Showing papers on "Colossal magnetoresistance published in 1999"
TL;DR: A review of the literature on mixed-valence manganites, placing new results in the context of established knowledge of these materials, and other magnetic semiconductors, is given in this paper.
Abstract: Mixed-valence manganese oxides (R1-χAχ)MnO3 (R=rare-earth cation, A=alkali or alkaline earth cation), with a structure similar to that of perovskite CaTiO3, exhibit a rich variety of crystallographic, electronic and magnetic phases. Historically they led to the formulation of new physical concepts such as double exchange and the Jahn-Teller polaron. More recent work on thin films has revealed new phenomena, including colossal magnetoresistance near the Curie temperature, dense granular magnetoresistance and optically-induced magnetic phase transitions. This review gives an account of the literature on mixed-valence manganites, placing new results in the context of established knowledge of these materials, and other magnetic semiconductors. Issues addressed include the nature of the electronic ground states, the metal-insulator transition as a function of temperature, pressure and applied magnetic field, the electronic transport mechanisms, dielectric and magnetic polaron formation, magnetic localization, ...
1,757 citations
TL;DR: In this article, it was shown that the magnetoresistive response increases dramatically when the Curie temperature (T C) is reduced, and that the massive magnetoresistance in low-T C systems can be explained by percolative transport through the ferromagnetic domains; this depends sensitively on the relative spin orientation of adjacent magnetoric domains which can be controlled by applied magnetic fields.
Abstract: Colossal magnetoresistance1—an unusually large change of resistivity observed in certain materials following application of magnetic field—has been extensively researched in ferromagnetic perovskite manganites. But it remains unclear why the magnetoresistive response increases dramatically when the Curie temperature (T C) is reduced. In these materials, T C varies sensitively with changing chemical pressure; this can be achieved by introducing trivalent rare-earth ions of differing size into the perovskite structure2,3,4, without affecting the valency of the Mn ions. The chemical pressure modifies local structural parameters such as the Mn–O bond distance and Mn–O–Mn bond angle, which directly influence the case of electron hopping between Mn ions (that is, the electronic bandwidth). But these effects cannot satisfactorily explain the dependence of magnetoresistance on T C. Here we demonstrate, using electron microscopy data, that the prototypical (La,Pr,Ca)MnO3 system is electronically phase-separated into a sub-micrometre-scale mixture of insulating regions (with a particular type of charge-ordering) and metallic, ferromagnetic domains. We find that the colossal magnetoresistive effect in low-T C systems can be explained by percolative transport through the ferromagnetic domains; this depends sensitively on the relative spin orientation of adjacent ferromagnetic domains which can be controlled by applied magnetic fields.
1,417 citations
TL;DR: In this paper, the magnetic properties of perovskite-type manganites are overviewed in the light of the mechanism of the colossal magnetoresistance (CMR).
896 citations
TL;DR: The results suggest that the transition and the associated magnetoresistance behavior should be viewed as a percolation of metallic ferromagnetic domains.
Abstract: Scanning tunneling spectroscopy was used to investigate single crystals and thin films of La1– x Ca x MnO3(with x of about 0.3), which exhibit colossal magnetoresistance. The different spectroscopic signatures of the insulating (paramagnetic) and metallic (ferromagnetic) phases enable their spatial extent to be imaged down to a lateral scale of the order of 10 nanometers. Above the bulk transition temperature T c, the images show mostly insulating behavior. Below T c, a phase separation is observed where inhomogeneous structures of metallic and more insulating areas coexist and are strongly field dependent in their size and structure. Insulating areas are found to persist far below T c. These results suggest that the transition and the associated magnetoresistance behavior should be viewed as a percolation of metallic ferromagnetic domains.
650 citations
TL;DR: In this paper, the anomalous Hall effect (AHE) observed in colossal magnetoresistance manganites is a manifestation of Berry phase effects caused by carrier hopping in a nontrivial spin background.
Abstract: We show that the anomalous Hall effect (AHE) observed in colossal magnetoresistance manganites is a manifestation of Berry phase effects caused by carrier hopping in a nontrivial spin background. We determine the magnitude and temperature dependence of the Berry phase contribution to the AHE, finding that it increases rapidly in magnitude as the temperature is raised from zero through the magnetic transition temperature ${T}_{c}$, peaks at a temperature ${T}_{\mathrm{max}}g{T}_{c}$, and decays as a power of $T$, in agreement with experimental data. We suggest that our theory may be relevant to the anomalous Hall effect in conventional ferromagnets as well.
