Showing papers on "Colossal magnetoresistance published in 2019"

Room-Temperature Low-Field Colossal Magnetoresistance in Double-Perovskite Manganite.

Abstract: We have discovered room-temperature low-field colossal magnetoresistance (CMR) in an $A$-site ordered ${\mathrm{NdBaMn}}_{2}{\mathrm{O}}_{6}$ crystal. The resistance changes more than 2 orders of magnitude at a magnetic field lower than 2 T near 300 K. When the temperature and magnetic field sweep from an insulating (metallic) phase to a metallic (insulating) phase, the insulating (metallic) conduction changes to the metallic (insulating) conduction within 1 K and 0.5 T, respectively. The CMR is ascribed to the melting of the charge and orbital ordering. The entropy change which is estimated from the $B\text{\ensuremath{-}}T$ phase diagram is smaller than what is expected for the charge and orbital ordering. The suppression of the entropy change is attributable to the loss of the short-range ferromagnetic fluctuation of Mn spin moments, which is an important key of the high temperature and low magnetic field CMR effect.

Thermodynamic stability of non-stoichiometric SrFeO3−δ: a hybrid DFT study

Abstract: SrFeO3−δ is a mixed ionic-electronic conductor with a complex magnetic structure that reveals a colossal magnetoresistance effect. This material and its solid solutions are attractive for various spintronic, catalytic and electrochemical applications, including cathodes for solid oxide fuel cells and permeation membranes. Its properties strongly depend on oxygen non-stoichiometry. Ab initio hybrid functional approach was applied herein to study the thermodynamic stability of a series of SrFeO3−δ compositions with several non-stoichiometries δ, ranging from 0 to 0.5 (SrFeO3–SrFeO2.875–SrFeO2.75–SrFeO2.5) as a function of temperature and oxygen pressure. The results obtained by two approaches, in which either (i) all electrons at Fe atoms explicitly described or (ii) inner core electrons at Fe atoms are replaced by effective core potential, are compared. Based on our calculations, phase diagrams were constructed, allowing the determination of environmental conditions for the existence of stable phases. It is shown that (within an employed model) only the SrFeO2.5 phase appears to be stable. The stability region for this phase was re-drawn on the contour map of oxygen chemical potential, presented as a function of temperature and oxygen partial pressure. A similar analysis was also performed using experimental Gibbs energies of perovskite formation from the elements. The present modelling strongly suggest a significant attraction between neutral oxygen vacancies. These vacancies are created during a series of the abovementioned SrFeO3−δ mutual transformations accompanied by oxygen release.

Magnetic-Field-Induced Suppression of Jahn-Teller Phonon Bands in (La0.6Pr0.4)0.7Ca0.3MnO3: the Mechanism of Colossal Magnetoresistance shown by Raman Spectroscopy.

Abstract: A long-standing issue in the physics of the colossal magnetoresistance is the role of electron-phonon coupling, which manifests itself as Jahn-Teller polarons. The origin and architecture of polarons makes it possible to study their behavior by Raman spectroscopy, which allows to analyze the polaronic behavior in an applied magnetic field. We performed magnetic-field-dependent Raman spectroscopy on thin films of (La0.6Pr0.4)0.7Ca0.3MnO3 in a range of H = 0–50 kOe and compared the obtained Raman spectra with the magnetic field behavior of the electrical resistivity. In the vicinity of the Curie temperature, TC = 197 K, the intensity of the Jahn-Teller stretching mode at 614 cm−1 and of the bending mode at 443 cm−1 was found to be suppressed and enhanced, respectively. This observed behavior has a remarkable similarity with the field and temperature dependence of the colossal magnetoresistance in (La0.6Pr0.4)0.7Ca0.3MnO3. Our work provides direct evidence that the reduction of the amount of Jahn-Teller polarons at the phase transition is the main mechanism underlying the colossal magnetoresistance.

