Showing papers on "Colossal magnetoresistance published in 2001"

The physics of manganites: Structure and transport

Abstract: The fundamental physical properties of doped ${\mathrm{LaMnO}}_{3},$ generically termed ``manganites,'' and much of the underlying physics, were known more than 40 years ago. This article first reviews progress made at that time, the concept of double exchange in particular, and points out the missing elements that have led to a massive resurgence of interest in these and related materials. More recent research is then described, treating first the ground states that emerge as divalent atoms are substituted for trivalent La. A wide range of ground states appear, including ferromagnetic metals, orbital- and charge-ordered antiferromagnets, and more complex stripe and spin-glass states. Because of the interest in so-called colossal magnetoresistance that occurs in the ferromagnetic/metallic composition range, a section is devoted to reviewing the atypical properties of that phase. Next the high-temperature phase is examined, in particular, evidence for the formation of self-trapped small polarons and the importance of Jahn-Teller coupling in this process. The transitions between the high-temperature polaronic phase and the ferromagnetic and charge-ordered states are treated in a fourth section. In each section, the authors stress the competition among charge, spin, and lattice coupling and review the current state of theoretical understanding. They conclude with some comments on the impact that research on these materials has on our understanding of doped oxides and other strongly correlated electronic materials.

Colossal effects in transition metal oxides caused by intrinsic inhomogeneities.

Abstract: The influence of quenched disorder on the competition between ordered states separated by a first-order transition is investigated. A phase diagram with features resembling quantum-critical behavior is observed, even using classical models. The low-temperature paramagnetic regime consists of coexisting ordered clusters, with randomly oriented order parameters. Extended to manganites, this state is argued to have a colossal magnetoresistance effect. A scale T(*) for cluster formation is discussed. This is the analog of the Griffiths temperature, but for the case of two competing orders, producing a strong susceptibility to external fields. Cuprates may have similar features, compatible with the large proximity effect of the very underdoped regime.

Electroresistance and electronic phase separation in mixed-valent manganites.

Abstract: The sensitivity of transport in colossal magnetoresistance (CMR) manganites to external electric and magnetic fields is examined using field effect configurations with La(0.7)Ca(0.3)MnO(3) (LCMO), Na(0.7)Sr(0.3)MnO(3), La(0.7)Ba(0.3)MnO(3), and La(0.5)Ca(0.5)MnO(3) (0.5-doped LCMO) channels, and ferroelectric PbZr(0.2)Ti(0.8)O(3) (PZT) or dielectric (SrTiO(3)) gates. A large electroresistance (ER) of approximately 76% at 4 x 10(5) V/cm is found in LCMO with PZT-ferroelectric gate, but the magnitude of the effect is much smaller (a few percent) in the other three channels. The ER and CMR effects are remarkably complimentary. The size and systematics of the effect strongly favor a percolative phase separation picture.

Gd5(SixGe1–x)4: An Extremum Material

Abstract: The large shear displacements of atomic layers in Gd5(SixGe1–x)4 materials, coupled with the change of crystallographic symmetry and magnetic order, characterizes these transformations as magnetic–martensitic, which are extremely rare. The start and the end of the magnetic–martensitic transitions depends strongly on the direction of change (i.e., increasing or decreasing) of either or both the temperature and magnetic field. These profound bonding, structural, electronic, and magnetic changes, which occur in the Gd5(SixGe1–x)4 system, bring about some extreme changes of the materials' behavior resulting in a rich variety of unusually powerful magneto-responsive properties, such as the giant magnetocaloric effect, colossal magnetostriction, and giant magnetoresistance.

Colossal magnetoresistive manganite thin films

Abstract: Mixed-valence perovskite manganites (Re1-xAxMnO3 where Re = rare earth, A = alkaline earth) provide a unique opportunity to study the relationships between the structure and the magnetotransport properties due to an interplay among charge carriers, magnetic coupling, orbital ordering and structural distortion. This makes these compounds very exciting from both the basic research and from the technological viewpoint. As the technology pursued with these materials requires film growth, extensive studies have been made on materials synthesis, structural and physical characterization and device fabrication. In this article, the results from the different experimental techniques and the effects of the deposition procedure of the manganite thin films are first reviewed. Second, the relation between the structural and the physical properties is mentioned, and the influence of strains discussed. Finally, possible applications of manganite thin films in spin electronics are presented.