432 citations
TL;DR: The magnetoresistance of La2/3Sr1/3MnO3/CeO2 composites is explored as a function of the metal/insulator composition, temperature, and magnetic field as mentioned in this paper.
Abstract: The magnetoresistance of La2/3Sr1/3MnO3/CeO2 composites is explored as a function of the metal/insulator composition, temperature, and magnetic field. An important enhancement of the low-field magnetoresistance is observed for samples close to metallic percolation threshold. The improved field sensitivity is still fairly large at room temperature. Manganese perovskites composites may constitute a new alternative for the development of magnetoresistive devices.
239 citations
TL;DR: In this article, the authors reported significant enhancements of magnetoresistance in granular (La0.67Ca0.33MnO3)x/(SrTiO31−x) systems with high (around 5 T) and low (a few hundred Oersted) fields.
Abstract: We report significant enhancements of magnetoresistance in granular (La0.67Ca0.33MnO3)x/(SrTiO3)1−x. The system exhibits a conduction threshold at x=xc∼60%, around which magnetoresistance versus x has a maximum. The composition xc at which maximum enhancement in magnetoresistance is observed is the same at high (around 5 T) and at low (a few hundred Oersted) fields. The enhancement is consistent with the disorder-driven amplification of spin-dependent transport at the structural boundaries of the mixture.
203 citations
TL;DR: In this paper, a novel ferromagnetic transition accompanied by carrier density collapse is found in doped charge-transfer insulators with strong electron-phonon coupling, driven by an exchange interaction of polaronic carriers with localized spins; the strength of the interaction determines whether the transition is first or second order.
Abstract: A novel ferromagnetic transition, accompanied by carrier density collapse, is found in doped charge-transfer insulators with strong electron-phonon coupling. The transition is driven by an exchange interaction of polaronic carriers with localized spins; the strength of the interaction determines whether the transition is first or second order. A giant drop in the number of current carriers during the transition, which is a consequence of bound pair formation in the paramagnetic phase close to the transition, is extremely sensitive to an external magnetic field. This carrier density collapse describes the resistivity peak and the colossal magnetoresistance of doped manganites.
200 citations
TL;DR: In this article, the authors reported significant intergrain magnetoresistance (IMR) in polycrystalline double perovskites of SrFe1−x(Mo1/2O3) at room temperature, indicating that a new avenue for spin-polarized tunneling junctions is to utilize insulating interface layers with ferromagnetic or ferrimagnetic coupling.
Abstract: We report significant intergrain magnetoresistance (IMR) in polycrystalline double perovskites of SrFe1−x(Mo or Re)xO3 at room temperature. Systematics of the temperature dependence of IMR indicates that the observed large room-temperature IMR in SrFe1/2Mo1/2O3 originates from the ferrimagnetic nature of insulating grain boundaries as well as the half-metallicity of this perovskite. Our results indicate that a new avenue for spin-polarized tunneling junctions is to utilize insulating interface layers with ferromagnetic or ferrimagnetic coupling.
197 citations
TL;DR: In this paper, the authors performed magnetic and transport measurements on La0.7Ca0.3MnO3 polycrystalline pellets and thin films and found that high-field magnetoresistance is independent of grain size, at least down to 12 nm, but depends strongly on the connectivity between grains.
Abstract: We have performed magnetic and transport measurements on La0.7Ca0.3MnO3 polycrystalline pellets and thin films. We propose a model that reproduces very well the low-field and high-field magnetoresistance at low temperature. The decrease of the resistance at high fields is modeled by the opening of new conduction channels in parallel due to the ordering of Mn spins blocked at the grain surface. We found that high-field magnetoresistance is independent of grain size, at least down to 12 nm, but depends strongly on the connectivity between grains.
127 citations
TL;DR: In this article, a large tunneling magnetoresistance of 150% was observed for a junction with a thin barrier layer (1.6 nm) under a low switching field (<10 Oe) at 5 K.
Abstract: Magnetic tunneling junctions are fabricated from epitaxially grown La0.8Sr0.2MnO3/SrTiO3/La0.8Sr0.2MnO3 trilayers. A large tunneling magnetoresistance of 150% is observed for a junction with a thin barrier layer (1.6 nm) under a low switching field (<10 Oe) at 5 K. A small tunneling magnetoresistance is observed even at 270 K, which is close to the ferromagnetic Curie temperature (290 K) of the La0.8Sr0.2MnO3 film. The large magnetoresistance and high operating temperature are attributed to the sufficiently thin and uniform barrier layer of SrTiO3.
