Session R17: Focus Session: Manganite Heterostructures

Interface effects in oxide heterostructures combining superconductors, ferromagnets and ferroelectrics

In oxide heterostructures, the interactions at the interfaces often yield novel physical properties, which radically differ from the individual constituents' and provide with new functionalities. Oxide perovskites offer much potential for this, because a variety of isostructural materials exist with very different ground states (superconductors, ferromagnets, ferroelectrics, etc). One interesting possibility is to locally couple one of the heterostructure constituents' sensitivity to an external stimulus (e.g. the electric field for ferroelectrics) to a physical property of the second constituent (e.g. the magnetization in a ferromagnet, or the critical temperature in a superconductor). Such local coupling can be achieved via nanoscale field-effect doping. Through this mechanism, a form of magneto-electric coupling between the local electric polarization in the ferroelectric and the local magnetic induction in the superconductor can be obtained, which allows the electrostatic manipulation of magnetic flux quanta [1]. Another interesting possibility is to intertwine the most distinctive properties from each of the heterostructure constituents. As an example of this, we show how to unite the phase-coherent dissipationless charge transport characteristic of superconductivity and the spin-polarized charge transport characteristic of ferromagnetism [2], which may open the door to novel spintronic devices [3].\\[4pt] [1] A. Crassous, R. Bernard, S. Fusil, K. Bouzehouane, D. Le Bourdais, S. Enouz-Vedrenne, J. Briatico, M. Bibes, A. Barth\'{e}l\'{e}my, and Javier E. Villegas, Phys. Rev. Lett. 107, 247002 (2011).\\[0pt] [2] C. Visani, Z. Sefrioui, J. Tornos, C. Le\'{o}n, J. Briatico, M. Bibes, A. Barth\'{e}l\'{e}my, J. Santamar\'ia and Javier E. Villegas, Nature Physics (2012), doi:10.1038/nphys2318.\\[0pt] [3] M. Eschrig, Phys. Today 64, 43 (2011).   

Influence of Interface Engineering on the Magnetization in La$_{0.67}$Sr$_{0.33}$MnO$_3$/SrTiO$_3$ Heterostructures

Rich new phenomena have been observed at the interfaces between of complex oxides with different electronic and magnetic properties. In particular electronic reconstruction may occur at epitaxial oxide interfaces because of the broken transitional symmetry, leading to new properties, some of which are in fact less desirable. At the La$_{0.67}$Sr$_{0.33}$MnO$_3$(LSMO) - SrTiO$_3$(STO) interface, it is thought electronic reconstruction, driven by the potential build-up at the interface, results in a degradation of the magnetization of LSMO. To explore this, we have studied LSMO/STO heterostructures with interfaces engineered to avoid this interfacial magnetization suppression [1]. In our case, this engineered interface refers to a La$_{0.33}$Sr$_{0.67}$O monolayer replacing a La$_{0.67}$Sr$_{0.33}$O monolayer at each interface. Depth-dependent magnetization profiles in the heterostructures, determined using polarized neutron reflectometry, show that indeed the interfacial magnetization of LSMO improves with interface engineering. [1] H. Boschker et al., Adv. Funct Mater 22, 2235 (2012).   

Strain control of electronic structure in La$_{2/3}$Sr$_{1/3}$MnO$_{3}$

Introducing biaxial strain into complex oxide thin films by epitaxial growth on lattice mismatched substrates is a powerful approach to engineering electronic and magnetic properties not attainable in bulk materials. Due to the strong many-body interactions characteristic of transition metal oxides, a microscopic understanding of the mechanisms underlying strain-driven phase transitions remains unclear. Here we utilize an integrated oxide molecular-beam epitaxy and angle-resolved photoelectron spectroscopy system to directly measure the electronic structure of colossal magnetoresistive La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ on four substrates, spanning -2.3\% to +1.6\% biaxial strain and two strain driven metal-insulator transitions. Contrary to conventional expectations of a bandwidth driven metal-insulator transition in strongly correlated systems, we find widely dispersive states in both insulating phases with finite weight at the Fermi level under compressive strain and a narrow gap under tensile strain. Our results point to two distinct mechanisms behind the metal-insulator transitions, and highlight the importance of phase coexistence and charge or orbital ordering in oxide thin films.   

Tuning out-of-plane strain in epitaxial La[1-x]Sr[x]MnO[3] thin films with noble ion implantation

Strongly correlated materials, such as cuprates, manganites, and heavy-fermions, have a wealth of exotic properties and are often associated with the coexistence of competing nearly degenerate states which couple simultaneously active degrees of freedom---charge, lattice, orbital, and spin states. To understand correlated electronic materials, we must begin to disentangle the underlying correlations and find novel methods to tune individual order parameters to recognize how mesoscopic interactions drive emergent behaviors. In this work, we will discuss recent progress on controlling the strain along the out-of-plane direction in epitaxial [LaSr]MnO3 films through implantation of noble ions. This technique allows for very fine manipulation of the lattice parameter in a manner that effectively gives us a novel means of controlling orbital overlaps without hole/carrier doping the sample. We observe that films can remain epitaxially lattice locked to the substrate while accommodating more than 1{\%} lattice expansion out-of-plane. We will present phase diagrams based on this new type of ``strain doping'' and discuss the implications. Supported by the US DOE Office of Basic Energy Sciences, Materials Sciences and Engineering Division.   

