Session Q37: Focus Session: Complex Oxide Thin Films -- Conductivity and Metal-Insulator Transition II

Misfit strain accommodation in epitaxial \textit{AB}O$_{3}$ perovskites: lattice distortions and lattice modulations

Transition-metal oxides exhibit variety of magnetic, electronic and structural properties due to the presence of strong electron-electron and electron-lattice correlations. For epitaxial \textit{AB}O$_{3}$ films substrate-induced biaxial stress is an effective tool to modify the electron-lattice coupling. We present a microstructural study of the lattice effects in SrRuO$_{3}$ and La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ thin films grown under different tensile and compressive stresses. Due to the symmetry constraints, the ``pseudocubic'' perovskite unit cell does not reveal the diversity of distortions and tilts of $B$O$_{6}$ octahedra which play a significant role in magnetic and electronic properties of the \textit{AB}O$_{3}$ perovskites. We show that the lattice distortions in perovskite thin films under misfit stress can be quantitatively described by assuming a lower symmetry unit cell: tetragonal, orthorhombic or monoclinic. The results demonstrate that the misfit strain modifies the degree and direction of $B$O$_{6}$ octahedra distortions and rotations via structural transitions between tetragonal and orthorhombic unit cells as well as lattice modulations. The coherently strained films exhibit stress relief mechanism that is highly anisotropic along perpendicular in-plane directions. Such anisotropic stress accommodation is believed to affect anisotropic magnetic or electronic properties.   

Optical conductivity and magnetic properties of ultrathin epitaxial LaNiO$_{3}$ under compressive and tesile strain.

The perovskite oxides RNiO$_{3,}$ (R = rare earth) exhibit a charge transfer metal-insulator (MI) transition to an antiferromagnetic state as a function of temperature and rare earth ionic radius. Bulk, stochiometric LaNiO$_{3}$ is the only member of the series to exhibit metallic conductivity at all temperatures (4-300 K). Motivated by the expectation that reduced dimensionality and strain will alter the nickelate series MI transition, we have grown ultrathin stoichiometric epitaxial films of LaNiO$_{3}$ on LaAlO$_{3 }$and (LaSr) (Al,Ta)O$_{3}$ substrates by rf magnetron sputtering. We present magnetic susceptibility and optical conductivity measurements up to 3 eV, and from 2 to 300 K on films as thin as 3 nm. Optical conductivity is measured by near normal incidence reflectivity and indicates strong thickness and strain dependent deviations from the Drude model. Low frequency reflectivity data extrapolates to measured DC conductivity which indicates a thickness and strain dependent MI transition. We compare our results to bulk measurements and to models of the ground state and transport properties of the nickelates.   

Strain-controlled electronic properties of NdNiO3 ultrathin films

Complex oxides are known for a rich diversity of tunable intriguing phases due to strong couplings among multiple degrees of freedom of $d$ electrons. Recent advances in epitaxial growth of complex oxides with atomic-scale precision have opened new possibilities to stabilize unusual phases in nanostructures. Understanding strongly correlated electron behavior in the ultrathin limit becomes fundamentally critical for building heterostructures with engineered properties. In particular, epitaxial strain is a potent control parameter which can profoundly affect the electronic structure of correlated materials. But its effect is difficult to quantify due to relaxation that may occur during the growth. To this end, we have grown perfectly strained atomic layers of NdNiO3 by PLD on a series of substrates with large variation in lattice mismatch. Our detailed study including electric and thermo-transport, XRD and XAS, shows dramatic modifications of electronic properties induced by strain and possible mechanisms will be discussed.   

Determining oxygen octahedral rotations in strained perovskite films using x-ray diffraction

While strain is known to alter the octahedral distortions and rotations in perovskite films, the details of how the local atomic structure accommodates strain are poorly understood due to the difficulty of measuring oxygen positions in thin films. Using epitaxial LaNiO$_{3}$ as a model system, we present a general strategy for determining the atomic structure of strained perovskites via x-ray diffraction. The oxygen positions have been determined by comparing the intensities of half-order Bragg peaks, arising from the two unit cell periodicity of the octahedral rotations, with calculated structure factors. The bond angles and lengths have been determined for LaNiO$_{3}$ films grown on SrTiO$_{3}$ (tensile strain) and LaAlO$_{3}$ (compressive strain), respectively, and are found to depend strongly on the strain state. These diffraction-based results are in excellent agreement with \textit{ab initio} density functional calculations.   

