Session Q18: Focus Session: Interfaces in Complex Oxides - Electronic, Magnetic and Optical Properties

First principles insights into electronic, magnetic, and dynamic effects at and across oxide interfaces

The recent experimental capacity to create high quality epitaxial oxide/oxide interfaces has opened new avenues for research and provides examples of novel materials properties that emerge at the interfaces and in some cases only exist at the interfaces. Furthermore, a coherent, high-quality interface allows degrees of freedom in the two materials to be coupled to each other across the interface thereby creating artificial multi-functional materials systems. Ab initio theoretical approaches can provide key understanding of these complex systems as they can directly describe the interfacial chemical and structural effects on the electronic properties without assumptions or empirical parameters that are derived from bulk properties. Here, we will provide recent examples from our work showing how the presence and structure of the interface can modify the electronic, magnetic, or transport properties. For example, at a ferroelectric/manganite interface we see how the ferroelectric polarization couples to and strongly modifies the magnetism in the manganite. Another example involves dynamic coupling across an insulator/manganite interface where structural fluctuations in the insulator modify the conductivity in the manganite. In part, we will be focusing on the types of structural distortions present at such interfaces, how they are different from or non-existent in the bulk, and which type of distortions create uniquely interfacial phenomena.   

Digital Layered-Manganites: Madelung Energy Effects on Atomic Structure and Properties

The atomic monolayer control of molecular beam epitaxy allows one to explore non-equilibrium dopant-cation configurations in correlated oxide thin films, enabling new electronic phases and magnetic properties to emerge. We report the effects of digital A-site cation doping on electronic phase stability and competition in perovskite-derived manganites. In such digitally synthesized films, Madelung energy constraints are expected to play a primary role in minimizing local Coulomb forces. We correlate changes in the electrical and magnetic properties of the ordered manganites and the resulting crystal structures (Mn-O bond lengths and O-Mn-O bond angles) to the compositionally equivalent control films, i.e. those with randomly distributed A-site dopant-cations. DFT calculations predict that these different layering patterns produce different local and extended crystallographic distortions, including a potential multiferroicity induced by a hitherto unknown mechanism. Synchrotron surface X-ray diffraction measurements in combination with COherant Bragg Rod Analysis at the Advanced Photon Source is used to investigate the resulting atomic structure of the different layering variants, while Second Harmonic Generation and Piezoforce Microscopy investigate the ferroelectric properties.   

Reduced dimensionality and pseudogap formation in (LaMnO$_{3}$)$_{2n}$(SrMnO$_{3}$)$_{n}$ superlattices

(LaMnO$_{3}$)$_{2n}$(SrMnO$_{3}$)$_{n}$ superlattices, composed of the antiferromagnetic insulators LaMnO$_{3}$ (LMO) and SrMnO$_{3}$ (SMO), are ferromagnetic and metallic for n $<$ 3. By increasing the separation between LMO/SMO interfaces for n $\ge$ 3, the system goes through a transition from a metallic to insulating ground state whose origin remains unresolved. We present ARPES measurements of LMO$_{2n}$SMO$_{n}$ superlattices grown by MBE. The electronic structure of states near the Fermi level is similar to the random alloy La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ for small n, but as n is increased we observe the formation of a more 2D state with a preferential occupation of $x^2-y^2$ orbitals. As the system passes into the insulating state at n = 3, a pseudogap forms at the Fermi level: charge carriers are suppressed over a scale of hundreds of meV but without substantial changes to the overall bandstructure. This pseudogap begins to fill as the temperature is increased, but a large suppression in spectral weight at the Fermi level remains at room temperature. Our observations indicate that the insulating state for large-n superlattices is related to strong many-body effects within this system, enhanced by the reduced dimensionality of an interfacial two-dimensional electron liquid.   

Cross-sectional Scanning Tunneling Microscopy (XSTM) Investigation on (SrMnO$_{3})_{n}$/( LaMnO$_{3})_{n}$ Heterostructures

Recent advances in obtaining heterostructures between dissimilar transition-metal oxides with atomically sharp interfaces have revealed emerging exotic electronic and magnetic phases in the vicinity of the interface, which are qualitatively different from the parent compounds. In this study, we have grown (SrMnO$_{3})_{n}$/( LaMnO$_{3})_{n}$ heterostrcuture superlattice, with $n$ being varied from 2 to 8, by laser MBE system with the assistance of the reflection high-energy electron diffraction (RHEED) intensity oscillations on (001) SrTiO$_{3}$ single-crystal substrates. The variation of the density of states across the vicinity of the SrMnO$_{3}$-LaMnO$_{3}$ interfaces were investigated by scanning tunneling microscopy (STM) to reveal the spatial modulation of the electronic properties arising from formation of the two-dimensional electron system at the interfaces. The effects of the layer number, $n$, on this electronically-induced contrast, which is generally ascribed to result from the differences in the energy band gaps, carrier concentrations, as well as electron affinities between SrMnO$_{3 }$and LaMnO$_{3}$, will be discussed.   

