Showing papers on "Colossal magnetoresistance published in 2005"
TL;DR: The spontaneous emergence of electronic nanometer-scale structures in transition metal oxides, and the existence of many competing states, are properties often associated with complex matter where nonlinearities dominate, such as soft materials and biological systems.
Abstract: A wide variety of experimental results and theoretical investigations in recent years have convincingly demonstrated that several transition metal oxides and other materials have dominant states that are not spatially homogeneous. This occurs in cases in which several physical interactions-spin, charge, lattice, and/or orbital-are simultaneously active. This phenomenon causes interesting effects, such as colossal magnetoresistance, and it also appears crucial to understand the high-temperature superconductors. The spontaneous emergence of electronic nanometer-scale structures in transition metal oxides, and the existence of many competing states, are properties often associated with complex matter where nonlinearities dominate, such as soft materials and biological systems. This electronic complexity could have potential consequences for applications of correlated electronic materials, because not only charge (semiconducting electronic), or charge and spin (spintronics) are of relevance, but in addition the lattice and orbital degrees of freedom are active, leading to giant responses to small perturbations. Moreover, several metallic and insulating phases compete, increasing the potential for novel behavior.
1,714 citations
TL;DR: In this paper, the authors performed a comprehensive magnetoresistance study on a set of organic semiconductor sandwich devices made from different conjugated polymers and small molecules, including a range of materials that show greatly different chemical structure, mobility, and spin-orbit coupling strength.
Abstract: Following the recent observation of large magnetoresistance at room temperature in polyfluorene sandwich devices, we have performed a comprehensive magnetoresistance study on a set of organic semiconductor sandwich devices made from different $\ensuremath{\pi}$-conjugated polymers and small molecules The study includes a range of materials that show greatly different chemical structure, mobility, and spin-orbit coupling strength We study both hole and electron transporters at temperatures ranging from 10 K to 300 K We observe large negative or positive magnetoresistance (up to 10% at 300 K and 10 mT) depending on material and device operating conditions We discuss our results in the framework of known magnetoresistance mechanisms and find that none of the existing models can explain our results
350 citations
TL;DR: In this paper, a list of 'things we know' and 'thing we do not know' in manganites and other compounds is presented. But the list is limited to compounds that do not involve ferromagnets.
Abstract: This is an informal paper that contains a list of 'things we know' and 'things we do not know' in manganites and other compounds. It is adapted from the Conclusions chapter of a recent book by the author, Nanoscale Phase Separation and Colossal Magnetoresistance. The Physics of Manganites and Related Compounds (Berlin: Springer-Verlag; 2002), but it also contains a summary of some of the most important recent results in the field. It is argued that the current main theoretical and experimental frameworks to rationalize the results of recent manganite investigations are based on the discovery of tendencies towards nanoscale inhomogeneous states, both in experiments and in simulations of models. The colossal magnetoresistance effect appears to be closely linked to these mixed-phase tendencies, although considerably more work is needed to fully confirm these ideas. The paper also includes information on cuprates, diluted magnetic semiconductors, relaxor ferroelectrics, cobaltites and organic and heavy fermion superconductors. These materials potentially share some common phenomenology with the manganites, such as a temperature scale T* above the ordering temperature where anomalous behaviour starts. Many of these materials also present low-temperature phase competition. The possibility of colossal-like effects in compounds that do not involve ferromagnets is briefly discussed. In particular, colossal effects in cuprates are explained. Overall, it is concluded that inhomogeneous 'clustered' states should be considered as a new paradigm in condensed matter physics, since their presence appears to be far more common than previously anticipated.
320 citations
TL;DR: In this article, a gate-field-controlled magnetoresistance response in carbon nanotubes connected by ferromagnetic leads is reported, which can be tuned in a predictable manner.