Relation between thickness, crystallite size and magnetoresistance of nanostructured La1−xSrxMnyO3±δ films for magnetic field sensors

Abstract: In the present study the advantageous pulsed-injection metal organic chemical vapour deposition (PI-MOCVD) technique was used for the growth of nanostructured La1-x Sr x Mn y O3±δ (LSMO) films on ceramic Al2O3 substrates. The compositional, structural and magnetoresistive properties of the nanostructured manganite were changed by variation of the processing conditions: precursor solution concentration, supply frequency and number of supply sources during the PI-MOCVD growth process. The results showed that the thick (≈400 nm) nanostructured LSMO films, grown using an additional supply source of precursor solution in an exponentially decreasing manner, exhibit the highest magnetoresistance and the lowest magnetoresistance anisotropy. The possibility to use these films for the development of magnetic field sensors operating at room temperature is discussed.

Structural and electrical transport properties of (La 0.7- x Y x )Ca 0.3 MnO 3 manganites

Abstract: The effect of yttrium (Y) substitution at La-site on structural, microstructural and electrical transport properties of (La0.7-xYx)Ca0.3MnO3 (x = 0.00, 0.05, 0.10, 0.15 and 0.20) were investigated. Samples were prepared by conventional solid state reaction method and structural properties of the samples were obtained using the Rietveld refinement of X-ray diffraction data. X-ray diffraction patterns show orthorhombic distorted perovskite structures. Resistivity measurement results of the LaYCaMnO3 compounds show that metal–insulator transition temperature, TMI shifts to lower temperature while the resistivity increases by increasing Y3+ substitution. In the metallic region (T TMI), resistivity data were fitted to model of small polaron hopping conduction mechanism and the activation energy, Ea increased as Y substitutions increases.

Room temperature Co-doped manganite/graphene sensor operating at high pulsed magnetic fields.

Abstract: The demand to increase the sensitivity to magnetic field in a broad magnetic field ranges has led to the research of novel materials for sensor applications. Therefore, the hybrid system consisting of two different magnetoresistive materials – nanostructured Co-doped manganite La1−xSrx(Mn1−yCoy)zO3 and single- and few-layer graphene – were combined and investigated as potential system for magnetic field sensing. The negative colossal magnetoresistance (CMR) of manganite-cobaltite and positive one of graphene gives the possibility to increase the sensitivity to magnetic field of the hybrid sensor. The performed magnetoresistance (MR) measurements of individual few layer (n = 1–5) graphene structures revealed the highest MR values for three-layer graphene (3LG), whereas additional Co-doping increased the MR values of nanostructured manganite films. The connection of 3LG graphene and Co-doped magnanite film in a voltage divider configuration significantly increased the sensitivity of the hybrid sensor at low and intermediate magnetic fields (1–2 T): 70 mV/VT of hybrid sensor in comparison with 56 mV/VT for 3LG and 12 mV/VT for Co-doped magnanite film, respectively, and broadened the magnetic field operation range (0.1–20) T of the produced sensor prototype.

Hidden metal-insulator transition in manganites synthesized via a controllable oxidation

Abstract: Oxygen usually plays crucial roles in tuning the phase structures and functionalities of complex oxides such as high temperature superconductivity, colossal magnetoresistance, catalysis, etc. Effective and considerable control of the oxygen content in those functional oxides could be highly desired. Here, using perovskite manganite (La0.5Sr0.5)MnO3 as a paradigm, we develop a new pathway to synthesize the epitaxial thin films assisted by an in-situ chemical process, where the oxygen content can be precisely controlled by varying oxidative activity tuned by the atmospheric temperature (Tatm) during the growth. A hidden metal-insulator transition (MIT) emerges due to the phase competition, which is never shown in the phase diagram of this classic manganite. The oxygen-mediated interaction between Mn ions together with the change of carrier density might be responsible for this emerging phase, which is compatible with the results of first-principle calculations. This work demonstrates that, apart from traditional cation doping, a precise modulation of anion (O2−, S2−, etc.) may provide a new strategy to control phase structures and functionalities of epitaxial compound thin films.

