Showing papers on "Colossal magnetoresistance published in 2009"
TL;DR: In this article, the Mott transition between a low-temperature insulating phase and a high temperature metallic phase usually occurs at 341 K in VO(2), but the active control of strain allows us to reduce this transition temperature to room temperature.
Abstract: Correlated electron materials can undergo a variety of phase transitions, including superconductivity, the metal-insulator transition and colossal magnetoresistance. Moreover, multiple physical phases or domains with dimensions of nanometres to micrometres can coexist in these materials at temperatures where a pure phase is expected. Making use of the properties of correlated electron materials in device applications will require the ability to control domain structures and phase transitions in these materials. Lattice strain has been shown to cause the coexistence of metallic and insulating phases in the Mott insulator VO(2). Here, we show that we can nucleate and manipulate ordered arrays of metallic and insulating domains along single-crystal beams of VO(2) by continuously tuning the strain over a wide range of values. The Mott transition between a low-temperature insulating phase and a high-temperature metallic phase usually occurs at 341 K in VO(2), but the active control of strain allows us to reduce this transition temperature to room temperature. In addition to device applications, the ability to control the phase structure of VO(2) with strain could lead to a deeper understanding of the correlated electron materials in general.
560 citations
TL;DR: The observation of an electronic conductor-insulator transition by control of band-filling in the model antiferromagnetic ferroelectric BiFeO3 through Ca doping opens the door to merging magnetoelectrics and Magnetoelectronics at room temperature by combining electronic conduction with electric and magnetic degrees of freedom already present in the multiferroic Bi FeO3.
Abstract: Many interesting materials phenomena such as the emergence of high-Tc superconductivity in the cuprates and colossal magnetoresistance in the manganites arise out of a doping-driven competition between energetically similar ground states. Doped multiferroics present a tantalizing evolution of this generic concept of phase competition. Here, we present the observation of an electronic conductor-insulator transition by control of band-filling in the model antiferromagnetic ferroelectric BiFeO3 through Ca doping. Application of electric field enables us to control and manipulate this electronic transition to the extent that a p-n junction can be created, erased and inverted in this material. A 'dome-like' feature in the doping dependence of the ferroelectric transition is observed around a Ca concentration of approximately 1/8, where a new pseudo-tetragonal phase appears and the electric modulation of conduction is optimized. Possible mechanisms for the observed effects are discussed on the basis of the interplay of ionic and electronic conduction. This observation opens the door to merging magnetoelectrics and magnetoelectronics at room temperature by combining electronic conduction with electric and magnetic degrees of freedom already present in the multiferroic BiFeO3.
466 citations
TL;DR: In this article, a direct charge-mediated magnetoelectric coupling in a heterogeneous multiferroic that takes advantage of the sensitivity of a strongly correlated magnetic system to competing electronic ground states is described.
Abstract: The drive to develop materials with new multifunctional capabilities has rekindled interest in multiferroics—systems which are characterized by the simultaneous presence of, and coupling between, magnetic and electric order parameters. In naturally occurring multiferroics the magnetoelectric coupling is often weak, and new classes of artificially structured composite materials that combine dissimilar magnetic and ferroelectric systems are being developed to optimize order parameter coupling. [1–6] Here, we describe direct, charge-mediated magnetoelectric coupling in a heterogeneous multiferroic that takes advantage of the sensitivity of a strongly correlated magnetic system to competing electronic ground states. Using magneto-optic Kerr effect magnetometry, we observe large magnetoelectric coupling in ferroelectric/lanthanum manganite heterostructures, including electric field-controlled on/off switching of magnetism. These results open a new vista for the development of novel magnetoelectric devices with large charge coupling between electric and magnetic degrees of freedom. Doped lanthanum manganites are complex oxides characterized by a strong interplay between electron transport, magnetism, and crystal lattice distortions, leading to a rich variety of electronic behavior, including magnetic and charge-ordered states, colossal magnetoresistance (CMR), and a diversity of electron transport behavior. Underlying the competition between these ground states is the prominent role of charge in double exchange, hopping, and orbital overlap. [7,8] To date, controlling charge as a parameter has most often been achieved using chemical doping, which is robust, and permanent. An alternative approach to
392 citations
TL;DR: Unexpected temperature-induced valence changes at the A and B sites in the A-site-ordered double perovskite LaCu3Fe4O12 are reported; the underlying intersite charge transfer is accompanied by considerable changes in the material’s structural, magnetic and transport properties.