Abstract: Spintronics aims to develop electronic devices whose resistance is controlled by the spin of the charge carriers that flow through them1,2,3. This approach is illustrated by the operation of the most basic spintronic device, the spin valve4,5,6, which can be formed if two ferromagnetic electrodes are separated by a thin tunnelling barrier. In most cases, its resistance is greater when the two electrodes are magnetized in opposite directions than when they are magnetized in the same direction7,8. The relative difference in resistance, the so-called magnetoresistance, is then positive. However, if the transport of carriers inside the device is spin- or energy-dependent3, the opposite can occur and the magnetoresistance is negative9. The next step is to construct an analogous device to a field-effect transistor by using this effect to control spin transport and magnetoresistance with a voltage applied to a gate10,11. In practice though, implementing such a device has proved difficult. Here, we report on a pronounced gate-field-controlled magnetoresistance response in carbon nanotubes connected by ferromagnetic leads. Both the magnitude and the sign of the magnetoresistance in the resulting devices can be tuned in a predictable manner. This opens an important route to the realization of multifunctional spintronic devices.
296 citations
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TL;DR: The perovskite manganites of general formula RE_1-xAe_xMnO_3 (RE= rare earth,AE=Ca, Sr, Ba and Pb) have drawn considerable attention, especially following the discovery of colossal magnetoresistance (CMR) as mentioned in this paper.
Abstract: The perovskite manganites of general formula RE_1-xAe_xMnO_3 (RE= rare earth,AE=Ca, Sr, Ba and Pb)have drawn considerable attention, especially following the discovery of colossal magnetoresistance (CMR). They exhibit extraordinary large magnetoresistance pronounced as CMR in the vicinity of insulator-metal/paramagnetic-ferromagnetic transition at a relatively large applied magnetic fields. However, for applied aspectes, occurence of significant CMR at low applied magnetic fields would be required. This review consists of of two sections: In the first section we have extensively reviewed the salient features e.g. structure, phase diagram, double exchange mechansim, Jahn Teller effect, different types of ordering and phase separation of CMR mangnaites. The second is devoted to an overview of experimental results on CMR and related magnetotransport characteristics at low magnetic fields for doped manganites such as polycrystalline La_0.67Ca_0.33MnO_3 films, Ag admixed La_0.67Ca_0.33MnO_3 films, polycrystalline (La_0.7Ca_0.2Ba_0.1MnO_3)and epitaxial (La_0.67Ca_0.33MnO_3) films on different substrates, nanophasic La_0.7Ca_0.3MnO_3, mangnaite-polymer composites (La_0.7Ba_0.2Sr_0.1MnO_3-PMMA and La_0.67Ca_0.33MnO_3-PMMA)and double layered polycrystalline (La_1.4Ca_1.6-xBa_xMn_2O_7) and films (La_1.4Ca_1.6Mn_2O_7). Some other potential magnetoresistive materials e.g. pyrochlores, chalcogenides, ruthenates, diluted magnetic semiconductors, magnetic tunnel junctions, nanocontacts etc have aslo been briefly dealt with. The review concludes with the summary of results for low field magnetotransport behaviour and prospectes for applications.
211 citations
TL;DR: This work argues that magnetic and charge modulation coexist in new thermodynamic phases, and argues for a fundamental reinterpretation of the nature of charge modulation in these materials, from a localized to a more extended ‘charge-density wave’ picture.
Abstract: The phenomenon of colossal magnetoresistance in manganites1 is generally agreed to be a result of competition between crystal phases with different electronic, magnetic and structural order; a competition which can be strong enough to cause phase separation between metallic ferromagnetic and insulating charge-modulated states2,3,4,5. Nevertheless, closer inspection of phase diagrams in many manganites reveals complex phases where the two order parameters of magnetism and charge modulation unexpectedly coexist6,7. Here we show that such experiments can be naturally explained within a phenomenological Ginzburg–Landau theory. In contrast to models where phase separation originates from disorder8 or as a strain-induced kinetic phenomenon9, we argue that magnetic and charge modulation coexist in new thermodynamic phases. This leads to a rich diagram of equilibrium phases, qualitatively similar to those seen experimentally. The success of this model argues for a fundamental reinterpretation of the nature of charge modulation in these materials, from a localized to a more extended ‘charge-density wave’ picture. The same symmetry considerations that favour textured coexistence of charge and magnetic order may apply to many electronic systems with competing phases. The resulting ‘electronically soft’ phases of matter with incommensurate, inhomogeneous and mixed order may be general phenomena in correlated systems.
