Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session H37: Focus Session: Complex Oxide Thin Films -- Manganites |
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Sponsoring Units: DMP GMAG Chair: John Freeland, Argonne National Laboratory Room: E147-E148 |
Tuesday, March 16, 2010 8:00AM - 8:12AM |
H37.00001: Anisotropic magnetoresistance in antiferromagnetic La$_{1-x}$Sr$_{x}$MnO$_{3}$ films Anand Bhattacharya, Tiffany Santos Anisotropic magnetoresistance (AMR) is commonly used to probe magnetocrystalline anisotropy in conducting ferromagnets. Here, we have measured AMR in antiferromagnetic thin films of La$_{1-x}$Sr$_{x}$MnO$_{3}$ near $x$=0.5. These epitaxial films were grown using ozone-assisted oxide MBE. They exhibit $A$-type magnetic order where the spins are aligned ferromagnetically within the planes and coupled antiferromagnetically between neighboring planes. The transport within these films is presumed to be highly two-dimensional as a result of $d_{x}2_{-y}2$ orbital occupancy. Upon cooling below the magnetic ordering (N\'{e}el) temperature, the AMR signal is found to be modulated with a periodicity of $\pi $, while at lower temperatures, a $\pi $/2 periodicity emerges. The amplitude of the $\pi $/2 modulation increases as the temperature is lowered. We shall discuss the data and possible explanations will be presented. [Preview Abstract] |
Tuesday, March 16, 2010 8:12AM - 8:24AM |
H37.00002: The micro-twinning in La$_{1-x}$Sr$_{x}$MnO$_{3}$ (LSMO) epitaxial thin films on SrTiO$_{3}$ (001) Lifeng Yin, Zheng Gai, Thomas Zac Ward, Jianxin Ma, DaLi Sun, Hangwen Guo, John D. Budai, Jian Shen We use variable-temperature scanning tunneling microscope to study the micro-twinning in La$_{1-x}$Sr$_{x}$MnO$_{3}$ (LSMO) epitaxial thin films grown on SrTiO$_{3}$ (001) substrates (STO). At room temperature, when the LSMO film thickness lager than 10 unit cell, micro-twinning patterns with long periodicity are formed, due to the shear strain between the rhombohedral LSMO film and the cubic perovskite STO substrate. Upon cooling, atomically flat regions start to appear at about 175 K and become dominant at 108 K. This morphological evolution is believed to be associated with the temperature driven structural transition of the STO substrate that leads to a shear strain relaxation in the LSMO films. During this transition, the twinned and un-twinned regions coexist and lead to large-scale spatially inhomogeneous distributions. [Preview Abstract] |
Tuesday, March 16, 2010 8:24AM - 8:36AM |
H37.00003: Interplay of strain and magnetism in manganese perovskites from first-principles calculations Alessio Filippetti, Giuseppe Colizzi, Vincenzo Fiorentini An exciting frontier for spintronic applications is the field of magnetic multilayered heterostructures capable of cleverly exploiting the coupling of strain and magnetism. A striking example is the piezoelectric/piezomagnetic heterojunction (e.g. La$_{1-x}$Sr$_{x}$MnO$_{3}$ (LSMO) grown on PZT) for which large resistance modulations have been observed. We have employed three different methodologies (GGA, GGA+$U, $and pSIC) to determine structural, electronic, and magnetic properties of LSMO at x=0.375 doping, and analyze a series of magnetic phase transitions caused by longitudinal strain applied on LSMO whose in-plane lattice constant is kept fixed at the value of several representative substrates. Overall we found LSMO quite sensitive to the applied strain, with highly anisotropic response in terms of both structural (i.e. Jahn-Teller) distortions and change in magnetization. In particular, the choice of two independent strain parameters (substrate and uniaxial strain) enables a nice tunability of the magnetic properties, which in turn, may dramatically alter the conductivity, even turning LSMO in insulator. [Preview Abstract] |
Tuesday, March 16, 2010 8:36AM - 8:48AM |
