Large Magnetoresistance Effect Research Articles

Extremely Large Magnetoresistance in Boron-Doped Silicon

Boron-doped Si-SiO2-Al structures are fabricated to study extremely large magnetoresistance (MR) effects. Current-voltage characteristics show a nonlinear behavior, dominated by an autocatalytic process of impact ionization. At low temperatures, the magnetic field postpones the onset of impact ionization to higher electric fields. This results in a symmetric positive MR of over 10,000% at 400 kA/m. Applying a magnetic field leads to an increase of the acceptor level compared to the valence band as deduced by admittance spectroscopy. A macroscopic transport model is introduced to describe how the MR is controlled by voltage, electrode spacing, and oxide thickness.

Structures and Physical Properties of n = 3 Ruddlesden–Popper Compounds Ca4Mn3−xNbxO10 (0 ≤ x ≤ 0.2)

The Ruddlesden–Popper series of compounds Ca4Mn3−xNbxO10 (x = 0–0.2) have been prepared by solid-state methods. Structural, magnetic, electrical, and magnetoresistive studies were performed on the compounds. Nb doping caused increases in both unit cell volume and octahedral distortion. The magnetization measurements indicated that the doped samples displayed ferromagnetism-like behavior, which could be explained by the double-exchange interaction between Mn4+ and Mn3+ induced by the charge-compensation effect. A phase-separation picture with the appearance of ferromagnetic clusters embedded in an antiferromagnetic matrix described the magnetic ground state of the doped samples well. All of the samples exhibited semiconducting behavior over the whole measured temperature range, with the high-temperature behavior better described by the small polaron conduction model and the low-temperature behavior by the two-dimensional version of Mott’s variable range hopping model. A large magnetoresistance effect was observed for all of the doped samples, and its value rose with decreasing temperature and increasing Nb content, attaining 51% at 40 K under a magnetic field of 5 T for the x = 0.2 sample. Calculated results for Ca4Mn3O10 and Ca4Mn2.8Nb0.2O10 samples further verified the corresponding experimental determinations.

Observation of large low field magnetoresistance and large magnetocaloric effects in polycrystalline Pr0.65(Ca0.7Sr0.3)0.35MnO3

Magnetotransport and magnetocaloric properties of polycrystalline Pr0.65(Ca0.7Sr0.3)MnO3 have been studied. Large low field magnetoresistance (∼100% at 75K for 1T field) with high magnetic field sensitivity (∼5000%T−1) has been observed due to the melting of charge ordered state. The system also shows large magnetocaloric effect at ∼215 and 90K. The magnetic entropy change is ∼8 and 5.5J∕kgK, respectively, around these temperatures for a field change from 0to7T. Such huge low field magnetoresistance along with large magnetocaloric effect exhibited by the material is interesting from the application point of view.

Tunneling Magneto-Resistance Effect of Fe<sub>4</sub>N with MgO Barrier

Gamma'-Fe4N is the ferromagnetic material with high Curie temperature and relatively high electrical conductivity. First principle calculation showed that the ganma'-Fe4N was a half-metal by 100% spin polarized conduction electrons with minority spins. So we expected large tunneling magneto-resistance effect in the Fe4N/MgO/Fe4N junctions. Gamma'-Fe4N was prepared by sintering Fe3O4 nanoparticles in the atmosphere of NH3/H2 gas mixture at 673 K. Then MgO was added to the gamma'-Fe4N powder and sintered at about 473 K in pure Ar gas. Magneto-resistance was measured by 4-terminals method for x=0-0.8 samples of (MgO)x(Fe4N)1-x, and magneto-resistance ratios of -1.2~-1 % were observed for x=0.3~0.6 samples at 300K. It is the first observation oftunneling magneto-resistance effect for gamma'-Fe4N granular system.