208 citations
TL;DR: In this article, the magnetocaloric effect in La 0.7 Sr 0.3 MnO 3 was studied in terms of magnetoelastic and electron interaction contribution to the magnetic entropy.
174 citations
TL;DR: Experimental evidence is reported that a very similar pseudogap state with a nodal–antinodal dichotomous character exists in a system that is markedly different from a superconductor: the ferromagnetic metallic groundstate of the colossal magnetoresistive bilayer manganite La1.2Sr1.8Mn2O7.
Abstract: A surprise finding has united the physics of two of the most important classes of materials in condensed-matter physics, but it seems to raise more questions than it answers. High-temperature copper oxide superconductors display what was thought to be a characteristic feature, a difference between electronic excitations in directions parallel and diagonal to the bonds between copper and oxygen atoms. This gap structure or ‘pseudogap’ state seemed to be unique to these compounds, so it was natural to assume it had something to do with superconductivity. But now a similar state has been identified in a ferromagnetic bilayer manganite, a ‘colossal magnetoresistive material’ that is very different from a superconductor. The ‘enigmatic’ pseudogap structure has become a bit more enigmatic, and as a result researchers are likely to reconsider the nature of the link to superconductivity. A characteristic feature of the copper oxide high-temperature superconductors is the dichotomy between the electronic excitations along the nodal (diagonal) and antinodal (parallel to the Cu–O bonds) directions in momentum space, generally assumed to be linked to the ‘d-wave’ symmetry of the superconducting state. Angle-resolved photoemission measurements in the superconducting state have revealed a quasiparticle spectrum with a d-wave gap structure that exhibits a maximum along the antinodal direction and vanishes along the nodal direction1. Subsequent measurements have shown that, at low doping levels, this gap structure persists even in the high-temperature metallic state, although the nodal points of the superconducting state spread out in finite ‘Fermi arcs’2. This is the so-called pseudogap phase, and it has been assumed that it is closely linked to the superconducting state, either by assigning it to fluctuating superconductivity3 or by invoking orders which are natural competitors of d-wave superconductors4,5. Here we report experimental evidence that a very similar pseudogap state with a nodal–antinodal dichotomous character exists in a system that is markedly different from a superconductor: the ferromagnetic metallic groundstate of the colossal magnetoresistive bilayer manganite La1.2Sr1.8Mn2O7. Our findings therefore cast doubt on the assumption that the pseudogap state in the copper oxides and the nodal-antinodal dichotomy are hallmarks of the superconductivity state.
167 citations
TL;DR: It is argued that this system is a natural analog to the artificial structures fabricated by depositing nanoscale ferromagnetic particles in a metallic or insulating matrix; i.e., this material displays a GMR effect without the deliberate introduction of chemical interfaces.
Abstract: We present small-angle neutron scattering data proving that, on the insulating side of the metal-insulator transition, the doped perovskite cobaltite ${\mathrm{L}\mathrm{a}}_{1\ensuremath{-}x}{\mathrm{S}\mathrm{r}}_{x}{\mathrm{C}\mathrm{o}\mathrm{O}}_{3}$ phase separates into ferromagnetic metallic clusters embedded in a nonferromagnetic matrix. This induces a hysteretic magnetoresistance, with temperature and field dependence characteristic of intergranular giant magnetoresistance (GMR). We argue that this system is a natural analog to the artificial structures fabricated by depositing nanoscale ferromagnetic particles in a metallic or insulating matrix; i.e., this material displays a GMR effect without the deliberate introduction of chemical interfaces.
