Session P9: Focus Session: Magnetic Oxide Thin Films And Heterostructures: Manganite Thin Films

Magnetic non-uniformity in (La$_{0.4}$Pr$_{0.6})_{0.67}$Ca$_{0.33}$MnO$_{3}$ films and measurement of the strain-magnetization coupling coefficient

We have characterized the non-uniformity of chemical and magnetic properties of (La$_{0.4}$Pr$_{0.6})_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LPCMO) films grown on NdGaO$_{3}$ using polarized neutron reflectometry (PNR). Our data indicate that the films exhibit coexistence of different magnetic phases as a function of depth. The variation of magnetism with depth is correlated with a variation of chemical composition with depth. Using PNR we also measured the magnetization depth profile of the LPCMO film as a function of applied bending stress. From these measurements we were able to obtain values for the coupling coefficients relating strain to the variation of the magnetization depth profile. Our results suggest that application of compressive (tensile) bending stress increases (suppresses) magnetization. We will discuss the implications of our results on the prevailing theories of the role of strain on phase separation in manganites.   

The dead layer in La0.67Sr0.33MnO3 thin films

La0.67Sr0.33MnO3 (LSMO) films have an interfacial dead layer that is attributed to either changes in the Mn valence state at the interface, a change in the Mn orbital ordering at the interface or a change in the magnetic exchange interaction due to a structural reconstruction at the interface. We studied the dead layer in LSMO films grown on SrTiO3 (STO) substrates. To directly compare the effect of the polar discontinuity on the dead layer, we removed the polar discontinuity by compositional interface engineering. We also studied films in the (110) direction. We found the presence of the dead layer in all types of films and interface configurations. The LSMO (001) samples with compositional interface engineering have the thinnest dead layer (2 nm). Using electron energy loss spectroscopy, we found no deviations in the Mn valence state at the interface. Using linear dichroism in x-ray absorption spectroscopy, we also found no deviations in the orbital ordering. Therefore, we suggest that the dead layer is caused by the structural reconstruction at the interface. An outlook towards preventing the structural reconstruction and further improving the interfacial properties of the LSMO thin films will be given.   

Angular dependence of the anomalous Hall effect in La$_{0.8}$Sr$_{0.2}$MnO$_3$ films

The anomalous Hall effect (AHE) is an intriguing magnetotransport phenomenon linked to various intrinsic and extrinsic mechanisms. While for some conductors quantitative understanding of this phenomenon has been achieved, the understanding of the AHE in the manganites is far from comprehensive. We measured the transverse resistivity ($\rho_{xy}$) of thin films of La$_{0.8}$Sr$_{0.2}$MnO$_3$ at temperatures between 5 to 200 K and magnetic fields up to 9 T as a function of the angle $\theta$ between the film normal and the magnetic field. We find that for fields above 4 T, for which the magnetization (M) is practically parallel to the magnetic field, $\rho_{xy}=A\cos\theta+B\cos(3\theta)$. The first term is attributed to the ordinary and anomalous Hall effect, and the unexpected $\cos(3\theta)$ term is attributed only to the anomalous Hall effect. We show that the angular dependence of the longitudinal resistivity, $\rho_{xx}$, and of the magnitude of M cannot explain the existence of a $\cos(3\theta)$ term. We discuss the implication of this term on the possible mechanisms of the anomalous Hall effect in this compound.   

Effect of electronic reconstruction on the superconducting properties in high $T_C$ superconducting spin valve structures

We have studied the angular dependence of the magnetoresistance (MR) and magnetization alignment in La$_{0.7}$Ca$_{0.3}$MnO$_3$ (LCMO)/YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO)/LCMO trilayers in the superconducting transition region. The top and bottom LCMOs have different easy-axis coercivities (H$_C$'s) so that the mutual alignment between the two LCMO's magnetizations is tunable with in-plane rotation of the magnetic field. When the amplitude of the applied field is between the two H$_C$'s, the MR shows a quasi-four-fold symmetry, an angular hysteresis between clockwise and anticlockwise rotations, and a unidirectional offset along the initial saturation direction. We find that the MR is not correlated with the LCMO's magnetization alignment. More interestingly, the angular dependence of the MR is understandable by the alignment between the applied magnetic field and the (exponential tail of the) induced exchange fields in YBCO, the latter of which originate from the electronic reconstruction at the LCMO/YBCO interfaces. Our results support the scenario recently proposed by Salafranca and Okamoto [Phys. Rev. Lett. \textbf{105}, 256804 (2010)], which explains the inverse superconducting spin switch effect in this system.   