Abstract: The introduction of 'foreign' elements into transition-metal oxides (called chemical doping) can change the valence state of the metal's cations and hence modify the physical properties of the material as a whole. These changes can be dramatic, for example causing high-temperature superconductivity in copper oxides and colossal magnetoresistance in manganese oxides. Youwen Long et al. have identified an oxide system, the perovskite LaCu3Fe4O12, in which changes in valence state occur when charge is shuffled between different cations (iron and copper) in the host structure, rather than via doping. As a result, the material can be reversibly transformed from one possessing iron in an unusually high Fe3.75+ state (partnered with fairly common Cu2+ ions) to one possessing rare Cu3+ ions. These changes are reflected in the magnetic and electronic properties of the material and, intriguingly, the material contracts slightly on being warmed through the transition. The temperature sensitivity of this effect makes it a strong candidate for technological applications. This paper identifies an oxide system where changes in valence state occur as a result of charge being shuffled between different cations in the host structure, rather than via doping, this charge transfer being sensitive to temperature. As a result, the material can be reversibly transformed from one possessing iron in an unusually high Fe3.75+ state to one possessing rare Cu3+ ions. These changes are reflected in the magnetic and electronic properties of the material and, intriguingly, are accompanied by negative thermal expansion. Changes of valence states in transition-metal oxides often cause significant changes in their structural and physical properties1,2. Chemical doping is the conventional way of modulating these valence states. In ABO3 perovskite and/or perovskite-like oxides, chemical doping at the A site can introduce holes or electrons at the B site, giving rise to exotic physical properties like high-transition-temperature superconductivity and colossal magnetoresistance3,4. When valence-variable transition metals at two different atomic sites are involved simultaneously, we expect to be able to induce charge transfer—and, hence, valence changes—by using a small external stimulus rather than by introducing a doping element. Materials showing this type of charge transfer are very rare, however, and such externally induced valence changes have been observed only under extreme conditions like high pressure5,6. Here we report unusual temperature-induced valence changes at the A and B sites in the A-site-ordered double perovskite LaCu3Fe4O12; the underlying intersite charge transfer is accompanied by considerable changes in the material’s structural, magnetic and transport properties. When cooled, the compound shows a first-order, reversible transition at 393 K from LaCu2+3Fe3.75+4O12 with Fe3.75+ ions at the B site to LaCu3+3Fe3+4O12 with rare Cu3+ ions at the A site. Intersite charge transfer between the A-site Cu and B-site Fe ions leads to paramagnetism-to-antiferromagnetism and metal-to-insulator isostructural phase transitions. What is more interesting in relation to technological applications is that this above-room-temperature transition is associated with a large negative thermal expansion.
305 citations
TL;DR: It is proposed that the observed large magnetoresistance can be explained by quasi-neutrality breaking of the space-charge effect, where insufficient charge is present to compensate the electrons injected into the device, and could be used to develop new magnetic devices from silicon, which may further advance silicon technology.
Abstract: Recent discoveries of large magnetoresistance in non-magnetic semiconductors have gained much attention because the size of the effect is comparable to, or even larger than, that of magnetoresistance in magnetic systems. Conventional magnetoresistance in doped semiconductors is straightforwardly explained as the effect of the Lorentz force on the carrier motion, but the reported unusually large effects imply that the underlying mechanisms have not yet been fully explored. Here we report that a simple device, based on a lightly doped silicon substrate between two metallic contacts, shows a large positive magnetoresistance of more than 1,000 per cent at room temperature (300 K) and 10,000 per cent at 25 K, for magnetic fields between 0 and 3 T. A high electric field is applied to the device, so that conduction is space-charge limited. For substrates with a charge carrier density below approximately 10(13) cm(-3), the magnetoresistance exhibits a linear dependence on the magnetic field between 3 and 9 T. We propose that the observed large magnetoresistance can be explained by quasi-neutrality breaking of the space-charge effect, where insufficient charge is present to compensate the electrons injected into the device. This introduces an electric field inhomogeneity, analogous to the situation in other semiconductors in which a large, non-saturating magnetoresistance was observed. In this regime, the motions of electrons become correlated, and thus become dependent on magnetic field. Although large positive magnetoresistance at room temperature has been achieved in metal-semiconductor hybrid devices, we have now realized it in a simpler structure and in a way different from other known magnetoresistive effects. It could be used to develop new magnetic devices from silicon, which may further advance silicon technology.
