Session H30: Focus Session: Superconducting and Magnetic Oxide Superlattices and Films

Magnetism in Complex Oxide Heterostructures Determined with Neutron Scattering

With the creation of high quality superlattices consisting of complex oxide materials novel materials exhibiting a wide range of interesting phenomena are emerging. Due to the diverse physical properties of complex oxides, (e.g., ferromagnetism, antiferromagnetism, superconductivity), some of which can be varied by doping, the versatility in their applications is large. The physical properties in these new materials, often is tied to the behavior at the interfaces between the different components of the superlattice, and therefore requires detailed knowledge of the relationship between the chemical and electronic composition. Polarized neutron reflectometry (PNR) provides access to the depth-dependent magnitude and orientation of the magnetization and can therefore link the magnetic to the electronic and chemical properties, especially close to these interfaces. Several examples of our work will be presented, including that on La$_{0.7}$Ca$_{0.3}$MnO$_{3}$/ YBa$_{2}$Cu$_{3}$O$_{7-\delta}$/ La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ trilayers which exhibit the inverse superconducting spin switch behavior, and where suppression of the magnetization close to the interface, as well as a varying anisotropy axis have been determined [1]. Another example is work on digitally layered analogs of La$_{1-x}$Sr$_{x}$MnO$_{3}$, where PNR reveals an asymmetric distribution of the magnetization across the two components (antiferromagnetic) LaMnO$_{3}$and SrMnO$_{3}$, which has been linked to structural properties at the interfaces [2]. \\[4pt] [1] V. Pe\~{n}a, Z. Sefrioui, D. Arias, C. Leon, J. Santamaria, J. L. Martinez, S. G. E. te Velthuis, A. Hoffmann, Phys. Rev. Lett. 94 (2005) 057002. \\[0pt] [2] S. J. May, A. B. Shah, S. G. E. te Velthuis, M. R. Fitzsimmons, J. M. Zuo, X. Zhai, J. N. Eckstein, S. D. Bader, and A. Bhattacharya, Phys. Rev. B 77 (2008) 174409.   

Spatial Mapping of the Interface Orbital Reconstruction in LaCaMnO$_3$/YBa$_2$Cu$_3$O$_7$ Heterostructures

Interfaces between strongly correlated electron materials is an exciting area for exploring new phenomena as these states are altered in the proximity of the interface. In recent work, we have shown that at the interface between the ferromagnetic metal LaCaMnO$_3$ and the superconductor YBa$_2$Cu$_3$O$_7$, the electronic state of Cu undergoes an orbital reconstruction[1,2]. Here we present results using polarization-dependent resonant scattering at the Cu L edge to probe depth dependence of the orbital occupancies in the YBCO layer. By modeling the scattering in the region of the YBCO (001) Bragg peak in LCMO/YBCO heterostructures, we can work to extract a picture of the orbital occupancies in the interface region. Work at Argonne is supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357. JC is funded by U.S. DOD-ARO under Contract No. 0402-17291 [1] J. Chakhalian, J.W. Freeland, et. al. Nature Physics {\bf 2}, 244 (2006). [2] J. Chakhalian, J.W. Freeland, et. al. Science {\bf 318}, 1114 (2007).   

Electronic and Magnetic Properties of the Interface Between a High-Tc Cuprate and CMR Manganite

Atomically controlled interfaces between two materials can give rise to novel physical phenomena and functionalities. Modern synthesis methods have yielded high-quality hetero-junctions of oxide materials with competing order parameters. Orbital reconstructions and covalent bonding has been shown to be important factors in the rational design of oxide heterostructures$^{1}$. To clarify the role of superconductivity we study the interface between a high-temperature superconductor (PrY)Ba2Cu3O7 and CMR manganite La2/3Ca1/3MnO3 by resonant x-ray spectroscopy, magneto-optics and neutron reflectivity. The resulting data provide a hint of orbital changes and strong modification of magnetic structure in the heterojuction. $^{1}$J. Chakhalian et al, Science, v. 318, 1155 (2007).   