TL;DR: In this paper, X-ray and neutron scattering measurements directly demonstrate the existence of polarons in the paramagnetic phase of optimally doped colossal magnetoresistive oxides.
Abstract: X-ray and neutron scattering measurements directly demonstrate the existence of polarons in the paramagnetic phase of optimally doped colossal magnetoresistive oxides. The polarons exhibit short-range correlations that grow with decreasing temperature, but disappear abruptly at the ferromagnetic transition because of the sudden charge delocalization. The ``melting'' of the charge ordering as we cool through ${T}_{C}$ occurs with the collapse of the quasistatic polaron scattering, and provides important new insights into the relation of polarons to colossal magnetoresistance.
TL;DR: In this paper, the colossal magnetoresistance effect (CMR) can be tuned in polycrystalline mixed valence manganite La 0.67Ca 0.33MnO3 via changing grain size by means of a sol-gel method.
Abstract: In this article, we show how colossal magnetoresistance effect (CMR) can be tuned in polycrystalline mixed valence manganite La0.67Ca0.33MnO3 via changing grain size by means of a sol-gel method. Below a critical diameter (150 nm), CMR disappears, but large intergrain MR remains even well above Tc (1.2Tc for ≈95 nm particles). Possible explanation for this effect involves single magnetic domain behavior in samples annealed at low temperature.
TL;DR: Magnetoresistance observed in pressed powder compacts of half-metallic ferromagnetic oxides is reviewed in this article, where the main, isotropic negative magnetoresistance, which exceeds 50% in CrO2 at low temperature, is due to alignment of the moments of contiguous Ferromagnetic grains. But the effect is related to interparticle tunelling and shows hysteresis similar to that of the bulk magnetization.
Abstract: Magnetoresistance observed in pressed powder compacts of half-metallic ferromagnetic oxides is reviewed. The main, isotropic negative magnetoresistance, which exceeds 50% in CrO2 at low temperature, is due to alignment of the the ferromagnetic moments of contiguous ferromagnetic grains. The effect is related to interparticle tunelling and shows hysteresis similar to that of the bulk magnetization. Spin-dependent Coulomb blocade observed in the smallest particles. There is also, an anisotropic magnetoresistance of up to 1% and a high-field increase in conductivity of order 1%/T in the powder compacts.
TL;DR: In this article, the magnetoresistance of compressively strained La0.7Sr0.3MnO3 (LSMO) films in various magnetic states was studied to understand the role of magnetic domain structure on magnetotransport.
Abstract: We have studied the magnetoresistance (MR) of compressively strained La0.7Sr0.3MnO3 (LSMO) films in various magnetic states in order to understand the role of magnetic domain structure on magnetotransport. In thin films of LSMO on (100) LaAlO3, the perpendicular magnetic anisotropy results in perpendicularly magnetized domains with fine scale ∼200 nm domain subdivision, which we image directly at room temperature using magnetic force microscopy. The main MR effects can be understood in terms of bulk colossal MR and anisotropic MR. We also find evidence for a small domain wall contribution to the MR, which is an order of magnitude larger than expected from a double exchange model.
TL;DR: In this paper, the electrical transport properties of grain boundaries in epitaxial La2/3Ca1/3MnO3 − δ thin films have been studied as a function of temperature and applied magnetic field below the Curie temperature TC.
Abstract: The electrical transport properties of grain boundaries in the epitaxial La2/3Ca1/3MnO3 − δ thin films have been studied as a function of temperature and applied magnetic field Below the Curie temperature TC an additional grain boundary resistance, highly non-linear current-voltage curves, and a large magnetoresistive effect in the whole temperature regime below TC are found The results can be explained consistently by the presence of a disordered, a few nm wide paramagnetic grain boundary layer that is depleted below TC due to an increase of the work function of the ferromagnetic grain material adjacent to this layer The related band bending and space charge effects are important for the physics of grain boundaries in the manganites
TL;DR: In this article, the effects of Ce doping in R1−xAxMnO3 (R=La, Ce, and A=Sr, Ce) on the magnetic and transport properties of this system were investigated.