147 citations
TL;DR: In this article, anisotropic electronic domain formation along one axis of a pseudocubic perovskite single-crystal thin-film manganite was induced by epitaxially locking it to an orthorhombic substrate.
Abstract: Complex oxide films are highly anisotropic in the way they conduct electricity, which is due to phase separation. However, the origin of this metal–insulator phase coexistence has been unclear. Transport measurements now show that strain, rather than chemical inhomogeneity, is mainly responsible. The presence of electronic phase separation in complex materials has been linked to many types of exotic behaviour, such as colossal magnetoresistance, the metal–insulator transition and high-temperature superconductivity1,2,3,4; however, the mechanisms that drive the formation of coexisting electronic phases are still debated5,6,7,8. Here we report transport measurements that show a preferential orientation of electronic phase domains driven by anisotropic long-range elastic coupling between a complex oxide film and substrate. We induce anisotropic electronic-domain formation along one axis of a pseudocubic perovskite single-crystal thin-film manganite by epitaxially locking it to an orthorhombic substrate. Simultaneous temperature-dependent resistivity measurements along the two perpendicular in-plane axes show substantial differences in the metal–insulator transition temperature and extraordinarily high anisotropic resistivities, which indicate that percolative conduction channels open more readily along one axis. These findings suggest that the origin of phase coexistence is much more strongly influenced by strain than by local chemical inhomogeneity.
145 citations
TL;DR: Scanning-electron-nanodiffraction images of nanoclusters as they form and evolve with temperature in La(1-x)Ca(x)MnO(3), x = 0.45 are provided.
Abstract: A nanoscale phase is known to coincide with colossal magnetoresistance (CMR) in manganites, but its volume fraction is believed to be too small to affect CMR. Here we provide scanning-electron-nanodiffraction images of nanoclusters as they form and evolve with temperature in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$, $x=0.45$. They are not doping inhomogeneities, and their structure is that of the bulk compound at $x=0.60$, which at low temperatures is insulating. Their volume fraction peaks at the CMR critical temperature and is estimated to be 22% at finite magnetic fields. In view of the known dependence of the nanoscale phase on magnetic fields, such a volume fraction can make a significant contribution to the CMR peak.
105 citations
TL;DR: In this paper, a ternary full Heusler alloys exhibiting substantial current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) has been theoretically proposed and experimentally realized.
Abstract: A materials system of ternary full Heusler alloys exhibiting substantial current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) has been theoretically proposed and experimentally realized. Observed trends in magnetoresistance are broadly consistent with the modeling results. A CPP-GMR of 6.7% and ΔRA of 4 mΩ μm2 have been demonstrated in the bottom spin-valve configuration. The spin-stand testing of narrow-track recording heads confirmed compatibility of these materials with hard disk drive reader technology.
100 citations
98 citations
TL;DR: The atomic-force-microscope-compatible microwave probe is able to perform a quantitative subsurface electrical study in a noninvasive manner and the phase change memory function in In(2)Se(3) nanoribbon devices can be locally recorded with big signals of opposite signs.
Abstract: Driven by interactions due to the charge, spin, orbital, and lattice degrees of freedom, nanoscale inhomogeneity has emerged as a new theme for materials with novel properties near multiphase boundaries. As vividly demonstrated in complex metal oxides (see refs 1-5) and chalcogenides (see refs 6 and 7), these microscopic phases are of great scientific and technological importance for research in high-temperature superconductors (see refs 1 and 2), colossal magnetoresistance effect (see ref 4), phase-change memories (see refs 5 and 6), and domain switching operations (see refs 7-9). Direct imaging on dielectric properties of these local phases, however, presents a big challenge for existing scanning probe techniques. Here, we report the observation of electronic inhomogeneity in indium selenide (In(2)Se(3)) nanoribbons (see ref 10) by near-field scanning microwave impedance microscopy (see refs 11-13). Multiple phases with local resistivity spanning 6 orders of magnitude are identified as the coexistence of superlattice, simple hexagonal lattice and amorphous structures with approximately 100 nm inhomogeneous length scale, consistent with high-resolution transmission electron microscope studies. The atomic-force-microscope-compatible microwave probe is able to perform a quantitative subsurface electrical study in a noninvasive manner. Finally, the phase change memory function in In(2)Se(3) nanoribbon devices can be locally recorded with big signals of opposite signs.