144 citations
TL;DR: In this paper, a positive colossal magnetoresistance (CMR) was discovered at low applied magnetic field and high temperature in the epitaxial p-n heterostructure fabricated with Sr-doped LaMnO3 and Nb-Doped SrTiO3 by laser molecular-beam epitaxy.
Abstract: Different from the negative colossal magnetoresistance (CMR) of the LaMnO3 compound family, a positive CMR has been discovered at low applied magnetic field and high temperature in the epitaxial p-n heterostructure we fabricated with Sr-doped LaMnO3 and Nb-doped SrTiO3 by laser molecular-beam epitaxy. The mechanism causing the unusual positive CMR is proposed as a interface effect, i.e., the creation of a space charge region at the interface with different electron filling in bands comparing to that in the homogeneous region in Sr-doped LaMnO3. Other puzzling CMR features with bias voltage, temperature, and even composition are well explained by the present scenario.
143 citations
TL;DR: In this article, a systematic investigation of lanthanum-based manganite, La0.67Ca0.33MnO3, has been undertaken with a view to understand the influence of varying crystallite size, in the nanoscale, on various physical properties.
Abstract: A systematic investigation of lanthanum-based manganite, La0.67Ca0.33MnO3, has been undertaken with a view to understand the influence of varying crystallite size, in the nanoscale, on various physical properties. The materials were prepared by the sol–gel route by sintering at four different temperatures starting from 800 to 1100 °C, with an interval of 100 °C. After the usual characterization of these materials structurally by XRD, their metal-insulator transition (TP) as well as magnetic transition (TC) temperatures were determined. Surprisingly these materials are found to exhibit two different types of behaviors, viz, while TC is found to decrease from 253 to 219 K, TP is increasing from 145 to 195 K with increasing sintering temperature. A systematic study of electrical conductivity of all four materials was undertaken not only as a function of temperature (80–300 K), but also as a function of magnetic field up to 7 T mainly to understand the detailed conduction mechanism in these materials. On analyzing the data by using several theoretical models, it has been concluded that the metallic (ferromagnetic) part of the resistivity (ρ) (below TP) fits well with the equation ρ ( T ) = ρ 0 + ρ 2.5 T 2.5 , indicating the importance of grain/domain boundary effects (ρ0) and electron–magnon scattering processes (∼T2.5). On the other hand, in the high temperature (T>TP) paramagnetic insulating regime, the adiabatic small polaron and VRH models fit well in different temperature regions, thereby indicating that polaron hopping might be responsible for the conduction mechanism.
TL;DR: In this article, a detailed investigation of the valence of manganese sites at the surface of colossal magnetoresistance La 0.7Sr 0.3MnO3 (LSMO) thin films by x-ray absorption spectroscopy (XAS) is presented.
Abstract: We present a detailed investigation of the valence of manganese sites at the surface of colossal magnetoresistance La0.7Sr 0.3MnO3 (LSMO) thin films by x-ray absorption spectroscopy (XAS). The XAS Mn L-edge spectra of epitaxial LSMO films usually show a peak or shoulder at 640 eV. Differences in the intensity of this feature are commonly attributed to slight changes in the Mn3+/Mn4+ ratio or the crystal field strength. By comparison of different XAS spectra of LSMO thin films with the known multiplet structure of Mn2+ in a cubic crystal field, we assign this 640-eV feature to Mn2+ ions. XAS with increased surface sensitivity, combined with photon energy-dependent photoelectron spectroscopy measurements of the Mn(3s) exchange splitting, show that the Mn2+ species are mainly located at the surface. The Mn2+ scenario indicates significant modification of the LSMO surface with respect to the bulk properties that should be taken into account in all the charge and spin tunneling and injection experiments. © 2005 The American Physical Society.
