Showing papers on "Colossal magnetoresistance published in 2007"

Oxide Spintronics

Abstract: Concomitant with the development of metal-based spintronics in the late 1980s and 1990s, important advances were made on the growth of high-quality oxide thin films and heterostructures. While this was at first motivated by the discovery of high-temperature superconductivity in perovskite Cu oxides, this technological breakthrough was soon applied to other transition-metal oxides and, notably, mixed-valence manganites. The discovery of colossal magnetoresistance in manganite films triggered intense research activity on these materials, but the first notable impact of magnetic oxides in the field of spintronics was the use of such manganites as electrodes in magnetic tunnel junctions, yielding tunnel magnetoresistance ratios that are one order of magnitude larger than what had been obtained with transition-metal electrodes. Since then, research on oxide spintronics has been intense, with the latest developments focused on diluted magnetic oxides and, more recently, on multiferroics. In this paper, the most important results on oxide spintronics was reviewed, emphasizing materials physics as well as spin-dependent transport phenomena, and finally give some perspectives on how the flurry of new magnetic oxides could be useful for next-generation spintronics devices

Oxide spintronics

Abstract: Concomitant with the development of metal-based spintronics in the late 1980's and 1990's, important advances were made on the growth of high-quality oxide thin films and heterostructures While this was at first motivated by the discovery of high-temperature superconductivity in perovskite Cu oxides, this technological breakthrough was soon applied to other transition metal oxides, and notably mixed-valence manganites The discovery of colossal magnetoresistance in manganite films triggered an intense research activity on these materials, but the first notable impact of magnetic oxides in the field of spintronics was the use of such manganites as electrodes in magnetic tunnel junctions, yielding tunnel magnetoresistance ratios one order of magnitude larger than what had been obtained with transition metal electrodes Since then, the research on oxide spintronics has been intense with the latest developments focused on diluted magnetic oxides and more recently on multiferroics In this paper, we will review the most important results on oxide spintronics, emphasizing materials physics as well as spin-dependent transport phenomena, and finally give some perspectives on how the flurry of new magnetic oxides could be useful for next-generation spintronics devices

Trapping, self-trapping and the polaron family

Abstract: The earliest ideas of the polaron recognized that the coupling of an electron to ionic vibrations would affect its apparent mass and could effectively immobilize the carrier (self-trapping). We discuss how these basic ideas have been generalized to recognize new materials and new phenomena. First, there is an interplay between self-trapping and trapping associated with defects or with fluctuations in an amorphous solid. In high dielectric constant oxides, like HfO2, this leads to oxygen vacancies having as many as five charge states. In colossal magnetoresistance manganites, this interplay makes possible the scanning tunnelling microscopy ( STM) observation of polarons. Second, excitons can self-trap and, by doing so, localize energy in ways that can modify the material properties. Third, new materials introduce new features, with polaron-related ideas emerging for uranium dioxide, gate dielectric oxides, Jahn-Teller systems, semiconducting polymers and biological systems. The phonon modes that initiate self-trapping can be quite different from the longitudinal optic modes usually assumed to dominate. Fourth, there are new phenomena, like possible magnetism in simple oxides, or with the evolution of short-lived polarons, like muons or excitons. The central idea remains that of a particle whose properties are modified by polarizing or deforming its host solid, sometimes profoundly. However, some of the simpler standard assumptions can give a limited, indeed misleading, description of real systems, with qualitative inconsistencies. We discuss representative cases for which theory and experiment can be compared in detail.

Competing ferromagnetic and charge-ordered states in models for manganites: the origin of the colossal magnetoresistance effect.

Abstract: The one-orbital model for manganites with cooperative phonons and superexchange coupling ${J}_{\mathrm{AF}}$ is investigated via large-scale Monte Carlo simulations. The results for two orbitals are also briefly discussed. Focusing on the electron density $n=0.75$, a regime of competition between ferromagnetic metallic and charge-ordered (CO) insulating states is identified. In the vicinity of the associated bicritical point, colossal magnetoresistance (CMR) effects are observed. The CMR is associated with the development of short-distance correlations among polarons, above the spin ordering temperatures, resembling the charge arrangement of the low-temperature CO state.

