1,720,962 research outputs found
In situ TEM analysis of resistive switching in manganite based thin-film heterostructures
No full textThe mechanism of the electric-pulse induced resistance change effect in Au/Pr0.65Ca0.35MnO3/SrTi0.99Nb0.01O3 thin-film samples is studied by means of in situ electrical stimulation inside a transmission electron microscope. A detailed equivalent-circuit model analysis of the measured current voltage characteristics provides crucial information for the proper interpretation of the microscopy results. The electrical transport data of the electron-transparent samples used for the in situ investigations is verified by comparison to measurements of unpatterned thin-film samples. We find comprehensive evidence for electrochemical oxygen vacancy migration affecting the potential barrier of the pn junction between Pr0.65Ca0.35MnO3 and SrTi0.99Nb0.01O3 as well as the resistance of the manganite bulk. The high-resistance state formation in the Pr0.65Ca0.35MnO3 bulk is frequently accompanied by structural transformations, namely detwinning and superstructure formation, most likely as the result of the joint impact of dynamic charge inhomogenities by oxygen vacancy migration and injection of high carrier densities at the electrodes.Deutsche Forschungsgemeinschaft (DFG) [JO 348/10-1, SFB1073
Role of oxygen vacancies for resistive switching in noble metal sandwiched Pr<sub>0.67</sub>Ca<sub>0.33</sub>MnO<sub>3-δ</sub>
No full textNon-volatile resistance change under electric stimulation in oxides is a promising path to next generation memory devices. However, the underlying mechanisms are still not fully understood. We report here on the study of switching in Pr0.67Ca0.33MnO3-delta (PCMO) films sandwiched by noble metal Pt electrodes, where electrode oxidation can be excluded. In order to develop an understanding of the switching induced oxygen migration, its initial concentration is modified by post-annealing of the deposited PCMO films. The oxygen distribution is obtained by manganese valence determination using spatially resolved electron energy loss spectroscopy in scanning transmission electron microscopy mode. We observe correlations between virgin state resistance, resistive switching properties, oxygen vacancy distribution, and stress/strain state of the PCMO films and propose a simplified interface resistance model based on the measured valence distribution. It assumes a linear correlation of oxygen vacancy concentration with conductivity and a metal to insulator transition above a critical vacancy concentration threshold. Our results suggest that resistance changes can take place at both interfaces of symmetric devices and only requires small changes in oxygen vacancy concentration. (C) 2017 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).DFG [Jo 348/10-01
Polarity reversal in bipolar resistive switching in Pr<sub>0.7</sub>Ca<sub>0.3</sub>MnO<sub>3</sub> noble metal sandwich structures
No full textThe electrically induced persistent resistance change in perovskite Pr0.7Ca0.3MnO3 films sandwiched by metallic electrodes is analyzed with respect to noble electrode materials (Pt, Au, and Ag) and geometric arrangement by electrical transport measurements. Comparing switching behavior in symmetric and asymmetric electrode interfaces gives evidence for identifying the active, single interface in the switching process. The interaction of two opposing interfaces can lead to an observed switching polarity inversion in different current density regimes in the otherwise well defined bipolar behavior. The different noble metals exhibit a quite similar switching behavior, but a lower interfacial resistance seems to favor switching. (C) 2011 American Institute of Physics. [doi:10.1063/1.3610429
Developing an in situ environmental TEM set up for investigations of resistive switching mechanisms in Pt-Pr1-xCaxMnO3-δ-Pt sandwich structures
No full textPulse length and amplitude dependent resistive switching mechanisms in Pt-Pr0.67Ca0.33MnO3-Pt sandwich structures