The Nanoscale Phase Separation in Hole-Doped Manganites

A macroscopic phase separation, in which ferromagnetic clusters are observed in an insulating matrix, is sometimes observed, and believed to be essential to the colossal magnetoresistive (CMR) properties of manganese oxides. The application of a magnetic field may indeed trigger large magnetoresistance effects due to the percolation between clusters allowing the movement of the charge carriers. However, this macroscopic phase separation is mainly related to extrinsic defects or impurities, which hinder the long-ranged charge–orbital order of the system. We show in the present article that rather than the macroscopic phase separation, an homogeneous short-ranged charge–orbital order accompanied by a spin glass state occurs, as an intrinsic result of the uniformity of the random potential perturbation induced by the solid solution of the cations on the A -sites of the structure of these materials. Hence the phase separation does occur, but in a more subtle and interesting nanoscopic form, here referred as “homogeneous”. Remarkably, this “nanoscale phase separation” alone is able to bring forth the colossal magnetoresistance in the perovskite manganites, and is potentially relevant to a wide variety of other magnetic and/or electrical properties of manganites, as well as many other transition metal oxides, in bulk or thin film form as we exemplify throughout the article.

Magnetoresistive properties of Zn 1−xCo xFe 2O 4 ferrites

The magnetic and magnetoresistive properties of spinel-type Zn 1− x Co x Fe 2O 4 ( x=0, 0.2 and 0.4) ferrites are extensively investigated in this study. A large negative magnetoresistance (MR) effect is observed in Zn 1− x Co x Fe 2O 4 ferrites of spinel structure. These materials are either ferrimagnetic or paramagnetic at room temperature, and show a spin-(cluster) glass transition at low temperatures, depending on the chemical compositions. The MR curves as a function of magnetic fields, MR( H), are parabolic at all temperatures for paramagnetic polycrystalline ZnFe 2O 4. The MR for ZnFe 2O 4 at 110 K in the presence of 9 T applied magnetic field is 30%. On the other hand, MR( H) are linear for x=0.2 and 0.4 ferrimagnetic Zn 1− x Co x Fe 2O 4 samples up to 9 T. The MR effect is independent of the sintering temperatures, and can be explained with the help of the spin-dependent scattering and the Yafet–Kittel angle of Zn 1− x Co x Fe 2O 4 mixed ferrites.

Extremely strong domain-wall pinning and spontaneous demagnetization in MnAs(0001) films onGaAs(111)B

We demonstrate a dramatic dependence of the magnetic properties of epitaxial layers on the substrate orientation. MnAs(0001) layers on $\mathrm{Ga}\mathrm{As}(111)B$ contain magnetic moments that cannot be aligned by a magnetic field of $140\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$, and produce a hysteretic behavior over the entire field range. Depinned magnetic moments demagnetize spontaneously at weak fields. These peculiar properties are related to an antiferromagnetic coupling that emerges at low temperatures, whose origin we attribute to antiphase boundaries. Despite the tiny fraction of the anomalous moments, they give rise to large magnetoresistance effects.

The crucial role of mixed valence in the magnetoresistance properties of manganites and cobaltites

The mixed valence Mn3+/Mn4+ and Co3+/Co4+ in manganites and cobaltites with the perovskite structure is absolutely necessary for the appearance of magnetotransport properties. It is shown that in these systems the Jahn Teller effect of the transition element, the charge and orbital ordering and the oxygen stoichiometry play a key role in the appearance of large and even colossal magnetoresistance. It has been discovered that these oxides exhibit a new phenomenon, the crystallographic and electronic phase separation. It is this phenomenon that is at the origin of the competition between ferromagnetism and antiferromagnetism or spin glass behaviour and which leads to the negative magnetoresistance (MR). The doping of these materials at different sites appears then to be a means of inducing large MR effects.

A novel spin transfer torque effect in Ag2Co granular films

We studied a spin transfer torque (STT) effect in Ag2Co granular films, induced by a high current density 108 - 9 A cm− 2 injected via a point contact. The system consists of single domain Co nanoparticles randomly embedded in Ag matrix, with a mean distance corresponding to the typical quantity of layer thicknesses used for the nonmagnetic space in multilayer nanopillars. A large giant magnetoresistance (GMR) effect of 55% measured at 4.2 K is an indication for high spin scattering anisotropy which is required for STT observations. Supposedly, a certain amount of large-sized Co particles saturated in external magnetic field H acts here as the spin polarizer for the injected current I and small-sized particles unsaturated at 4.2 K and Hmax (90 kOe) even act as the detector for switching. A novel STT effect was observed thereby as I rises across a threshold value Ic, showing a sharp decrease in R (ΔR/R = 130% with two steps), which arises accordingly from further alignment of the small-sized Co granules. The behavior is polar and hysteretic, similar to properties measured for multilayer nanopillars. The two-step behavior could be an effect related to a lognormal particle size distribution. Depending on the spin polarization, Ic is found to be field disproportional, indicating a larger STT efficiency at a higher H.