STEM-EELS and theoretical analysis of the electronic structure in cuprate-manganite heterostructures

Scanning transmission electron microscopy in combination with electron energy loss spectroscopy allows sub-nanometer scale resolution mapping of the formal oxidation state of the transition metal ions in YBa$_2$Cu$_3$O$_{7-\delta}$/La$_{0.7}$Ca$_{0.3}$MnO$_3$ superlattices. The experiments show an unexpected excess of valence electrons near the interface. We compare these results with tight binding model calculations where Coulomb interactions are included within Hartree approximation. Neither the polar catastrophe mechanism nor the mismatch of chemical potentials between the two materials are sufficient to account for the observed profile. We study the effect of oxygen vacancies near the interface and find that they can explain the measured electronic structure.   

Phase Diagram of Thin Film Oxides Growth by Pulsed Laser Deposition

We present a qualitative analysis of the microscopic thermo-dynamical origin of thin film oxides growth using the pulsed laser deposition technique. A phase diagram containing different growth mechanisms has been established. By tuning growth parameters experimentally in [LaSr]MnO3/SrTiO3 system, we observe an excellent fit of thin film morphologies to our growth phase diagram. Our results offer guidance on controlling morphology, stoichiometry and crystallinity of oxides thin films.   

Domain Structures in Perovskite Oxide Superlattices

Perovskite oxides possess a wide range of technologically relevant functional properties including ferromagnetism, ferroelectricity, and superconductivity. Furthermore, the interfaces of perovskite oxides have been shown to exhibit unexpected functional properties not found in the constituent materials. These functional properties arise due to various structural and chemical changes as well as electronic and/or magnetic interactions occurring over nanometer length scales at the interfaces. In order to understand how these interfacial effects impact the ferromagnetic (FM) properties of the half metal La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO), we have examined superlattices composed of LSMO sublayers alternating with either the antiferromagnetic (AFM) insulator La$_{0.7}$Sr$_{0.3}$FeO$_{3}$ (LSFO) or the non-magnetic metal La$_{0.5}$Sr$_{0.5}$TiO$_{3}$ (LSTO). A comprehensive approach consisting of bulk magnetization, magneto-transport measurements, and scanning transmission electron microscopy as well as soft x-ray magnetic spectroscopy and microscopy has been used to fully characterize the properties of the interfaces. We find that the nature of the charge transfer across the interfaces affects the FM properties of LSMO, such that at a given sublayer thickness, the LSMO/LSTO system displays a similar Curie temperature but a higher saturation magnetization than the LSMO/LSFO system. For a specific range of sublayer thicknesses, the LSMO/LSFO system displays a unique spin-flop coupling where the FM moments and the AFM spin axis maintain a perpendicular orientation relative to one another. Through this coupling mechanism, the direction of the AFM spin axis can be reoriented with an applied magnetic field. In this talk, I will discuss how these interfacial phenomena contribute to the types of FM and AFM domain patterns observed in the individual layers in the superlattices.   

Temperature dependent optical properties of thin films of the doped manganite La$_{0.67}$Ca$_{0.33}$MnO$_{3}$

Reflectivity as a function of temperature has been measured for thin film samples of the manganite La$_{0.67 }$Ca$_{0.33 }$MnO$_{3 }$across the metal-insulator transition. The optical properties in the infrared and visible range were determined by fits to a Drude-Lorentz model, using exact formulas for the thin film optics and the measured properties of the substrate. The phonon modes were identified and verified with lattice dynamical calculations for distorted orthorhombic crystal structure of the material. The reflectance has a strong temperature dependence in the far infrared and in the region of the phonons, rising as the temperature is lowered and the film becomes metallic. In the near-infrared and visible range, there are conductivity peaks due to electronic band transition shifts to the lower frequencies with decreasing temperature. We also observe the spectral weight shift with temperature.   

Induced ferromagnetism in La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/LaFeO$_{3}$ interfaces and its role on magnetic tunnel junctions

Magnetic tunnel junctions with antiferromagnetic barriers have so far been poorly studied. We have investigated La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO)/LaFeO$_{3}$ (LFO)/LSMO magnetic tunnel junctions(MTJ) where LFO is an antiferromagnetic tunnel barrier. We examined the tunneling magnetoresistance (TMR) behavior of junctions as a function of temperature finding a 30\% maximum at 100K. If the top ferromagnetic electrode is replaced by a non-ferromagnetic metal (Au) we still observe a TMR effect, which we attribute to spin filtering. We will show that this effect is connected to the presence of an induced (ferro)magnetic moment in the nominally antiferromagnetic LFO barrier at the interface with LSMO, which was detected by XMCD measurements. Finally, we will conclude discussing the new opportunities offered by such type of interfaces to obtain large spin filtering effects.   