TL;DR: In this paper, the authors studied the low-frequency dielectric properties of phase-separated manganite as a function of applied magnetic field in the low temperature phase separated state.
Abstract: We have studied the low-frequency dielectric properties of the phase-separated manganite ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ as a function of applied magnetic field in the low temperature phase-separated state. The dielectric constant is strongly field dependent and also depends on the magnetic field history of the sample. The dielectric behavior appears to be associated with the hopping of polaronic charge carriers, and we can derive the field dependent hopping energy barrier from the frequency dependence of the dielectric constant. This analysis allows us to associate the metal-insulator transition observed in this material with the field-induced suppression of the polaron activation energy.
TL;DR: In this paper, a colossal magnetoresistance material, Nd0.67Sr0.33MnO3 was prepared by the sol-gel technique by sintering at four different temperatures between 800 and 1100°C mainly to understand the influence of varying crystallite size (in the nanorange) on various physical properties.
TL;DR: In this paper, a systematic investigation of Neodymium based colossal magnetoresistive manganites with general formula Nd 0.67 A 0.33 MnO 3, (A=Ca, Sr, Pb and Ba) has been undertaken mainly to understand their structural, magnetic as well as electrical behavior.
TL;DR: In this paper, a thorough study of the magnetocaloric effect (MCE) in a colossal magnetoresistive compound of La 0.7Sr0.3Mn0.98Ni0.02O3 has been made.
Abstract: A thorough study of the magnetocaloric effect (MCE) in a colossal magnetoresistive compound of La0.7Sr0.3Mn0.98Ni0.02O3 has been made. The large magnetic entropy change of 7.65 J∕kg K upon an applied field of 70 kOe has been found to occur at 350 K, which allows magnetic refrigeration at room temperature. It is interesting to note that, even in high magnetic fields, the magnetic entropy change versus temperature distribution is much more uniform than that of gadolinium and several polycrystalline perovskite manganites, which is desirable for an Ericson-cycle magnetic refrigerator. It is found that such a small amount (∼2%) of substitution of Mn3+ by a magnetic ion (Ni3+ or Co3+) in the perovskite manganite can favor the spin order and hence the MCE. Undoubtedly, this observation opens a window to explore the active magnetic refrigeration at high temperatures.
TL;DR: The effect of TiO2 doping on the structure and magnetic and magnetotransport properties of La0.75Sr0.25MnO3 (LSMO)∕xTiO2 has been investigated in this paper.
Abstract: The effect of TiO2 doping on the structure and magnetic and magnetotransport properties of La0.75Sr0.25MnO3 (LSMO)∕xTiO2 has been investigated. These studies show that at low doping level (x⩽2) TiO2 mainly goes into the grain-boundary region, but at high doping level (x⩾3), some part of the TiO2 goes into the perovskite lattice substituting Mn in LSMO and the remainder segregates as a separate phase at the grain boundaries. Results also show that the TiO2 doping has an important effect on a low-field magnetoresistance. In the magnetic field of 8000Oe and at 77K a magnetoresistance value of ∼20% was observed for the composite with a TiO2 doping level of x=2.
TL;DR: Current-induced switching from a metallic to an insulating state is observed in phase-separated states of (La(1-y)Pr(y), and this oscillation is well reproduced by a simple model of local switching of a percolative conduction path.
Abstract: Current-induced switching from a metallic to an insulating state is observed in phase-separated states of (La(1-y)Pr(y))0.7Ca0.3MnO3 (y=0.7) and Nd(0.5)Ca(0.5)Mn(1-z)Cr(z)O3 (z=0.03) crystals. The application of magnetic fields to this current-induced insulating state causes a pronounced low-field negative magnetoresistance effect [rho(H)/rho(0)=10(-3) at H=1 kOe]. The application of a constant voltage also causes the breakdown of the Ohmic relation above a threshold voltage. At voltages higher than this threshold value, oscillations in currents are observed. This oscillation is well reproduced by a simple model of local switching of a percolative conduction path.