Dynamics of Proximate Order Parameters Measured by the Time-Resolved Magneto-Optical Kerr Effect in SrRuO$_{3}$ /YBa$_{2}$Cu$_{3}$O$_{7-x }$Heterostructures

The interaction between ferromagnetic and superconducting complex oxides in a heterostructure is a subject of great interest. The recent observation of ferromagnetic resonance in SrRuO$_{3}$ by the time-resolved magneto-optical Kerr effect (TRMOKE) presents a new method for insight into such a system. TRMOKE has been used to compare the temperature dependence of magnetization dynamics in SrRuO$_{3}$ grown on insulating substrates and ferromagnetic SrRuO$_{3}$/superconducting YBa$_{2}$Cu$_{3}$O$_{7-x }$heterostructures. The substantial differences between SRO grown on an insulating substrate compared to YBCO as well as the effects seen upon passing through the YBCO transition temperature are reported.   

Competition between high Tc superconductivity and ferromagnetism in oxide multilayers

Artificial multilayers offer unique opportunities for combining materials with antagonistic orders such as superconductivity and ferromagnetism. It was previously shown for multilayers of classical superconductors and ferromagnets that fascinating new quantum states can be realized by tuning the layer thicknesses or the interface properties. In my talk, I will show that multilayers of cuprate high T$_{c}$ superconductors (HTSC) and oxide-based ferromagnets provide an equally fascinating playground for studying the competition between the superconducting and ferromagnetic orders under the condition that both opponents are of comparable strength. I will present experimental results from neutron reflectometry and low energy muon spin rotation ($\mu $SR) measurements on thin film superlattices that were grown by pulsed laser deposition (PLD). These measurements establish that the interaction between superconductivity and ferromagnetism is surprisingly strong and gives rise to a number of unexpected and very unusual phenomena. In particular, our data provide evidence that a giant superconductivity-induced modulation of the vertical profile of the ferromagnetic magnetization takes place in some of these superlattices.   

Enhanced Superconductivity in Superlattices of High-$T_c$ Cuprates

We investigate the electronic properties of multilayers of strongly correlated models for cuprate superconductors using cluster dynamical mean-field techniques. We focus on combinations of underdoped and overdoped layers and find that the superconducting order parameter in the overdoped layers is enhanced by the proximity effect of the strong pairing scale originating from the underdoped layers. The enhanced order parameter can even exceed the maximum value in uniform systems. This behavior is well reproduced in slave-boson mean-field calculations which also find higher transition temperatures than in the uniform system. This work was supported by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy. A portion of this research at Oak Ridge National Laboratory's Center for Nanophase Materials Sciences was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.   

Anomalous Expansion of the Cu-Apical O Distance in Superconducting Cuprate Oxide Bilayers

Interfaces between complex oxides have received considerable attention due to the observation of fascinating quasi two- dimensional phenomena such as a high-mobility electron gas, interfacial ferromagnetism, and, recently, the observation of interfacial high-temperature superconductivity in epitaxially grown bilayers of metallic La$_{1.55}$Sr$_{0.45}$CuO$_{4}$ and insulating La$_{2}$CuO$_{4}$ on LaSrAlO$_{4}$ substrates$^{1}$. To help understand the mechanism underlying the observed interfacial superconductivity, we directly measured the 3D electron density of this epitaxial bilayer system with sub- atomic resolution using the Coherent Bragg Rod Analysis phase retrieval method$^{2}$. The Cu to apical O distance, which is believed to be a critical parameter controlling T$_{C}$, was found to expand dramatically from the substrate/film interface towards the surface. The correlation between structural features and interface transport properties will be discussed. $^{1}$A. Gozar et al., Nature, 455, 782(2008). $^{2}$Y. Yacoby et al., Phys. Rev. B, 77, 195426(2008).   

