Showing papers on "Colossal magnetoresistance published in 2006"

Critical features of colossal magnetoresistive manganites

Abstract: Colossal magnetoresistance (CMR) phenomena are observed in the perovskite-type hole-doped manganites in which the double-exchange ferromagnetic metal phase and the charge–orbital ordered antiferromagnetic phase compete with each other. The quenched disorder arising from the inherent chemical randomness or the intentional impurity doping may cause major modifications in the electronic phase diagram as well as in the magnetoelectronic properties near the bicritical point that is formed by such a competition of the two phases. One is the phase separation phenomenon on various time-scales (from static to dynamic) and on various length-scales (from glass-like nano to grain-like micron). The other is the enhanced phase fluctuation with anomalous reduction in the transition temperatures of the competing phases (and hence in the bicritical-point temperature). The highly effective suppression of such a phase fluctuation by an external magnetic field is assigned here to the most essential ingredient of the CMR physics. Such profound and dramatic features as appearing in the bicritical region are extensively discussed in this paper with ample examples of the material systems specially designed for this purpose. The unconventional phase-controls over the competing phases in terms of magnetic/electric fields and photo-excitations are also exemplified.

Large enhancement of the thermopower in NaxCoO2 at high Na doping

Abstract: Research on the oxide perovskites has uncovered electronic properties that are strikingly enhanced compared with those in conventional metals. Examples are the high critical temperatures of the cuprate superconductors and the colossal magnetoresistance in the manganites. The conducting layered cobaltate Na(x)CoO2 exhibits several interesting electronic phases as the Na content x is varied, including water-induced superconductivity and an insulating state that is destroyed by field. Initial measurements showed that, in the as-grown composition, Na(x)CoO2 has moderately large thermopower S and conductivity sigma. However, the prospects for thermoelectric cooling applications faded when the figure of merit Z was found to be small at this composition (0.6 0.75, S undergoes an even steeper enhancement. At the critical doping x(p) approximately 0.85, Z (at 80 K) reaches values approximately 40 times larger than in the as-grown crystals. We discuss prospects for low-temperature thermoelectric applications.

Large enhancement of the thermopower in Na$_x$CoO$_2$ at high Na doping

Abstract: Research on the oxide perovskites has uncovered electronic properties that are strikingly enhanced compared with those in conventional metals. Examples are the high critical temperatures of the cuprate superconductors and the colossal magnetoresistance in the manganites. The conducting layered cobaltate $\rm Na_xCoO_2$ displays several interesting electronic phases as $x$ is varied including water-induced superconductivity and an insulating state that is destroyed by field. Initial measurements showed that, in the as-grown composition, $\rm Na_xCoO_2$ displays moderately large thermopower $S$ and conductivity $\sigma$. However, the prospects for thermoelectric cooling applications faded when the figure of merit $Z$ was found to be small at this composition (0.6$ $0.75, $S$ undergoes an even steeper enhancement. At the critical doping $x_p\sim$ 0.85, $Z$ (at 80 K) reaches values $\sim$40 times larger than in the as-grown crystals. We discuss prospects for low-temperature thermoelectric applications.

Observation of large magnetoresistance of magnetic Heusler alloy Ni50Mn36Sn14 in high magnetic fields

Abstract: The magnetic and electrical properties on magnetic Heusler alloy Ni50Mn36Sn14 were studied in magnetic fields up to 18T in 4.2–270K temperature range. It was found that at the vicinity of 160K the resistivity jump of 46% is accompanied by the magnetic phase transition. Furthermore, the large magnetoresistance effect of 50% by the magnetic field induced magnetic phase transition was observed.

Composite-hydroxide-mediated approach for the synthesis of nanostructures of complex functional-oxides.