Physical and chemical strains co-tuned magnetic properties of double perovskite PrBaMn2O5.5+δ epitaxial films

Abstract: Different layered perovskite-related oxides are known to exhibit important electronic, magnetic, and electrochemical properties including metal-insulator transition, colossal magnetoresistance, excellent mixed ionic/electronic conductivity, and especially their flexible tunability by external or internal stimuli. Here, we show that the microstructure and magnetic properties of double perovskite PrBaMn2O5.5+δ (PBMO) epitaxial films can be co-tuned by the physical strain via a proper choice of substrate and film thickness and the chemical strain from the concentration of oxygen vacancies. It is surprisingly found that the films with more oxygen vacancies reveal more Mn4+ formed along with Mn2+ under the influence of interface strain, and meanwhile, Mn4+ exhibits a thickness-dependent distribution with a high amount at the interface. Consequently, the increased proportion of Mn4+ diminishes the saturation magnetization and decreases the Curie temperature of PrBaMn2O5.5+δ epitaxial films, revealing the availability of physical and chemical strains tuning the properties of highly epitaxial double perovskite films.

Defects induced huge magnetoresistance in epitaxial La1–xSrxMnO3 thin films deposited by magnetic sputtering

Abstract: In this work, epitaxial La1–xSrxMnO3 (LSMO) films were fabricated on SrTiO3 substrates at temperatures (Ts) ranging from 550 to 750 °C by RF magnetron sputtering. Significant Ts-dependent structural, magnetic, and magnetotransport properties were observed. The LSMO (Ts = 750 °C) film exhibits the colossal magnetoresistance (CMR) of −47% under the magnetic field (H) of 5 T. In contrast, the LSMO (Ts = 650 °C) film demonstrates a huge magnetoresistance (MR) of −98% (H = 5 T) around the metal-insulator transition temperature and –59% at 5 K. The spin-glass-like behaviors indicate that the defects, particularly the oxygen vacancies, in the epitaxial LSMO (Ts = 650 °C) films destroy the double exchange. The huge MR is related to the defect modulated magnetic structures and spin-dependent magnetotransport properties. Our work helps to understand the physical mechanism of the CMR and provides a way for tuning the magnetotransport properties of the perovskite films.

Quantum effects in graphitic materials: Colossal magnetoresistance, Andreev reflections, Little-Parks effect, ferromagnetism, and granular superconductivity

Abstract: Unlike the more common local conductance spectroscopy, nonlocal conductance can differentiate between nontopological zero-energy modes localized around inhomogeneities, and true Majorana edge modes in the topological phase. In particular, negative nonlocal conductance is dominated by the crossed Andreev reflection. In graphene, the Andreev reflection and the inter-band Klein tunneling couple electron-like and hole-like states through the action of either a superconducting (SC) pair potential or an electrostatic potential. We are here probing quantum phenomena in modified graphitic samples. Four-point contact transport measurements at cryogenic to room temperatures were conducted using a Quantum Design Physical Property Measurement System. The observed negative nonlocal differential conductance Gdiff probes the Andreev reflection at the walls of the SC grains coupled by Josephson effect through the semiconducting matrix. In addition, Gdiff shows the butterfly shape that is characteristic to resistive random-access memory devices. In a magnetic field, the Andreev reflection counters the effect of the otherwise lowered conduction. At low temperatures, the magnetoresistance shows irreversible yet strong colossal oscillations that are known to be quantum in nature. In addition, we have found evidence for seemingly granular SC as well as ferromagnetism. Moreover, the Little-Parks effect is revealed in both the classical small-amplitude and the phase-slip driven large-amplitude oscillations in the magnetoresistance. Thus, graphitic materials show potential for quantum electronics applications, including rectification and topological states.