H37.00004: Two-dimensional ~La$_{2/3}$Sr$_{4/3}$MnO$_{4}$ Manganite Films Probed by Epitaxial Strain and Cation Ordering Brittany Nelson-Cheeseman, Tiffany Santos, Anand Bhattacharya Dimensionality is known to play a central role in the properties of strongly correlated systems. Here we investigate magnetism and transport in thin films of the Ruddlesden-Popper n=1 phase, La$_{1-x}$Sr$_{1+x}$MnO$_{4}$. Within this material, the~MnO$_{6}$-octahedra~form two-dimensional perovskite sheets~separated by an extra rocksalt layer. By fabricating high quality thin films with ozone-assisted molecular beam epitaxy, we study how the effects of epitaxial strain and intentional cation ordering, known as digital synthesis, influence the properties of this 2-dimensional manganite. For example, at the same Mn$^{3+}$:Mn$^{4+}$ ratio (2:1) as its fully spin-polarized 3D manganite counterpart, this two dimensional analog at x=1/3 only displays a spin glass phase below 20K in bulk.~This is believed to result from a competition between superexchange and double exchange, as well as disordered Jahn-Teller distortions.~ However, in our films we find weak ferromagnetic order up to much higher temperatures in addition to a low temperature spin glass phase. We will discuss how strain and cation order effect the presence of this weak ferromagnetism. [Preview Abstract] |
Tuesday, March 16, 2010 8:48AM - 9:00AM |
H37.00005: In-plane magnetic anisotropy and domain transition in phase separated (La$_{0.4}$Pr$_{0.6})_{0.67}$Ca$_{0.33}$MnO$_{3}$ thin films on orthorhombic NdGaO$_{3}$ substrates Hyoung Jeen Jeen, Amlan Biswas Phase separated manganites are a model system to study the interplay among crystal structure, transport and magnetism of materials, since it is believed that the free energies of the insulating and metallic phases are similar, but their crystal structures, magnetic and electronic properties are quite different. Thus, strain effects may play a critical role in determining the magnetic properties in manganite thin films. However, it is still unclear how the ferromagnetic metallic phase evolves in the temperature region where the insulator to metal transition occurs. We will present extensive studies of magnetic properties of phase separated (La$_{0.4}$Pr$_{0.6})_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LPCMO) thin films. Atomically smooth LPCMO thin films were grown on NdGaO$_{3}$ substrates by pulsed laser deposition technique. The observed change in coercive field as a function of temperature is similar to that of the coercivity as a function of grain size observed in fine ferromagnetic particles. Also, an in-plane magnetic anisotropy is observed from the magnetization hysteresis loops at different angles and temperatures. [Preview Abstract] |
Tuesday, March 16, 2010 9:00AM - 9:12AM |
H37.00006: Epitaxial-strain effect on charge- and orbital-ordered Pr0.5Ca0.5MnO3 films Daisuke Okuyama, Masao Nakamura, Yasujiro Taguchi, Yusuke Wakabayashi, Hirotake Itoh, Reiji Kumai, Hiroyuki Yamada, Takahisa Arima, Masashi Kawasaki, Yoshinori Tokura Transition from charge- and orbital-ordered (CO-OO) state to disordered state has been studied extensively in mixed valence manganites. An epitaxial strain effect from a substrate is one way to control the phases. To clarify the effect of epitaxial strain on CO-OO states, we have studied the growth-orientation dependence of epitaxial-strain effects by using thin films of CO-OO material, Pr0.5Ca0.5MnO3 (PCMO). We have fabricated PCMO thin films with two different growth orientations; one is made on a (011)-orientated substrate [(011)-film] and the other is deposited on a (001)-orientated substrate [(001)-film]. The resistivity of (011)-film shows a clear anomaly around 220 K, while that of (001)-film does not. We have performed synchrotron x-ray diffraction experiments on these films. In the (011)-film, the temperature dependence of the intensity of a superlattice reflection of (1/4 7/4 2) disappears at 220 K in accord with the anomaly in resistivity. By contrast, the reflection in the (001)-film subsists around 300 K. From these experimental results, we conclude that the transition temperature of CO-OO in PCMO film can be controlled by the growth orientation. [Preview Abstract] |
Tuesday, March 16, 2010 9:12AM - 9:24AM |