Abstract: We demonstrate a generic approach for the synthesis of single-crystal complex oxide nanostructures of various structure types, such as perovskites, spinels, monoclinic, corundum, CaF(2) structured, tetragonal, and even metal hydroxides. The method is based on a reaction between a metallic salt and a metallic oxide in a solution of composite-hydroxide eutectic at approximately 200 degrees C and normal atmosphere without using an organic dispersant or capping agent. The synthesis technique is cost-effective, one-step, easy to control, and is performed at low temperature and normal atomospheric pressure. The technique can be expanded to many material systems, and it provides a general, simple, convenient, and innovative strategy for the synthesis of nanostructures of complex oxides with important scientific and technological applications in ferroelectricity, ferromagnetism, colossal magnetoresistance, fuel cell, optics, and more.

Colossal Magnetocapacitance and Colossal Magnetoresistance in HgCr2S4

Abstract: We present a detailed study of the dielectric and charge transport properties of the antiferromagnetic cubic spinel ${\mathrm{HgCr}}_{2}{\mathrm{S}}_{4}$. Similar to the findings in ferromagnetic ${\mathrm{CdCr}}_{2}{\mathrm{S}}_{4}$, the dielectric constant of ${\mathrm{HgCr}}_{2}{\mathrm{S}}_{4}$ becomes strongly enhanced in the region below 60--80 K, which can be ascribed to polar relaxational dynamics triggered by the onset of ferromagnetic correlations. In addition, the observation of polarization hysteresis curves indicates the development of ferroelectric order below about 70 K. Moreover, our investigations in external magnetic fields up to 5 T reveal the simultaneous occurrence of magnetocapacitance and magnetoresistance of truly colossal magnitudes in this material.

Coulomb blockade anisotropic magnetoresistance effect in a (Ga,Mn)As single-electron transistor.

Abstract: We observe low-field hysteretic magnetoresistance in a $(\mathrm{Ga},\mathrm{Mn})\mathrm{As}$ single-electron transistor which can exceed 3 orders of magnitude. The sign and size of the magnetoresistance signal are controlled by the gate voltage. Experimental data are interpreted in terms of electrochemical shifts associated with magnetization rotations. This Coulomb blockade anisotropic magnetoresistance is distinct from previously observed anisotropic magnetoresistance effects as it occurs when the anisotropy in a band structure derived parameter is comparable to an independent scale, the single-electron charging energy. Effective kinetic-exchange model calculations in $(\mathrm{Ga},\mathrm{Mn})\mathrm{As}$ show chemical potential anisotropies consistent with experiment and ab initio calculations in transition metal systems suggest that this generic effect persists to high temperatures in metal ferromagnets with strong spin-orbit coupling.

Local switching of two-dimensional superconductivity using the ferroelectric field effect.

Abstract: Correlated oxides display a variety of extraordinary physical properties including high-temperature superconductivity and colossal magnetoresistance. In these materials, strong electronic correlations often lead to competing ground states that are sensitive to many parameters--in particular the doping level--so that complex phase diagrams are observed. A flexible way to explore the role of doping is to tune the electron or hole concentration with electric fields, as is done in standard semiconductor field effect transistors. Here we demonstrate a model oxide system based on high-quality heterostructures in which the ferroelectric field effect approach can be studied. We use a single-crystal film of the perovskite superconductor Nb-doped SrTiO3 as the superconducting channel and ferroelectric Pb(Zr,Ti)O3 as the gate oxide. Atomic force microscopy is used to locally reverse the ferroelectric polarization, thus inducing large resistivity and carrier modulations, resulting in a clear shift in the superconducting critical temperature. Field-induced switching from the normal state to the (zero resistance) superconducting state was achieved at a well-defined temperature. This unique system could lead to a field of research in which devices are realized by locally defining in the same material superconducting and normal regions with 'perfect' interfaces, the interface being purely electronic. Using this approach, one could potentially design one-dimensional superconducting wires, superconducting rings and junctions, superconducting quantum interference devices (SQUIDs) or arrays of pinning centres.

Giant discrete steps in metal-insulator transition in perovskite manganite wires.

