Showing papers on "Colossal magnetoresistance published in 1998"
TL;DR: A review of the metal-insulator transition can be found in this article, where a pedagogical introduction to the subject is given, as well as a comparison between experimental results and theoretical achievements.
Abstract: Metal-insulator transitions are accompanied by huge resistivity changes, even over tens of orders of magnitude, and are widely observed in condensed-matter systems. This article presents the observations and current understanding of the metal-insulator transition with a pedagogical introduction to the subject. Especially important are the transitions driven by correlation effects associated with the electron-electron interaction. The insulating phase caused by the correlation effects is categorized as the Mott Insulator. Near the transition point the metallic state shows fluctuations and orderings in the spin, charge, and orbital degrees of freedom. The properties of these metals are frequently quite different from those of ordinary metals, as measured by transport, optical, and magnetic probes. The review first describes theoretical approaches to the unusual metallic states and to the metal-insulator transition. The Fermi-liquid theory treats the correlations that can be adiabatically connected with the noninteracting picture. Strong-coupling models that do not require Fermi-liquid behavior have also been developed. Much work has also been done on the scaling theory of the transition. A central issue for this review is the evaluation of these approaches in simple theoretical systems such as the Hubbard model and $t\ensuremath{-}J$ models. Another key issue is strong competition among various orderings as in the interplay of spin and orbital fluctuations. Experimentally, the unusual properties of the metallic state near the insulating transition have been most extensively studied in $d$-electron systems. In particular, there is revived interest in transition-metal oxides, motivated by the epoch-making findings of high-temperature superconductivity in cuprates and colossal magnetoresistance in manganites. The article reviews the rich phenomena of anomalous metallicity, taking as examples Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Ru compounds. The diverse phenomena include strong spin and orbital fluctuations, mass renormalization effects, incoherence of charge dynamics, and phase transitions under control of key parameters such as band filling, bandwidth, and dimensionality. These parameters are experimentally varied by doping, pressure, chemical composition, and magnetic fields. Much of the observed behavior can be described by the current theory. Open questions and future problems are also extracted from comparison between experimental results and theoretical achievements.
5,781 citations
TL;DR: In this paper, an ordered double perovskite (Sr2FeMoO) was shown to exhibit intrinsic tunnelling-type magnetoresistance at room temperature.
Abstract: Colossal magnetoresistance—a huge decrease in resistance in response to a magnetic field—has recently been observed in manganese oxides with perovskite structure. This effect is attracting considerable interest from both fundamental and practical points of view1. In the context of using this effect in practical devices, a noteworthy feature of these materials is the high degree of spin polarization of the charge carriers, caused by the half-metallic nature of these materials20,21; this in principle allows spin-dependent carrier scattering processes, and hence the resistance, to be strongly influenced by low magnetic fields. This type of field control has been demonstrated for charge-carrier scattering at tunnelling junctions2,3 and at crystal-twin or ceramic grain boundaries4,5, although the operating temperature of such structures is still too low (⩽150 K) for most applications. Here we report a material—Sr2FeMoO6, an ordered double perovskite6—exhibiting intrinsic tunnelling-type magnetoresistance at room temperature. We explain the origin of this behaviour with electronic-structure calculations that indicate the material to be half-metallic. Our results show promise for the development of ordered perovskite magnetoresistive devices that are operable at room temperature.
2,065 citations
01 May 1998
TL;DR: A review of the topics of colossal magnetoresistance, charge ordering and related phenomena exhibited by oxides, and contributions covering the present status of the subject can be found in this paper,.
Abstract: An aspect of metal oxides, colossal magnetoresistance exhibited by certain manganese oxides, in particular rare-earth manganates of perovskite structure, has received much attention in recent years. Some of these oxides show 100 per cent magnetoresistance, and have potential for technological applications. This text begins with a review of the topics of colossal magnetoresistance, charge ordering and related phenomena exhibited by oxides, and presents contributions covering the present status of the subject.
1,094 citations
TL;DR: In this article, a different pattern of charge localization in the charge-ordered phase of the manganese oxide La1−xCaxMnO3 (x ⩾ 0.5) was reported.