Abstract: In this study we investigate the effects of Ce doping in R1−xAxMnO3 (R=La, Ce, and A=Sr, Ce) on the magnetic and transport properties of this system. For La1−xCexMnO3 (LCMO), an increase in Ce concentration is accompanied by an increase in TC from 225 to 236 K, as well as an increase in the electrical resistivity. An extremely high resistivity is observed in the new system Ce1−xSrxMnO3 (CSMO) which becomes insulating below its Curie temperature of 43 K. A maximum magnetoresistance (MR) ratio of 40% for CSMO and 53% for LCMO is observed. A larger change in resistivity is seen to correspond to an increase in the Ce concentration, however this is offset by an overall resistivity increase which keeps the MR ratio low. The high resistivity may be due to unreacted oxides in the samples. If true, the amount of impurity appears to be proportional to the Ce doping. If this impurity level can be reduced, a significant colossal magnetoresistance effect could be exhibited by these systems.
TL;DR: In this article, thin films of (001)-oriented Sr2FeMoO6 have been epitaxially deposited on LaAlO3 and SrTiO3 (001) substrates.
Abstract: Thin films of (001)-oriented Sr2FeMoO6 have been epitaxially deposited on LaAlO3 and SrTiO3 (001) substrates. Comparison of their transport and magnetic properties with those of polycrystalline ceramic samples shows a metallic versus semiconductor temperature dependence and a saturation magnetization Ms at 10 K of 3.2 μB/f.u. in the film as against 3.0 for a tetragonal polycrystalline sample. However, the Curie temperature TC≈389 K is reduced from 415 K found for the tetragonal ceramic, which lowers Ms at 300 K in the thin films to 2.0 μB/f.u. compared to 2.2 μB/f.u. in the ceramics. A Wheatstone bridge arrangement straddling a bicrystal boundary has been used to verify that spin-dependent electron transfer through a grain boundary is responsible for the low-field magnetoresistance found in polycrystalline samples below TC.
TL;DR: In this paper, the temperature and field dependence of microwave losses in micron-size powders (∼3 μm) of the manganites Nd0.7Sr0.3MnO3, La0.8Ba0.2Mn O3, and La1.4Ca1.
Abstract: We report systematic measurements of the temperature and field dependence of microwave losses in micron-size powders (∼3 μm) of the manganites Nd0.7Sr0.3MnO3, La0.7Ca0.3MnO3, La0.8Ba0.2MnO3, and La1.4Ca1.6Mn2O7. We fully confirm previous findings of a large increase in zero-field absorption, in La0.7Ba0.3MnO3 and La0.7Sr0.3MnO3, accompanying the onset of ferromagnetism and a concomitant colossal low-field magnetoimpedance (up to 80% at 600 Oe at room temperature). An oversimplified set of computations renders plausible support for interpreting these observations as arising from absorption in the spin system. Possible applications of these novel phenomena are also described.
TL;DR: In this article, the authors show that an intrinsic colossal magnetoresistance (CMR) occurs in an O-orthorhombic phase near an O′-orthoric/O-orth-hombric phase boundary in pseudoperovskites, where the volume of the more conductive clusters increases from below to beyond a percolation threshold.
Abstract: In the Ln1-xAxMnO3 pseudoperovskites in which Ln is a lanthanide and A an alkaline-earth atom, an intrinsic colossal magnetoresistance (CMR) occurs in an O-orthorhombic phase near an O′-orthorhombic/O-orthorhombic phase boundary. For a fixed ratio Mn(IV)/Mn = 0·3, the transition through the O phase from localised-electron behaviour and orbital ordering in the O′ phase to itinerant-electron behaviour in an R-rhombohedral phase occurs with increasing geometric tolerance factor t ≡ 〈A-O〉/√2〈Mn-O〉, where 〈A-O〉 and 〈Mn-O〉 are mean equilibrium bond lengths. The CMR occurs in the temperature interval Tc ≤ T < Ts where there is a segregation, via cooperative oxygen displacements, into a Mn(IV)-rich ferromagnetic phase imbedded in a paramagnetic phase. The volume of the ferromagnetic, more conductive clusters increases from below to beyond a percolation threshold in response, above Tc, to an applied magnetic field and, below Tc, to a Weiss molecular field. In the O phase, the magnetic transition at Tc decreases on the exchange of 18O/16O and increases under hydrostatic pressure. Charge and orbital ordering below a Tco ≤ Tc is found in compositions with x ≈ f or x ≈ ½. With x ≈ ½, the charge-ordered phase CE is tetragonal and antiferromagnetic. An applied magnetic field stabilises the ferromagnetic, conductive phase relative to the insulator phase CE to give a second type of intrinsic CMR. For x ≈ 0·3, there is no static charge and orbital ordering; but for smaller t, strong electron-lattice coupling gives a ‘bad metal’ behaviour below Tc indicative of a dynamic phase segregation as in a traveling charge-density wave. In La1-xCaxMnO3 with ½ ≤ x ≤ f, segregation of the CE x = 0·5 phase and the all-Mn(IV) x = 1 phase has been reported to take the form of a static charge-density wave. The origins of this complex behaviour are discussed.