Magnetoresistance and field-induced structural transitions in Ni 50Mn 50−xSn x Heusler alloys

Large magnetoresistance effects were observed in Ni 50Mn 50− x Sn x Heusler alloys for concentrations 13⩽ x⩽16. The effects occur at the respective martensitic transition temperatures ( T M), which can be tuned in temperature through variations in Sn concentration. The largest observed values of the magnetoresistance (Δ ρ/ ρ o) were −42% for x=14 at T≈250 K, and −23% above room temperature ( T≈320 K) for the sample with x=13, where the magnetic field change was Δ H=5 T. Magnetization and X-ray diffraction measurements indicate that the origin of the observed magnetoresistance is a magnetically induced magnetostructural transition from a complex 10 M modulated orthorhombic phase containing some antiferromagnetic coupling to a ferromagnetic cubic L2 1 structure.

Large magnetoresistance effects in diluted magnetic semiconductors in low-dimensional geometries

We have introduced the conductivity in the variable ranging hopping condition in quasi-one-dimensional system of diluted magnetic semiconductors (DMSs), by using the path-integral method and the supersymmetric sigma formula. A large magnetoresistance mechanism is proposed.

Double Perovskites with Ferromagnetism Above Room Temperature

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Large positive magnetoresistance effect below Curie temperature in In1.90−xMn0.1SnxO3

We report on the magnetoresistance (MR) and magnetic properties of In1.90−xMn0.1SnxO3 (0⩽x⩽0.06) oxide. All samples were found to be ferromagnetic below TC=46K. Sn doping changed In1.90Mn0.1O3 from an insulator to a highly conducting phase at 300K. A positive MR effect was observed over a wide temperature range just below TC. Calculated MR values reached a maximum of 20% at 5K. A change in the MR effect, from positive to negative, occurred under magnetic field H>4T at 5K. The results of x-ray absorption near-edge spectroscopy indicated that Mn ions are present both as Mn2+ and Mn4+.

Large inverse tunneling magnetoresistance in Co2Cr0.6Fe0.4Al∕MgO∕Co80Fe20 magnetic tunnel junctions

Magnetic tunnel junctions with the layer sequence Co2Cr0.6Fe0.4Al∕MgO∕Co80Fe20 were fabricated by magnetron sputtering at room temperature (RT). The samples exhibit a large inverse tunneling magnetoresistance (TMR) effect of up to −66% at RT. The largest value of −84% at 20K reflects a rather weak influence of temperature. The dependence on the voltage drop shows an unusual behavior with two almost symmetric peaks at ±600mV with large inverse TMR ratios and small positive values around zero bias.

Large tunneling magnetoresistance effect at high voltage drop for Co-based Heusler alloy∕MgO∕CoFe junctions

Growth and magnetic characterization of thin films of Co2Cr0.6Fe0.4Al and Co2MnSi full-Heusler compounds are investigated. Thin films were deposited by magnetron sputtering at room temperature directly onto oxidized Si wafers. These Heusler films are magnetically very soft and ferromagnetic with Curie temperatures well above room temperature. Polycrystalline Co2Cr0.6Fe0.4Al Heusler films combined with MgO barriers and CoFe counter electrodes are structured to magnetic tunnel junctions and yield almost 50% magnetoresistance at room temperature. The magnetoresistance shows a strong bias dependence with the maximum occurring at a voltage drop well above 1V. This special feature is accompanied by only a moderate temperature dependence of the tunnel magnetoresistance.

Preparation and magnetoresistance of Ag2+xSe thin films deposited via pulsed laser deposition.