Abstract: The propensity of systems of charge and spin to form, under certain conditions, ‘stripe’ phases has recently attracted much attention, as it has been suggested that dynamically fluctuating stripe phases may be of central importance for an understanding of the physics of high-temperature superconductors1,2,3,4,5. A related phenomenon — static charge stripes — characterizes6 the insulating antiferromagnetic ground state of the manganese oxides, a class of materials which (like the copper oxide superconductors) have a perovskite structure, and are notable for their extraordinary electronic and magnetic properties, such as colossal magnetoresistance and charge ordering7,8. Here we report a different pattern of charge localization in the charge-ordered phase of the manganese oxide La1−xCaxMnO3 (x ⩾ 0.5). This pattern takes the form of extremely stable pairs of Mn3+O6 stripes, with associated large lattice contractions (due to the Jahn–Teller effect), separated periodically by stripes of non-distorted Mn4+O6 octahedra. These periodicities, which adopt integer values between 2 and 5 times the lattice parameter of the orthorhombic unit cell, correspond to the commensurate carrier concentrations (x = 1/2, 2/3, 3/4 and 4/5): for other values of x, the pattern of charge ordering is a mixture of the two adjacent commensurate configurations. These paired Jahn–Teller stripes appear therefore to be the fundamental building blocks of the charge-ordered state in the manganese oxides, and so may be expected to have profound implications for the magnetic and transport properties of these materials.
474 citations
TL;DR: Experimental, phenomenological, and theoretical analyses are given of the dependence on strain of the ferromagnetic Tc of the colossal magnetoresistance (CMR) rare earth manganese perovskites as discussed by the authors.
Abstract: Experimental, phenomenological, and theoretical analyses are given of the dependence on strain of the ferromagnetic Tc of the colossal magnetoresistance (CMR) rare earth manganese perovskites. It is found that Tc is extremely sensitive to biaxial strain; by implication other physical properties are also. The results indicate that biaxial strain is an important variable which must be considered in the design of devices based on thin films and provide evidence in favor of the relevance of the Jahn–Teller electron-phonon coupling to the CMR phenomenon.
369 citations
Journal Article•
TL;DR: In this paper, the first-principles band calculation predicts that the half metallic ground state with the ferrimagnetic coupling of Fe and Re spins is obtained for polycrystalline ceramics with Curie temperature above 400 K. The experimental results of electronic and magnetic properties are in accord with this picture.
Abstract: Intergrain tunneling magnetoresistance (TMR) subsisting up to room temperature has been observed for polycrystalline ceramics of ${\\mathrm{Sr}}_{2}{\\mathrm{FeReO}}_{6},$ which has ordered double perovskite structure with Curie temperature above 400 K. The first-principles band calculation predicts that ${\\mathrm{Sr}}_{2}{\\mathrm{FeReO}}_{6}$ shows the half metallic ground state with the ferrimagnetic coupling of Fe and Re spins. The experimental results of electronic and magnetic properties are in accord with this picture. In fact, the magnitude of intergrain TMR with the magnetic field of 7 T at 4.2 and 300 K is as large as 21 and 7%, respectively, reflecting high spin polarization of carriers.
341 citations
TL;DR: In this article, the half-metallic ferromagnetic powders of the halfmetallic granular metal are dielectric granular metals and the negative magnetoresistance is due to tunneling between contiguous magnetagnetic particles along a critical path with a spin-dependent Coulomb gap.
Abstract: Cold-pressed powders of the half-metallic ferromagnet ${\mathrm{CrO}}_{2}$ are dielectric granular metals. Hysteretic magnetoresistance with maxima at the coercive field arises from interparticle contacts. Dilution with insulating antiferromagnetic ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ powder reduces the conductivity by 3 orders of magnitude, but enhances the magnetoresistance ratio which reaches 50% at 5K. The negative magnetoresistance is due to tunneling between contiguous ferromagnetic particles along a critical path with a spin-dependent Coulomb gap.
329 citations
TL;DR: In this paper, the light-induced insulator-metal transition in the "colossal magnetoresistance" compound Pr0.7Ca0.3MnO3 is shown to generate a well-localized conducting path while the bulk of the sample remains insulating.
Abstract: The light-induced insulator-metal transition in the “colossal magnetoresistance” compound Pr0.7Ca0.3MnO3 is shown to generate a well-localized conducting path while the bulk of the sample remains insulating. The path can be visualized through a change of reflectivity that accompanies the phase transition. Its visibility provides a tool for gaining insight into electronic transport in materials with strong magnetic correlations. For example, a conducting path can be generated or removed at an arbitrary position just because of the presence of another path. Such manipulation may be useful in the construction of optical switches.