Epitaxial stabilization and crystal symmetry of ultra-thin heterostructures of (La,Nd)NiO3/LaAlO3

Recently, interface-controlled heterostructures containing ultra-thin layers of correlated oxides have attracted considerable attention. At the same time, novel synthesis methods based on epitaxial stabilization have enabled a route to stabilization of unusual phases of rare-earth nickelates far away from the bulk equilibrium. Specific features of atomic structure at the interface between rare-earth nickelates and LaAlO3 and SrTiO3, the role of strain and crystal geometry resulting in the formation of unusual phases of ultra-thin layers of nickelates will be discussed in detail. The experimental results have been deduced from a combination of synchrotron based XRD, RHEED, high-resolution electron diffraction and soft x-ray spectroscopy.   

Low dimensional Mott material: Transport in ultra thin epitaxial LaNiO$_{3}$

Ultrathin Mott materials, close to a metal-insulator transition, are expected to be sensitive to local bonding, coordination, strain and dimensionality. LaNiO$_{3}$ films have recently attracted interest because of theoretical predictions of antiferromagnetism and high-temperature superconductivity in superlattices. We have grown ultrathin, epitaxial LaNiO$_{3}$ on different substrates, (LaAlO$_{3})_{0.3}$(Sr$_{2}$AlTaO$_{6})_{0.7}$ (LSAT) and LaAlO$_{3}$ (LAO). High oxygen pressures were required for stoichiometric films. Atomic resolution Z-contrast imaging confirmed that all LaNiO$_{3}$ films were epitaxial and continuous down to 2.5 nm. Resistivity, magnetoresistance, Hall coefficient and mobility were measured between 2 and 300 K. The resistivity ($<$ 200 $\mu \Omega$ cm) was comparable to bulk for films down to 5 nm on LSAT and 3 nm on LAO, indicating good oxygen stoichiometry. All films showed temperature dependent Hall coefficients indicative of both electron and hole contributions. For 4 nm films on LSAT and 2.5 nm films on LAO, weak localization was observed. Films below 4 nm on LSAT (tensile stress) were strongly localized while those on LAO (compressive stress) remained metallic at thicknesses down to 2.5 nm. We will discuss these results in the context of confinement in ultrathin Mott materials.   

Optical Probe of Strong Correlations in LaNiO$_3$ Thin Films

We present an optical study of LaNiO$_3$ in the range between 10 meV and 6 eV. Thin films of varying thickness were epitaxially grown by pulsed laser deposition on LaAlO$_3$ and SrTiO$_3$ substrates. The samples were investigated using near-normal incidence reflectance and variable angle spectroscopic ellipsometry at temperatures ranging from 20 K to 298 K. Several interband transitions are evident in the optical conductivity above 1 eV, in good agreement with band structure calculations. No Drude peak is observed however, in stark contrast with theoretical works predicting a finite density of states at the Fermi energy. This experimental finding of a vanishing Drude spectral weight, compared to a finite electron kinetic energy obtained from band structure calculations, is indicative of strong electronic correlations in LaNiO$_3$. A resonance centered at 40 meV is observed, which appears to be caused by multiple, overlapping phonon modes.   

Magnetic Interactions in Strained Multiferroic EuTiO$_3$ Thin Films

Bulk EuTiO$_3$ possesses a paraelectric and antiferromagnetic ground but it has been predicted that under tensile strain the system would show spontaneous ferromagnetic and ferroelectric order[1]. Here we present a study of the magnetic interactions in thin films of EuTiO$_3$ grown on SrTiO$_3$(001) and DyScO$_3$(110) substrates by reactive molecular-beam epitaxy (MBE). Using magnetometry, x-ray magnetic circular dichroism, and magneto-capacitance, there clear evidence of ferromagnetic order under tensile strain while the unstrained films are anti-ferromagnetic. These results will be discussed in the context of the predicted behavior. Work at Argonne, including the Advanced Photon, is supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. [1] C.J. Fennie and K.M. Rabe, Phys. Rev. Lett. 97, 267602 (2006)].   

