Session P43: Manganite Films

Thickness dependent Mn valence in La$_{\mathrm{0.7}}$Sr$_{\mathrm{0.3}}$MnO$_{\mathrm{3}}$ thin films

The Mn valence in thin film La$_{\mathrm{0.7}}$Sr$_{\mathrm{0.3}}$MnO$_{\mathrm{3}}$ is studied as a function of film thickness in the range of 1-16u.c. Using a combination of bulk and surface sensitive x-ray absorption spectroscopy techniques, the layer-by-layer Mn valence is determined for these film thicknesses. It is found that while the bulk averaged valence hovers around its expected value of 3.3, a significant deviation occurs within several unit cells of the surface and interface, where the surface and interface valence are determined to be 4 and 2.68, respectively. These results are supported by theoretical calculations. The change in valence from the expected bulk value arises from the polar discontinuity at the film-substrate interface.   

Above room-temperature ferromagnetism in La1-xCaxMnO3 epitaxial thin films on SrTiO3(001) substrates

The colossal magnetoresistive (CMR) manganites are popular materials for spintronics applications due to their high spin polarization. Only a couple of manganites like La1-xSrxMnO3 have a Curie temperature (Tc) that is higher than room temperature. Finding methods to raise the Tc of manganites over room temperature is useful but challenging. In this work, we use the most intensively studied La1-xCaxMnO3 (LCMO) as the prototype system to demonstrate that Tc can be greatly enhanced by carefully tuning the electronic structure using doping and strain. Specifically, we grow LCMO films on SrTiO3 (001) substrates using pulsed laser deposition. Magnetic and transport measurements indicate a great enhancement of Tc over room temperature at x$=$0.2 doping. Theoretical calculations indicate that the combined effects from doping and strain give rise to a new electronic structure favoring ferromagnetism in LCMO system. Furthermore, using the La0.8Ca0.2MnO3 as ferromagnetic electrodes, we achieve finite tunneling magnetoresistance (TMR) above room temperature.   

Low spin to high spin phase transition in ultrathin film of La0.7Sr0.3MnO3/SrTiO3

Ultrathin films of manganites often exhibit dramatically different physical properties with respect to the thicker films and the constituent bulk material. Here, we have studied the spin structure of La0.7Sr0.3MnO3 (LSMO) untrathin films grown on SrTiO3 (001) substrate, which remains unresolved till date. Combing results from thickness-dependent magneto-optical Kerr effect and scanning tunneling spectroscopy measurements, we have obtained a full phase diagram of the spin structure of the LSMO ultrathin films. Specifically, the LSMO films are in a low-spin magnetic phase from 4 unit cells to 7 unit cells. Above 9 unit cells , the whole films transform into ferromagnetic metallic state. At the critical thickness of 8 unit cell, the film exhibits an interesting surface spin state that mediates the low-spin to high-spin transition.   

Giant electroresistance in strained ultrathin La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ films

We investigated the effect of an electric current on the transport properties of microstructured La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO) thin films. Pulsed laser deposition was used to grow atomically smooth thin films of LSMO on singly terminated SrTiO$_{3}$ (STO) substrates. The microstructure pattern was designed to restrict conduction either in the direction or across the unit cell steps on the atomically smooth surfaces. Previous experiments on these thin films had suggested possible phase separation due to charge ordering near the step edges. We will present evidence that this charge ordered state can be modified by an electric current leading to large electroresistance of upto 95{\%} for a 1 \textmu A current which is comparable to magnetoresistance values at 4 T. Interestingly, the electoresistance was large (about 65 {\%}) even at room temperature when the current was applied along the step directions. Our results suggest possible use of ultrathin LSMO films as resistance switching devices at room temperature.   

Anisotropy change of the magnetization-direction dependence of the density of states as a function of the electron energy in La$_{\mathrm{2/3}}$Sr$_{\mathrm{1/3}}$MnO$_{\mathrm{3}}$

Recent studies of the tunnel anisotropic magnetoresistance for single-crystal ferromagnetic materials have shown that the magnetic-field direction dependence of the density of states (DOS) is related to the electronic structure via the spin-orbit interaction and can largely change depending on the electron energy. This fact will provide us a novel way to control the magnetic anisotropy. Here, we have investigated the magnetic-field direction dependence of \textit{dI}/\textit{dV} in a tunnel diode consisting of La$_{\mathrm{2/3}}$Sr$_{\mathrm{1/3}}$MnO$_{\mathrm{3}}$ (LSMO, 40 uc)/ LaAlO$_{\mathrm{3}}$ (4 uc) grown on a Nb-doped SrTiO$_{\mathrm{3}}$(001) substrate by molecular beam epitaxy. We applied an in-plane magnetic field of 1 T and a voltage $V$ to the LSMO electrode with respect to the substrate at 4 K. At $V$ ranging from --0.2 to $+$0.4 V, \textit{dI}/\textit{dV} showed two-fold symmetries along the [100] and [110] axes in addition to a weak four-fold symmetry along the \textless 110\textgreater axes. However, when $V$ was decreased from --0.2 V to --0.4 V, these symmetries were gradually rotated by 90 degrees. This large change of the anisotropy is probably induced by the emergence of the $t_{\mathrm{2}}_{g}$ state just below the Fermi level.   