267 citations
TL;DR: In this paper, the authors investigated the magnetic-field-induced phase transitions of the charge carriers and found that the destruction of the real-space ordering is accompanied with a structural phase transition as well as with the magnetic phase transition and the colossal magnetoresistance effect.
Abstract: We have investigated the magnetic-field-induced phase transitions of ${R}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ ($R=\mathrm{Pr}$ and Nd, $x=0.50,$ 0.45 and 0.50, 0.45, 0.40) by measurements of magnetization, magnetoresistance, and magnetostriction utilizing a nondestructive long-pulse magnet (generating up to 40 T). We observed processes where magnetic fields destroy the real-space ordering of the charge carriers and cause insulator-to-metal phase transitions over the whole temperature region below about 250 K. We found that the destruction of the charge ordering is accompanied with a structural phase transition as well as with the magnetic phase transition and the colossal magnetoresistance effect. The different profiles of the temperature vs transition field curve depending on the carrier concentration $x$ may be ascribed to the difference in the entropy between the commensurate and the discommensurate charge-ordered state. It turned out that the stability of the charge-ordered state is strongly correlated with the colinear antiferromagnetic ordering of the localized Mn moments.
267 citations
TL;DR: In this paper, the authors discuss the different factors which govern the magnetic and transport properties of manganites with perovskite structure: carrier concentration, average size of the interpolated cation, and mismatch effect on the A-site.
Abstract: Manganites with the perovskite structure represent a very important family of oxides which are extensively studied for their colossal magnetoresistance (CMR) properties In the present review we discuss the different factors which govern the magnetic and transport properties of these materials: carrier concentration, average size of the interpolated cation, and mismatch effect on the A-site Three types of oxides are mainly examined: (i) the hole doped manganites Ln07A03MnO3 (A = Ca, Sr, Ba), (ii) the “charge ordered” Ln05A05MnO3 manganites, and (iii) the electron doped manganites Ca1-xLnxMnO3 and Ca1-xThxMnO3 The relationships between structural and magnetic transitions are discussed, and particular attention is paid to charge ordering phenomena The doping of the Mn sites by various elements (Al, Ga, In, Ti, Sn, Fe, Cr, Co, Ni) is systematically examined The beneficial effect of “Cr, Co, Ni” elements, which induce CMR properties in these perovskites, is emphasized
218 citations
TL;DR: In this paper, a first-order magnetostructural transition at TC≅240 K, from a high temperature paramagnetic to a low-temperature ferromagnetic phase, accompanied by a large drop in the resistivity was observed.
Abstract: Zero-field electrical resistivity over the temperature range of 4–300 K and magnetoresistance in magnetic fields of up to 12 T have been measured in Gd5(Si1.8Ge2.2). This system undergoes a first-order magnetostructural transition at TC≅240 K, from a high-temperature paramagnetic to a low-temperature ferromagnetic phase, accompanied by a large drop in the resistivity. The application of an external magnetic field above TC can induce this transition, and a giant negative magnetoresistance effect (Δρ/ρ≅−20%) is observed associated with this first-order field-induced transition.
TL;DR: In this paper, the electron concentration of perovskites has been investigated and the effect of electron concentration on magnetoresistance has been shown to be the predominant one by comparing the effects of Ln3+ and Th4+ on the magnetoreduction.
Abstract: Magnetotransport properties of the Mn(IV)-rich perovskites Ca1-xLnxMnO3 and Ca1-xThxMnO3 (0 ≤ x ≤ 0.20) have been systematically investigated. Among the different factors governing their colossal magnetoresistance (CMR), we demonstrate that the electron concentration is the predominant one by comparing the effects of Ln3+ and Th4+, the optimal x value giving the highest magnetoresistance being twice for the former compared to the latter, in accordance with their respective oxidation states. CMR is linked to the ferromagnetism−antiferromagnetism competition that is maximum for xopt = 0.08 in the case of Th, whereas xopt increases from x = 0.135 for Pr to x = 0.16 for Ho.
TL;DR: In this article, a modification of Mott's original model was proposed by taking into account that the hopping barrier depends on the misorientation between the spins of electrons at an initial and a final state in an elementary hopping process.