H37.00007: Mesoscopic percolation network in a manganite thin film revealed by microwave impedance microscopy Keji Lai, Worasom Kundhikanjana, Michael Kelly, Zhi-Xun Shen, Masao Nakamura, Masashi Kawasaki, Yoshi Tokura Using a microwave impedance microscope implemented under variable temperatures and magnetic fields, we discovered a pronounced percolating network with a period of 100nm in Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ thin films. The spatially resolved impedance maps vividly demonstrate the microscopic origin of the colossal magnetoresistance effect. Strikingly, the filamentary ferromagnetic metallic domains emergent from the antiferromagnetic charge/orbital-ordered insulating background align preferentially along certain crystal axes of the (110) SrTiO$_{3}$ substrate. Such mesoscopic glassy orders, partially smeared out by disorder effect, indicate that the substrate-induced anisotropic strain rather than the Coulomb interaction plays the dominant role in the phase separation. The microwave images also revealed drastically different domain structures between the zero-field-cool and field-cool processes, consistent with the macroscopic transport measurements. [Preview Abstract] |
Tuesday, March 16, 2010 9:24AM - 9:36AM |
H37.00008: Phase coexistence and collapse of charge ordering in low dimensional (La,Pr)CaMnO$_{3}$ N. Laurita, S. Chandra, N.S. Bingham, M.H. Phan, H. Srikanth, T.H. Hoang, H.D. Chinh, T.Z. Ward, J. Shen La$_{5/8}$Pr$_{3/8-x}$Ca$_{x}$MnO$_{3 }$(LPCMO) manganites exhibit a complex phase diagram due to coexisting and competing magnetic and electronic phases. Of particular interest is the charge-ordered (CO) phase that is unstable under various perturbations, such as carrier doping, strain, magnetic and electric field. Our systematic magnetic, transverse susceptibility (TS) and magnetocaloric studies on LPCMO nanocrystalline and thin film materials reveal that the long-range CO is largely suppressed and the ferromagnetic (FM) order is established in these systems. The nanocrystalline samples ($\sim $50nm in diameter) were prepared by sol-gel method, while the thin films ($\sim $70 nm in thickness) were epitaxially grown on LaAlO$_{3}$ and SrLaGaO$_{4}$ substrates. The magnetic anisotropy of the FM phase and the metamagnetic transition are probed by TS experiments. These results point to the importance of the particle-size and strain effects on the CO and FM states in low-dimensional LPCMO systems. [Preview Abstract] |
Tuesday, March 16, 2010 9:36AM - 9:48AM |
H37.00009: Competing Dielectric Phases and Detailed Balance in Thin Film Manganites Patrick Mickel, Amlan Biswas, Arthur F. Hebard Mixed-valence manganites exhibit a rich variety of crystallographic, electronic, and magnetic phases that compete in phase-separated ground states. We present frequency-dependent measurements of the complex capacitance of thin La$_{1-y}$Pr$_{y}$Ca$_{0.33}$MnO$_{3}$ films together with an analysis that enables the simultaneous characterization of the competing paramagnetic insulating and the charge-ordered insulating phases. Resistance measurements in thin films are incapable of distinguishing these phases. Our analysis provides the individual time-scales and relative amplitudes of each phase at fixed temperatures in the range 100K-300K, allowing the determination of the activation energies and transition temperatures of each phase. Analysis of the time-scales and the relative proportions of each phase uncover a detailed balance equation describing the dynamic conversion between the phases during the polarization process. The competition of these dielectric phases is also studied as a function of external strain and magnetic field, demonstrating tunable dielectric constants and magnetic anisotropy in their transition temperatures. [Preview Abstract] |
Tuesday, March 16, 2010 9:48AM - 10:24AM |
H37.00010: Direct Imaging of Nanoscale Phase Separation in La$_{1-x}$Ca$_{x}$MnO$_{3}$ and Its Relation with Colossal Magnetoresistance Invited Speaker: Colossal magnetoresistance (CMR) that occurs at intermediate temperatures in doped manganites has been widely believed to arise from inhomogeneous phases. However, convincing evidence is missing to link the structural inhomogeneity, especially at nanoscale, to the CMR effect without direct observation. Here we employed a newly developed technique, scanning electron nanodiffraction, to map in real space the distribution of the charge-ordered (CO) phase, $\sim $3-4 nm in size, with its characteristic structure as a function of temperature in La$_{1-x}$Ca$_{x}$MnO$_{3}$. The direct observation enables us to establish that the nanoclusters are not due to chemical inhomogeneities and quantify the volume fraction of the nanoscale CO phase. Our results show that the volume fraction of the nanoscale CO phase is significant enough to contribute to the transport property through the phase transition. Correlated with the property measurements from the bulk materials, the magnetic and transport properties of the nanoscale CO phase will also be discussed to establish the possible role of the nanoscale phase in the CMR effect.$^{1,2}$ [1]. J. Tao, et al.,\textit{ Phys. Rev. Lett.}, \textbf{103}, 097202 (2009). [2]. Research was sponsored by the U.S. DOE/BES, Division of Materials Sciences and Engineering. [Preview Abstract] |