New routes to multiferroics

Abstract: Multiferroic materials are those which possess both ferroelectric and ferromagnetic properties. Clearly, there is a contradiction here since ferromagnetism requires d-electrons while ferroelectricity generally occurs only in the absence of d-electrons. Several multiferroics demonstrating magnetoelectric coupling effects have, however, been discovered in the past few years, but they generally make use of alternative mechanisms in attaining these properties. Several new ideas and concepts have emerged in the past two years, typical of them being magnetic ferroelectricity induced by frustrated magnetism, lone pair effect, charge-ordering and local non-centrosymmetry. Charge-order driven magnetic ferroelectricity is interesting in that it would be expected to occur in a large number of rare earth manganites, $Ln_{1-x}A_xMnO_3$ (A = alkaline earth), well known for colossal magnetoresistance, electronic phase separation and other properties. In this article, we discuss novel routes to multiferroics, giving specific examples of materials along with their characteristics.

Origin of ferromagnetic response in diluted magnetic semiconductors and oxides

Abstract: This paper reviews the present understanding of the origin of ferromagnetic response that has been detected in a number of diluted magnetic semiconductors (DMSs) and diluted magnetic oxides (DMOs) as well as in some nominally magnetically undoped materials. It is argued that these systems can be grouped into four classes. To the first belong composite materials in which precipitations of a known ferromagnetic, ferrimagnetic or antiferromagnetic compound account for magnetic characteristics at high temperatures. The second class forms alloys showing chemical nanoscale phase separation into the regions with small and large concentrations of the magnetic constituent. Here, high-temperature magnetic properties are determined by the regions with high magnetic ion concentrations, whose crystal structure is imposed by the host. Novel methods enabling a control of this spinodal decomposition and possible functionalities of these systems are described. To the third class belong (Ga, Mn)As, heavily doped p-(Zn, Mn)Te, and related semiconductors. In these solid solutions the theory built on the p–d Zener model of hole-mediated ferromagnetism and on either the Kohn–Luttinger kp theory or the multi-orbital tight-binding approach describes qualitatively, and often quantitatively, thermodynamic, micromagnetic, optical, and transport properties. Moreover, the understanding of these materials has provided a basis for the development of novel methods, enabling magnetization manipulation and switching. Finally, in a number of carrier-doped DMSs and DMOs a competition between long-range ferromagnetic and short-range antiferromagnetic interactions and/or the proximity of the localization boundary lead to an electronic nanoscale phase separation. These materials exhibit characteristics similar to colossal magnetoresistance oxides.

Intergranular Magnetoresistance in Sr 2 FeMoO 6 from a Magnetic Tunnel Barrier Mechanism across Grain Boundaries

Abstract: We present magnetization (M) and magnetoresistance (MR) data for a series of ${\mathrm{Sr}}_{2}{\mathrm{FeMoO}}_{6}$ samples with independent control on antisite defect and grain-boundary densities, which reveal several unexpected features, including a novel switching-like behavior of MR with M. These, in conjunction with model calculations, establish that the MR in ${\mathrm{Sr}}_{2}{\mathrm{FeMoO}}_{6}$ is dominantly controlled by a new mechanism, derived from the magnetic polarization of grain-boundary regions acting like spin valves, leading to behavior qualitatively different from that usually encountered in tunneling MR. We show that a simple and useful experimental signature for the presence of this spin-valve-type MR (SVMR) is a wider hysteresis in MR compared to that in M.

Non-Griffiths-like clustered phase above the Curie temperature of the doped perovskite cobaltite La{sub x-1}Sr{sub x}CoO{sub 3}.

Abstract: The existence of preformed clusters above the Curie temperature of the doped perovskite manganites is well established and, in many cases, conforms to the expectations for a Griffiths phase. We show here that the canonical perovskite cobaltite (La{sub 1?x}Sr{sub x}CoO{sub 3}) also exhibits a clustered state above the Curie point in the ferromagnetic phase. The formation of magnetic clusters at a well-defined temperature (T*) is revealed in the small-angle neutron scattering and dc susceptibility. Remarkably, the characteristics of this clustered state appear quite unlike those of a Griffiths phase; the deviation from Curie-Weiss behavior is opposite to expectations and is field independent, while T* does not correspond to the undiluted Curie temperature. These results demonstrate that, although the Griffiths model may apply to many systems with quenched disorder, it is not universally applicable to randomly doped transition metal oxides.

Negative refraction observed in a metallic ferromagnet in the gigahertz frequency range.