Abstract: The low carrier mobility of the magnetic perovskite ${\mathrm{Nd}}_{0.52}{\mathrm{Sr}}_{0.48}{\mathrm{MnO}}_{3}$ implies that the dominant conductivity mechanism is related to Mott hopping. We propose a modification of Mott's original model by taking into account that the hopping barrier depends on the misorientation between the spins of electrons at an initial and a final state in an elementary hopping process. Using this model we deduce a negative-magnetoresistivity scaling proportional to the Brillouin function $B$ in the ferromagnetic state and to ${B}^{2}$ in the paramagnetic state. Both predictions are in full agreement with the magnetoresistivity measured in pulsed magnetic fields up to 50 T.
TL;DR: In this paper, the results of magnetization measurements on pseudomorphic (fully strained) c-axis oriented colossal magnetoresistance manganite thin films grown by molecular beam epitaxy were reported.
Abstract: We report the results of magnetization measurements on pseudomorphic (fully strained) c-axis oriented colossal magnetoresistance manganite thin films grown by molecular beam epitaxy. We observe uniaxial magnetic anisotropy (hard axis/easy plane) with the easy plane being the film plane. Within the plane a weaker biaxial anisotropy is observed with [100] (Mn–O bond direction) easy axes. The magnetization dependence of the uniaxial anisotropy constant follows the predicted magnetization dependence of the magnetostriction constants within single-ion models indicating that the anisotropy energy is dominated by strain-induced anisotropy from the lattice constant mismatch with the SrTiO3 substrate. These results indicate a magnetostriction constant λ100≈+7×10−5, and an induced orbital moment of at least 0.02μb/Mn ion. We predict that by appropriate substrate selection an equilibrium out-of-plane magnetization can be produced.
15 Jul 1998-Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences
TL;DR: Recent measurements of the magnetoresistance of artificial grain boundaries in thin–film bicrystals suggest that spin–polarized tunnelling and spin scattering at interfaces play a relatively minor role, the dominant low–field contribution to magnetores resistance coming from the mesoscale response of magnetic inhomogeneity induced by the grain boundary.
Abstract: Although theoretical understanding of doped mixedvalence manganites that exhibit colossal magnetoresistance (CMR) is still incomplete, the general observation of a systematic correlation at a given...
TL;DR: In this paper, the authors show that the bulk low-field magnetoresistance (near and above T c) is determined by a single parameter: the charge-carrier density.
Abstract: Magnetoresistance—the field-dependent change in the electrical resistance of a ferromagnetic material—finds applications in technologies such as magnetic recording. Near and above the Curie point, T c, corresponding to the onset of magnetic order, scattering of charge carriers by magnetic fluctuations can substantially increase the electrical resistance1,2. These fluctuations can be suppressed3 by a magnetic field, leading to a negative magnetoresistance. Magnetic scattering might also have a role in the ‘colossal’ magnetoresistance observed in some perovskite manganese oxides4,5,6, but is it not yet clear how to reconcile this behaviour with that of the conventional ferromagnetic materials. Here we show that, in generic models of magnetic scattering, the bulk low-field magnetoresistance (near and above T c) is determined by a single parameter: the charge-carrier density. In agreement with experiment3,7,8, the low-field magnetoresistance scales with the square of the ratio of the field-induced magnetization to the saturation magnetization. The scaling factor is C ≈ x −2/3, where x is the number of charge carriers per magnetic unit cell. Data from very different ferromagnetic metals and doped semiconductors are in broad quantitative agreement with this relationship, with the notable exception of the perovskite manganese oxides (in which dynamic lattice distortions complicate and enhance4,9,10,11,12 the effects of pure magnetic scattering). Our results might facilitate searches for new materials with large bulk magnetoresistive properties.
TL;DR: In this paper, off-axis radio frequency magnetron sputtering was employed to grow La0.7Ca0.3MnOz (LCMO) thin films onto three different types of substrates.