Magnetoelectric coupling at the interface of manganite thin films grown on strontium titanate

Oxide heterostructures of PbZr0.2Ti0.8O3/La0.8Sr0.2MnO3/SrTiO3 (PZT/LSMO/STO) have been shown to undergo a large charge-driven magnetoelectric coupling. Switching the polarization state of the ferroelectric PZT layer induces a spin reconstruction in the top atomic layer of the magnetic LSMO layer, changing from ferromagnetic to antiferromagnetic ordering [1]. In this work, the LSMO layer is several unit cells thick. In order to enhance this magnetic switching effect, we have reduced the thickness of the active LSMO layer, replacing part of the LSMO film with LaMnO3 (LMO), a bulk antiferromagnetic insulator. To carry out this approach, we use oxide molecular beam epitaxy (MBE) to control the composition of thin films of LMO and LSMO/LMO heterostructures on SrTiO3(001) with atomic layer precision. Heterostructures grown in this way show a large deviation from the expected behavior for a simple combination of the individual components. We compare measurements of the magnetization for epitaxial LMO on SrTiO3 and heterostructures of LSMO/LMO grown on SrTiO3. Using this approach, one can optimize the properties of the ferromagnetic layer and improve the magnetoelectric switching properties of the PZT/LSMO/STO system. [1] CAF Vaz, et al. Phys. Rev. Lett. 104, 127202 (2010); doi:10.1103/PhysRevLett.104.127202   

Density-functional Study of Suppressed Magnetism at La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/SrTiO$_{3}$ Interfaces

The experimentally observed magnetism suppression at interfaces of La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ has attracted increasing attention. Here we report density-functional calculations for the interface systems of La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/SrTiO$_{3}$. Two interface models are employed to isolate and identify different effects coming from epitaxial strain, symmetry-breaking, charge redistribution, and oxygen vacancy segregation. We found that the strain effect from SrTiO$_{3}$ substrate is not significant enough to cause magnetism suppression at the interface. Although the symmetry is broken at interfaces, this effect leads only to a local ground state and does not cause the observed suppression either. The choice of interface termination does have an effect: moderate magnetism suppression is found for SrO/MnO$_{2}$ termination. Finally, we considered the effect of oxygen vacancy segregation at the interface. In the scenarios we have tested, oxygen vacancies do not suppress the interfacial magnetism. Thus, a complicated mechanism is needed to explain the suppressed magnetism at La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/SrTiO$_{3}$ interfaces.   

Optical Measurements on Magnetoelectric LSMO/PZT Bilayers

Fairly weak magnetoelectric coupling observed in the only single phase material (BeFiO$_{3})$ exhibiting magnetism and ferroelectricity at room temperature has pushed scientists to consider alternative systems. Multilayers prove promising theoretically and experimentally, however, most modern techniques are blind to the interfacial mechanisms causing the coupling. Without a full understanding of the physical mechanism for these effects, significant improvements in the design and multiple potential applications of magnetoelectric coupling will be difficult to achieve. Optical measurements including second harmonic generation are crucial tools to solve this problem, as they provide complementary insight into the magnetic/ferroelectric properties and resulting carrier dynamics. For example, angular dependence SHG of magnetic LaSrMnO$_{3}$ and ferroelectric PbZrTiO$_{3}$ bilayers indicates the symmetry and magnetization as we vary thicknesses of the magnetic and ferroelectric layers and its implication to magnetoelectric coupling.   

Mn valences in La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$ Heterostructures

Magnetoelectric (ME) coupling---the electrical control of magnetic properties or vice versa---has promising applications in computer memory and logic, magnetic sensing and energy scavenging. Understanding the coupling mechanisms in a variety of magnetoelectric material systems is an important step as it will allow us to design better magnetoelectric systems. Our group studies the interfacial properties of the known magnetoelectric system of a ferromagnetic La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) and a ferroelectric PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$ (PZT). Through photoemission electron microscopy imagining, ME coupling was confirmed at the interface. X-ray absorption spectroscopy of Mn was taken across wedged samples of varying ferroelectric and ferromagnetic thicknesses. Here, we will show the changes of Mn valences at different thicknesses of LSMO and PZT, which helps to understand ME coupling and impact of thickness on the ME properties.   