Probing non-collinear magnetism in Ca$_{1-x}$Sr$_{x}$Mn$_{7}$O$_{12}$ films by neutron scattering

CaMn$_{7}$O$_{12}$ has been reported to be a single-phase multiferroic quadruple manganite that exhibits both ferroelectricity and helical magnetism below 90 K, but presently no experimental data from bulk or thin films have demonstrated coupling between these two ordering types. Herein, we synthesized epitaxial Ca$_{1-x}$Sr$_{x}$Mn$_{7}$O$_{12}$ thin films grown by oxide molecular beam epitaxy and pulsed laser deposition. We utilized neutrons to map out the non-collinear magnetic wavevectors as a function of temperature. To verify whether this coupling is present in our thin films we performed both magnetic and electric field studies. The results highlight the scientific opportunities in using chemical pressure and strain to modify non-collinear magnetism and better understand the link between ferroelectricity and helical magnetism. \textit{This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists, Office of Science Graduate Student Research (SCGSR) program. The SCGSR program is administered by the Oak Ridge Institute for Science and Education for the DOE under contract number DE-SC0014664.}   

A Strain Mediated Ferromagnetic to Antiferromagnetic Transition in ESMO Thin Films

Strain engineering in complex oxide heterostructures has recently emerged as a valuable tool for tuning magnetic and electrical properties of thin films. The mixed-valence manganite compound Eu$_{\mathrm{0.7}}$Sr$_{\mathrm{0.3}}$MnO$_{\mathrm{3\thinspace \thinspace }}$(ESMO) is ideally suited for strain engineering, as the bulk tolerance factors places it squarely between a ferromagnetic and antiferromagnetic ground state for doping level. We find a magnetic state which is very sensitive to substrate-induced strain, ranging from a fully saturated ferromagnetic state to antiferromagnetism, as well as intermediate phase-separated states characterized by partial ferromagnetic ordering. We find conclusive evidence that the antiferromagnetic structure is G-type with an ordering above 200 K, well above the observed ferromagnetic transition temperature of the remaining films. Potential mechanisms for this behavior based on variations of the octahedral connectivity will be discussed.   

Tailoring Spin Textures in Patterned Perovskite Oxide Heterostructures

Engineered topological spin textures in magnetic materials have emerged in recent years as the building blocks for various spin-based memory devices. Examples of these magnetic configurations include magnetic skyrmions, vortices, and domain walls. In this work, we explore how the unique properties of perovskite oxide heterostructures provide additional means to control the spin textures found in patterned micromagnets. In particular, interfaces of perovskite oxides have been shown to exhibit unexpected functional properties not found in the constituent materials. These 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. The resulting spin textures originate from the delicate balance between exchange interactions, as well as shape and magnetocrystalline anisotropy energies and demonstrate the tunability of magnetic parameters in perovskite oxides through methods such as nanostructuring and local strain engineering.   

Visualization of a stable intermediate phase in photoinduced metal-to-insulator transition in manganites

First order metal-insulator transition, accounting for various intriguing phenomena, is one of the most important phase transitions in condensed matter systems. Aside from the initial and final states, i.e. the metallic and insulating phases, no stable intermediate phase has been experimentally identified in such first order phase transition, though some transient phases do exist at the ultrafast time scale. Here, using our unique low-temperature, high-field magnetic force microscopy with photoexcitation, we directly observed a stable intermediate phase emerging and mediating the photoinduced first order metal-insulator transition in manganites. This phase is characteristic of low net magnetization and high resistivity. Our observations unveil the microscopic details of the photoinduced metal-insulator transition in manganites, which may be insightful to study first order metal-insulator transition in other condensed matter systems.   