Abstract: Off-axis radio frequency magnetron sputtering was employed to grow La0.7Ca0.3MnOz (LCMO) thin films onto three different types of substrates. The substrate strongly influences the structure and the colossal magnetoresistance effect of the obtained films. Single-crystalline thin films were prepared on LaAlO3 (100) substrates, showing a low value of residual resistivity and a metal–insulator transition at a temperature of up to Tpeak=290 K. The latter value of the transition temperature is one of the highest reported so far on thin films of the La–Ca–Mn–O system. Films deposited onto Y-stabilized ZrO2 substrates and onto MgO substrates are polycrystalline and less textured. These films are characterized by a large negative magnetoresistance ratio MR=[R(H)−R(0)]/R(0) measured for small values of the magnetic field H. For H=1.5 kOe, the MR was found to be approximately −30%, −20%, and −8% at temperatures of 20, 77, and 180 K, respectively. The magnetoresistance of polycrystalline LCMO films shows two contribu...
TL;DR: In this article, a phenomenological model to explain the magnetic field dependence of resistance in granular CMR materials taking into account the spin polarized tunneling at the grain boundaries was developed, which was fitted to two systems, namely, La0.55Ho0.15Sr0.3MnO3 and La1.8Y0.5Ca0.7Mn2O7.
Abstract: It has been observed that in bulk and polycrystalline thin films of colossal magnetoresistive (CMR) materials the magnetoresistance follows a different behavior compared to single crystals or single crystalline films below the ferromagnetic transition temperature Tc. In this paper we develop a phenomenological model to explain the magnetic field dependence of resistance in granular CMR materials taking into account the spin polarized tunneling at the grain boundaries. The model has been fitted to two systems, namely, La0.55Ho0.15Sr0.3MnO3 and La1.8Y0.5Ca0.7Mn2O7. From the fitted result we have separated out, in La0.55Ho0.15Sr0.3MnO3, the intrinsic contribution from the intergranular contribution to the magnetoresistance coming from spin polarized tunneling at the grain boundaries. It is observed that the temperature dependence of the intrinsic contribution to the magnetoresistance in La0.55Ho0.15Sr0.3MnO3 follows the prediction of the double exchange model for all values of field.
TL;DR: In this article, it was shown that perovskite manganese oxide materials exhibit anomalously large 1/f noise and temperature-dependent ferromagnetic resonance (FMR) linewidths.
Abstract: Perovskite manganese oxide materials known for the phenomenon of colossal magnetoresistance often exhibit anomalously large 1/f noise and large, temperature-dependent ferromagnetic resonance (FMR) linewidths. We show that in epitaxial films, these anomalies are very sensitive to oxygen partial pressure during film growth and to postdeposition thermal processing in oxygen, suggesting that oxygen stoichiometry plays a key role. We find that the temperature coefficient of resistance (TCR) at the metal–insulator transition increases and the FMR linewidth decreases as we increase the oxygen partial pressure during growth. Postdeposition heat treatment in oxygen leads to further increase in TCR and decrease in FMR linewidth, accompanied by a dramatic reduction in 1/f noise magnitudes.
15 Jul 1998-Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences
TL;DR: This review explores the various interesting technological avenues that are being pursued and addresses the uniqueness of the material that may enable a given technology as well as the various bottlenecks that will have to be overcome in order to successfully compete with the existing technologies.
Abstract: Since the rejuvenation of interest in the rare earth manganites owing to their potential use as magnetoresistive sensors, there has been adequate research to arrive at some evaluation of the potent...
TL;DR: In this article, magnetoresistance measurements in very thin Pr0.67Sr0.33MnO3 films (5-15 nm) grown on LaAlO3 (001) substrates are reported, where the films are under compressive strain imposed by the lattice mismatch with the substrate.
Abstract: We report magnetoresistance (MR) measurements in very thin Pr0.67Sr0.33MnO3 films (5–15 nm) grown on LaAlO3 (001) substrates. The films are under compressive strain imposed by the lattice mismatch with the substrate. The MR ratio [R(H)−R0]/R0 is ∼92% at H=800 Oe and T=70 K when the magnetic field is applied perpendicular to the film plane and is much smaller when the magnetic field is parallel to the film plane. We suggest that the large low-field MR is due to strain-induced magnetic anisotropy and spin-dependent scattering at domain boundaries.
TL;DR: In this paper, the low-field magnetoresistance of as-deposited films on and Si is measured and shown to be temperature independent at low temperatures with a value of about.