Magnetic coupling at vertical perovskite-spinel epitaxial interface

Interfaces in complex oxides have been the focus of scientists because of their intriguing and unique properties that cannot be found in bulk. Vertical nanostructure is one of the most interesting heterostructure that has been studied for interface phenomena due to its high interface-volume ratio. In this study, (La,Ca)MnO3 (LCMO) (perovskite, matrix) / CoFe2O4 (CFO) (spinel, pillars, 50-200 nm in size) vertical nanostructure has been taken as model system to investigate the interface magnetic coupling with X-ray absorption spectroscopy and photoemission electron microscopy (PEEM), taking the advantages of their element sensitivity and spatial resolution. Matrix and pillars were studied separated with the photon energy set to different absorption edges. The magnetic order and valence states as well as site occupancy in CFO pillars were characterized by XMCD measurements at Co and Fe L-edges with the application of external magnetic fields. In order to investigate the exchange coupling at the interface, we combined this XMCD study with angular dependent XMLD measurements at Fe and Mn L-edges, which give us the information of orbital order in LCMO, while CFO pillars are magnetized in different directions. Similarly, XMCD studies at the Mn L-edges provide detailed insights into the magnetic order of the LCMO matrix, the Mn valence state and elucidate the impact of the CFO pillars. In addition, PEEM measurement provides spatially resolved XMCD/XMLD images that give more insight of the magnetic coupling at the matrix-pillar interface.   

Transport Properties of Films of Prussian Blue Analogues and Manganites

The CoFe Prussian blue analogue (PBA) is a coordination polymer, A$_j$Co$_k$[Fe(CN)$_6$] (A = K, Rb, Cs), that exhibits a photoinduced charge transfer.\footnote{O. Sato \emph{et al}., Science \textbf{272} (1996) 704.} The resulting changes in the magnetization and in the lattice parameter have been used to successfully apply stress on other pressure-sensitive materials, achieving photocontrol of the magnetic response.\footnote{ D.M. Pajerowski \emph{et al}., J. Am. Chem. Soc. \textbf{132} (2010) 4058.} The metal-to-insulator transition (MIT) is a prominent feature of the manganites, whose transition temperature can be tuned by applying various stimuli such as pressure or magnetic field.\footnote{H. J. Jeen and A. Biswas, Phys. Rev. B \textbf{83} (2011) 064408.} By coupling a layer of PBA over a thin film of manganite, the temperature for the MIT can be altered by the light-induced changes in the CoFe PBA.   

Enhanced magnetoresistivity of modulation-doped manganite superlattices

Modulation-doped epitaxial superlattices of La$_{1-x}$Ca$_{x}$MnO$_{3}$ and La$_{1-y}$Ca$_{y}$MnO$_{3}$ layers alternating with x=1/3 and y=1/2 on (001) LaAlO$_{3}$ substrates have been grown by magnetron sputtering. The magnetoresistivity (MR) has been greatly enhanced over a wide temperature range in comparison with the parent compounds. The MR is -50{\%} at 0. 5 Tesla and -90{\%} at 4 Tesla. At low temperatures, a quantum interference effect (QIE) was observed, as manifested by a resistivity minimum considered to be incurred by the 3D e-e interactions. The modulation-doping approach is an enabling strategy to boost the sensitivity of colossal magnetoresistive oxides in response to an applied magnetic field.   

Induced magnetism at complex oxide interfaces

Modified bonding at complex oxide interfaces may be at the bottom of the appearance of interesting novel behaviours not appearing in the bulk constituents. The possibility of tailoring the electronic structure of interfaces has driven an important effort towards the design of interfaces with specific functionalities. We have examined novel interfacial magnetic states originating at the modification of the orbital occupancy resulting from the modified bonding at the interface. We discuss the effect of these low dimensional magnetic states in determining the macroscopic magnetic response and in tailoring specific functionalities of heterostructures.   

Photostriction-Magnetostriction Coupled Epitaxial Nanostructures

Extensive research on complex oxide thin films and heterostructures suggest new possibilities to create and design devices with tantalizing functionalities by taking the advantages of the interplay between lattice, charge, orbital, and spin degrees of freedom. Recently, the self-assemble vertical nanostructures have drawn considerable attentions due to the strong coupling mediated by high interface-to-volume ratio to tailor the properties of oxide nanostructures. However, most studies have been stressing on the controllability of heterostructues through external electric or magnetic fields. In this study, we have synthesized SrRuO3 (SRO)-CoFe2O4 (CFO) nanostructures to introduce light as other external control parameter, where the light-controlled properties are enabled by ultrafast photostriction of SRO and the magnetostriction of CFO. Through a combination of ultrafast-optics, magnetic force microscopy, and soft Xray absorption spectroscopy, the coupling between SRO and CFO is clearly revealed. When illuminating CFO-SRO nanostructures, SRO matrix inflates its volume via expanding its c-axis; the elongated SRO matrix relaxes the out-of-plane compressive strain in CFO pillars effectively reduces the magnetic anisotropy thereof; the reduce magnetic anisotropy resets the preferred magnetization direction of CFO pillars; after removing the illumination, the magnetic anisotropy is re-installed and CFO pillars choose to reverse their magnetization. Our study paves a way to ultrafast optical-coupled functionalities.