Magnetic studies on thin films of La$_{0.65}$Pb$_{0.35}$MnO$_{3}$

Magnetic and structural investigations on thin films of La$_{0.65}$Pb$_{0.35}$MnO$_{3}$ deposited on a LaAlO$_{3}$ substrate are reported. Transmission electron microscopy showed an almost epitaxial growth of the perovskite film, indicating fourfold symmetry for both substrate and thin film. Low Energy Electron Diffraction and Wide Angle X-Ray Scattering support transmission electron microscopy and scanning tunneling microscopy results. Magneto-optical Kerr effect data are consistent with the fourfold symmetry. Ferromagnetic Resonance experiments performed in the X band revealed a more complex structure. The angular dependence of the resonance line width, resonance line intensity, and double integral of the resonance line support a slightly distorted four-fold symmetry whereas the angular dependence of the resonance line position has a two-fold symmetry. This discrepancy was ascribed to the mismatch between the film and the substrate and it is considered as a proof of the sensitivity of ferromagnetic resonance.   

Using ultrafast optical pump-probe spectroscopy to reveal coexisting magnetic orders in epitaxial $R$MnO$_{3}$ films

Recent discoveries of spin-driven ferroelectricity in perovskite manganites, $R$MnO$_{3}$ ($R$=rare-earth ions), have attracted enormous interest in the research of multiferroics. Although extensive experimental and theoretical studies have already been done on single crystal $R$MnO$_{3}$, there are only a few reports describing the properties of $R$MnO$_{3}$ thin films. Here, we choose two typical materials in $R$MnO$_{3}$ manganites as examples: SmMnO$_{3}$ and TbMnO$_{3}$. Previously, ultrafast optical pump-probe spectroscopy has proven to be an ideal technique for unraveling the interplay between different orders in the time domain. In this work, we used this technique to study ultrafast dynamics in epitaxial SmMnO$_{3}$ and TbMnO$_{3}$ films grown on SrTiO$_{3}$ substrates. At low temperatures, we observed an extraordinarily slow rising process, with a timescale of tens of picoseconds, followed by another decay process with a relaxation time of hundreds of picoseconds. Analysis of the time constants associated with these two processes as a function of temperature reveals that antiferromagnetic, ferromagnetic, and ferroelectric orders can coexist in these materials.   

Influence of growth mode \& substrate doping on the reversed remanent magnetic configuration in La$_{0.7}$Sr$_{0.3}$MnO$_3$ films

Charge transfer effects which can occur at oxide interfaces can modify the properties of oxide thin films. In such a system, an unusual reversed orientation of the remanent magnetic state was observed recently for La$_{0.7}$Sr$_{0.3}$MnO$_3$ on Nb-doped SrTiO$_3$(001) deposited via pulsed laser deposition\footnote{J.-S. Lee \emph{et al., Phys. Rev. Lett. } {\bf 105}, 257204 (2010)}. We observe a similar effect for La$_{0.7}$Sr$_{0.3}$MnO$_3$ grown via molecular beam epitaxy, a deposition method with different growth kinetics, onto both Nb-doped and undoped SrTiO$_3$ (STO) substrates. The reversed magnetic state occurrs in both samples, and a region of slightly increased charge density was revealed with x-ray reflectivity. Intriguingly, the onset of the reversed remanent state occurred at different temperatures: $\sim$125K for the Nb:STO substrate and $\sim$240K for the undoped STO substrate. High resolution x-ray diffraction reveals a subtle relationship with the cubic-to-tetragonal structural transition of the STO substrate at $\sim$105 K. Our results point to an additional mechanism for controlling the magnetism in mixed-valence oxide films\footnote{J.-S. Lee \emph{et al., J. Phys. D: Appl. Phys}. {\bf 44}, 245002 (2011)}.   

Magnetic Exchange Between Superconducting and Ferromagnetic Oxide Layers

The origins of high temperature superconductivity and the rich phase diagrams in complex oxides are still a matter of contention that have stimulated many novel experimental studies and observations. Recently the improvement of layer by layer growth techniques of thin films has enabled investigations of both bulk and surface properties. For most common superconductors the order parameter is thought to be antagonistic to that of the exchange mechanism in ferromagnets. Accurately grown thin fllms have enabled these competing interactions to be probed experimentally. In particular, the growth of epitaxial oxide layers, with well-characterized atomically flat interfaces, consisting of superconducting layers of YBa$_2$Cu$_3$O$_7$ (YBCO) and lattice-matched ferromagnetic La$_{2/3}$Ca$_{1/3}$MnO$_3$ (LCMO) has flourished. Using XMCD we demonstrate that the known superexchange between Mn and Cu across the YBCO/LCMO is modified when an apparent critical thickness of the superconducting layer is reduced. All samples show an apparent exchange below the superconducting transition but above it is dependent on the YBCO thickness. Possible origins of this behaviour will be discussed.