Abstract: The low-field magnetoresistance of films deposited on different substrates has been measured. Whereas post-annealed films on and exhibit a clear anisotropic magnetoresistance (AMR), the low-field magnetoresistance of as-deposited films on and Si is dominated by grain-boundary magnetoresistance. At low temperatures the anisotropic magnetoresistance is temperature independent with a value of about . A simple atomic d-state model can explain the sign of the anisotropic magnetoresistance.
TL;DR: In this article, a quasi-one-dimensional model of spin transport in heterogeneous media based on the Boltzmann equation is presented in order to define the basic properties characterizing perpendicular spin transport.
Abstract: A quasi one-dimensional model of spin transport in heterogeneous media based on the Boltzmann equation is presented in order to define the basic properties characterizing perpendicular spin transport in nanostructures. Experimental results are reviewed, first on the giant magnetoresistance of magnetic multilayers: spin dependent scattering in bulk and at interfaces, spin-diffusion length in the ferromagnetic layers and in the non-magnetic spacers. The observations of magnetoresistance associated with spin-scattering at Bloch walls are summarized. The junction magnetoresistance of ferromagnetic-insulator-ferromagnetic structures are reviewed, including the tunnel junctions produced with materials which present colossal magnetoresistance. An outlook on possible devices and novel structures is given.
TL;DR: In this paper, the effects of Cr doping on the magnetic and nuclear structures are studied by neutron diffraction for Pr0.5Ca 0.95Cr 0.05O3. And it is shown that, at low temperature, the strongly distorted charge ordered structure usually observed in Pr 0.5 Ca 0.
Abstract: Cr doping in Pr0.5Ca0.5MnO3 is known to destroy the low-temperature antiferromagnetic charge-ordered state and to induce a ferromagnetic component as well as colossal magnetoresistance properties. Effects of Cr doping on the magnetic and nuclear structures are studied here by neutron diffraction for Pr0.5Ca0.5Mn0.95Cr0.05O3. It is shown that, at low temperature, the strongly distorted charge ordered structure usually observed in Pr0.5Ca0.5MnO3 is replaced by a ferromagnetic phase in which the basal-plane distortion of the MnO6 octahedra has disappeared, in agreement with the restoration of the double–exchange interactions.
TL;DR: The magnetization and resistivity of single crystals of the Zintl compound were measured as functions of temperature and applied magnetic field as discussed by the authors, and the magnetization data showed an apparent antiferromagnetic transition at 32 K, even though the high temperature susceptibility suggests that the exchange coupling is ferromagnetic in nature.
Abstract: The magnetization and resistivity of single crystals of the Zintl compound ${\mathrm{Eu}}_{14}{\mathrm{MnBi}}_{11}$ are measured as functions of temperature and applied magnetic field. The magnetization data show an apparent antiferromagnetic transition at ${T}_{N}=32\mathrm{K}$ even though the high-temperature susceptibility suggests that the exchange coupling is ferromagnetic in nature. The zero-field resistivity is approximately independent of temperature above 32 K. Below 32 K the resistivity increases slightly and peaks at about 20 K before decreasing as the temperature is decreased. This temperature dependence is fairly normal for an antiferromagnetic metal. On the other hand, in contrast to other antiferromagnets, the single-crystal magnetoresistance is large and negative at all temperatures below about ${3T}_{N}.$ In addition, the dependence of the resistivity upon the magnetization is quite similar to the colossal-magnetoresistance materials.
TL;DR: In this article, a La0.7Ca0.3MnO3 thin film made by pulsed laser deposition (PLD) and another film of the same composition made by metal organic chemical vapor deposition (MOCVD), both on single crystal LaAlO3, were subject to a series of six, short, controlled anneals.
Abstract: A La0.7Ca0.3MnO3 thin film made by pulsed laser deposition (PLD) and another film of the same composition made by metal organic chemical vapor deposition (MOCVD), both on single crystal LaAlO3, were subject to a series of six, short, controlled anneals. The oxygen content was purposely not changed in the films from the first anneal to subsequent anneals. After each anneal, the film microstructures were characterized to determine average grain size, lattice constants, nonuniform strain, and crystalline mosaic spread, and these parameters were correlated with the magnetotransport properties. For both sets of films, the influence of annealing was to both increase the temperature at which the maximum in the magnetoresistance occurs (Tm) and the maximum magnetoresistance (MR) value. The improvement in film properties occurred in conjunction with stress relaxation and improved crystallinity, as a result of grain growth. The MOCVD films showed poorer grain coupling and poorer epitaxy compared to the PLD films. These features did not significantly influence the absolute values of the resistivity, but did produce spin canting in the MOCVD film, as seen in magnetization and resistivity versus field data. The canting resulted in a lower Tm and depressed MR value for the MOCVD film which increased only marginally with annealing. The work highlights the importance of controlling microstructure for optimizing properties of colossal magnetoresistance films.