Local structure of potassium doped nickel oxide: A combined experimental-theoretical study

Abstract: The electronic structure of Mott and charge-transfer insulators can be tuned through charge doping to achieve a variety of fascinating physical properties, e.g., superconductivity, colossal magnetoresistance, and metal-to-insulator transitions. Strong correlations between $d$ electrons give rise to these properties but they are also the reason why they are inherently difficult to model. This holds true especially for the evolution of properties upon charge doping. Here, we hole-dope nickel oxide with potassium and elucidate the resulting structure by using a range of experimental and theoretical tools; potassium is twice as big as nickel and is expected to lead to distortions in its vicinity. Our measurements of the x-ray absorption fine structure (XAFS) show a significant distortion around the dopant and that the dopant is fully incorporated in the nickel oxide matrix. In parallel, the theoretical investigations include developing a Gaussian process for quantum Monte Carlo calculations to determine the lowest energy local structure around the potassium dopant. While the optimal structures determined from density functional theory and quantum Monte Carlo calculations agree very well, we find a large discrepancy between the experimentally determined structures and the theoretical doped structures. Further modeling indicates that the discrepancy is likely due to vacancy defects. Our work shows that potassium doping is a possible avenue to doping NiO, in spite of the size of the potassium dopant. In addition, the Gaussian process opens up a new route towards obtaining structure predictions outside of density functional theory.

Unusual electrical behaviour in sol-gel-synthesised PKMO nano-sized manganite

Abstract: Mixed-valence manganites have gained tremendous attention in the scientific research for its colossal magnetoresistance phenomenon. Nevertheless, the study devoted to praseodymium-based manganites is still limited to date. The present work aims to investigate the grain size effect on sol–gel grown nano-sized Pr0.85K0.15MnO3 (PKMO). The grain size has been modified by heat treatment ranging from 600 °C to 1000 °C. PKMO samples have been studied in detail to elucidate the correlation of spin, charge, orbital and lattice degrees of freedom. The X-ray diffraction analysis revealed that all samples exhibit in single orthorhombic phase with the space group of Pnma (62). The obtained crystallite size and average grain size are in the range of 45–115 nm and 51–210 nm, respectively. The evolution of grains intensively affects the electrical and magneto-transport properties of PKMO. The temperature dependence of the resistivity has been fitted with theoretical expressions in different temperature regimes to investigate their conduction mechanisms. The resistivity exhibits an unusual trend when the grain size increases where a similar pattern also been observed in metal–insulator transition temperature (TMI). This behaviour can be ascribed to the grain size distribution, grain formation and also the occurrence of oxygen vacancies at the grain boundaries. Enhancement of high field magnetoresistance has been discovered below 180 K, whereas low field magnetoresistance is suppressed as the temperature increases and almost vanished at 300 K. The PKMO study demonstrated here is clearly dominated by extrinsic properties (grain evolution) from the evidence of electrical and magneto-transport measurements.

Polaronic effect in the x-ray absorption spectra of La 1-x Ca x MnO 3 manganites

Abstract: X-ray absorption spectroscopy (XAS) is performed to study changes in the electronic structures of colossal magnetoresistance (CMR) and charged ordered (CO) La1-x Ca x MnO3 manganites with respect to temperature. The pre-edge features in O and Mn K-edge XAS spectra, which are highly sensitive to the local distortion of MnO6 octahedral, exhibit contrasting temperature dependence between CMR and CO samples. The seemingly counter-intuitive XAS temperature dependence can be reconciled in the context of polarons. These results help identify the most relevant orbital states associated with polarons and highlight the crucial role played by polarons in understanding the electronic structures of manganites.

Probing Phase Separation and Local Lattice Distortions in Cuprates by Raman Spectroscopy

Abstract: It is generally accepted that high temperature superconductors emerge when extra carriers are introduced in the parent state, which looks like a Mott insulator. Competition of the order parameters drives the system into a poorly defined pseudogap state before acquiring the normal Fermi liquid behavior with further doping. Within the low doping level, the system has the tendency for mesoscopic phase separation, which seems to be a general characteristic in all high Tc compounds, but also in the materials of colossal magnetoresistance or the relaxor ferroelectrics. In all these systems, metastable phases can be created by tuning physical variables, such as doping or pressure, and the competing order parameters can drive the compound to various states. Structural instabilities are expected at critical points and Raman spectroscopy is ideal for detecting them, since it is a very sensitive technique for detecting small lattice modifications and instabilities. In this article, phase separation and lattice distortions are examined on the most characteristic family of high temperature superconductors, the cuprates. The effect of doping or atomic substitutions on cuprates is examined concerning the induced phase separation and hydrostatic pressure for activating small local lattice distortions at the edge of lattice instability.