96 citations
TL;DR: In this article, the authors reported the observation of electronic inhomogeneity in indium selenide (In2Se3) nanoribbons by near-field scanning microwave impedance microscopy.
Abstract: Driven by interactions due to the charge, spin, orbital, and lattice degrees of freedom, nanoscale inhomogeneity has emerged as a new theme for materials with novel properties near multiphase boundaries. As vividly demonstrated in complex metal oxides and chalcogenides, these microscopic phases are of great scientific and technological importance for research in high-temperature superconductors, colossal magnetoresistance effect, phase-change memories, and domain switching operations. Direct imaging on dielectric properties of these local phases, however, presents a big challenge for existing scanning probe techniques. Here, we report the observation of electronic inhomogeneity in indium selenide (In2Se3) nanoribbons by near-field scanning microwave impedance microscopy. Multiple phases with local resistivity spanning six orders of magnitude are identified as the coexistence of superlattice, simple hexagonal lattice and amorphous structures with 100nm inhomogeneous length scale, consistent with high-resolution transmission electron microscope studies. The atomic-force-microscope-compatible microwave probe is able to perform quantitative sub-surface electronic study in a noninvasive manner. Finally, the phase change memory function in In2Se3 nanoribbon devices can be locally recorded with big signal of opposite signs.
TL;DR: In this article, the authors reported the discovery of colossal resistance switching effect in polycrystalline spinel-like structure MgZnO thin films with high Mg contents sandwiched by Pt electrodes.
Abstract: We reported the discovery of colossal resistance switching effect in polycrystalline spinel-like structure MgZnO thin films with high Mg contents sandwiched by Pt electrodes. The ultrahigh resistance ratio of high resistance state to low resistance state of about seven to nine orders of magnitude with a low reset voltage of less than 1 V was obtained in this thin film system. The resistance ratio shows an increase of several orders of magnitude compared with those of previously reported resistance switching material systems including metal oxides, semiconductors, and organic molecules. This colossal resistance switching effect will greatly improve the signal-to-noise ratio and simplify the process of reading memory state for nonvolatile memory applications. Our study also provides a material base for studying the origin of resistance switching phenomenon.
TL;DR: In this paper, a single-phase La 0.7 Sr 0.3 MnO 3 nanoparticles were prepared by a simple chemical coprecipitation route and their structural, magnetoresistance, and magnetic properties were investigated.
TL;DR: In this paper, a systematic investigation of magnetization as a function of both magnetic field and temperature has been undertaken, and it has been found from the magnetization versus temperature plot of PCMO that it exhibits reentrant type of spin glass behavior at 44 K.
TL;DR: In this paper, the Schottky junctions Au/SrTiO3:0.5 Wtwt % Nb and Au/srNiO3 :0.05 Wt % nb were experimentally investigated in the temperature range from 293 to 454 K. Both junctions show electron tunneling-dominated transport behavior.
Abstract: Current-voltage characteristics and colossal electroresistance (CER) have been experimentally investigated in the temperature range from 293 to 454 K for the Schottky junctions Au/SrTiO3:0.5 wt % Nb and Au/SrTiO3:0.05 wt % Nb. Both junctions show electron tunneling-dominated transport behavior. Postannealing of SrTiO3:0.05 wt % Nb in oxygen atmosphere causes a transition of the transport behavior from electron tunneling to thermionic emission. The CER effect appears in the junctions with the transport behavior dominated by electron tunneling and greatly weakens when thermionic emission prevails after postannealing. This result reveals the presence of a close relation between CER and electron tunneling.
TL;DR: In this article, the authors show that the large positive magnetoresistance in nonmagnetic silicon devices can be controlled by a current applied in the non-Ohmic transport regime.
Abstract: We show that the large positive magnetoresistance in nonmagnetic silicon devices can be controlled by a current applied in the non-Ohmic transport regime. The experimental results indicate that the carrier transport in this regime is dominated by the space-charge effect, where the magnetoresistance effect is greatly enhanced. We propose a device concept based on the space-charge-induced magnetoresistance effect in silicon that is controlled by both the current and the magnetic field, which looks similar to the characteristics of the field-effect transistors.
TL;DR: In this paper, a composite series consisting of colossal magnetoresistive and ferroelectric materials, with chemical formula La 0.67 Ca 0.33 MnO 3 (LCMO) and BaTiO3 (BTO), has been prepared by the solid state route.