Abstract: Optical lithography is used to fabricate LPCMO wires starting from a single La5=80:3Pr0:3Ca3=8MnO3 (LPCMO) film epitaxially grown on a LaAlO3100 substrate As the width of the wires is decreased, the resistivity of the LPCMO wires exhibits giant and ultrasharp steps upon varying temperature and magnetic field in the vicinity of the metal-insulator transition The origin of the ultrasharp transitions is attributed to the effect of spatial confinement on the percolative transport in manganites The strong spin-charge-lattice interaction in transition metal oxides (TMOs) often leads to a striking phenomenon called electronic phase separation (PS), which is identified with the coexistence of a range of exotic electronic and magnetic phases despite the single crystalline structure [1] The role of the PS in the related physical properties such as high-T c superconducting and colossal magnetoresistance (CMR) is a hotly debated issue in the field of condensed matter physics Spatial confinement is a very useful route to gain deeper insight into the nature of PS In particular, when the spatial dimension of the TMOs is reduced to the characteristic PS length scale, one would expect that changes in the transport properties of the TMOs could be quite dramatic A model system for this study is La5=8xPrxCa3=8MnO3 (LPCMO), a perovskite manganite that is famously known for its large-scale PS Transmission electron microscopy has revealed submicrometer-scaled ferromagnetic (FM) and charge ordered (CO) domains [2] The coexistence of FM=CO phases in LPCMO reflects the competition between the intrinsic properties of its two starting materials, ie, FM La5=8Ca3=8MnO3 (TC 275 K) and CO antiferro

Coexisting tunable fractions of glassy and equilibrium long-range-order phases in manganites

Abstract: Antiferromagnetic-insulating (AF-I) and ferromagnetic-metallic (FM-M) phases coexist in various half-doped manganites over a range of temperature and magnetic field, and this is often believed to be an essential ingredient of their colossal magnetoresistance We present magnetization and resistivity measurements on Pr 05 Ca 05 Mn 0975 Al 0025 O 3 and Pr 05 Sr 05 MnO 3 showing that the fraction of the two coexisting phases at low temperature in any specified measuring field, H, can be continuously controlled by following designed protocols traversing field-temperature space; for both materials the FM-M fraction rises under similar cooling paths Constant-field temperature variations, however, show that the former sample undergoes a first-order transition from AF-I to FM-M with decreasing T, while the latter undergoes the reverse transition We suggest that the observed path-dependent phase-separated states result from the low-T equilibrium phase coexisting with supercooled glass-like high-temperature phase, where the low-T equilibrium phases are actually homogeneous FM-M and AF-I phases respectively for the two materials

Colossal magnetoresistance study in nanophasic La0.7Ca0.3MnO3 manganite

Abstract: In this study we report the effect of sintering temperature on the low field magnetotransport properties of La07Ca03MnO3 manganite synthesized through the polymeric precursor route The La07Ca03MnO3 has been sintered at 600 °C (S6), 700 °C (S7), 800 °C (S8), 900 °C (S9) and 1000 °C (S10) X-ray diffraction confirms that phase formation starts at 600 °C All the samples are single phasic having an orthorhombic unit cell The lattice parameters decrease on lowering the sintering temperature (TS) The crystallite as well as the grain size also show strong dependence on the sintering temperature All the samples possess characteristic insulator–metal (TIM) as well as paramagnetic–ferromagnetic (PM–FM) (TC) transitions The TC varies in a small range [272–256 K] as a function of sintering temperature whereas the TIM goes down from 267 K (S10) to 138 K (S6), a strong decrease of 129 K This TC–TIM discrepancy is due to the fact that whereas the former is an intrinsic characteristic, the latter depends strongly on the extrinsic factors eg synthesis conditions, grain boundaries and associated disorders For all the samples magnetoresistance (MR) shows strong dependence on TS The MR increases on lowering the temperature as well as on increasing the field with the occurrence of an intrinsic contribution around TC These variations of MR for all the samples have been explained in terms of the microstructural variations and spin-polarized tunnelling at low temperatures