TL;DR: Evidence for unconventional magnetoresistance of double-walled carbon nanotubes under a magnetic field as large as 50 T is presented and interpreted by means of theoretical calculations.
Abstract: We report on the first experimental study of the magnetoresistance of double-walled carbon nanotubes under magnetic field as large as 50 Tesla. By varying the field orientation with respect to the tube axis, or by gate-mediated shifting the Fermi level position, evidences for unconventional magnetoresistance are presented and interpreted by means of theoretical calculations.
TL;DR: In this article, the integration of epitaxial colossal magnetoresistive La0.67Ba0.33MnO films on Si(100) semiconductor using SrTiO3 template layer by pulsed-laser deposition was reported.
Abstract: We report on the integration of epitaxial colossal magnetoresistive La0.67Ba0.33MnO films on Si(100) semiconductor using SrTiO3 template layer by pulsed-laser deposition. X-ray diffraction reveals the superior quality of the manganite film that grows epitaxially on heteroepitaxially grown SrTiO3 template layer on Si substrate. The epitaxial films demonstrate remarkable surface morphology, magnetic transition and hysteresis, magnetoresistance, and ferromagnetic resonance, illustrating the ferromagnetic nature of the film and possible device applications at room temperature.
TL;DR: In this article, the influence of the Joule effect on the nonlinear behavior of the transport I-V curves in polycrystalline samples of the manganite Pr0.8Ca0.2MnO3 was studied.
Abstract: We study the influence of the Joule effect on the nonlinear behavior of the transport I-V curves in polycrystalline samples of the manganite Pr0.8Ca0.2MnO3 by using the crystalline unit-cell parameters as an internal thermometer in x-ray and neutron diffractions. We develop a simple analytical model to estimate the temperature profile in the samples. Under the actual experimental conditions we show that the internal temperature gradient or the difference between the temperature of the sample and that of the thermal bath is at the origin of the nonlinearity observed in the I-V curves. Consequences on other compounds with colossal magnetoresistance are also discussed.
TL;DR: In this paper, a massive magnetoresistive (CMR) system with Ni doping was studied, where the spin-wave stiffness constant D, characterising the magnon in the low-temperature (T p ) ferromagnetic phase, estimated from the magnetisation (M ) data, decreases with increasing Ni content.
TL;DR: In this article, three examples of ceramics are reviewed, which are all characterized by a mixed valence of the transition element, allowing electronic delocalization to be produced: high Tc superconducting cuprates (Cu2+/Cu3+), thermoelectric cobaltites (Co3+/Co4+) and colossal magnetoresistance manganites (Mn3++/Mn4+).
Abstract: Transition metal oxides represent a considerable potential for the generation of new frameworks with new magnetic and transport properties with a view of applications in the field of electronics. Three examples of ceramics are reviewed herein, which are all characterized by a mixed valence of the transition element, allowing electronic delocalization to be produced: high Tc superconducting cuprates (Cu2+/Cu3+), thermoelectric cobaltites (Co3+/Co4+) and colossal magnetoresistance manganites (Mn3+/Mn4+).
TL;DR: In this paper, the same authors showed that magnetic semiconductors with large magnetoresistance can be obtained based on conventional oxides with independent doping of spins and carriers, and they also found negative magnetoreduction in these compounds.
Abstract: We studied $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ into which spins and carriers are doped by Cr substitution for Ti and La substitution for Sr, respectively. It was found that Weiss temperatures vary from negative to positive values with carrier doping, indicating the appearance of a ferromagnetic interaction between Cr spins via the conduction carriers. We also found negative magnetoresistance in these compounds, whose magnitude amounts to 70% at low temperatures. These results demonstrate that magnetic semiconductors with large magnetoresistance can be obtained based on conventional oxides, like $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$, with independent doping of spins and carriers.
TL;DR: In this paper, a heterojunction has been fabricated by growing a La067Ca033MnO3 film on silicon with a buffer layer of yttrium-stabilized zirconia (YSZ).