TL;DR: In this article, the experimental situation is reviewed, and open questions are identified for spin-polarized carriers in high-temperature superconducting compounds and manganite perovskites.
TL;DR: In this article, the role of local lattice distortion in magnetic and charge transport properties was analyzed in terms of the atomic pair-density function to elucidate the role played by the Jahn-Teller distortions.
Abstract: Pulsed neutron-diffraction measurements were carried out on powder samples of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ $(0l~xl~0.5)$ in order to study the local atomic structure of this system known for its colossal magnetoresistance phenomenon. The results are analyzed in terms of the atomic pair-density function to elucidate the role of local lattice distortion in the magnetic and charge transport properties. It is shown that the Jahn-Teller (JT) distortions are locally present even when the crystallographic structure suggests otherwise, and indicate the formation of small lattice polarons in the insulating phase. Local JT distortions are observed even in the metallic phase up to $x=0.35,$ suggesting that the charge distribution in the metallic phase just above the metal-insulator transition may not be spatially uniform. It is possible that charges are partially confined by spin and lattice resulting in microscopic separation of the charge-rich and charge-poor regions.
TL;DR: In this paper, it was observed that crystallisation nature depends on y = (La + A)/Mn ratio in a melt and that stable growth of undoped, Sr substituted (up to x = 0.3) and Ca substituted (x < 0.5°) single crystals with mosaicity not more than 0.9-0.95° were produced.
TL;DR: In this paper, the crystal structure and magnetoresistance of La1-xNaxMnO3 were investigated and it was shown that Na doping tends to drive the system from a regime characterized by strong Hund coupling and strong electron-phonon coupling to one characterized by weak Hund coupled and weak electron-on coupling.
Abstract: The crystal structure and magnetoresistance of are investigated. La1-xNaxMnO3 crystallizes in a rhombohedrally distorted perovskite structure and exhibits a sharp ferromagnetic transition as well as a negative magnetoresistance at around room temperature. On the basis of alternating-current susceptibility and resistivity measurements as well as a comparison with La1-xNaxMnO3 compounds, it is proposed that Na doping tends to drive the system from a regime characterized by strong Hund coupling and strong electron-phonon coupling to one characterized by weak Hund coupling and weak electron-phonon coupling.
TL;DR: In this paper, a resonant method based on a tunnel-diode oscillator for precision measurements of relative impedance changes in materials is described, which consists of an effective self-resonant LC tank circuit driven by a forward-biased tunnel diode operating in its negative resistance region.
Abstract: A resonant method based on a tunnel-diode oscillator for precision measurements of relative impedance changes in materials is described. The system consists of an effective self-resonant LC tank circuit driven by a forward-biased tunnel diode operating in its negative resistance region. Samples under investigation are placed in the core of an inductive coil and impedance changes are determined directly from the measured shift in resonance frequency. A customized low temperature insert is used to integrate this experiment with a commercial Model 6000 Physical Property Measurement System (Quantum Design). Test measurements on a manganese-based perovskite sample exhibiting colossal magnetoresistance indicate that this method is well suited to study the magnetoimpedance in these materials.
TL;DR: In this paper, the superlattice properties of La 0.74Sr 0.26MnO3 were investigated using x-ray diffraction and electron microscopy, and it was shown that increasing Λ (to 21 ap) leads to a decrease in the Tc, the low-temperature magnetization, the magnetoresistance and the TMI.
Abstract: Artificial (LaMnO3)m(SrMnO3)n superlattices, n/(m+n)=0.26, were grown with pulsed-laser deposition, and the superlattice period Λ was varied. Their structural characteristics were investigated using x-ray diffraction and electron microscopy. When Λ
TL;DR: In this paper, the magnetotransport properties of laser ablated La07Ce03MnO3 films on LaAlO3, and the effect of varying the ambient oxygen pressure during growth and the film thickness were investigated.