Colossal Magnetoresistance in a Rare Earth Zintl Compound with a New Structure Type: EuIn2P2

Abstract: Single crystals of a new Zintl compound, EuIn2P2, were grown from indium metal as a flux solvent. The compound crystallizes in the hexagonal P6(3)/mmc space group with a unit cell of a = 4.0829(6) A, c = 17.595(4) A, and Z = 2. It contains alternating Eu2+ layers and [In2P2]2- layers. This compound is paramagnetic at high temperatures with a magnetic transition at 24 K. In the magnetically ordered state, it shows large magnetic anisotropy. The temperature-dependent resistivity of this compound suggests interaction between conduction electrons and local spins. Negative colossal magnetoresistance of up to −398% (MR = {[ρ(H) − ρ(0)]/ρ(H)} × 100%) at 5 T is observed at 24 K.

Low field magnetotransport properties of (La0.7Sr0.3MnO3)0.5:(ZnO)0.5 nanocomposite films

Abstract: (La0.7Sr0.3MnO3)0.5:(ZnO)0.5 nanocomposite thin films were deposited on c-cut sapphire substrates via pulsed laser deposition. The as-grown films were composed of fine grains of 20–50nm size. The epitaxial orientation relationships between the La0.7Sr0.3MnO3 (LSMO) and the sapphire was (111)LSMO∕∕(0003)Al2O3 and ⟨112¯⟩LSMO∕∕⟨101¯0⟩Al2O3. A low field magnetoresistance (LFMR) of ∼12% was achieved at an external magnetic field of H=1T at 77K, possibly due to enhanced grain boundary effects. The postannealed film had columnar structures with well-crystallized large grains (∼200nm), and showed a low resistivity and consequently negligible LFMR similar to that of single crystal LSMO.

Electric control of room temperature ferromagnetism in a Pb(Zr0.2Ti0.8)O3∕La0.85Ba0.15MnO3 field-effect transistor

Abstract: Spintronics, which takes advantage of both spin and charge degrees of freedom, is a promising key technique relevant to future applications of information and data storage. Ferromagnetic transition metal oxides, including perovskite manganites, represent the most promising materials for use as devices controlling magnetic states by an electric field at high temperature with high efficiency. This is because these materials possess a strong intrinsic relationship between charge and magnetism, showing ferromagnetism above room temperature by adjustment of carrier filling, in addition, particular magnetoelectric properties such as a colossal magnetoresistance phenomenon. Nevertheless, the device operation such a field control of magnetism has not been verified so far in manganites. It is essential to determine whether the magnetism of manganites can be controlled via carriers modulated by an electric field in these applications. Here the authors report on the direct demonstration of a simultaneous change in t...

Phase Coexistence and Resistivity near the Ferromagnetic Transition of Manganites

Abstract: Pairing of oxygen holes into heavy bipolarons in the paramagnetic phase and their magnetic pair breaking in the ferromagnetic phase (the so-called current-carrier density collapse) has accounted for the first-order ferromagnetic-phase transition, colossal magnetoresistance, isotope effect, and pseudogap in doped manganites. Here we propose an explanation of the phase coexistence and describe the magnetization and resistivity of manganites near the ferromagnetic transition in the framework of the current-carrier density collapse. The present quantitative description of resistivity is obtained without any fitting parameters, by using the experimental resistivities far away from the transition and the experimental magnetization, and is essentially model-independent.

Polaronic excitations in colossal magnetoresistance manganite films

Abstract: In the colossal magnetoresistance manganites polarons have been proposed as the charge carrier state which localizes across the metal-insulator transition. The character of the polarons is still under debate. We present an assessment of measurements which identify polarons in the metallic state of ${\mathrm{La}}_{2∕3}{\mathrm{Sr}}_{1∕3}{\mathrm{MnO}}_{3}$ (LSMO) and ${\mathrm{La}}_{2∕3}{\mathrm{Ca}}_{1∕3}{\mathrm{MnO}}_{3}$ (LCMO) thin films. We focus on optical spectroscopy in these films which displays a pronounced resonance in the mid-infrared. The temperature-dependent resonance has been previously assigned to polaron excitations. These polaronic resonances are qualitatively distinct in LSMO and LCMO and we discuss large and small polaron scenarios which have been proposed so far. There is evidence for a large polaron excitation in LSMO and small polarons in LCMO. These scenarios are examined with respect to further experimental probes, specifically charge carrier mobility (Hall-effect measurements) and high-temperature dc resistivity.

