Session W13: Focus Session: Thin Films and Superlattices

Fabrication and Funcntionality of Complex Oxide Superlattices

In this talk, we focus on three-constituent magnetic oxide superlattice (`tricolor' superlattice), where the asymmetric stacking like $\cdot \cdot$ ABCABC $\cdot \cdot$ artificially breaks the space-inversion symmetry. The purpose of the study is to realize artificial polar ferromagnets. The polar/noncentrosymmetric magnets, represented by multiferroic compounds, recently attract a considerable interest, because we can expect novel phenomena, such as magneto-electric (ME) effect, or magnetization-induced second harmonic generation (MSHG). In the artificial superlattice, the gigantic MSHG was first realized with a `tricolor' consisting of a ferromagnet La$_{0.6}$Sr$_{0.4}$MnO$_{3}$ and band insulators LaAlO$_{3}$ (LAO) and SrTiO$_{3}$ [H. Yamada,\textit{ et al.}, APL \textbf{81}, 4793 (2002), Y. Ogawa, \textit{et al.}, PRL \textbf{90}, 217403 (2003)]. From the temperature dependence of MSHG in this superlattice, we found that the MSHG originate from the interface magnetism. By utilizing the MSHG as a probe for interface magnetism, we optimized the various oxide interfaces, leading to the discoveries of huge tunneling magnetoresistance in a junction with engineered interfaces [H. Yamada \textit{et al.}, Science \textbf{305}, 646 (2004)], or enhanced optical ME effect in the `tricolor' superlattice composed of LaMnO$_{3}$, SrMnO$_{3}$ and LAO. In those functionalities, crucial roles are played by the interface effects characteristic of correlated electron oxides, such as charge transfer or orbital-state-mediated magnetism [H. Yamada, et al., APL\textbf{89}, 052506 (2006)].   

Physical properties of epitaxial n-type La$_{0.7}$Ce$_{0.3}$MnO$_{3}$ films.

Electronic doped Colossal Magnetoresistance (CMR) Materials have been long for spintronic applications. Since the n-type CMR material can exist only in a metastable state rather than in a thermodynamic equilibrium state [1], \textit{in situ} growth of epitaxial films is regarded as an efficient way for forming single n-type CMR films. In this study, La$_{0.7}$Ce$_{0.3}$MnO$_{3}$ (LCeMO) films were grown on LaAlO$_{3}$ and SrTiO$_{3}$ substrates at various growth conditions. Earlier reports of LCeMO films exhibits a single T$_{p}$, the metal-insulator like transition temperature, at 250K. It is found in our studies, all films shows higher T$_{p}$ and T$_{C}$ between 260 and 300K. Hall measurements indicate, only specific growth conditions within a narrow growth window can approach to the metastable state and results in n-type LCeMO films. Because the Ce and La are next to each other in the periodic table, it is not easy to identify the composition of films by either the energy dispersion spectrum or the Rutherford backscattering spectrum. We are unable to tell the exact compositions of our films. The phase separation that usually occurred when the films approaches to a thermodynamic equilibrium state does not happen for our films. The detail of our results will be reported in the presentation. Reference [1]: H. Chou, C. B. Wu, S. G. Hsu, and C. Y. Wu, Phys. Rev. B \textbf{74}, 174405 (2006).   

Influence of $B$-site cationic ordering on the magnetic properties of La$_{2}$NiMnO$_{6}$ epitaxial thin films.

Monoclinic (0 0 1)-oriented La$_{2}$NiMnO$_{6}$ thin films were grown on (0 0 1) SrTiO$_{3}$ and (0 0 1) LAST substrates by pulsed-laser deposition. The crystal structures, magnetic properties, and the Raman spectrum have been studied for films with different growth conditions. The magnetic properties were found to be very sensitive to the growth conditions and to the substrate. Analysis of the magnetization, x-ray diffraction, and the Raman spectroscopy measurements demonstrate that the $B$-site cationic ordering, which is sensitive to both the growth conditions and the lattice mismatch with the substrate, affects the magnetic properties.   

Structural and Magnetic Characterization of Fe-doped La$_{2/3}$Ca$_{1/3}$MnO$_{3}$ Films

We have investigated pure and $^{57}$Fe-doped La$_{2/3}$Ca$_{1/3}$MnO$_{3}$ thin films, which were prepared via high O$_{2}$-pressure (500~mTorr) by magnetron DC sputtering on (100)~LaAlO$_{3}$, (100)~SrTiO$_{3}$ and (100)~MgO substrates. The $^{57}$Fe-doped samples contained 1\% and 3\% $^{57}$Fe per Mn. The structural and magnetic properties of the films and targets were characterized using X-ray diffraction(XRD) and reflectivity, M\"{o}ssbauer spectroscopy and magnetometry measurements. XRD shows that films are single phase and epitaxially oriented, and have negligible structural changes upon $^{57}$Fe-doping. The M\"{o}ssbauer spectra measured at room temperature exhibit one doublet with an isomer shift of 0.320$\pm$0.003~mms$^{-1}$, indicating the presence of the Fe$^{3+}$ ion at room temperature in the sample, which is a typical value of the high-spin of Fe$^{3+}$ with octahedral coordination. The quadrupole splitting value was 0.210$\pm$0.006~mms$^{-1}$. This clearly indicates that Fe is incorporated into the structure by substituting Mn. We will furthermore discuss the influence of $^{57}$Fe-doping on magnetic and magnetotransport properties.   