Abstract: It is generally believed that nature does not provide materials with negative refraction. Here we demonstrate experimentally that such materials do exist at least at GHz frequencies: ferromagnetic metals reveal a negative refraction index close to the frequency of the ferromagnetic resonance. The experimental realization utilizes a colossal magnetoresistance manganite ${\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_{3}$ as an example. In this material the negative refractive index can be achieved even at room temperature using external magnetic fields.

Transition metal oxides: The exciting world of orbitals

Abstract: Transition metal oxides show many interesting phenomena from high-Tc superconductivity to colossal magnetoresistance and multiferroicity, triggering intensive studies. However, we have barely tapped into their potential for applications.

Modelling colossal magnetoresistance manganites

Abstract: Rare-earth (Re) manganites (with alkaline-earth (Ak) ions partially substituting them), i.e. Re1−xAkxMnO3, have been intensively explored for the last decade or more because of the promise of magnetoelectronic applications as well as because of complex and unusual phenomena in which electronic, structural and magnetic effects are intertwined. A brief survey of these and a description of the three strong local interactions of the eg electrons (in two different orbital states at each site), namely with Jahn–Teller phonon modes (strength g), with resident t2g spins (ferromagnetic Hund's rule coupling JH) and between each other (the Mott–Hubbard correlation U) form the background against which efforts at modelling manganite behaviour are described. A new two-fluid model of nearly localized (l) polarons and band (b) electrons for low-energy behaviour is hypothesized for large g; some of its applications are mentioned here. First I describe some results of large U, JH calculations in single-site DMFT (dynamical mean field theory) which includes the effect of all the strong local correlations. These results are directly appropriate for the orbital liquid regime, found typically for 0.2

Effect of nanometric grain size on room temperature magnetoimpedance, magnetoresistance, and magnetic properties of La0.7Sr0.3MnO3 nanoparticles

Abstract: In this paper we have investigated the effect of nanometric grain size modulation on the behavior of magnetoimpedance (MI), magnetoresistance, and magnetic properties of a series of single-phase nanocrystalline colossal magnetoresistance La0.7Sr0.3MnO3 (LSMO) manganites all synthesized through chemical route “pyrophoric reaction process.” MI measurements were carried out at room temperature (T=300K) in the ac signal frequency (f) range of 40kHz-110MHz and in dc magnetic field (H) range of −2.5kOe

Polaron coherence condensation as the mechanism for colossal magnetoresistance in layered manganites

Abstract: Angle-resolved photoemission spectroscopy data for the bilayer manganite La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} show that, upon lowering the temperature below the Curie point, a coherent polaronic metallic groundstate emerges very rapidly with well defined quasiparticles which track remarkably well the electrical conductivity, consistent with macroscopic transport properties. Our data suggest that the mechanism leading to the insulator-to-metal transition in La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} can be regarded as a polaron coherence condensation process acting in concert with the Double Exchange interaction.

Colossal magnetocapacitance and scale-invariant dielectric response in phase-separated manganites

Abstract: Afull characterization of phase separation and the competition between phases is necessary for a comprehensive understanding of strongly correlated electron materials, such as under-doped high-temperature superconductors1,2, complex oxide heterojunctions3, spinels4,5, multiferroics6,7, rare-earth ferroelectric manganites8 and mixed-valence manganites in which phase competition is the dominant mechanism governing the insulator–metal (IM) transition and the associated colossal magnetoresistance effect9,10,11. Thin films of strongly correlated electron materials are often grown epitaxially on planar substrates and typically have anisotropic properties that are usually not captured by edge-mounted four-terminal electrical measurements, which are primarily sensitive to in-plane conduction paths. We present an experimental study of anisotropic transport in phase-separated manganite thin films by using an unconventional arrangement of contact electrodes that enables the simultaneous determination of colossal magnetoresistance associated with d.c. transport parallel to the film substrate and colossal magnetocapacitance associated with a.c. transport in the perpendicular direction. We distinguish two distinct direction-dependent IM transitions and find a dielectric response that collapses onto a scale-invariant dependence over a large range of frequency, temperature and magnetic field.