TL;DR: It is concluded that it must be induced by a single ferromagnetic contact, and that spin transport can be ruled out as the origin, and the importance of a systematic investigation of spin-valve devices in order to discriminate between ambiguous interpretations.
Abstract: We demonstrate electric field control of sign and magnitude of the magnetoresistance in InP nanowires with ferromagnetic contacts. The sign change in the magnetoresistance is directly correlated with a sign change in the transconductance. Additionally, the magnetoresistance is shown to persist at such a high bias that Coulomb blockade has been lifted. We also observe the magnetoresistance when one of the ferromagnets is replaced by a nonmagnetic metal. We conclude that it must be induced by a single ferromagnetic contact, and that spin transport can be ruled out as the origin. Our results emphasize the importance of a systematic investigation of spin-valve devices in order to discriminate between ambiguous interpretations.
TL;DR: In this paper, tunneling magnetoresistance is observed across the naturally occurring intrinsic insulating strips (tunnel barriers) spanning the width of the bridges, and the presence of such intrinsic tunnel barriers introduces an alternative approach to fabricating novel nanoscale magnetic tunnel junctions.
Abstract: The manganite $(\text{La},\text{Pr},\text{Ca}){\text{MnO}}_{3}$ is well known for its micrometer-scale phase separation into coexisting ferromagnetic metallic (FMM) and insulating regions. Fabrication of bridges with widths smaller than the phase-separation length scale has allowed us to probe the magnetic properties of individual phase-separated regions. At the onset of phase separation, a magnetic field induced insulator-to-metal transition among a discrete number of domains within the narrow bridges gives rise to abrupt, low-field colossal magnetoresistance steps at well-defined switching fields. At lower temperatures when the FMM phase becomes energetically favorable, the insulating regions shrink to form thin insulating strips separating adjacent FMM regions with different coercive fields. Tunneling magnetoresistance is observed across the naturally occurring intrinsic insulating strips (tunnel barriers) spanning the width of the bridges. The presence of such intrinsic tunnel barriers introduces an alternative approach to fabricating novel nanoscale magnetic tunnel junctions.
TL;DR: In this article, the giant magnetoresistance characteristics of magnetic III-V semiconductor heterojunctions are described, and it is shown that the magnetocurrent ratio of the junction saturates with magnetic field.
Abstract: The giant magnetoresistance characteristics of magnetic III--V semiconductor $p\text{\ensuremath{-}}n$ heterojunctions are described. The origin of the extremely large positive magnetoresistance (2680%) observed at room temperature and at a field of 18 T is attributed to efficient spin-polarized carrier transport. The magnetocurrent ratio of the junction saturates with magnetic field. The field dependence of the magnetoresistance points to the existence of a paramagnetic component, which determines the degree of spin polarization of the junction current. This work indicates that highly spin-polarized magnetic semiconductor heterojunction devices that operate at room temperature can be realized.
TL;DR: In this paper, the anisotropy of the Compton profile in bilayer manganite bilayer bilayers has been studied and the number and type of electrons involved in the metal-insulator transition is estimated.
Abstract: We have studied the [100]-[110] anisotropy of the Compton profile in the bilayer manganite. Quantitative agreement is found between theory and experiment with respect to the anisotropy in the two metallic phases (i.e., the low temperature ferromagnetic and the colossal magnetoresistant phase under a magnetic field of 7 T). Robust signatures of the metal-insulator transition are identified in the momentum density for the paramagnetic phase above the Curie temperature. We interpret our results as providing direct evidence for the transition from the metalliclike to the admixed ionic-covalent bonding accompanying the magnetic transition. The number of electrons involved in this phase transition is estimated. Our study demonstrates the sensitivity of the Compton scattering technique for identifying the number and type of electrons involved in the metal-insulator transition.
TL;DR: In this article, a series of samples were prepared by polyvinyl alcohol (PVA) gel route, and the electrical resistivity vs. temperature plot of the sample x = 0.10 is found to exhibit an insulating behavior below 36 K, while the sample, x =0.20 exhibits two peaks.
TL;DR: In this paper, the resistivity data of nanocrystalline samples of sodium doped manganites with compositional formula La1−xNaxMnO3 (0.025⩽x ⩽0.25) were analyzed using effective medium approximation.
