Ferroelectricity Modulates Polaronic Coupling at Multiferroic Interfaces

Physics of the multiferroic interfaces is currently understood mostly within a phenomenological framework including screening of the polarization field and depolarizing charges. Largely unexplored still remains the band dependence of the interfacial charge modulation, as well as the associated changes of the electron-phonon interaction, coupling the charge and lattice degrees of freedom. Here, multiferroic heterostructures of the colossal-magnetoresistance manganite La1-xSrxMnO3 buried under ferroelectric BaTiO3 and PbZrxTi1-xO3 are explored using soft-X-ray angle-resolved photoemission. The experimental band dispersions from the buried La1-xSrxMnO3 identify coexisting two-dimensional hole and three-dimensional electron charge carriers. The ferroelectric polarization modulates their charge density, changing the band filling and orbital occupation in the interfacial region. Furthermore, these changes in the carrier density affect the coupling of the 2D holes and 3D electrons with the lattice which forms large Froelich polarons inherently reducing mobility of the charge carriers. We find that the fast dynamic response of electrons makes them much more efficient in screening of the electron-lattice interaction compared to the holes. Our k-resolved results on the orbital occupancy, band filling and electron-lattice interaction in multiferroic oxide heterostructures modulated by the ferroelectric polarization disclose most fundamental physics of these systems needed for further progress of beyond-CMOS ferro-functional electronics.

Nanostructured Manganite Films Grown by Pulsed Injection MOCVD: Tuning Low- and High-Field Magnetoresistive Properties for Sensors Applications

The results of colossal magnetoresistance (CMR) properties of La0.83Sr0.17Mn1.21O3 (LSMO) films grown by pulsed injection MOCVD technique onto various substrates are presented. The films with thicknesses of 360 nm and 60 nm grown on AT-cut single crystal quartz, polycrystalline Al2O3, and amorphous Si/SiO2 substrates were nanostructured with column-shaped crystallites spread perpendicular to the film plane. It was found that morphology, microstructure, and magnetoresistive properties of the films strongly depend on the substrate used. The low-field MR at low temperatures (25 K) showed twice higher values (−31% at 0.7 T) for LSMO/quartz in comparison to films grown on the other substrates (−15%). This value is high in comparison to results published in literature for manganite films prepared without additional insulating oxides. The high-field MR measured up to 20 T at 80 K was also the highest for LSMO/quartz films (−56%) and demonstrated the highest sensitivity S = 0.28 V/T at B = 0.25 T (voltage supply 2.5 V), which is promising for magnetic sensor applications. It was demonstrated that Mn excess Mn/(La + Sr) = 1.21 increases the metal-insulator transition temperature of the films up to 285 K, allowing the increase in the operation temperature of magnetic sensors up to 363 K. These results allow us to fabricate CMR sensors with predetermined parameters in a wide range of magnetic fields and temperatures.

John Goodenough and the Many Lives of Transition-Metal Oxides

In honor of John Goodenough's centennial birthday, I discuss some of his insights into magnetism and the role of mixed valence in transition-metal oxides. His ideas form an important part of the continuing evolution of our understanding of these fascinating materials with a wide range of technologically-important functionalities. In particular, will mention connections to phenomena such as colossal magnetoresistance, enhanced thermopower, and high-temperature superconductivity.

Influence of Thickness on the Magnetic and Magnetotransport Properties of Epitaxial La0.7Sr0.3MnO3 Films Deposited on STO (0 0 1)

Epitaxial La0.7Sr0.3MnO3 films with different thicknesses (9–90 nm) were deposited on SrTiO3 (0 0 1) substrates by pulsed laser deposition. The films have been investigated with respect to morpho-structural, magnetic, and magneto-transport properties, which have been proven to be thickness dependent. Magnetic contributions with different switching mechanisms were evidenced, depending on the perovskite film thickness. The Curie temperature increases with the film thickness. In addition, colossal magnetoresistance effects of up to 29% above room temperature were evidenced and discussed in respect to the magnetic behavior and film thickness.