Electron spin resonance of proton-irradiated graphite.

Abstract: In the case of colossal magnetoresistance in the perovskite manganites, ``double exchange'' mediated by the itinerant spins is believed to play a key role in the ferromagnetism. In contrast, the conventional ``Heisenberg'' interaction, i.e., direct (unmediated) interaction between the localized spins produced by the proton irradiation, is identified as the origin of proton irradiation-induced ferromagnetism in graphite.

Microstructural and magnetotransport properties of La0.7Ca0.3MnO3/BaTiO3 and La0.7Sr0.3MnO3/BaTiO3 bilayered films

Abstract: The microstructural and the magnetotransport properties of La0.7Ca0.3MnO3 and La0.7Sr0.3MnO3 films, deposited on a BaTiO3 layer LCMO/BTO and LSMO/BTO, respectively, and on LaAlO3 and SrTiO3 001 single crystals LCMO/LAO, LSMO/LAO and LSMO/STO by rf-magnetron sputtering using the “soft” or powder targets, have been investigated. The films grown on BTO demonstrate biaxial tensile in-plane and compressive out-of-plane strains, while those grown on LAO show the opposite trend, i.e., compressive in-plane and tensile out-of-plane strains. The films with a biaxial tensile in-plane strain undergo the magnetic transition at a higher temperature than those with a biaxial compressive one. This implies that the variation of Mn-O-Mn bond angle, controlled by the lattice strain, plays a more important role in the formation of spin ordering in the manganite film than the modification in the Mn-O bond length does. It was shown that the magnetic inhomogeneity, observed through the difference between field-cooled and zero-field-cooled temperature-dependent magnetization, is not greatly relevant to the electronic nature, but is controlled by the lattice distortion and the microstructural defects. The observed enhancement of magnetoresistance for the LSMO/BTO bilayer at room temperature makes this material system promising in the development of new hybrid ferromagnetic/ferroelectric devices.

Large tunnel magnetoresistance in tunnel junctions with Co2MnSi∕Co2FeSi multilayer electrode

Abstract: Two kinds of magnetic tunnel junctions with Co2FeSi electrodes are compared. Using Co2FeSi single layers a tunnel magnetoresistance of 52% is reached, whereas the magnetization of the Co2FeSi is only 75% of the expected value. By using [Co2MnSi∕Co2FeSi]x10 multilayer electrodes the magnetoresistance can be increased to 114% and the full bulk magnetization is reached. All junctions show an inverse tunnel magnetoresistance, when the electrons are tunneling from the Co–Fe into the Heusler compound electrode. This results from a special band structure feature of full Heusler compounds, which is robust even for atomic disorder in the films.

Influence of different substrates on phase separation in La1−x−yPryCaxMnO3 thin films

Abstract: Large scale phase separation between ferromagnetic metallic and charge-ordered insulating states in La1−x−yPryCaxMnO3 (LPCMO) crystals and thin films is very sensitive to structural and magnetic changes and is responsible for the enhanced magnetoresistance in LPCMO compared to its parent compounds. By epitaxially growing LPCMO thin films on different substrates, the strain on the LPCMO thin films can be changed, thereby controlling the energy balance between the two phases. LPCMO films of several different thicknesses have been grown on NdGaO3 (NGO), SrTiO3 (STO), SrLaGaO4 (SLGO), and LaAlO3 (LAO) substrates. The compressive strain from the LAO and SLGO substrates suppresses the long-range charge ordering in these samples and enhances magnetoresistance and magnetic hystereses. Conversely, the tensile strain from the STO and NGO substrates enhances the long-range charge ordering and reduces the magnetoresistance and magnetic hystereses.