Abstract: Nanocrystalline samples of sodium doped manganites with compositional formula La1−xNaxMnO3 (0.025⩽x⩽0.25) were prepared by polyvinyl alcohol assisted precursor method. After characterizing the samples by x-ray diffraction and transmission electron microscopy a systematic investigation of electrical, magnetic, and thermopower properties has been undertaken. The resistivity data were analyzed using effective medium approximation. From the analysis it has been found that the metallic fraction is increasing up to x=0.10 and remains constant with further doping. A close examination of the resistivity data clearly indicates that the sodium doped samples are slowly transformed from colossal magnetoresistance behavior to charge ordering behavior. Thermoelectric power data at low temperatures were analyzed by considering the magnon drag concept, while the high temperature data were explained by small polaron conduction mechanism.
TL;DR: In this article, the magnetocaloric effect in the La0.67Ca0.33Mn0.9Fe0.1O3 perovskite has been investigated and a peak in the magnetic entropy change [ΔSM(H)] versus T curve, centered around 113 K has been observed.
Abstract: We report the magnetocaloric effect in the La0.67Ca0.33Mn0.9Fe0.1O3 perovskite. A peak in the magnetic entropy change [ΔSM(H)] versus T curve, centered around 113 K, has been observed. −ΔSM(H) increases with an increasing applied magnetic field. The maximum value of −ΔSM(H) for ΔH=3 T field has been found to be 1.18 J kg−1 K−1 at 113 K with a relative cooling power of ∼87 J kg−1 and a quite broad operative temperature range 65–160 K. The hysteresis loss has been found to be very small. A large magnetocaloric effect with a relatively low material cost makes the studied perovskite attractive for magnetic refrigeration.
TL;DR: In this article, the temperature dependence of the ac resistance and the inductive reactance (X) of the complex impedance (Z = R+jX) under different dc bias magnetic fields (Hdc = 0-1 kOe) were studied for different frequencies f = 0.1 to 5 MHz of alternating current.
Abstract: We report dynamical magnetotransport in a ferromagnetic metallic oxide, La0.7Sr0.3MnO3 using the ac impedance technique. The temperature dependence of the ac resistance(R) and the inductive reactance (X) of the complex impedance (Z = R+jX) under different dc bias magnetic fields (Hdc = 0-1 kOe) were studied for different frequencies f = 0.1 to 5 MHz of alternating current. The zero field R, which decreases smoothly around the Curie temperature TC for f = 100 kHz, transforms into a peak for f = 0.5-5 MHz. The peak decreases in amplitude, broadens and shifts downward in temperature as the bias field increases. A huge ac magnetoresistance (= 45 % at f = 2 MHz) in a field of Hdc = 1 kOe is found and we attribute it to the magnetic field- induced enhancement in the skin depth and concomitant suppression of magnetic fluctuations near TC. Our study suggests that radio frequency magnetotransport provides an alternative strategy to enhance the magnetoresistance and probe the spin-charge coupling in manganites.
TL;DR: In this paper, the authors investigated the hysteretic behavior of granular high-temperature superconductors (HTSCs) for transport current densities lower and higher than the critical density (at H = 0).
Abstract: The hysteretic behavior of the magnetoresistance R(H) of granular high-temperature superconductors (HTSCs) of the Y-Ba-Cu-O, Bi-Ca-Sr-Cu-O, and La-Sr-Cu-O classical systems is investigated for transport current densities lower and higher than the critical density (at H = 0). All systems exhibit universal behavior of the width of the magnetoresistance hysteresis loop: independence of transport current under identical external conditions. This means that flux trapping in HTSC grains is the main mechanism controlling the hysteretic behavior of the magnetoresistance of granular HTSCs, while pinning of Josephson vortices in the intragranular medium makes no appreciable contribution to the formation of magnetoresistance hysteresis (when transport current flows through the sample). Experimental data on relaxation of residual resistance after the action of a magnetic field also confirm this conclusion.
TL;DR: In this paper, the temperature and magnetic field dependence of angular dependent magnetoresistance (AMR) along two orthogonal directions were investigated in a charge-orbital-ordered Sm0.5Ca 0.5MnO3 film.