Andreev Nanoprobe of Half-Metallic Oxides Using Superconducting Cuprate Tips

Andreev reflection has been extensively used to determine the spin polarization of various ferromagnetic materials, with conventional $s$-wave superconductors as a counterelectrode. In this work, we use the high-$T_{c}$ superconductor YBa$_{2} $Cu$_{3}$O$_{7-\delta }$ (YBCO) as point-contact tips to probe the half-metallic ferromagnets CrO$_{2}$ and La$_{0.66}$Ca$_ {0.33}$MnO$_{3}$ (LCMO) in thin-film form. High-impedance point-contact junctions are made and their differential conductance spectra are measured at 4.2K. Strong suppression of the $d$-wave Andreev reflection characteristics is observed, in contrast to spectra taken on Au films, indicating strong spin polarization in the CrO$_{2}$ and LCMO films. Our point contacts are estimated to range between several nanometers to tens of nanometers in size, attesting to their ballistic and microscopic nature. These results demonstrate the feasibility of using superconducting cuprate tips as spin-sensitive nanoprobes of itinerant ferromagnets.   

Magnetic and structural properties of half-metallic Sr$_{2}$FeMoO$_{6}$ epitaxial films fabricated by ultra-high vacuum sputtering

Sr$_{2}$FeMoO$_{6}$, a double-perovskite half-metallic ferromagnet, has attracted much attention because of its high Tc of 420 K. However, the fabrication of Sr$_{2}$FeMoO$_{6}$ epitaxial films has been challenging due to impurity phases and disorder. Using ultrahigh vacuum off-axis RF sputtering with precisely controlled low-concentration H$_{2}$ in Ar, we have fabricated phase-pure Sr$_{2}$FeMoO$_{6}$ epitaxial films on SrTiO$_{3}$ (001) and (111) substrates. X-ray diffraction confirms pure phase with double perovskite ordering. The phase purity and magnetic moments are highly sensitive to the H$_{2}$ partial pressure. The optimal range for the H$_{2}$ concentration is 0.4{\%} to 0.6{\%} in Ar with 70 mTorr total pressure. The saturation magnetization of the Sr$_{2}$FeMoO$_{6}$ films grown in this range is 1.5 $\mu _{B}$ per formula unit at 5 K, which is a strong magnetization considering the epitaxial strain. Aberration-corrected HAADF TEM images reveal atomically sharp interface between Sr$_{2}$FeMoO$_{6}$ and SrTiO$_{3}$.   

Atomic structure of the polar Fe$_2$O$_3$(0001)/MgO(111) interface

We present a first-principles investigation of the stability and structural properties of layer-by-layer growth of thin films of Fe$_2$O$_3$(0001) (hematite) on polar MgO(111). The interface is ``oxide-like'', atomically abrupt, and stabilized by significant structural relaxations. The electronic and magnetic properties are found to vary as a function of hematite film thickness. In contrast to the insulating and antiferromagnetic nature of bulk hematite, the heterointerface is half-metallic and ferromagnetic. Drastic structural rearrangements of the Fe$_2$O$_3$ overlayer are observed at a critical thickness of three Fe bilayers, resulting in an effective expulsion of oxygen from the hematite film. To clarify the effect of the MgO(111) substrate polarity on the nature and growth of the Fe$_2$O$_3$ films, comparisons will be made to unsupported hematite slabs and to Fe$_2$O$_3$/Ti(0001) interfaces.   

Investigation of Electrically Driven Phase Transition in Magnetite Thin Films

Magnetite, Fe$_{3}$O$_{4}$, is an example of strongly electronically correlated system. It undergoes so called Verwey transition at T$_{V}\sim $122 K accompanied both by structural distortion and drastic decrease in electrical conductivity, i.e. metal-insulator transition. Recently, we discovered a new electrically driven phase transition in magnetite nanoparticles and thin films. We observed that a low-temperature (T below T$_{V})$ insulating state is broken upon applying an electric field, resulting in a sharp transition to the state with much higher conductivity. We report on further electrical characterization of this newly discovered state. There is a question whether this state is the same as high-temperature phase above T$_{V}$ or this is a new state of magnetite. In standard two-terminal measurement dominant contribution of contact resistance impedes intrinsic electrical properties. Thus, four-terminal configuration is necessary. Electrical and magnetoresistance properties are measured in challenging four-terminal geometry at nanoscale.