Anisotropic magnetoresistance in colossal magnetoresistive La1-xSrxMnO3 thin films

Abstract: We report on magnetic field and temperature-dependent measurements of the anisotropic magnetoresistance (AMR) in epitaxial La1−xSrxMnO3 (LSMO) thin films. While in 3d ferromagnetic alloys increasing the magnetization, either by reducing the temperature or increasing the magnetic field, increases the AMR, we find that in LSMO films the AMR dependence on magnetization displays nonmonotonic behavior which becomes particularly pronounced in lightly doped compounds. We believe that this behavior is related to the inhomogeneity exhibited by these materials.

Linear Positive Magnetoresistance and Quantum Interference in Ferromagnetic Metals

Abstract: Positive, linear in field, and isotropic magnetoresistance in fields up to 60 T is found in geometrically constrained ferromagnets, such as thin films of iron, nickel, and cobalt and their granular mixtures with nonmagnetic materials. The resistivity measured as a function of temperature shows a minimum at temperatures reaching a remarkably high 92 K, followed by logarithmic dependence at low temperatures. We propose to explain both phenomena by a modified version of the quantum electron-electron interaction theory. The agreement is only qualitative while the observed magnitude of the magnetoresistance slope is much larger than the calculated one.

Oscillation of giant tunneling magnetoresistance with respect to tunneling barrier thickness in fully epitaxial Fe∕MgO∕Fe magnetic tunnel junctions

Abstract: The authors fabricated fully epitaxial Fe(001)∕MgO(001)∕Fe(001) magnetic tunnel junctions (MTJs) with various MgO thicknesses (tMgO) and investigated spin-dependent transport properties. Both the tunneling resistance in the parallel magnetic state (RP) and that in the antiparallel magnetic state (RAP) exhibited short-period oscillations as functions of tMgO with the same period of 3.2A and different phases. RAP also showed a long-period oscillation with a period of 9.9A. As a result, tMgO dependence of magnetoresistance is expressed as a superposition of the short- and long-period oscillations. These results provide important clues for understanding the oscillatory tMgO dependence of the tunneling magnetoresistance effect.

Highly Local Measurements of Strong Transient Magnetic Fields During Railgun Experiments Using CMR-Based Sensors

Abstract: The accurate measurement of the magnetic field distribution during electromagnetic launch experiments is an ambitious task. Loop sensors are widely used for detecting the change in magnetic flux. However, this technique is mostly used only for qualitative purposes, e.g., for triggering various devices. This paper deals with the use of another type of high magnetic field sensor based on thin (<1 mum) manganite films, which exhibit the colossal magnetoresistance (CMR) effect. This sensor measures the magnitude of the magnetic induction and can have very small sensitive areas (e.g., 0.5 mmtimes50 mum). Some basics about CMR and the design of the sensor are given. Several sensors were used in experiments performed with the ISL-launcher EMA3 (Eprim =0.6 MJ, l=3 m, cal=15 mmtimes30 mm). Transient magnetic field profiles with rise times of approximately 50 mus and amplitudes up to 4 T were recorded. The results obtained with the CMR sensors are compared with those of conventional loop sensors. Also, some metrological peculiarities due to high-frequency coupling to the detector circuit are mentioned. The highly local measurements of these CMR sensors were validated by results obtained from 3-D finite element (FE) calculations of the magnetic field distributions

Magnons in ferromagnetic metallic manganites.

Abstract: Ferromagnetic (FM) manganites, a group of likely half-metallic oxides, are of special interest not only because they are a testing ground for the classical double-exchange interaction mechanism for the 'colossal' magnetoresistance, but also because they exhibit an extraordinary arena of emergent phenomena. These emergent phenomena are related to the complexity associated with strong interplay between charge, spin, orbital, and lattice. In this review, we focus on the use of inelastic neutron scattering to study the spin dynamics, mainly the magnon excitations in this class of FM metallic materials. In particular, we discuss the unusual magnon softening and damping near the Brillouin zone boundary in relatively narrow-band compounds with strong Jahn-Teller lattice distortion and charge-orbital correlations. The anomalous behaviours of magnons in these compounds indicate the likelihood of cooperative excitations involving spin and lattice as well as orbital degrees of freedom.