TL;DR: In this paper, the authors present neutron-diffraction data which show that the magnetic structure of this material switches from antiferromagnetic stacking of the (ferromagnetically ordered) sheets in zero field to ferromagnetic stack in a field of 1.5 T. The data are the first to be collected on any manganite as a function of applied field, exactly as the magnetoresistance data themselves are collected.
Abstract: The layered compound ${\mathrm{La}}_{2\ensuremath{-}2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ $(x=0.3)$ consists of bilayers of metallic ${\mathrm{MnO}}_{2}$ sheets separated by insulating material. The compound exhibits markedly anisotropic magnetoresistance at temperatures well below the three-dimensional magnetic ordering temperature ${T}_{c}=90\mathrm{K}$ in addition to colossal magnetoresistance around ${T}_{c}.$ We present neutron-diffraction data which show that the magnetic structure of this material switches from antiferromagnetic stacking of the (ferromagnetically ordered) sheets in zero field to ferromagnetic stacking in a field of 1.5 T. To our knowledge, the data are the first to be collected on any manganite as a function of applied field, exactly as the magnetoresistance data themselves are collected. They provide a natural explanation of the low-field magnetoresistance in the ordered phase in terms of spin-polarized tunneling between the magnetic layers and suggest that the material is a bulk stack of spin-valve devices.
TL;DR: In this article, it was shown that these ceramics become magnetically harder when reducing the particle size, exhibiting large magnetic anisotropy that also increases with the grain size, in concomitance with this enhancement of the magnetic hardness a gradual increase of the low-field magnetoresistance is also detected.
Abstract: The low-field magnetoresistance and the magnetization of ceramic oxides have been studied as a function of the grain size. It is shown that these ceramics become magnetically harder when reducing the particle size, exhibiting large magnetic anisotropy that also increases when reducing the grain size. In concomitance with this enhancement of the magnetic hardness a gradual increase of the low-field magnetoresistance is also detected. We suggest that both phenomena are closely related and associated with the existence of some degree of spin disorder at the grain boundaries. Implications of these findings for improvements of the field response sensitivity of these materials are discussed.
TL;DR: In this article, a two-component signal was seen consisting of a fast positive transient associated with the paramagnetic insulating state and a slower negative signal associated with ferromagnetic metallic state.
Abstract: Pulsed laser excitation induced conductance changes in colossal magnetoresistance material ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ were studied on the picosecond time scale. A two-component signal was seen consisting of a fast positive transient associated with the paramagnetic insulating state and a slower negative signal associated with the ferromagnetic metallic state. The fast component corresponds to the photoionization of the Jahn-Teller small polaron. The slow component is explained in terms of the reduced carrier mobility due to photogenerated magnetic excitations.
TL;DR: In this article, the La1−xLixMnO3 (x=0, 0.2 and 0.3) compounds were determined to be of rhombohedral structure by x-ray powder diffraction.
Abstract: La1−xLixMnO3 (x=0, 0.1, 0.2, and 0.3) were synthesized by using conventional solid state reaction and partial melting technique. The compounds were determined to be of rhombohedral structure by x-ray powder diffraction. For the x=0.2 and x=0.3 samples, room temperature neutron powder diffraction patterns were collected and analyzed with the Rietveld method. The analysis showed that the Li substitutes onto the La site in the rhombohedral perovskite lattice. Resistivities were measured between 4.2 K and room temperature in the magnetic fields of 0, 1, 2, 4, 8, and 13 T. A wide semiconductor-metal transition at temperatures between 160 and 210 K or between 200–230 K were observed for both samples with x=0.2 and 0.3, respectively. Large magnetoresistances above 20%–80% were achieved at fields from null to 13 T over a wide temperature range from the ferromagnetic transitions down to 4.2 K. However, the x=0 and 0.1 samples show a semiconducting behavior in resistivity measured between 77 and 300 K.