Low temperature magnetic imaging of strained multiferroic EuTiO$_3$ thin films

It has been predicted that the competition between paraelectric antiferromagnetic state and ferroelectric ferromagnetic state in perovskite EuTiO$_3$ can be tuned by epitaxial strain.\footnote{C.J. Fennie and K.M. Rabe, Phys. Rev. Lett. 97, 267602 (2006).} Tensile-strained EuTiO$_3$ thin films grown on DyScO$_3$ (110) substrate by molecular-beam epitaxy are confirmed to be ferromagnetic at low temperature by magnetometry and magneto-capacitance. Here we present magnetic imaging of EuTiO$_3$/DyScO$_3$ thin film using low temperature magnetic force microscopy (LT-MFM). Temperature dependence of MFM contrast confirms the ferromagnetic ground state. The magnetic field dependence of MFM images will be discussed in conjunction with magnetometry and magneto-capacitance measurements.   

Strain effects in spinel ferrite thin films from density functional theory calculations

Spinel ferrites CoFe$_2$O$_4$ (CFO) and NiFe$_2$O$_4$ (NFO) are insulating magnetic oxides with high magnetic ordering temperatures and large saturation magnetization which are of interest for spintronics applications and as building blocks of multiferroic heterostructures [1]. In both cases, strain-induced changes of the magnetic properties are particularly important. Here we present results of density functional theory calculations for the structural and magnetic properties of strained bulk CFO and NFO, with special emphasis on strain-induced changes in the magneto-crystalline anisotropy energy (MAE). Our results are representative for (001)-oriented thin films of CFO and NFO, grown on different lattice-mismatched substrates. We find a large and strongly strain-dependent MAE for CFO, and a significantly smaller but also strongly strain-dependent MAE for NFO. We discuss the influence of cation order within the inverse spinel structure and analyze the effect of different exchange correlation functionals on the structural and magnetic properties. [1] H. Zheng et al., Science 303, 661 (2004).   

Cobaltite thin films: $^{59}$Co NMR in epitaxial La_${0.5}$Sr$_{0.5}$CoO$_{3}$ (001)

Considerable progress has been made in investigating bulk samples of CMR transition metal oxides and attention has turned to epitaxially grown thin films whose physical properties are considerably modified compared to bulk. Strain-induced effects in cobaltite films have been shown to be important. The present work involves low temperature (T$\sim$1 K) zero field $^{59}$Co NMR measurements on stacks of eight epitaxial thin film La$_{0.5}$Sr$_{0.5}$CoO$_{3}$ (001) samples, grown on SrTiO$_{3}$ (001) (1.9\% lattice mismatch), with film thicknesses 60, 20 and 10 nm, and on an x = 0.5 polycrystalline sample for comparison. All of the film spectra show a large hyperfine field distribution with a half-height width to peak frequency ratio of 0.3 and are shifted in frequency by 50\% compared to bulk. The dramatic increase in the hyperfine field in films compared to bulk is due to changes in electronic structure linked to strain and/or oxygen defects.   

Magnetism in LaCoO$_{3}$ epitaxial thin films

Bulk LaCoO$_{3 }$(LCO) exhibits a competition between diamagnetic and paramagnetic behavior without any magnetic ordering. Recent studies of epitaxial LCO thin films surprisingly show ferromagnetism. While some have attributed this to tetragonal distortions from the substrate or to a cation or oxygen non-stoichiometry, many discrepancies and open questions still persist as to the precise mechanism underlying the magnetism. To address this we have synthesized and characterized epitaxial LCO films grown on SrTiO$_{3}$, LaAlO$_{3}$, (La,Sr)(Al,Ta)O$_{3}$, and YAlO$_{3}$ substrates inducing different strain states. The films grown in oxygen rich atmospheres are ferromagnetic with a Curie temperature of 75K-85K. The observed magnetic moment varies with strain but remains consistently below 2$\mu $B/Co ion indicative of an intermediate Co spin state. Synchrotron x-ray techniques reveal a small orbital contribution to the total Co magnetic moment.   