Fabrication and enhanced magnetoresistance of SiO2-coated Fe3O4 nanosphere compact

Abstract: Magnetoresistance (MR) of a nanostructured material, monodisperse Fe3O4 nanospheres of about 200nm coated with thin SiO2 and compactly cold pressed and sintered, has been investigated. A high MR, up to 10.8% at 1T and 17% at 8T, has been observed at 100K. This enhanced MR is likely arising from the spin-polarized tunneling of conducting electrons through the insulating SiO2 boundaries. The decrease of the MR with the temperature increasing was attributed to the existence of the local spins in the grain boundaries. In addition, the MR ratio decreases with the SiO2 thickness increasing and sintering temperature decreasing. It suggests that the magnetic property of the insulating barrier is crucial for enhancing the low-field MR at a high temperature.

Transport in superlattices of magnetic nanoparticles: coulomb blockade, hysteresis, and switching induced by a magnetic field.

Abstract: We report on magnetotransport measurements on millimetric superlattices of Co-Fe nanoparticles surrounded by an organic layer. At low temperature, the transition between the Coulomb blockade and the conductive regime becomes abrupt and hysteretic. The transition between both regimes can be induced by a magnetic field, leading to a novel mechanism of magnetoresistance. Between 1.8 and 10 K, a high-field magnetoresistance attributed to magnetic disorder at the surface of the particles is also observed. Below 1.8 K, this magnetoresistance abruptly collapses and a low-field magnetoresistance is observed.

Colossal electroresistance and electron instability in strongly correlated electron systems

Abstract: Studies of the electron instability effects (EIEs) in structures based in strongly correlated electron systems (SCESs), such as high-temperature superconductors (HTSCs) and doped manganites (compounds with colossal magnetoresistance), are reviewed. These effects manifest themselves in a change of several orders of magnitude in the resistive state of the normal metal–HTSC or normal metal–DM (doped manganite) interface in an electric field under significant current injection conditions. The results of studying HTSC- and doped-manganite-based heterojunctions are considered. EIEs in heterostructures are compared with the electric field effect on the properties of an SCES in thin fllms and gate-containing devices. The general features and distinctions in the physics of these ðhenomena are analyzed.

Imaging of time evolution of the first-order magneto-structural transition in Fe-Rh alloy using magnetic force microscopy

Abstract: Cooperative lattice distortion can lead to phase coexistence at micrometer scales across a first-order magneto-structural transition. Using magnetic force microscopy we show such a phase coexistence across an antiferromagnetic to ferromagnetic transition coupled with a structural distortion in the intermetallic alloy of Fe-Rh. Our results provide direct evidence of strong coupling between the elastic and electronic degrees of freedom seen across this first-order magneto-structural transition. Theoretical frameworks based on long-range strain disorder coupling proposed originally for explaining phase coexistence in manganese oxide compounds with colossal magnetoresistance also appear to be applicable for other systems, like intermetallics, undergoing magneto-structural transitions.

Relationship between macroscopic physical properties and local distortions of low-doping La 1 − x Ca x MnO 3 : An EXAFS study

Abstract: A temperature-dependent extended x-ray-absorption fine structure investigation of La1xCaxMnO3 is presented for the concentration range that spans the ferromagnetic-insulator FMI to ferromagnetic-metal FMM transition region, x=016, 018, 020, and 022; the titrated hole concentrations are slightly higher, y=02, 022, 024, and 025, respectively For this range of Ca concentrations the samples are insulating for x=016‐02 and show a metal/insulator M /I transition for x=022 All samples are ferromagnetic although the saturation magnetization for the 16% Ca sample is only 70% of the expected value at 04 T This raises a question as to the nature of the ferromagnetic coupling mechanism in such insulating samples We find that the FMI samples have similar correlations between changes in the local Mn-O distortions and the magnetization as observed previously for the colossal magnetoresistance CMR samples 02x05—except that the FMI samples never become fully magnetized The data show that there are at least two distinct types of distortions The initial distortions removed as the insulating sample becomes magnetized are small and provide direct evidence that roughly 50% of the Mn sites associated with the hole charge carriers have a small average distortion/site and are magnetized first The large Mn-O distortions that remain at low T are attributed to a small fraction 30% of fully Jahn-Teller-distorted Mn sites that are either unmagnetized or antiferromagnetically ordered Thus the insulating samples are very similar to the behavior of the CMR samples up to the point at which the M /I transition occurs for the CMR materials The lack of metallic conductivity for x 02, when 50% or more of the sample is magnetic, implies that there must be preferred magnetized Mn sites that involve holes and that such sites do not percolate at these concentrations