Superconductivity Mediated by Charge Fluctuations in Layered Molecular Crystals

Abstract: The issue of the interplay of superconductivity, mag-netism, and charge ordering is relevant to a wide rangeof strongly correlated electron materials. Examples in-clude the copper-oxide (high-temperature) superconduc-tors [1], colossal magnetoresistance materials [2], heavyfermion compounds [3], vanadium oxides [4], and organicmolecular crystals [5–7]. In particular, for the cuprate su-perconductors there is controversy about the relative im-portance of charge fluctuations (associated with dynamical“stripes”) and antiferromagnetic spin fluctuations (associ-ated with the Mott insulator which occurs when there is anaverage of one electron or hole for every lattice site). Forsome heavy fermion compounds recent experiments sup-port the idea that the superconductivity is mediated by spinfluctuations [3].The family k- BEDT-TTF

Strong ferromagnetism and weak antiferromagnetism in double perovskites: Sr2FeMO6 (M = Mo, W, and Re)

Abstract: Double perovskites Sr2FeMO6 (M = Mo and Re) exhibit significant colossal magnetoresistance even at room temperature due to the high Curie temperatures (419 and 401 K). However, such a high Curie temperature is puzzling, given the large separation between

Raman phonons and Raman Jahn-Teller bands in perovskite-like manganites

Abstract: The perovskite-like manganites RAMnO, where R is a trivalent rare earth or Y and A is a divalent alkaline earth element, are characterized by a strong interplay of magnetism, electric transport and crystallographic distortion At doping levels 015 < x < 045 the materials exhibit colossal magnetoresistance near the concomitant ferromagnetic and insulator-metal transitions At a fractional doping level, such as x = 05, the crystallographic and magnetic environment is strongly modified and charge ordering between Mn and Mn or phase separation takes place In this work, the polarized Raman spectra of the orthorhombic and rhombohedral phases of parent RMnO compound were analyzed in close comparison with results of lattice dynamic calculations We argue that the strong high-wavenumber bands between 400 and 700 cm, which dominate the Raman spectra of rhombohedral RMnO and magnetoresistive LaAMnO are not proper Raman modes for the R3c or Pnma structures Rather, the bands are of phonon density-of-states origin and correspond to oxygen phonon branches activated by the non-coherent Jahn-Teller distortions of the MnO octahedra The reduction of these bands upon doping of LaAMnO and their disappearance in the ferromagnetic metallic phase support the model The variation with temperature of the Raman spectra of LaCaMnO is also discussed The results give a strong indication for charge and orbital ordering and formation of superstructure at low temperatures

Enhanced room-temperature magnetoresistance in La0.7Sr0.3MnO3-glass composites

Abstract: In this letter, we report the improved magnetotransport properties of La0.7Sr0.3MnO3-borosilicate glass composite with different weight percents of glass. All the composites showed ferromagnetic nature at room temperature. The microstructure of these composites was seen using a scanning electron microscope. The microstructure was reconfirmed using spot energy dispersive x-ray analysis. We observe an enhancement of the low-field magnetoresistance (<200 Oe) at room temperature for the optimal composition of 25 wt % of glass. It is argued that glass layer separating the grain boundaries may be acting as barrier for spin-polarized tunneling, thereby enhancing the low-field magnetoresistance.

Ultrafast conductivity dynamics in colossal magnetoresistance manganites.