No full textWe report here on the presence of two different nonvolatile resistive switching mechanisms in Pt-Pr0.67Ca0.33MnO3-Pt sandwich structures based on pulsed electrical transport measurements. As a function of pulse length, amplitude and temperature, the devices show two different switching regimes. The first is positive switching (PS) where a high resistance state (HRS) evolves at positive bias at the top electrode in the voltage range of U approximate to 0.5-1.2 V and pulse lengths t(p) approximate to 10(-7) s. In addition, we observe a cross over to negative switching (NS) for U > 1 V and t(p) approximate to 10(-3) s. Here, the HRS evolves at negative bias applied at the top electrode. We present strong evidence that both switching mechanisms take place at the interface between Pr0.67Ca0.33MnO3 and the top electrode. Based on finite element simulations of the temperature evolution during the electrical pulses, we show that the onset of Joule heating is characteristic of the PS regime, whereas drastic temperature increases of several hundred Kelvin evolve during NS. Based on the observed different timescales, pulse amplitudes and temperature dependences of PS and NS, respectively, we suggest that two different switching mechanisms are involved: a fast, short range exchange of oxygen at the interface with the metallic electrode for PS and a slower, long range redistribution of oxygen in the entire PCMO film for the NS.Open Access Publikationsfonds 201
Strain Driven Phase Decomposition in Ion-Beam Sputtered Pr1-XCaXMnO3 Films
Full text linkThe deposition of heteroepitaxial thin films on single crystalline substrates by means of physical deposition methods is commonly accompanied by mechanical strain due to lattice mismatch and defect generation. Here we present a detailed analysis of the influence of strain on the Mn solubility of Pr1-xCaXMnO3 thin films prepared by ion-beam sputtering. Combining results from X-ray diffraction, transmission electron microscopy and in situ hot-stage stress measurements, we give strong evidence that large tensile strain during deposition limits the Mn solubility range of the Perovskite phase to near-stoichiometric composition. Mn excess gives rise to MnO2. precipitates and the precipitation seems to represent a stress relaxation path. With respect to size and density of the precipitates, the relaxation process can be affected by the choice of substrate and the deposition parameters, that is, the deposition temperature and the used sputter gas.DFG [SFB 1073
Magnetization dynamics in optically excited nanostructured nickel films
No full textIn this work, the laser-induced magnetization dynamics of nanostructured nickel films is investigated. The influence of the nanosize is discussed considering the timescale of hundreds of femtoseconds as well as the GHz regime. While no nanosize effect is observed on the short timescale, the excited magnetic mode in the GHz regime can be identified by comparison with micromagnetic simulations. The thickness dependence reveals insight into the dipole interaction between single nickel structures. Also, transient reflectivity changes are discussed.Deutsche Forschungsgemeinschaf
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Interplay of cross-plane polaronic transport and resistive switching in Pt-Pr0.67Ca0.33MnO3-Pt heterostructures
No full textThe identification of the cross-plane electric transport mechanisms in different resistance states of metal-oxide sandwich structures is essential for gaining insights into the mechanisms of resistive switching (RS). Here, we present a systematic study of cross-plane electric transport properties of Pr0.67Ca0.33MnO3 (PCMO) thin films sandwiched by precious Pt metal electrodes. We observe three different transport regimes: ohmic, nonlinear and RS. The nonlinear regime is associated with colossal magneto-resistance (CMR) and colossal electro-resistance (CER) effects. In contrast to RS, the CMR and CER are volatile resistance effects which persist only during application of strong magnetic or electric fields and they are restricted to low temperatures. At low current densities, the device resistance is dominated by small polaron hopping transport of the PCMO film. At higher electric current densities near the switching threshold, the interface resistance starts to dominate and remarkably also exhibits thermally activated transport properties. Our studies also shed light onto the interplay of colossal resistance effects and RS: at low temperatures, RS can be only induced by reduction of the PCMO resistivity through CMR and CER. This clearly demonstrates the key role of the current density for controlling the amplitude of non-volatile resistive changes. Conversely, the CMR can be used as a probe for the switching induced changes in disorder and correlations. At small switching amplitudes, we observe slight changes in polaron activation energy which can be attributed to changes at the interface. If the switching amplitude exceeds 1000% and more, the CMR effect in the device can be reversibly changed. This indicates persistent changes in electronic or lattice structure of large regions within the PCMO film.DFG [JO 348/10-1]Open-Access-Publikationsfonds 201
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