Orientation dependence of the interfacial magnetic phase separation in epitaxial La$_{1-x}$Sr$_{x}$CoO$_{3}$ films on SrTiO$_{3}$ and LaAlO$_{3}$ substrates

We have observed interfacial magneto-electronic phase separation in epitaxial films of La$_{1-x}$Sr$_{x}$CoO$_{3}$ (x $>$ 0.18) on SrTiO$_{3}$ (001) substrates, where no such phase separation occurs in the bulk. This magnetic phase separation was detected indirectly via reduced magnetization, insulating transport, and the presence of intercluster type GMR, and subsequently verified directly by small-angle neutron scattering. Z contrast STEM/EELS results reveal that the effect originates from a reduction in local hole doping near the interface, due to combined and subtle effects of slight Sr and O deficiency. Surprisingly, films on SrTiO$_{3}$ (110) and LaAlO$_{3}$ (001) surfaces show a large reduction in the thickness of this interfacial phase separated region. The origin of this suppression, which is important for heterostructured devices, will be discussed in light of detailed magnetotransport, Z contrast STEM/EELS and polarized neutron reflectivity measurements in different orientations.   

Pulsed-laser deposition of crystalline cobalt ferrite thin films at lower temperatures

Cobalt ferrite thin films have been proposed for various engineering applications due to their exceptional magnetic, magnetoelastic, magnetotransport, magnetooptical properties. In this research, cobalt ferrite thin films were grown on SiO$_{2}$/Si(100) substrates using pulsed-laser deposition (PLD) technique at substrate temperatures ranging from 250\r{ }C to 600\r{ }C. It has been shown in this study, that polycrystalline films with (111)-preferred orientation can be prepared at substrate temperatures as low as 250\r{ }C, as opposed to a report of optimum 600\r{ }C substrate temperature [1]. Thermal expansion mismatch between the film and substrate was found to have a substantial effect on the magnetic properties of the cobalt ferrite films, due to the large magnetoelastic coupling of cobalt ferrite. The growth of crystalline cobalt ferrite films at such low temperatures indicates the potential to use cobalt ferrite for MEMS devices and sensor applications [2] including integration with a wider range of multilayered device structures. This research was supported by the UK EPSRC (EP/D057094) and the US NSF (DMR-0402716). [1] J. Zhou et. al, \textit{Applied Surface Sciences}, 253 (2007), p. 7456. [2] J. A. Paulsen et. al., \textit{Journal of Applied Physics}, 97 (2005), p. 044502.   

Effect of strain on the magnetic domain transition in (La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$(MnO$_{3})$ thin films grown on tetragonal (001) SrLaGaO$_{4}$ substrates.

The perovskite manganese oxide (La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$(MnO$_{3})$ (LPCMO) exhibits a coexistence of a ferromagnetic metallic phase and a charge ordered insulating phase in certain temperature ranges. In LPCMO thin films, substrate induced strain modifies this phase coexistence. An SrLaGaO$_{4 }$(SLGO)$_{ }$substrate has negligible strain effects on LPCMO due to similar lattice spacing. In addition SLGO is non-magnetic, which allows clear magnetization measurements unlike LPCMO films grown on paramagnetic NdGaO$_{3}$ substrates . The films were grown using Pulsed Laser Deposition allowing for control of composition and thickness. X-ray diffraction measurements verify epitaxial growth and the chemical composition. Atomic force microscopy shows that the LPCMO forms an atomically smooth surface. The metal to insulator transition is clearly observed near 130K. SQUID measurements show that the magnetization does not saturate at 10 K in a magnetic field of 2000 Oe, and that the ferromagnetic to paramagnetic transition occurs at around 150K.