Abstract: A heterojunction has been fabricated by growing a La067Ca033MnO3 film on silicon with a buffer layer of yttrium-stabilized zirconia (YSZ) The current-voltage measurement shows that it is a diode with a good rectifying property At low positive bias voltage, temperature dependence of the junction resistance shows a peak at a certain temperature, which shifts to low temperatures when the voltage is increased from 03Vto07V This behavior is quite different from the previous reports on p-n junctions composed of manganites and Nb-doped SrTiO3 The heterojunction shows remarkable magnetoresistance for both positive and negative biases The results were discussed by considering the depletion layers in both La067Ca033MnO3 and Si, and the tunneling through YSZ This work shows the potential application of integrating manganite-based devices and semiconductor circuits
TL;DR: In this article, the magnetotransport properties of La1−xCaxMnO3 (x=0.33, 0.5) thin films on different substrates are systematically studied.
Abstract: Magnetotransport properties of La1−xCaxMnO3 (x=0.33, 0.5) thin films on different substrates are systematically studied. Due to the different lattice-substrate mismatch between films and substrates, compressive, and tensile epitaxial strain can be induced, respectively. The structure distortion induced by the strain with different sign influences the transport property and magnetoresistance. The tensile strain induces an increase of resistivity and magnetoresistance, while the resistivity peak temperature decreases. Compressive strain leads to a contrary behavior. The microstructure of films is also dominated by the lattice-substrate mismatch strongly. The inhomogeneities and different mixed domain structure induce the broadening of magnetic transition in films on SrTiO3 and NdGaO3, and a sharp increase in magnetoresistance in La0.67Ca0.33MnO3 films with increasing field. In addition, the low temperature metal–insulator transition in the resistivity of La0.5Ca0.5MnO3 film on LaAlO3 also indicates the samp...
TL;DR: In this paper, the authors studied the spin excitation spectrum in the presence of disorder in the double exchange model and compared the results with experimental results in manganese oxides and showed that spatially correlated or mesoscopic scale disorder is more relevant in real compounds than local or atomic-scale disorder.
Abstract: Spin excitation spectrum is studied in the double-exchange model in the presence of disorder. Spin wave approximation is applied in the lowest order of $1∕S$ expansion. The disorder causes anomalies in the spin excitation spectrum such as broadening, branching, anticrossing with gap opening. The origin of the anomalies is the Friedel oscillation, in which the perfectly polarized electrons form the charge density wave to screen the disorder effect. Near the zone center $q=0$, the linewidth has a $q$ linear component while the excitation energy scales to ${q}^{2}$, which indicates that the magnon excitation is incoherent. As $q$ increases, there appears a crossover from this incoherent behavior to the marginally coherent one in which both the linewidth and the excitation energy are proportional to ${q}^{2}$. The results are compared with experimental results in colossal magnetoresistance manganese oxides. Quantitative comparison of the linewidth suggests that spatially-correlated or mesoscopic-scale disorder is more relevant in real compounds than local or atomic-scale disorder. Comparison with other theoretical proposals is also discussed. Experimental tests are proposed for the relevance of disorder.
TL;DR: In this paper, the transport properties of mechanically stable Fe3O4 and La0.7Sr0.3MnO3 nanoconstrictions patterned by focused ion-beam milling were studied.
Abstract: We have studied the transport properties of mechanically stable Fe3O4 and La0.7Sr0.3MnO3 nanoconstrictions patterned by focused ion-beam milling. The magnetoresistance decreases with the square of the applied voltage and scales with the resistance of the constriction, with values up to 8000% for magnetite and 100% for La0.7Sr0.3MnO3. These results are interpreted within a model for domain-wall magnetoresistance. Some samples exhibit electrical hysteresis with discrete changes of resistance that disappear in the presence of a magnetic field, indicating domain-wall displacement driven by a spin-polarized current.