Abstract: La07Ce03MnO3 is a relatively new addition to the family of colossal magnetoresistive manganites, in which the cerium ion is believed to be in the Ce4+ state In this article, we report the magnetotransport properties of laser ablated La07Ce03MnO3 films on LaAlO3, and the effect of varying the ambient oxygen pressure during growth and the film thickness We observe that the transport and magnetic properties of the film depend on the oxygen pressure, surface morphology, film thickness, and epitaxial strain The films were characterized by x-ray diffraction using a four-circle goniometer We observe an increase in the metal-insulator transition temperature with decreasing oxygen pressure This is in direct contrast to the oxygen pressure dependence of La07Ca03MnO3 films and suggests the electron doped nature of the La07Ce03MnO3 system With decreasing film thickness we observe an increase in the metal-insulator transition temperature This is associated with a compression of the unit cell in the a-b plane due to epitaxial strain On codoping with 50% Ca at the Ce site, the system (La07Ca015Ce015MnO3) is driven into an insulating state suggesting that the electrons generated by Ce4+ are compensated by the holes generated by Ca2+, thus making the average valence at the rare-earth site 3+ as in the parent material LaMnO3La07Ce03MnO3 is a relatively new addition to the family of colossal magnetoresistive manganites, in which the cerium ion is believed to be in the Ce4+ state In this article, we report the magnetotransport properties of laser ablated La07Ce03MnO3 films on LaAlO3, and the effect of varying the ambient oxygen pressure during growth and the film thickness We observe that the transport and magnetic properties of the film depend on the oxygen pressure, surface morphology, film thickness, and epitaxial strain The films were characterized by x-ray diffraction using a four-circle goniometer We observe an increase in the metal-insulator transition temperature with decreasing oxygen pressure This is in direct contrast to the oxygen pressure dependence of La07Ca03MnO3 films and suggests the electron doped nature of the La07Ce03MnO3 system With decreasing film thickness we observe an increase in the metal-insulator transition temperature This is associated with a compression of the unit cell in the a-b
TL;DR: In this article, the magnetic properties of AMnO3 perovskites have been investigated and the substitutions of rare earths for La have been shown to increase the coercive field Hc and magnetization M, while decreasing Tc.
Abstract: The substitutions of rare earths for La on the magnetic properties of the perovskites La0.7−xRxA0.3MnO3 (0
TL;DR: In this article, it was shown that the carrier density collapse explains the colossal magnetoresistance of doped manganites close to the transition, which is a consequence of local bound-pair formation in the paramagnetic phase, is extremely sensitive to an external magnetic field.
Abstract: The exchange interaction of polaronic carriers with localized spins leads to a ferromagnetic/paramagnetic transition in doped charge-transfer insulators with strong electron-phonon coupling. The relative strength of the exchange and electron-phonon interactions determines whether the transition is first or second order. A giant drop in the number of current carriers during the transition, which is a consequence of local bound-pair (bipolaron) formation in the paramagnetic phase, is extremely sensitive to an external magnetic field. Below the critical temperature of the transition, , the binding of the polarons into immobile pairs competes with the ferromagnetic exchange between polarons and the localized spins on Mn ions, which tends to align the polaron moments and, therefore, breaks up those pairs. The number of carriers abruptly increases below leading to a sudden drop in resistivity. We show that the carrier-density collapse explains the colossal magnetoresistance of doped manganites close to the transition. Below , transport occurs by polaronic tunnelling, whereas at high temperatures the transport is by hopping processes. The transition is accompanied by a spike in the specific heat, as experimentally observed. The gap feature in tunnelling spectroscopy is related to the bipolaron binding energy, which depends on the ion mass. This dependence explains the giant isotope effect of the magnetization and resistivity upon substitution of for . It is shown also that the localization of polaronic carriers by disorder cannot explain the observed huge sensitivity of the transport properties to the magnetic field in doped manganites.
TL;DR: In this article, the electrical transport properties of heteroepitaxial La0.7Ca0.3MnO3(LCMO)/SrTiO3 multilayers were investigated.
Abstract: We have investigated the electrical transport properties of heteroepitaxial La0.7Ca0.3MnO3(LCMO)/SrTiO3 multilayers; as the LCMO layer thickness was varied from 25 to 2.5 nm, the metallic transition was suppressed and enhanced magnetoresistance extended over low temperatures. The results of transport and magnetic measurements imply a vertically inhomogeneous magnetic structure in the LCMO layers, with magnetically disordered interfaces. Although strain is clearly implicated in this surface disorder, we show that intrinsic magnetic disorder must also be associated with the interfaces.