Abstract: The temperature and magnetic field dependence of angular dependent magnetoresistance (AMR) along two orthogonal directions ([100] and [01¯1]) was investigated in a charge-orbital-ordered Sm0.5Ca0.5MnO3 (SCMO) film grown on (011)-oriented SrTiO3 substrates. A dramatic decrease of AMR magnitude in both directions was observed with the appearance of magnetic-field-induced metal-insulator transition, which further led to a sign crossover in the AMR effect. The AMR crossover may give a direct evidence of the drastic modification of electronic structure or possible orbital reconstruction with the magnetic-destruction of charge/orbital ordering in SCMO films.
TL;DR: Comparisons between the latter magnetic data provide the first unequivocal demonstration that the presence of Griffiths-phase-like (GP) features do not guarantee colossal magnetoresistance (CMR), while confirming that neither are the appearance of such features a prerequisite for CMR.
Abstract: Detailed measurements of the magnetic and transport properties of single crystals of La(1-x)Ca(x)MnO(3) (0.18 ≤ x ≤ 0.27) are summarized, and lead to the following conclusions. While temperature-dependent (magneto-) resistance measurements narrow the compositionally modulated metal-insulator (M-I) transition to lie between 0.19 ≤ x(c) ≤ 0.20 in the series studied, comparisons between the latter magnetic data provide the first unequivocal demonstration that (i) the presence of Griffiths-phase-like (GP) features do not guarantee colossal magnetoresistance (CMR), while confirming (ii) that neither are the appearance of such features a prerequisite for CMR. These data also reveal that (iii) whereas continuous magnetic transitions occur for 0.18 ≤ x ≤ 0.25, the universality class of these transitions belongs to that of a nearest-neighbour 3D Heisenberg model only for x≤0.20, beyond which complications due to GP-like behaviour occur. The implications of the variation (or lack thereof) in critical exponents and particularly critical amplitudes and temperatures across the compositionally mediated M-I transition support the assertion that the dominant mechanism underlying ferromagnetism across the M-I transition changes from ferromagnetic super-exchange (SE) stabilized by orbital ordering in the insulating phase to double-exchange (DE) in the orbitally disordered metallic regime. The variations in the acoustic spin-wave stiffness, D, and the coercive field, H(C), support this conclusion. These SE and DE interaction mechanisms are demonstrated to not only belong to the same universality class but are also characterized by comparable coupling strengths. Nevertheless, their percolation thresholds are manifestly different in this system.
TL;DR: Results of inelastic neutron scattering measurements show that inside the metallic phase polarons remain as fluctuations that strongly broaden and soften certain phonons near the wavevectors where the charge-order peaks appeared in the insulating phase, implying that polaronic signatures in metals may generally come from a competing insulating charge-ordered phase.
Abstract: Charge carriers in low-doped semiconductors may distort the atomic lattice around them and through this interaction form so-called small polarons. High carrier concentrations on the other hand can lead to short-range ordered polarons (large polarons) and even to a long-range charge and orbital order. These ordered systems should be insulating with a large electrical resistivity. However, recently a polaronic pseudogap was found in a metallic phase of La(2-2x)Sr(1+2x)Mn(2)O(7) (ref. 7). This layered manganite is famous for colossal magnetoresistance associated with a phase transition from this low-temperature metallic phase to a high-temperature insulating phase. Broad charge-order peaks due to large polarons in the insulating phase disappear when La(2-2x)Sr(1+2x)Mn(2)O(7) becomes metallic. Investigating how polaronic features survive in the metallic phase, here we report the results of inelastic neutron scattering measurements showing that inside the metallic phase polarons remain as fluctuations that strongly broaden and soften certain phonons near the wavevectors where the charge-order peaks appeared in the insulating phase. Our findings imply that polaronic signatures in metals may generally come from a competing insulating charge-ordered phase. Our findings are highly relevant to cuprate superconductors with both a pseudogap and a similar phonon effect associated with a competing stripe order.
TL;DR: In this article, the magneto-optical response in the visible spectrum of thin-film ferromagnetic manganites was investigated and it was found that the unusually large MRE at visible frequencies is accompanied by an increase in the magnetic field of the optical conductivity.
Abstract: We have investigated the magneto-optical response in the visible spectrum of thin-film ferromagnetic manganites. We observe a huge enhancement of the magnetorefractive effect (MRE) around the Curie temperature, which is linked to the colossal magnetoresistance. It is found that the unusually large MRE at visible frequencies is accompanied by an increase in the magnetic field of the optical conductivity. We argue that these remarkable phenomena are related to the field-induced suppression of Jahn-Teller dynamical charge localization.