Electrical Transport and Magnetic Behavior of Pr$_{0.5}$Ca$_{0.5}$MnO$_{3}$ Film Grown by Chemical Solution Deposition Technique

Rare-earth manganites {\{}R$_{1-x}$A$_{x}$MnO$_{3}$ (R = rare earth, e.g. La, Pr, A = alkaline earth metal, e. g. Ca, Sr){\}} have attracted much interest because of their rich phenomena like colossal magnetoresistance and charge ordering (CO). For certain values of $x$, close to 0.5, these compounds undergo a first-order CO transition. The CO state is characterized by the real space ordering of Mn$^{3+}$/Mn$^{4+}$ in the mixed valent manganite. The CO in bulk Pr$_{1-x}$Ca$_{x}$MnO$_{3}$ has been widely studied, however, there are very limited studies on thin films of this material. In this work, we have systematically studied the magnetic and electrical behaviors of the Pr$_{0.5}$Ca$_{0.5}$MnO$_{3}$ (PCMO) thin films grown by chemical solution deposition technique on LaAlO$_{3}$ substrates. The ground state in PCMO was found to be a charge-ordered antiferromagnetic insulator. The CO transition at 235 K was observed. With the application of magnetic field the resistivity of the film decreased at low temperatures. Detailed magnetic and electrical properties of these films will be presented.   

Characterization of Anisotropy in Manganite (LPCMO) Thin Films

Resistance measurements on thin films of strongly correlated electronic materials that have anisotropic properties due to atomic layering and/or substrate induced strain are primarily sensitive to in-plane conduction paths and therefore fail to capture any information about perpendicular transport. We present an experimental technique in which the films under investigation, pulse laser deposited (La$_{1-y}$Pr$_{y})_{5/8}$Ca$_{3/8}$MnO$_{3}$ (LPCMO) with thicknesses in the range 300-900 {\AA}, comprise the base electrodes of trilayer capacitor structures, thus allowing the simultaneous characterization of dc transport (resistance) in the parallel direction and ac transport (capacitance) in the perpendicular direction. For a given film, we find two distinct direction-dependent insulator-metal percolation transitions reflecting the competition between insulating and ferromagnetic metallic phases. With increasing thickness, the temperature difference between these transitions decreases. This decrease occurs because the presence of a strain-stabilized ferromagnetic metal phase at the LPCMO/substrate (NdGaO$_{3})$ has less of an effect on transport as the thickness increases and the LPCMO manifests isotropic bulk behavior.   

Plasmon Enhancement of Photoinduced Resistivity Changes in Bi$_{1-x}$Ca$_{x}$MnO$_{3 }$ Thin Films

Doped rare-earth manganese oxides (manganites) exhibit a wide variety of physical phenomena due to complex interplay of electronic, magnetic, orbital, and structural degrees of freedom and their sensitivity to external fields. A photoinduced insulator to conductor transition in charge-ordered 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 resistivity changes associated with melting of the charge ordering by visible light [1]. We have found a considerable increase of the photoinduced resistivity changes in the Bi$_{0.4}$Ca$_{0.6}$MnO$_{3}$ thin film after depositing metal nanoparticles on the surface. This increase can be explained by enhancement of local electromagnetic field in the vicinity of the gold nanoparticle due to the plasmon resonance. The changes in lifetime of the photoinduced state will be reported, and the possible origin of these effects will be discussed. [1] V. N. Smolyaninova at al., Appl. Phys. Lett. 86, 071922 (2005).   

Colossal Magneto Resistive (CMR) Bolometric Sensors as a Total Energy Monitor for the Linac Coherent Light Source (LCLS) Free Electron Laser (FEL)

We are developing a CMR based bolometric x-ray detector as a total energy monitor for the LCLS FEL to be built at the Stanford Linear Accelerator Center. The FEL will produce $\sim $200 femtosecond pulses in the energy range 0.8 to 8 keV with 10$^{12}$ photons per pulse. The bolometer is designed to measure the total energy of each laser pulse with repeatability below 1{\%} and an absolute accuracy below 10{\%}. The detector is fabricated using epitaxial thin films of Nd$_{0.67}$Sr$_{0.33}$MnO$_{3}$ grown on Si by Pulsed Laser Deposition. An epitaxial buffer layer of SrTiO$_{3}$ is used as a chemical barrier and an additional template layer of Bi$_{4}$Ti$_{3}$O$_{12}$ is employed to facilitate lattice match with the Nd$_{0.67}$Sr$_{0.33}$MnO$_{3}$ layer. We have been able to obtain good quality epitaxial thin films by this approach. Prototype detectors have been fabricated photolithographically, and are operated in a pulse tube refrigerator at temperatures between 100 to 150 K. Initial tests with a thermal heater pulse show that the film properties are sufficient for detector functionality. We will discuss the details of material optimization, characteristics of the sensor material such as the temperature coefficient of resistance and 1/f noise, and projections of the detector response under photon illumination.   

