and Transport in SrTiO$_{3}$(001) / La$_{1-x}$Sr$_{x}$CoO$_{3}$: Evidence for Interfacial Magnetic Phase Separation M.A. Torija, M. Sharma, C. He, J. Gazquez, M. Varela, M. Laver, B.B. Maranville, J.A. Borchers, C. Leighton Doped cobaltites have proven to be excellent choices for the study of the magneto-electronic phase separation phenomenon. Strong motivation exists for the study of these materials in films and heterostructures, the effect of dimensional confinement on this phase separation being a prime example. We investigated the structure, magnetism, and magnetotransport, in epitaxial La$_{1-x}$Sr$_{x}$CoO$_{3}$ on SrTiO$_{3}$ (001). We have observed deterioration in ferromagnetism and conductivity in the thin film limit (e.g. $<$ 8 nm at x = 0.50). We demonstrate that this can be definitively ascribed to interfacial magnetoelectronic phase separation. Key observations are the existence of an intercluster ``GMR'', anomalous multiterminal transport, strongly non-gaussian resistance fluctuations, and direct measurement of short-range ferromagnetic order by SANS. The thickness of the phase-separated region diverges as the doping is reduced from x = 0.50 to x = 0.18, and it can also be induced by deposition of SrTiO$_{3}$ overlayers. STEM/EELS data rule out the possibility of chemical phase separation proving that the deterioration in magnetic and electronic properties near the interface with SrTiO$_{3}$ is due to an intrinsic magnetic phase separation effect. [Supported by NSF and DOE]. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D30.00012: Electronic structure of La$_{1-x}$Sr$_{x}$CoO$_{3-\delta }$ in the presence of ordered oxygen vacancies Jaume Gazquez, M. Varela, M.P. Oxley, W. Luo, S.T. Pantelides, M.A. Torija, M. Sharma, C. Leighton, S.J. Pennycook Here we present a study of oxygen vacancy ordering in La$_{1-x}$Sr$_{x}$CoO$_{3-\delta }$ (LSCO) thin films using a combination of atomic resolution Z-contrast imaging and electron energy-loss spectroscopy (EELS). Substituting Sr$^{2+}$ for La$^{3+}$ in LaCoO$_{3}$ results in either the formation of oxygen vacancies or an increase in the mean cobalt valence in order to preserve charge neutrality. At large concentrations, oxygen vacancies in LSCO form ordered structures with orientations determined by epitaxial strain. This talk will show how different O $K$-edge fine structures can be observed in EEL spectra obtained from different sites of the superstructure, while the Co $L$-edges are unchanged. These results, together with density functional theory and dynamical scattering calculations, suggest that there is no charge ordering but a modulation of the hole doping in these systems. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D30.00013: Magnetic phase separation-induced coercivity enhancement in epitaxial Nd$_{0.5}$Sr$_{0.5}$CoO$_{3}$ films M. Sharma, J. Gazquez, M. Varela, C. Leighton Interfacial magneto-electronic phase separation has recently been observed in epitaxial thin films of the doped perovskite cobaltite La$_{1-x}$Sr$_{x}$CoO$_{3}$ at doping values where no such phase separation exists in bulk. Such systems also display anomalously large coercivity, which is not understood. To achieve a better understanding of this phenomenon we have extended this study to Nd$_{1-x}$Sr$_{x}$CoO$_{3}$ (x = 0.5), the perovskite cobaltite with the largest coercivity in bulk. Thin films of Nd$_{0.5}$Sr$_{0.5}$CoO$_{3}$ are grown via high pressure reactive sputtering on SrTiO$_{3}$ (001) substrates. We have observed a rapid deterioration in magnetization and onset of large intercluster-type magnetoresistance below a critical thickness of 80 {\AA}, signatures of interfacial magneto-electronic phase separation also seen in our earlier work on La$_{1-x}$Sr$_{x}$CoO$_{3}$. The temperature, angular, and thickness dependence of the coercivity ($H_{c})$ was studied using magnetoresistance. Low temperature $H_{C}$ values become very large (up to 3.6 Tesla) at low thickness, and a strong, superlinear $T$ dependence emerges. We propose that the coercivity enhancement arises due to efficient domain wall pinning by the inhomogeneous magnetically phase separated region near the SrTiO$_{3}$ substrate. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700