The preparation of Ag2+xSe thin films with thicknesses between 4 nm and 3 μm by pulsed laser deposition on single crystalline NaCl and MgO substrates is reported. The films are perfectly dense and show a good lateral uniformity with a small number of defects. The microstructure of the films corresponds to a nanoparquet, being composed of two different phases of silver selenide. One phase is identified as the Naumannite low temperature phase of silver selenide, the structure of the other phase has not been reported in detail before and probably represents a metastable phase. Silver-rich films contain silver precipitates with typical sizes on the nanoscale. Their presence and their size appears to be responsible for the large and linear magnetoresistance effect of silver-rich silver selenide.

Magnetic field effects on current, electroluminescence and photocurrent in organiclight-emitting diodes

We report on the experimental characterization of a recentlydiscovered large magnetoresistive effect in polyfluorene and inAlq3 organic light-emitting diodes. We also observe similar magnetic field effects (MFEs) ofcomparable magnitude in electroluminescence and photocurrent measurements. Weprovide a comprehensive overview of all these three types of MFE. To the best ofour knowledge, the mechanism causing these MFEs is not currently known withcertainty. Moreover, we show that experiments in bipolar, electroluminescent devicesdo not allow determination of whether the MFE acts on the carrier density orcarrier mobility, making any attempt at explaining it ambiguous. As a remedy, weperform magnetoresistance measurements in hole-only polyfluorene devices and showthat the MFE acts on the carrier mobility rather than carrier recombination.

Transformation of spin information into large electrical signals using carbon nanotubes

Spin electronics (spintronics) exploits the magnetic nature of electrons, and this principle is commercially applied in, for example, the spin valves of disk-drive read heads. There is currently widespread interest in developing new types of spintronic devices based on industrially relevant semiconductors, in which a spin-polarized current flows through a lateral channel between a spin-polarized source and drain. However, the transformation of spin information into large electrical signals is limited by spin relaxation, so that the magnetoresistive signals are below 1% (ref. 2). Here we report large magnetoresistance effects (61% at 5 K), which correspond to large output signals (65 mV), in devices where the non-magnetic channel is a multiwall carbon nanotube that spans a 1.5 microm gap between epitaxial electrodes of the highly spin polarized manganite La(0.7)Sr(0.3)MnO3. This spintronic system combines a number of favourable properties that enable this performance; the long spin lifetime in nanotubes due to the small spin-orbit coupling of carbon; the high Fermi velocity in nanotubes that limits the carrier dwell time; the high spin polarization in the manganite electrodes, which remains high right up to the manganite-nanotube interface; and the resistance of the interfacial barrier for spin injection. We support these conclusions regarding the interface using density functional theory calculations. The success of our experiments with such chemically and geometrically different materials should inspire new avenues in materials selection for future spintronics applications.

Competition between intragranular and intergranular tunneling magnetoresistance in polycrystallineSr2FeMoO6

Polycrystalline ${\mathrm{Sr}}_{2}\mathrm{Fe}\mathrm{Mo}{\mathrm{O}}_{6}$ (SFMO) samples with various grain sizes and densities of $\mathrm{Fe}∕\mathrm{Mo}$ disorder defects were synthesized from sol-gel-derived precursors by means of a sample-encapsulation technique. The samples with perfect $\mathrm{Fe}∕\mathrm{Mo}$ ordering exhibited metallic transport behavior and a large magnetoresistance (MR) effect, whereas the samples with a high-disorder density and small grain size showed a metal-insulator transition and lower MR values. It is suggested that a competition between intragranular and intergranular tunneling MR effects exists in the SFMO system. To confirm this competition, homocomposite samples consisting of two or three single-phase SFMO components with different grain sizes were designed. Large enhancement in low-field magnetoresistance (LFMR) was observed in the homocomposites, which could be attributed to the enhanced intergranular effect. It is thus concluded that the intergranular effect is more important to the LFMR than the intragranular effect.

Metal-insulator transitions and large magnetoresistance effects in diluted magnetic semiconductors

We have discussed the localization mechanism of hole-induced magnetic solitons and large magnetoresistance effects in diluted magnetic semiconductors, by using the effective Lagrangian and the supersymmetric sigma formula.