Observing Metal-Insulator Transitions in Spatially-Confined Perovskite Manganite Thin Films

Transition metal oxides (TMO) exhibit a strong spin-charge-lattice interaction that can lead to electronic phase separation (PS). This phenomenon carries a number of fascinating electronic and magnetic phases while maintaining a single crystalline structure. To better understand the nature of phase transition involving the coexistence of multiple phases, we have fabricated La$_{5/8-x}$Pr$_{x}$Ca$_{3/8}$MnO$_{3}$ (LPCMO) wires from single crystal LPCMO thin films using optical and E-beam lithographic techniques. These wires display giant and ultrasharp steps with varying temperature and magnetic field near the metal-insulator transition, which is believed to be a direct consequence of the influence of spatial confinement on percolative transport in these structures.   

Low-Electric-Field Tuned Mobile Carrier Density and Heat Conduction in SrMnO$_3$

Nominally undoped CaMnO$_3$ (CMO) and SrMnO$_3$ (SMO), both G-type antiferromagnets with orthorhombic and cubic structure, respectively, exhibit very different low-temperature thermal conductivities ($\kappa$), with $\kappa^{CMO}\gg\kappa^{SMO}$. These compounds are lightly electron doped due to oxygen vacancies, with $n\sim 10^{18}-10^{19}$~cm$^{-3}$ at room temperature. Measurements of the electrical conductivity and Hall coefficient indicate that the low-temperature mobile carrier density in SMO is larger by four orders of magnitude than that of CMO, suggesting that the disparity in $\kappa$ values reflects enhanced phonon-electron scattering in the former compound. We will report results of thermal conductivity measurements on SMO designed to test this hypothesis by using applied electric fields to vary the mobile carrier density at fixed temperature.   

Low-Electric-Field Tuned Impurity Conduction in Antiferromagnetic Manganites

Transport measurements for temperatures in the range $4.2~{\rm K}\leq T\leq 300$~K are reported for the semiconducting, antiferromagnetic manganites SrMnO$_3$ and CaMnO$_3$. At low $T$ where impurity conduction predominates, the electrical conductivity and Hall coefficient are found to be strongly electric-field dependent. For SrMnO$_3$, the mobile carrier density is continuously tunable over a range of more than three orders of magnitude in electric fields $F\leq 50$~V/cm. The conductivity and carrier density scale with field $\propto\exp(\sqrt{F})$, indicating Poole-Frenkel field-assisted ionization of bound carriers. The binding energy for SrMnO$_3$ ($\delta \sim 3.5$~meV) implies that electrons are ionized to more mobile states within the energy gap, rather than to the conduction band. This small energy scale correlates with the low-temperature onset of a small ferromagnetic moment in this compound, suggesting that bound electrons form ferromagnetic polarons. Strong electron correlation effects are suggested by the electric-field dependent Hall mobility.   

Voltage-induced Metal-Insulator Transitions in Perovskite Oxide Thin Films Doped with Strongly Correlelated Electrons

We have observed a reversible metal-insulator transition in perovskite oxide thin films that can be controlled by charge trapping pumped by a bipolar voltage bias. In the as-fabricated state, the thin film is metallic with a very low resistance comparable to that of the metallic bottom electrode, showing decreasing resistance with decreasing temperature. This metallic state switches to a high-resistance state after applying a voltage bias: such state is non-ohmic showing a negative temperature dependence of resistance. Switching at essentially the same voltage bias was observed down to 2K. The metal-insulator transition is attributed to charge trapping that disorders the energy of correlated electron states in the conduction band. By increasing the amount of charge trapped, which increases the disorder relative to the band width, increasingly more insulating states with a stronger temperature dependence of resistivity are accessed. This metal-insulator transition provides a platform to engineer new nonvolatile memory that does not require heat (as in phase transition) or dielectric breakdown (as in most other oxide resistance devices).   

Variable electron correlation in high-quality MBE- and PLD-grown SrRuO$_{3}$ thin films.

We show that systematic variations in the degree of correlation can occur within SrRuO$_{3}$ as a function of disorder/off-stoichiometry. In particular, we find that one source of disorder can be controlled in SrRuO$_{3}$ thin films by varying the deposition conditions or the deposition technique. Specifically, we clearly demonstrate that variation of vacancies on the ruthenium site gives rise to a variation in correlated behavior as seen in the photoemission spectra (XPS and UPS),. Moreover, the transport properties of our samples are clearly linked to their photoemission spectra, and independently the crystal unit cell parameters. SrRuO$_{3}$ appears to be a system where these effects can be studied in a more systematic fashion, usually not easily accessible, but we suspect that the underlying physics is generic in complex oxidesWork supported by the DoE BES and EPRI.