Tuesday, March 16, 2010 10:24AM - 10:36AM |
H37.00011: Studies of the intrinsic inhomogeneity and pseudogap phenomena of La0.7Ca0.3MnO3 epitaxial films using spin-polarized (SP) scanning tunneling microscopy (STM) C. R. Hughes, J. Shi, A. D. Beyer, N.-C. Yeh Spatially resolved tunneling spectra of La0.7Ca0.3MnO3 (Tc = 260K) epitaxial films are studied using STM under varying temperature, magnetic field, and the degree of spin polarization in the tunneling currents. With both regular and magnetic STM tips, conductance inhomogeneity related to the phase separation nature of manganites is observed below the Curie temperature and in zero fields, and the inhomogeneity decreases with magnetic field and above the Curie temperature. On the other hand, with magnetic tips the field-dependent tunneling spectra may be quantitatively explained by SP tunneling in a spin-valve configuration. In addition, a nearly temperature independent psuedogap ($\sim$ 0.4 eV) and a larger energy gap ($\sim$ 0.6 eV) at low temperatures are found; the former disappears upon the application of magnetic field, and the latter may be attributed to a surface ferromagnetic insulator phase because of the manifestation of spin filtering effect under SP tunneling in opposite field orientations. [Preview Abstract] |
Tuesday, March 16, 2010 10:36AM - 10:48AM |
H37.00012: Fabrication and Characterization of YBa$_{2}$Cu$_{3}$O$_{7}$ and La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ Nanostructures Yi-Tang Yen, John Y.T. Wei, Pai-Chia Kuo, Jessie Shiue, Maw-Kuen Wu We present a novel technique to fabricate nanostructures of superconducting YBa$_{2}$Cu$_{3}$O$_{7}$ and ferromagnetic La$_{0.67}$Ca$_{0.33}$MnO$_{3}$. The nanostructures are in the form of Aharonov-Bohm rings and ultralong wires with very high aspect ratios, intended to study quantum interference phenomena at mesoscopic length scales in these materials. The fabrication involves deep nano-patterning of SrTiO$_{3}$ substrates using focused ion beam (FIB) and epitaxial thin-film growth using pulsed laser-ablated deposition (PLD). The samples are characterized by electrical transport measurements under cryomagnetic conditions, and various high-resolution microscopies including AFM, SEM, TEM, and STM. Our fabrication technique combining FIB and PLD provides a reliable general method for nano-structuring complex oxide materials. [Preview Abstract] |
Tuesday, March 16, 2010 10:48AM - 11:00AM |
H37.00013: Investigations of low-energy excitations in half-doped Pr$_{0.5}$Sr$_{0.5}$MnO$_{3}$ thin films by the means of THz time-domain spectroscopy M. Tonouchi, D.S. Rana, K.R. Mavani, I. Kawayama, H. Murakami Epitaxial Thin films of A-type charge-ordered perovskite Pr$_{0.5}$Sr$_{0.5}$MnO$_{3}$ (PSMO) PSMO were deposited on (LaAlO$_{3})_{0.3}$(Sr$_{2}$AlTaO$_{6})_{0.7}$ along (100), (110) and (111) orientations. Terahertz (THz) time domain spectroscopy measurements were performed on all the films and their optical conductivity and dielectric constant were extracted from the complex optical spectra. THz-conductivity exhibits sharp jump for (110) and (111) films and a broader transition for (100) film at the Ne\'{e}l temperature of 150 K. These results were found to corroborate magnetization data which reveals a clear and sharp first-order at 150 K for PSMO (110) and (111) films, but a broader and smeared transition for the phase-separated (100) film. Several exotic features in the optical conductivity were observed, namely, i) thermal cycling across the phase-separated region to above charge-ordering (CO) transition for (100) films results in substantial drop in conductivity, ii) conductivity in metallic region does not obey the Drude law, thus, indicating the existence of pseudo-gap and, iii) a peak centered at $\sim $ 6meV in frequency dependent conductivity depicts the formation of charge-density waves in (110) films. [Preview Abstract] |
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