Abstract: Ultrafast picosecond measurements of optically induced changes in the absolute conductivity (0.4–1.0 THz) of La0.7M0.3MnO3 thin films (M Ca, Sr) from 10 K to 0.9Tc reveal a two-component relaxation. A fast, 2 ps, conductivity decrease arises from an optically induced modification of the effective phonon temperature. The slower component, related to spin-lattice relaxation, has a lifetime that increases upon approaching Tc from below in accordance with an increasing spin specific heat. We show that, for T o Tc, ≠s≠T is primarily determined by thermally disordered phonons while spin fluctuations dominate near Tc. The observation of “colossal” negative magnetoresistance (CMR) in the hole-doped manganite perovskites (R12xDxMnO3, where, e.g., R La, Nd and D Ca, Sr) demonstrates the sensitivity of electronic conduction to the underlying magnetic structure in these materials [1,2]. Experimental and theoretical work has also revealed the importance of the lattice and orbital degrees of freedom in determining the electronic properties of CMR materials above and below Tc [3,4]. Nonetheless, it is still not clear, especially for T o Tc, what the relative importance of phonons is in comparison to double exchange in determining s. Ultrafast optical spectroscopy has provided significant insight into electron dynamics in metals [5–7], and more recently, transition metal oxides [8 –10]. Using similar ultrafast techniques, we address the relative contributions of spin fluctuations and phonons in determining the conductivity in the manganites from 10 Kt o0.9Tc. Terahertz time-domain spectroscopy is an ultrafast optical technique in which electric field transients are used to measure the complex conductivity of a material. Since this is a coherent technique, a sample can be optically excited and then probed with a terahertz (THz) pulse to measure induced conductivity changes with picosecond (ps) resolution. We use this method, known as timeresolved terahertz spectroscopy (TRTS), to measure ps conductivity transients in La0.7Ca0.3MnO3 (LCMO) and La0.7Sr0.3MnO3 (LSMO) thin films. The dynamics occur on two time scales. A fast, 2 ps, conductivity decrease arises from optically induced modification of the effective phonon temperature. The slower component, related to spin-lattice relaxation, has a lifetime that increases upon approaching Tc from below in accordance with an increasing spin specific heat. Our results demonstrate that, at low temperatures, ≠s≠T is primarily determined by thermally disordered phonons while spin fluctuations dominate close to Tc. The TRTS experiments were performed on LCMO and LSMO epitaxial thin films grown on LaAlO3 substrates using pulsed laser deposition [11]. For very thin films (150 A), island growth can alter the film properties, but the thicker films used in these experiments (1000 A) display bulk behavior [12]. Magnetization measurements

Magnetocaloric effect in perovskite manganites La0.7−xNdxCa0.3MnO3 and La0.7Ca0.3MnO3

Abstract: In perovskite manganites La0.7−xNdxCa0.3MnO3 (x=0, 0.05, 0.1, 0.15, and 0.20) prepared by the sol–gel technique, large magnetocaloric effects have been observed. The maximum of the magnetic entropy change peaks at the magnetic ordering temperature. Compared to La0.7Ca0.3MnO3, an enhancement of the magnetocaloric effect has been obtained in the samples with Nd3+ dopant (for La0.5Nd0.2Ca0.3MnO3, the maximum of ΔSM is about −2.31 J/kg K).

Double exchange model and the unique properties of the manganites

Abstract: In this review the double exchange (DE) model forming a basis for the description of the physics of colossal magnetoresistance manganites is discussed. For a limiting case of exchange interaction which is large compared with the band width, the effective Hamiltonian of the DE model is derived from that of the sd-exchange model. Since this Hamiltonian is very complicated, the dynamical mean field approximation, successful for other strongly correlated systems, is found to be more suitable for describing the model of interest. Two simplified versions of the DE model, both capable of accounting for a wide range of physical properties, are proposed — one using classical localized spins and the other involving quantum spins but no transverse spin fluctuations. A temperature–electron concentration phase diagram for a system with consideration for the domain of phase separation is constructed, whose basic features are shown to be in qualitative agreement with experimental data for the manganites, as also are the temperature and electron concentration dependences of their electrical resistivity, magnetization, and spectral characteristics. At the quantitative level, introducing additional electron–lattice interaction yields a good agreement. A number of yet unresolved problems in the physics of manganites, including the mechanism of temperature- or doping-induced metal–insulator phase transition and the nature of charge ordering, are also discussed. By comparing predictions made by computing approach with the experimental data, the adequacy of the DE model is assessed and its drawbacks are analyzed. Numerous recent theoretical studies of the unique properties of this broad class of strongly correlated systems are summarized in this review.

Magnetoresistance in manganite/alumina nanocrystalline composites

Abstract: Magnetotransport properties of manganite/insulator composites are studied in this article. By merging the half metallic character of La2/3Ca1/3MnO3 and the mixed composition with alumina grains dispersed in the structure we have been able to increase three times intergranular magnetoresistance around the percolation threshold. The transport properties have been studied employing a theoretical model for ferromagnetic–insulator systems and a two channel equation in order to reproduce the behavior of resistivity in the whole temperature range. The percolation theory is introduced to try to understand and improve the fascinating properties of these mixed systems.