Effect of electric field doping on the anisotropic magnetoresistance in doped manganites

Abstract: We have modulated the anisotropic magnetoresistance (AMR) in $3--4\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ manganite films using the ferroelectric field effect---a method that electrostatically varies the carrier density without affecting the lattice distortion. While significant changes have been induced in ${T}_{C}$ and $\ensuremath{\rho}$, the AMR ratio remains the same when the magnetic state is not changed. This scaling behavior is in striking contrast to chemical doping results, where similar modulation of the carrier concentration $(\ensuremath{\sim}0.1∕\mathrm{Mn})$ changes the AMR ratio by $\ensuremath{\geqslant}30%$. The results reveal unambiguously the dominant role of chemical distortion in determining the AMR in manganites.

Fe/MgO interface engineering for high-output-voltage device applications

Abstract: The magnetotransport characteristics of Fe∕MgO∕Fe epitaxial tunnel junctions are reported. For clean Fe∕MgO interfaces, a tunnel magnetoresistance of 150% is measured. However, the magnetoresistance decreases rapidly with the applied voltage. Consequently, the main parameter to optimize for device application, namely the output voltage, remains relatively low. This limitation has been solved by interface engineering through the insertion of carbon impurities at the Fe∕MgO interface. Although the tunnel magnetoresistance amplitude is slightly reduced, its variation versus the applied voltage becomes strongly asymmetric with large magnetoresistance maintained up to 1.5V. This determines a large increase of the tunnel junction output voltage.

Competition between intragranular and intergranular tunneling magnetoresistance in polycrystalline Sr2FeMoO6

Abstract: Powered by TCPDF (www.tcpdf.org) This material is protected by copyright and other intellectual property rights, and duplication or sale of all or part of any of the repository collections is not permitted, except that material may be duplicated by you for your research use or educational purposes in electronic or print form. You must obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or otherwise to anyone who is not an authorised user. Huang, Y.H.; Yamauchi, H.; Karppinen, Maarit

Effect of ferroelectric layers on the magnetocapacitance properties of superlattices-based oxide multiferroics

Abstract: A series of superlattices composed of ferromagnetic La0.7Ca0.3MnO3 (LCMO) and ferroelectric/paraelectric Ba1−xSrxTiO3 (0⩽x⩽1) were deposited on SrTiO3 substrates using pulsed laser deposition. Magnetotransport properties of the films reveal a ferromagnetic Curie temperature in the range of 145–158K, and negative magnetoresistance as high as 30%, depending on the type of ferroelectric layers employed for their growth (i.e., “x” value). Ferroelectricity at temperatures ranging from 55Kto105K is also observed, depending on the barium content. More importantly, the multiferroic nature of the film is determined by the appearance of negative magnetocapacitance, which is maximum around the ferroelectric transition temperature (3% per tesla). These results are understood based on the role of the ferroelectric/paraelectric layers and strains in inducing the multiferroism.

A-site ordering versus electronic inhomogeneity in colossally magnetoresistive manganite films.

Abstract: Epitaxial La(3/4)Ca(1/4)MnO3/MgO(100) (LCMO) thin film shows an unusual rhombohedral (R-3c) structure with a new perovskite superstructure at room temperature due to the CE-type ordering of La and Ca with modulation vector q=1/4[011]. A-site ordered film was found to be electronically homogeneous down to the 1 nm scale as revealed by scanning tunnelling microscopy/spectroscopy. In contrast, orthorhombic and A-site disordered LCMO demonstrate a mesoscopic phase separation far below the Curie temperature (TC). Unique La/Ca ordering compensates the cation mismatch stress within one supercell, a(S) approximately 1.55 nm, and enhances the electronic homogeneity. The phase separation does not seem to be a unique mechanism for the colossal magnetoresistance (CMR) as very large CMR approximately 500% was also observed in A-site ordered films.

