Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session U23: Focus Session: Manganite Thin Films |
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Sponsoring Units: DMP GMAG Chair: Amlan Biswas, University of Florida Room: Morial Convention Center 215 |
Thursday, March 13, 2008 8:00AM - 8:12AM |
U23.00001: Thickness dependence of the exchange bias in epitaxial manganite bilayers Alexey Kobrinskii, Maria Varela, Allen Goldman A series of thin ferromagnetic/antiferromagnetic (F/AF) bilayers of doped lanthanum manganites La2/3Ca1/3MnO3 (F) and La1/3Ca2/3MnO3 (AF) have been grown by ozone-assisted molecular beam epitaxy (OAMBE). The lattice of the substrate material (001) SrTiO3 is a good match to that of the manganites. Growth by the OAMBE method results in samples with sharp interfaces, which are suitable systems to study the interfacial phenomenon of exchange bias (EB). We present STEM and high-resolution X-ray diffraction data that verify the high structural quality of the samples. We have studied EB as a function of the AF layer thickness and determined two critical values of the thickness for the onset and for the saturation of the hysteresis loop shift which is traditionally used to measure the effect. The observed dependence of EB on the AF layer thickness can be described within the original or generalized Meiklejohn-Bean model. Using this simple approach we have estimated the interfacial coupling energy and the antiferromagnetic anisotropy constant. [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U23.00002: Infrared studies of Phase Separated (La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3}$ Thin Films Naveen Margankunte, Tara Dhakal, Amlan Biswas, D.B. Tanner We report optical spectroscopy studies of phase separated (La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3}$ thin films grown on the substrate NdGaO$_{3}$. Reflectance measurements in the far and mid infrared were performed for a range of temperatures from 10 to 300 K. Particular attention was given to the narrow temperature range where the insulator-metal transition occurs. The optical constants were extracted by fitting the measured reflectance to a Drude-Lorentz dielectric function in conjunction with thin film optics and the measured properties of the substrate. Spectral weight analyzes show that the growth of low energy oscillator strength occurs well above the Curie temperature, indicating phase coexistence in the hysteresis regime seen in resistivity measurements. The optical conductivity results are contrasted with existing models for free carrier electrodynamics in manganites. [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U23.00003: Anisotropic Magnetoresistance in (La,Pr)$_{0.67}$Ca$_{0.33}$MnO$_{3}$ Films. Megumi Yamamoto, Chuhee Kwon, Anthony Davidson, Sanjay Adhikari, Rajeswari Kolagani The out-of-plane anisotropic magnetoresistance (AMR) was measured in mixed phase manganite (La,Pr)$_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LPCMO) films. Two samples with different film thicknesses ($\sim $30 nm and $\sim $150 nm) on LaAlO$_{3}$ substrate were compared for the effects of stress on AMR. The thicker sample exhibits an insulator-metal resistive transition with a hysteresis typical of LPCMO with the peak temperature of 175 K and 250 K at H = 0 T and 8 T, respectively. While the resistance of the thinner sample is too high for our system to measure (Quantum Design PPMS) below 140 K at H = 0 T, the peak temperature at 8 T is 105 K. AMR shows a sinusoidal angular dependence typical of a ferromagnet for both samples. In this talk, we will present systematic AMR measurements of the LPCMO samples. We found that the peak position of AMR depends both on film thickness and on temperature. In addition, we observed time-dependent changes in resistance at lower temperatures indicating a long relaxation time for spins. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 9:12AM |
U23.00004: Interface magnetism in complex oxide heterostructures Invited Speaker: Magnetic oxides are an important class of materials from the perspectives of fundamental physics and technological applications. Advances in growth of high quality thin films and epitaxial oxide heterostructures over the years, have led to the realization of ideal condensed matter systems in which the complex and rich physics associated with cooperative phenomena can be explored. Examples of coupled phenomena in heterostructures include exchange bias effects, magnetoelectric coupling and interplay between magnetism and superconductivity. In this talk, I will focus on three classes of oxide heterostructures --PLD-grown M-type barium hexaferrite(BaM)/barium strontium titanate(BST), CVD-grown CrO$_{2}$/Cr$_{2}$O$_{3}$ bilayers and high-pressure sputtered LCMO/YBCO films. The common theme is the magnetic coupling across the interfaces. I will demonstrate that dynamic susceptibility and kinetic inductance experiments using a sensitive tunnel-diode oscillator (TDO) are effective probes of such coupled effects. In the case of CrO$_{2}$/Cr$_{2}$O$_{3}$ and LCMO/YBCO, the interface coupling results in anomalous anisotropy, exchange bias in the former and complex interaction between the LCMO magnetism and YBCO vortex lattice in the latter. In BaM/BST heterostructures, I will discuss how interfacial coupling influences the microwave response that is both electrically and magnetically tunable. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U23.00005: Atomic resolution STM study of Perovskite Manganite Thin Films Kenji Fuchigami, Zheng Gai, T. Zac Ward, Lifeng Yin, E. Ward Plummer, Jian Shen The perovskite manganites have attracted huge interest due to their intriguing electronic inhomogeneous nature which is believed to be responsible for colossal magnetoresistance. Scanning tunneling microscope (STM) is one of the most promising techniques for studying such electronic inhomogeneity in real space. In order to investigate electronic property at the surface of non-layered perovskite manganite, we have synthesized single crystal La5/8Ca3/8MnO3 (LCMO) thin film by laser MBE technique. In-situ thin film growth enables us to obtain atomically resolved STM image which has c-2x2 superlattice unit cell. In this talk, we will discuss the electronic properties as well as lattice structures of the LCMO surfaces. This research is sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory (ORNL), managed by UT-Battelle, LLC for the US Department of Energy under Contract No. DEAC05-00OR22725. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U23.00006: Linear Crack Arrays and Resistive Anisotropy in Nd$_{0.2}$Sr$_{0.8}$Mn$O_3$ Thin Films Under Tensile Strain$^{\ast}$ Krishna Neupane, Joshua Cohn, John Neumeier The structure, morphology, and electrical properties of epitaxial $a$-axis oriented thin films of Nd$_{0.2}$Sr$_{0.8}$Mn$O_3$ are reported for thicknesses $10\leq t\leq 150$~nm. Films with $t\geq 20$~nm grown under tensile stress on NdGaO$_3$ (100) and LSAT (110) substrates develop uniform linear crack arrays (cracks running along film $c$ axis) with a crack spacing (0.3-10~$\mu$m) that decreases with increasing thickness. Films grown under compression on LaAlO$_3$(110) substrates exhibit no cracks. The room-temperature in-plane electrical resistance ratio, $\rho_b/\rho_c$, increases approximately exponentially with increasing film thickness to values of $\sim 1000$ in the thickest films studied. The temperature dependencies for $\rho_b$ and $\rho_c$ are essentially identical, suggesting that very long effective transport paths perpendicular to the cracks are responsible for enhanced values of $\rho_b$. \vskip .1in \noindent $^{\ast}$ This material is based upon work supported by the National Science Foundation under grants DMR-0072276 (Univ. Miami) and DMR-0504769 (Montana State Univ.), the Research Corporation (Univ. Miami), and the U.S. DOE Office of Basic Energy Sciences (Grant No. DE-FG-06ER46269). [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U23.00007: Effect of strain and disorder in manganite thin films Sung Hee Yun, Rajiv Misra, Ben deGlee, Jacob Tosado, Tara Dhakal, Arthur Hebard, Amlan Biswas We have studied the effect of strain and disorder on the phase separated state in thin films of the manganite (La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3}$(LPCMO, $y$ = 0.4, 0.5, 0.6) grown on (110) NdGaO$_{3}$ substrates using pulsed laser deposition. Due to the competition between the charge-ordered insulating and ferromagnetic metallic phases, thin films of LPCMO display a fluid-like phase separation (FPS) near the insulator-to-metal transition temperature. By applying direct mechanical stress on the LPCMO thin films using a three-point beam bending technique, we observed a colossal piezoresistance in the FPS state of these materials. Our observations show that a small amount of strain ($\sim $10$^{-4})$ can move the phase boundaries in the FPS state. We then modified the extrinsic disorder in the thin films by bombarding them with Ar-ions. Our measurements show a reduction of piezoresistance in the ion-bombarded samples which suggests that such extrinsic disorder can pin the phase boundaries and reduce the fluidity of the FPS state. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U23.00008: Low Temperature Magneto-transport Measurements on Multilayered Manganite Films M. Spencer, P. Bhupathi, S. H. Yun, A. Biswas, Y. Lee We have performed out-of-plane resistance measurements on a micro-fabricated multilayered manganite thin film structure down to 50 mK. The structure is composed of a 26 nm thick (La$_{0.4}$Pr$_{0.6})_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LPCMO) film on top of a 60 nm thick La$_{0.67}$Ca$_{0.33}$MnO$_{3 }$(LCMO) layer grown on an (110) NdGaO$_{3}$ (NGO) substrate. Two gold electrodes were deposited on the LPCMO layer and the exposed LPCMO layer was then etched by ion plasma etching technique. We observed an upturn in resistance below 30 K for various current excitations ranging from 0.2 to 100 $\mu $A. Based on the electric and magnetic field dependence of the resistance, we attribute the upturn to the disorder-induced static phase separation of the LPCMO thin film at low temperature. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U23.00009: The influence of growth temperature on the physical properties of La1-xSrxMnO3 thin film by rf magnetron sputtering Yonghang Pei, Jiwei Lu, Stuart Wolf Perovskite La$_{1}-$xSrxMnO$_{3}$ has been of interest for such diverse applications as a spin injector for spintronic devices and for fuel cell electrodes due to its magnetic and transport properties, namely high spin polarization and ionic conduction. In this work, we prepared La$_{1}-$xSrxMnO$_{3}$ (LSMO) thin films by rf-magnetron sputtering. LSMO films were deposited on single crystal SrTiO$_3$ (100) substrates and platinized Si wafers concurrently in the temperature range from 450 to 600 $^{\circ}$C. X-Ray diffraction (XRD) determined that the LSMO film was epitaxial on either substrate. Magnetic hystersis loops were measured at temperatures between 10 and 300K and the saturation moment was significantly improved by increasing the growth temperature. In addition, the Curie temperature of LSMO was between 150 and 250 K and was also strongly dependent on the growth temperature. We will also discuss the impact of growth temperature on the temperature dependent transport and magnetic properties of LSMO. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U23.00010: Current-induced and Photoinduced Effects in Annealed Bi$_{1-x}$Ca$_{x}$MnO$_{3}$ Thin Films Vera Smolyaninova, K. Karki, Rajeswari Kolagani, G. Yong, R. Kennedy Doped rare-earth manganese oxides exhibit a wide variety of physical phenomena. Application of magnetic field, electric field, or electromagnetic wave irradiation drastically modifies electrical conductivity and refraction index of these materials. A photoinduced and current-induced insulator to metal transition in charge-ordered (CO) manganese oxides is especially interesting from the point of view of photonic and opto-electronic device development. We have found that 50 nm Bi$_{0.4}$Ca$_{0.6}$MnO$_{3}$ thin film grown on NdCaAlO$_{4}$ substrate is very susceptible to increase of current and illumination with laser light ($\lambda \sim $ 500 nm) [1]. Application of these stimuli partially destroys CO and produces charge-disordered conductive phase. Combined application of illumination and current completely destroys the charge ordering in this material, while the application of one of these factors separately is not sufficient to produce such effect. Current-induced and photoinduced properties of the Bi$_{0.4}$Ca$_{0.6}$MnO$_{3 }$thin films grown on different substrates will also be presented and current-voltage characteristics will be discussed. [1] V. N. Smolyaninova at al., Phys. Rev. B 76, 104423 (2007). [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U23.00011: Enhancement of Photoinduced Effects in Annealed Bi$_{1-x}$Ca$_{x}$MnO$_{3 }$Thin Films. K. Karki, Rajeswari Kolagani, G. Yong, R. Kennedy, K. DeMarchi, Vera Smolyaninova Doped rare-earth manganese oxides (manganites) exhibit a rich variety of interesting physical phenomena including their sensitivity to application of magnetic, electric fields, and electromagnetic wave irradiation. A photoinduced insulator to conductor transition in charge-ordered (CO) manganites is especially interesting from the point of view of creating photonic devices. Thin films of Bi$_{0.4}$Ca$_{0.6}$MnO$_{3}$ exhibit large photoinduced effects associated with melting of the charge ordering by visible light and can support conducting and insulating phase coexistence on a submicron scale [1]. We have found significant increase of the photoinduced resistivity changes and the life time of the photoinduced conducting phase after annealing. The changes in current-induced effects in annealed films will be also reported, and the possible origin of these effects will be discussed. [1] V. N. Smolyaninova at al., Phys. Rev. B 76, 104423 (2007). [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U23.00012: Insulating Domain Walls in Mixed Phase Manganite Guneeta Singh Bhalla, S. Selcuk, T. Dhakal, A. Biswas, A.F. Hebard We present here evidence of \textit{novel }insulating domain walls (IDWs) which allow direct tunneling of spin polarized currents in thin films of the mixed phase ferromagnet (La,Pr,Ca)MnO$_{3}$. Elastic interactions in the distorted perovskite structure of (La,Pr,Ca)MnO$_{3}$ coupled with magnetostatic interactions give rise to coexisting ferromagnetic metallic and insulating regions near the Curie temperature, $T_{C}$. Well below $T_{C}$, magnetization measurements reveal that the mixed phase metal/insulator state evolves into a fully saturated ferromagnetic metallic state. However, when either the film thickness is reduced, or a thicker film is patterned into a nanometer wide bridge structure, the formation of domain structure is modified as theoretically predicted for mixed phase ferromagnets, resulting in thin IDWs separating adjacent half-metallic domains. Experimentally we observe that upon cooling below $T_{C}$, a predominantly ferromagnetic supercooled state persists where remnants of the insulating regions behave as IDWs within the bridge. Tunneling across IDWs results in metastable, temperature-independent, high-resistance plateaus over a large range of temperatures below $T_{C}$. Upon application of fields on the order of the coercive field, neighboring domains align and the IDWs are extinguished resulting in sharp, colossal resistance drops. The presence of IDWs offers rich physical insights into ferromagnetic domain formation in mixed phase systems. [Preview Abstract] |
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