Magnon-mediated magnetoresistance in layered manganites

Abstract: We describe here a type of magnetoresistance that takes place in naturally layered and outstanding ordered single phase manganites that may be mediated by magnon excitation. In particular, we show the effect for the Ruddlesden-Popper compound, LaSr2Mn2O7 synthesized by ceramic method. This material exhibits, besides the conventional colossal magnetoresistance, another type of magnetoresistance at low temperature, associated with breaking of the A-type antiferromagnetic coupling of Mn-containing planes. Excitation of magnons or application of a magnetic field breaks this antiparallel alignment so that some electrons, initially confined on the planes, become itinerant along the interplain directions through a double exchange mechanism, giving rise to resistance variations of the order of similar to 60% for polycrystalline samples. The effect described here might be present in other types of manganites exhibiting a natural layered structure, opening up the possibility of developing magnetoresistive devices based on antiferromagnetic oxide materials without requiring artificial multilayered structures.

Observation of Small Polaron and Acoustic Phonon Coupling in Ultrathin La 0.7 Sr 0.3 MnO 3 /SrTiO 3 Structures

Abstract: Understanding the underlying physics of interactions among various quasi-particles is a fundamental issue for the application of spintronics and photonics. Here the observation of a coupling between the small polarons in the nanoscale ultrathin La0.7Sr0.3MnO3 (LSMO) films and the acoustic phonons in the SrTiO3 (STO) substrate using ultrafast pump–probe spectroscopy has been reported. According to the temperature- and wavelength-dependent measurements, the amplitudes of the acoustic phonons are suppressed by tuning the small polarons absorption. This shows a coupled relationship between the acoustic phonons and the small polarons. At the probe photon energy of 1.55 eV where the polaron absorption is dominant, the acoustic phonons become unobservable. Furthermore, by performing the pump fluence dependent measurements on the LSMO films with different thicknesses, smaller acoustic phonon amplitudes are found in the thinner film with stronger small polaron binding energy. Such a coupled nature can be utilized to manipulate the small polarons using the acoustic phonons or vice versa, which is of great importance in device applications of colossal magnetoresistance materials.

Origins of the Appearance of Ferromagnetic State and Colossal Magnetoresistance in Cobaltites

Abstract: The crystal structure and magnetotransport properties of stoichiometric and anion-deficient Ba2+‑alloyed cobaltites with a perovskite structure have been studied. It was shown that the development of ferromagnetic state is related to the presence of cobalt ions mainly with single eg electron (intermediate spin IS state), whereas the existence of two eg electrons (high-spin HS state) leads to the antiferromagnetic state. The covalent component of the chemical bond stabilizes an electron configuration that is close to the intermediate spin state. The colossal magnetoresistance appears at either concentration or temperature boundary corresponding to the coexistence of ferromagnetic and antiferromagnetic phases or clusters and results from the filed-induced spin transition from the HS/LS mixture to the IS state. Cobalt ions in the IS state are responsible for the ferromagnetism and high resistivity.

Magnetic Doublon Bound States in the Kondo Lattice Model.

Abstract: We present a novel pairing mechanism for electrons, mediated by magnons. These paired bound states are termed ``magnetic doublons.'' Applying numerically exact techniques (full diagonalization and the density-matrix renormalization group, DMRG) to the Kondo lattice model at strong exchange coupling $J$ for different fillings and magnetic configurations, we demonstrate that magnetic doublon excitations exist as composite objects with very weak dispersion. They are highly stable, support a novel ``inverse'' colossal magnetoresistance and potentially other effects.