Heterogeneity in magnetic complex oxides

Heterogeneity of quantum materials on the nanoscale can result from the spontaneous formation of regions with distinct atomic, electronic and/or magnetic order, and indicates coexistence of competing quantum phases. In complex oxides, the subtle interplay of lattice, charge, orbital, and spin degrees of freedom gives rise to especially rich phase diagrams. For example, coexisting conducting and insulating phases can occur near metal-insulator transitions, colossal magnetoresistance can emerge where ferromagnetic and antiferromagnetic domains compete, and charge-ordered and superconducting regions are present simultaneously in materials exhibiting high-temperature superconductivity. Additionally, externally applied fields (electric, magnetic, or strain) or other external excitations (light or heat) can tip the energy balance towards one phase, or support heterogeneity and phase coexistence and provide the means to perturb and tailor quantum heterogeneity at the nanoscale. Engineering nanomaterials, with structural, electronic and magnetic characteristics beyond what is found in bulk materials, is possible today through the technique of thin film epitaxy, effectively a method of `spray painting' atoms on single crystalline substrates to create precisely customized layered structures with atomic arrangements defined by the underlying substrate. Charge transfer and spin polarization across interfaces as well as imprinting nanoscale heterogeneity between adjacent layers lead to intriguing and important new phenomena testing our understanding of basic physics and creating new functionalities. Moreover, the abrupt change of orientation of an order parameter between nanoscale domains can lead to unique phases that are localized at domain walls, including conducting domain walls in insulating ferroelectrics, and ferromagnetic domain walls in antiferromagnets. Here we present our recent results on tailoring the electronic anisotropy of multiferroic heterostructures by imprinting the BiFeO$_{3}$ domain pattern in an adjacent La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ layer, understanding the metal-insulator transition in strained VO$_{2}$ thin films and identifying a three-dimensional quasi-long-range electronic supermodulation in YBa$_{2}$Cu$_{3}$O$_{7-x}$/La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ heterostructures.   

Influence of bending stress on the saturation magnetization of La$_{\mathrm{0.8}}$Sr$_{\mathrm{0.2}}$MnO$_{\mathrm{3}}$

Using polarized neutron reflectometry, we measured the influence of elastic bending stress on the magnetization depth profile of a La$_{\mathrm{0.8}}$Sr$_{\mathrm{0.2}}$MnO$_{\mathrm{3\thinspace }}$(LSMO) epitaxial film grown on a SrTiO$_{\mathrm{3}}$ (STO) substrate. Despite the uniform chemical structure throughout the film, we observed strong variations of the saturation magnetization as function of depth. Most important, the elastic bending strain of $\pm $ 0.03{\%} has no obvious effect on the magnetization depth profile at saturation. This result is in stark contrast to that of (La$_{\mathrm{1-x}}$Pr$_{\mathrm{x}})_{\mathrm{1-y}}$Ca$_{\mathrm{y}}$MnO$_{\mathrm{3}}$ films for which strain of $\pm $ 0.01{\%} produced dramatic changes in the magnetization profile and Curie temperature. We attribute the difference between the influence of strain on the saturation magnetization in LSMO (weak or none) and LPCMO (strong) to a difference in the ability of LSMO (weak or none) and LPCMO (strong) to phase separate.   

Exploring Temperature Dependent Magnetization of La$_{\mathrm{2/3}}$Sr$_{\mathrm{1/3}}$MnO$_{\mathrm{3}}$ and Related Perovskite Heterostructures through Optical Probe Techniques

La$_{\mathrm{2/3}}$Sr$_{\mathrm{1/3}}$MnO$_{\mathrm{3}}$ (LSMO) and related perovskite oxides lend themselves to a wide range of applications due to their varied magnetic and insulating states that rely on doping, film thickness, or strain. By carefully tuning these properties, unique magnetic and electronic states can emerge at the interface and in the bulk of the material. LSMO, in particular, has drawn attention due to its room temperature ferromagnetism and half metallicity. Here, we present work detailing the optical response of thin film LSMO grown on SrTiO$_{\mathrm{3}}$ (001) (STO) substrates and related perovskite heterostructures as a function of temperature and applied magnetic field. Magneto-optical Kerr Effect (MOKE) measurements are taken at varying wavelengths in order to probe different energies of the LSMO film. By probing the LSMO films and heterostructures, we gain an insight into the magnetic nature of the material and discuss possible sources of the optical response.   

Self-assembled magnetic nano-structure with exchange bias and inverted hysteresis

The competing interactions in transition metal oxides generate rich magnetic phase diagram. In heterostructures, interface between film and substrate often is the key in discovering new functionality. Here we show that it is possible to fabricate and engineer a self-assembled interface in a single thin film material -- half metallic La$_{\mathrm{2/3}}$Sr$_{\mathrm{1/3}}$MnO$_{\mathrm{3}}$ --with unusual magnetic behavior by special processing. Transmission electron microscopy {\&} spectroscopy shows clear structural and compositional evidence of the new engineered inherent interface. Consequently, the engineered interface results in complex magnetic properties in a monolithic La$_{\mathrm{1-x}}$Sr$_{\mathrm{x}}$MnO$_{\mathrm{3}}$ thin film. The thin film exhibits unexpected negative and positive exchange bias that changes exponentially with temperature. Also, the magnetic coupling between layers at each sides of the interface creates a complex ferromagnetic heterostructure that exhibits inverted hysteresis. Our results shed light on a pathway towards fabricating self-assembled interface to manipulate novel physical properties in a monolithic thin film.