Critical Phenomena of Ferromagnetic Transition in Double-Exchange Systems

Abstract: Critical phenomena of ferromagnetic transition at finite temperatures are studied in double-exchange systems. In order to investigate strong interplay between charge and spin degrees of freedom, the Monte Carlo technique is applied to include fluctuations in a controlled and unbiased manner. By using finite-size scaling analysis, critical exponents and transition temperature are estimated for a model with Ising spin symmetry in two dimensions. The obtained exponents differ significantly from the mean-field values, but are consistent with those of spin models with short-range exchange interactions. The universality class of this transition belongs to that of short-range interaction with the same spin symmetry. We also discuss the case for three dimensions. The results are compared with experimental results in perovskite manganites which show colossal magnetoresistance.

Large room-temperature magnetoresistance and phase separation in La1−xNaxMnO3 with 0.1⩽x⩽0.3

Abstract: The structural, magnetic, and electronic properties of the polycrystalline La1−xNaxMnO3 (x=0.10, 0.15, 0.20, and 0.30) are investigated. The result of the Rietveld refinement of x-ray powder diffraction shows that these compounds crystallize in a rhombohedrally distorted structure with space group R3C. The magnetic measurement shows that Curie temperature TC of the studied samples is near or above room temperature. The temperature dependence of resistivity shows that all samples undergo a sharp transition accompanying a paramagnetic to ferromagnetic with the decrease of temperature, however, for x⩾0.15 samples, double transition peaks with a single ferromagnetic transition is observed. In the meanwhile, a large room-temperature magnetoresistance with low applied magnetic field is observed. The co-existing ferromagnetic metallic phases and ferromagnetic insulating (FMI) phases induced by the electronic inhomogeneity as well as the additional FMI phases caused by the presence of vacancies at the A sites, a...

Ballistic magnetoresistance in a magnetic nanometer sized contact: An effective gate for spintronics

Abstract: We present experimental results of unprecedented large magnetoresistance obtained in stable electrodeposited Ni–Ni nanocontacts 10–30 nm in diameter. The contacts exhibit magnetoresistance of up to 700% at room temperature and low applied fields and, therefore, act as very effective spin filters. These large values of the magnetoresistance are attributed to spin ballistic transport through a magnetic “dead layer” at the contact of width of about 1 nm or smaller. Nanometer sized, high sensitive magnetoresistive sensors could become key elements for magnetic storage in the terabit/in.2 range and in high density magnetic random access memories.

Large magnetoresistance effect due to spin injection into a nonmagnetic semiconductor.

Abstract: A novel magnetoresistance effect, due to the injection of a spin-polarized electron current from a dilute magnetic into a nonmagnetic semiconductor, is presented. The effect results from the suppression of a spin channel in the nonmagnetic semiconductor and can theoretically yield a positive magnetoresistance of $100%$, when the spin flip length in the nonmagnetic semiconductor is sufficiently large. Experimentally, our devices exhibit up to $25%$ magnetoresistance.

Substrate effect on the magnetic microstructure of La0.7Sr0.3MnO3 thin films studied by magnetic force microscopy

Abstract: Colossal magnetoresistive La0.7Sr0.3MnO3 thin films have been grown under tensile strains on (100)-SrTiO3 substrates and compressive strains on (100)-LaAlO3 and (110)-NdGaO3 substrates by pulsed laser deposition. Using magnetic force microscopy (MFM), a “feather-like” magnetic pattern, characteristic of films with an in-plane magnetization, is observed for films deposited on both SrTiO3 and NdGaO3 while a “bubble” magnetic pattern, typical of films with an out-of-plane magnetization, is recorded for LaAlO3. We show that the shape of the magnetic pattern imaged by MFM is fully correlated to the easy direction of the magnetization in the film.