Colossal Magnetoresistance Observed in Monte Carlo Simulations of the One- and Two-Orbital Models for Manganites

Abstract: The one- and two-orbital double-exchange models for manganites are studied using Monte Carlo computational techniques in the presence of a robust electron-phonon coupling (but neglecting the antiferromagnetic exchange ${J}_{\mathrm{AF}}$ between the localized spins). The focus in this effort is on the analysis of charge transport. Our results for the one-orbital case confirm and extend previous recent investigations that showed the presence of robust peaks in the resistivity versus temperature curves for this model. Quenched disorder substantially enhances the magnitude of the effect, while magnetic fields drastically reduce the resistivity. A simple picture for the origin of these results is presented. It is also shown that even for the case of just one electron, the resistance curves present metallic and insulating regions by varying the temperature, as it occurs at finite electronic density. Moreover, in the present study these investigations are extended to the more realistic two-orbital model for manganites. The transport results for this model show large peaks in the resistivity versus temperature curves, located at approximately the Curie temperature, and with associated large magnetoresistance factors. Overall, the magnitude and shape of the effects discussed here resemble experiments for materials such as ${\mathrm{La}}_{0.70}{\mathrm{Ca}}_{0.30}\mathrm{Mn}{\mathrm{O}}_{3}$, and they are in agreement with the current predominant theoretical view that competition between a metal and an insulator, enhanced by quenched disorder, is crucial to understanding the colossal magnetoresistance (CMR) phenomenon. However, it is argued that further work is still needed to fully grasp the experimentally observed CMR effect, since in several other Mn oxides an antiferromagnetic charge-ordered orbital-ordered state is the actual competitor of the ferromagnetic metal.

Magnetoresistance of atomic-scale electromigrated nickel nanocontacts

Abstract: We report measurements of the electron transport through atomic-scale constrictions and tunnel junctions between ferromagnetic electrodes. Structures are fabricated using a combination of e-beam lithography and controlled electromigration. Sample geometries are chosen to allow independent control of electrode bulk magnetizations. As junction size is decreased to the single channel limit, conventional anisotropic magnetoresistance (AMR) increases in magnitude, approaching the size expected for tunneling magnetoresistance (TMR) upon tunnel junction formation. Significant mesoscopic variations are seen in the magnitude and sign of the magnetoresistance, and no evidence is found of large ballistic magnetoresistance effects.

Colossal electroresistance and current induced switching in ferromagnetic insulating state of La0.82Ca0.18MnO3

Abstract: Colossal electroresistance and current induced resistivity switching have been measured in the ferromagnetic insulating (FMI) state of single crystal manganite La0.82Ca0.18MnO3. The sample has a Curie transition temperature TC = 165 K and the FMI state is realized for temperatures T<100 K. The electroresistance (ER), arising from a strong nonlinear resistivity, attains a large value ( ≈ 100%) in the FMI state. However, this is accompanied by a collapse of the magnetoresistance (MR) to a small value even in magnetic field (H) of 10 T. This demonstrates that the mechanisms that give rise to ER and MR are effectively decoupled.

A-site disorder induced collapse of charge-ordered state and phase separated phase in manganites

Abstract: The effects of A-site cational size mismatch (A-site disorder) on the stability of charge-ordered states and phase separated phase in a series of manganites with constant A-site ionic average radii =1.18 angstrom but different A-site ionic size mismatches sigma(2) are experimentally investigated. It is revealed that the charge/orbital ordered antiferromagnetic ground state becomes destabilized and eventually collapses into coexisting of the predominant ferromagnetic metal (FMM) state and short-range charge/orbital ordered state with increasing sigma(2), resulting in enhanced colossal magnetoresistance. However, further increasing A-site disorder will suppress the FMM state and seem to favor a cluster-glass insulating state due to the severe electronic localization. (c) 2006 American Institute of Physics.