Characteristics of a Mott field-effect transistor (MottFET) based on La$_{1-x}$Sr$_{x}$MnO$_{3}$

Recently, the metal-insulator transition (MIT) phenomenon shown in transition metal oxides has attracted much interest due to its superior characteristics such as fast switching speed ($\sim$ femtoseconds), high on/off ratio, and low power consumption. One example is the MottFET, which utilizes the MIT modulated by electric field through the band-filling in a Mott insulator. In this work, we examined MottFET devices based on La$_{1-x}$Sr$_{x}$MnO$_{3}$(LSMO), which is one of the mostly studied Mott insulators and attractive for the potential application in spintronic devices due to its intriguing properties such as colossal magnetoresistance (CMR) and half-metallicity. For the devices with the composition near the boundary of the metal-insulator transition, we confirmed that the conductivity of the channel could be modulated by a gate electric field of moderate strength. In addition, for the future application in spintronic devices, we investigated the dependence of device characteristics on the magnetic field. As the applied magnetic field increased, we found that the current-voltage characteristic showed anomalous behavior, which might be attributed to the electron-electron interaction, spin ordering, and the magnetic impurities in the channel.   

Structure/property relationship for ultrathin films of La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ on SrTiO$_{3}$ (001)

Dead layer, the insulating behavior in ultrathin films of metallic oxides, is an intriguing property of TMO films. Is this intrinsic effect caused by dimensionality effect, or by interface, segregation, or stoichiometry? We have studied thickness-dependence of structure/property relationship for thin films of La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ (LSMO) grown with PLD on SrTiO$_{3}$ (001) (STO) by using in-situ characterization such as LEED and STM, and ex-situ transport measurements. By minimizing oxygen deficiency, the thickness of dead layer is found to be as small as 6 u.c., which can be characterized as the intrinsic critical thickness. Our LEED-I(V) structural refinement shows non-monotonic lattice relaxation with thickness. The distortion of the $c$-axis bond length at surface reaches its maximum value for 6 u.c. film being 19\% smaller than the bulk value. Mn is no longer at oxygen octahedron center with Mn-O-Mn bond angles between 167$^{\circ}$ and 176$^{\circ}$ varying with film thickness. Regardless of the thickness, the surface La/Sr-O layer is almost all Sr due to surface segregation. Capping with STO overlayer reduces the critical thickness of dead layer, thus suggesting that LSMO/STO interface enhances the conductivity of LSMO.   

Ultrafast conductivity dynamics in epitaxially strained La$_{1-x}$Ca$_{x}$MnO$_{3}$ thin films

La$_{1-x}$Ca$_{x}$MnO$_{3}$ is a prototype colossal magnetoresistance (CMR) material where the conductivity displays a marked sensitivity to an external magnetic field for reasons that are not fully understood. The underlying rich physics is a result of strong coupling of the spin, lattice, orbital, and charge degrees of freedom. Optical spectroscopy provides experimental access to the underlying interactions in the manganites including spin and orbital ordering and the metal-insulator transition. Ultrafast spectroscopy can dynamically probe photo-induced changes that drive phase transitions. In this work we report on time-resolved terahertz spectroscopic studies of epitaxially strained La$_{1-x}$Ca$_{x}$MnO$_{3}$ thin films. In these films, the strain results in a robust antiferromagnetic insulating phase below 260K. Following 1.5 eV short pulse excitation the THz conductivity reveals a transition to a persistent metallic phase. This response is a result of competition in a dynamic phase fluctuation regime. We will describe, in detail, the observed differences in the conductivity dynamics as a function of lattice strain.   

Magnetic behavior of La$_{2/3}$Ca$_{1/3}$MnO$_{3}$ / BaTiO$_{3}$ bilayers

We have grown ferroelectric BaTiO$_{3}$(BTO) and ferromagnetic La$_{2/3}$Ca$_{1/3}$MnO$_{3}$ (LCMO) onto (001) SrTiO$_{3}$ and Nb:SrTiO$_{3}$ by pulsed laser deposition (PLD) at pure oxygen atmosphere, and a substrate temperature of 820$^{\circ}$ C, seeking for a multiferroic behavior in this structure. From x-ray diffraction (XRD) we found lattice parameter a$_{BTO}$=4.068 {\AA}, and a$_{LCMO}$=3.804 {\AA}, for each individual layer. In the BTO/LCMO bilayer, (002)-Bragg peak for BTO maintain its position whereas (002) LCMO peak shift to lower Bragg angle indicating a strained LCMO film. Magnetization measurements reveal an increase in the Curie temperature from 170 K to 220 K for the bilayer when LCMO (t = 47 nm) is deposited on BTO (t=52 nm) film, while depositing the BTO (50 nm) above LCMO (48 nm) the Curie temperature remains at values close to that obtained for a LCMO single layer ($\sim$175 K), deposited under identical growth parameters   