Optical Conductivity of Colossal Magnetoresistance Manganites

<p>The colossal magnetoresistance manganites are a group of materials whose unusual physical properties are a symptom of strongly interacting electrons and phonons. In order to elucidate some of these electronic and vibrational properties, an infrared optical investigation of manganites with a broad range of physical characteristics has been performed. Temperature-dependent normal incidence reflectivity measurements have been made on two samples of manganites, in the energy range of 60 cm-1 - 50000 cm-1, 1 for La0.9 Ca0.1 MnO3, an insulating ferromagnet, and 2 La0.735 Ca0.265 MnO3, a metallic ferromagnet. Temperature-dependent ellipsometric reflection measurements were performed in the energy range of 50 cm-1 - 5000 cm-1, on four faces of two samples of structurally anisotropic manganite, probing the 3. ab plane and c-axis of La1.2 Sr1.8 Mn2O7, a metallic ferromagnet, and 4. the ab plane and c-axis of PrSr2 Mn2O7, an insulating antiferromagnet. The optical conductivity for each of the first two samples has been deduced by a careful Kramers-Kronig analyis of the normal incidence reflectivity. For samples 3. and 4. the optical conductivity has been deduced by inversion of the ellipsometric constants, and a careful subsequent fitting to account for their anisotropy. The transition temperatures and types of magnetic order for all samples have also been characterised by magnetisation measurements. Treatment of the surface is shown to be critical in reflectivity measurements by the observation of hugely contrasting spectra, measured from a polished sample of metallic-like La0.735 Ca0.256 MnO3, before and after annealing. Several features observed in the measurements, especially for the layered materials, are consistent with the idea that a polaron, or electron-lattice interaction, is hugely important in a description of the electron dynamics of these materials. The correlation between spectral features and the structural and magnetic properties of the materials is investigated, finding that the cause of charge transport modification seen in the metallic-like materials could be explained by either a polaron or localisation due to disorder.</p>

Optical Conductivity of Colossal Magnetoresistance Manganites

<p>The colossal magnetoresistance manganites are a group of materials whose unusual physical properties are a symptom of strongly interacting electrons and phonons. In order to elucidate some of these electronic and vibrational properties, an infrared optical investigation of manganites with a broad range of physical characteristics has been performed. Temperature-dependent normal incidence reflectivity measurements have been made on two samples of manganites, in the energy range of 60 cm-1 - 50000 cm-1, 1 for La0.9 Ca0.1 MnO3, an insulating ferromagnet, and 2 La0.735 Ca0.265 MnO3, a metallic ferromagnet. Temperature-dependent ellipsometric reflection measurements were performed in the energy range of 50 cm-1 - 5000 cm-1, on four faces of two samples of structurally anisotropic manganite, probing the 3. ab plane and c-axis of La1.2 Sr1.8 Mn2O7, a metallic ferromagnet, and 4. the ab plane and c-axis of PrSr2 Mn2O7, an insulating antiferromagnet. The optical conductivity for each of the first two samples has been deduced by a careful Kramers-Kronig analyis of the normal incidence reflectivity. For samples 3. and 4. the optical conductivity has been deduced by inversion of the ellipsometric constants, and a careful subsequent fitting to account for their anisotropy. The transition temperatures and types of magnetic order for all samples have also been characterised by magnetisation measurements. Treatment of the surface is shown to be critical in reflectivity measurements by the observation of hugely contrasting spectra, measured from a polished sample of metallic-like La0.735 Ca0.256 MnO3, before and after annealing. Several features observed in the measurements, especially for the layered materials, are consistent with the idea that a polaron, or electron-lattice interaction, is hugely important in a description of the electron dynamics of these materials. The correlation between spectral features and the structural and magnetic properties of the materials is investigated, finding that the cause of charge transport modification seen in the metallic-like materials could be explained by either a polaron or localisation due to disorder.</p>

Striping of orbital-order with charge-disorder in optimally doped manganites

The phase diagrams of LaMnO3 perovskites have been intensely studied due to the colossal magnetoresistance (CMR) exhibited by compositions around the $${\frac{3}{8}}^{th}$$ 3 8 t h doping level. However, phase segregation between ferromagnetic (FM) metallic and antiferromagnetic (AFM) insulating states, which itself is believed to be responsible for the colossal change in resistance under applied magnetic field, has prevented an atomistic-level understanding of the orbital ordered (OO) state at this doping level. Here, through the detailed crystallographic analysis of the phase diagram of a prototype system (AMn$${}_{3}^{A^{\prime} }$$ 3 A ′ Mn$${}_{4}^{B}$$ 4 B O12), we show that the superposition of two distinct lattice modes gives rise to a striping of OO Jahn-Teller active Mn3+ and charge disordered (CD) Mn3.5+ layers in a 1:3 ratio. This superposition only gives a cancellation of the Jahn-Teller-like displacements at the critical doping level. This striping of CD Mn3.5+ with Mn3+ provides a natural mechanism though which long range OO can melt, giving way to a conducting state.