Charge Transport and Colossal Magnetoresistance Phenomenon in La1−xZrxMnO3

Abstract: In this study we have investigated the magnetic and electrical transport properties of Zr doped lanthanum manganite perovskite The structural, magnetic, and transport properties of the Zr doped compounds were determined using x-ray diffraction, dc magnetic susceptibility, and a four probe method for electrical resistivity and magnetoresistance measurements in the temperature range of 5–400 K The structure of the compounds was found to be rhombohedral The magnetization versus temperature curves show ferromagnetic regions with the magnetic transition temperatures getting saturated for x⩾007 compounds The resistivity curves show decreasing resistivity with increasing Zr content in the compound The resistivity of the compounds is very high and is explained as due to the localization tendency of the electrons The metal–insulator transition temperature shows a compositional dependence and has additional contributions apart from magnetism The results are explained by the double exchange interaction and M

Resistivity and magnetoresistance of ferromagnetic metals with localized spins

Abstract: Resistivity, due to spin-dependent scattering of carriers in ferromagnetic metals with localized spins, is studied to investigate the extraordinary behaviors of the resistivity, such as the giant magnetoresistance and the anomalous temperature dependence near the ferromagnetic transition temperature ${T}_{c}.$ The resistivity is calculated on the basis of the $s\ensuremath{-}d$ model by use of the Boltzmann theory. We aim to clarify the effects of the coexistence of spin-independent scattering of carriers, the smallness of the carrier number, the instability of the ferromagnetic state, and external magnetic fields on the resistivity. It is shown that the temperature dependence of the resistivity can exhibit a peak at ${T}_{c}$ even in the presence of spin-independent scatterings. This resistivity peak becomes sharp for an almost unstable ferromagnet with a small carrier number. This peak is easily suppressed by magnetic fields, to result in giant magnetoresistance. The effect of the appearance of a half-metallic state on the resistivity is also discussed.

Tuning colossal magnetoresistance response by Cr substitution in La0.67Sr0.33MnO3

Abstract: We have studied the effects of Cr substitution in the perovskite La067Sr033MnO3 on the magnetic, electrical transport, and magnetoresistance properties Cluster glass behaviors have been observed in the La067Sr033Mn1−xCrxO3 system The most interesting feature is that extraordinary transport and colossal magnetoresistance (CMR) behaviors, characterized by double peaks, were observed with Cr substitution When 01⩽x⩽02, the temperature range of CMR response is greatly broadened, from the lowest temperature to above room temperature These results suggest that Cr substitution can be a potent way in tuning CMR response and also imply a ferromagnetic interaction similar to double exchange occurs between Mn and Cr

Correlation between electroresistance and magnetoresistance in La0.82Ca0.18MnO3 single crystal

Abstract: The resistivity of La082Ca018MnO3 single crystal has been investigated as a function of external magnetic field and separately under an applied current flow The measurements were carried out at various temperatures below and above the ferromagnetic transition temperature TC It has been found that the dynamic electroresistance exhibits stunning similarities to the colossal magnetoresistance at the corresponding temperatures The correlation observed between the electric- and magnetic-field effects is attributed to electrically induced magnetoresistance

Magnetic field sensors from polycrystalline manganites

Abstract: We report on the fabrication of magnetic sensors based on bulk La 0.67 Sr 0.33 MnO 3 and La 0.67 Ba 0.33 MnO 3 material. The sensors were characterized at fields of 0–8 T and temperatures of 4.2–300 K. The sensors display a maximum sensitivity of ∼200%/T at room temperature within a field range of 1–3 mT. For the sensor geometry investigated here, the low-field magnetoresistance (MR) of the sensors measured in three field orientations with respect to the sensor plane is strongly anisotropic. The high-field MR, in contrast, is found to be field-orientation independent. Periodic response of the sensor’s resistance to the angle between the field and sensor-plane is demonstrated at room temperature.

Effect of magnetic coupling on the magnetoresistive properties in La0.67Sr0.33MnO3/BaFe11.3(ZnSn)0.7O19 composites

Abstract: The La0.67Sr0.33MnO3/BaFe11.3(ZnSn)0.7O19 (LSMO/BaM) composites have been prepared by cosintering the LSMO and the BaM powders. The microstructural, magnetic, and magnetoresistive (MR) properties of those systems were systematically studied. Due to the magnetic coupling between the LSMO (a soft magnet metal) and BaM (a hard magnet insulator) grains, the low-field MR sensitivity was reduced and the high-field MR slope was enhanced for the composites. In addition, an abnormal MR hysteresis (resistance reaches its maximum before field reversal) was identified for the composite when the field is applied vertical to the sample plane. Its origin was attributed to a coplay of the LSMO/BaM spin coupling and the demagnetization effect. Finally, an anisotropic magnetoresistance (AMR) effect was observed in the composites. The AMR value decreased with increasing the BaM content, which can be explained by the changes of the local effective field on the grain boundaries.