Canted Antiferromagnetism in Electron-Doped CaMnO$_3$ under Epitaxial Strain

CaMnO$_3$ (CMO) is a G-type antiferromagnetic (G-AFM) insulator at low temperature. A small amount of electron doping to CMO induces electronic and magnetic state change to a weak ferromagnetic (FM) metal. The recent experiment in thin-film [1] has revealed that the metallic character by electron-doping is sensitive to the strain exerted by the substrate. In this study, we clarify the electron-doping effect for CMO with the existence of epitaxial strain from substrates, by first-principles electronic structure calculation with noncollinear version of local spin density approximation. We show that a metallic character with a weak FM component is brought by the spin-canting from the G-AFM spin alignment (cG-AFM) by the double exchange effect. The canting angle becomes larger with increase of doping-amount and $c/a$, where $c$ and $a$ represent in-plane and out-of-plane lattice constants, respectively. We also show that a magnetic state change from cG-AFM state to C-AFM one takes place by further enhancement of compressive strain. We analyze our results by comparing with the experimental results.\\[4pt] [1] P.-H. Xiang et al., Adv. Mater. 23, 5822 (2011).   

Analysis of Stoichiometry Variations in La $_{\mathrm{1-x}}$Ba$_{\mathrm{x}}$MnO $_{\mathrm{y}}$ Thin Films using Laser-Ablation Inductively Plasma Mass Spectrometry and X-ray diffraction

Structural, electrical and magnetic properties of thin films of the doped rare earth manganese oxide material s are known to change dramatically by varying the oxygen partial pressure employed during Pulsed Laser Deposition. In contrast to the commonly accepted idea that such variation is solely due to the variation of the oxygen stoichiometry of the films, we find that varying the deposition oxygen partial pressure also results in the variation of the cation stoichiometry at the rare earth site. We also find that in addition to oxygen partial pressure, laser fluence is a determining factor for the stoichiometry. We have analyzed the composition, structure and properties of La $_{\mathrm{1-x}}$ Ba$_{\mathrm{x}}$ MnO$_{\mathrm{y}}$ thin films grown under a range of oxygen pressures. Cation composition is analyzed using the Laser-ablation Inductively Coupled Plasma Mass Spectroscopy technique (LA-ICPMS). LA-ICPMS results, coupled with structural information from 4-circle X-ray diffraction, allows us to delineate oxygen content variations from cation stoichiometry variations. We will correlate the changes in stoichiometry with surface morphology, and electrical and magneto-resistive properties.   

Structural, AFM, MFM and magnetic studies of LaMnO$_3$ thin films prepared by atomic layer deposition method

Structural, microstructural and magnetic properties of the thin films of LaMnO$_3$ (LMO) have been investigated and will be presented. Thin films were deposited by atomic layer deposition (ALD) method on silicon substrates. Effects of various process parameters have been studied on LMO thin films. Single phase perovskite crystal structure was confirmed from the X-ray diffraction and Raman spectra. SEM/AFM studies confirm the uniform and high quality films grown with grains in a range of 20-100 nm, depending on preparation conditions. MFM images measured at low temperature (65K), show different magnetic domains in films annealed in N$_2$ and O$_2$ environments. Stoichiometry, microstructure and magnetic properties of films strongly depend on annealing environments; however there was no change in their crystal structure. Curie temperature in those LMO thin films annealed in N$_2$ and O$_2$ atmospheres were 200 and 250K, respectively. Enhanced Curie temperature from the ideal value ($\sim$140 K) can be related to non-stoichiometry in our LMO films.   

The effects of annealing on the infrared and optical properties of La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ films

La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO) films grown by pulsed laser deposition on lanthanum aluminate substrates undergo a phase transition from ferromagnetic metallic state to paramagnetic insulating state at T$_{\mathrm{c}}$ of about 350 K. This second-order phase transition proceeds via a phase coexistence regime over an extended temperature range. Annealing affects the strain and oxygen content in films thereby causing significant changes to the magnetic properties, electronic structure, lattice distortion, and possibly the nanoscale properties of coexisting phases. We use ellipsometry and Fourier-transform infrared spectroscopy to investigate the effects of annealing on LSMO films over a broad spectral range from ultraviolet to far infrared. We deduce the Jahn-Teller energy splitting and the Hund's coupling energy from our data on annealed and unannealed films.