Huge Positive Magnetoresistance Research Articles

Effect of Etching Time to Tune Magnetoresistance Between Positive and Negative Values in p-Type Silicon Nanowires

Silicon nanowires (SiNWs) are formed by metal-assisted chemical etching of crystalline p-type silicon, in a mixture of aqueous HF and AgNO3 chemical solutions. The magnetic field effects on the current of Ag/SiNWs/Si/Al structures have been studied. At room temperature, magnetoresistance (MR) measurements revealed positive and negative MR depending on the etching time and on the applied voltage. Huge positive MR of about 1200% has been observed at the low field. The negative MR is attributed to the weak localization effect, while the positive one is related to the ohmic regime, where holes control the current. The MR effect depends on the applied voltage and on the SiNWs length. The huge MR effect on SiNWs can be exploited in magnetic-field sensor devices.

Huge positive magnetoresistance in antiferromagnetic double perovskite metals

Metals with large positive magnetoresistance are rare. We demonstrate that antiferromagnetic metallic states, as have been predicted for the double perovskites, are excellent candidates for huge positive magnetoresistance. An applied field suppresses long range antiferromagnetic order leading to a state with short range antiferromagnetic correlations and strong electronic scattering. The field induced resistance ratio can be more than tenfold, at moderate field, in a structurally ordered system, and continues to be almost twofold even in systems with ∼25% antisite disorder. Although our explicit demonstration is in the context of a two- dimensional spin-fermion model of the double perovskites, the mechanism we uncover is far more general, complementary to the colossal negative magnetoresistance process, and would operate in other local moment metals that show a field driven suppression of non-ferromagnetic order.

Electron heating and huge positive magnetoresistance in an AlGaAs∕GaAs high electron mobility transistor structure at high temperatures

We have performed magnetoresistivity measurements ρxx(B) on an AlGaAs∕GaAs high electron mobility transistor (HEMT) structure at high temperatures T. The observed positive magnetoresistance (MR) in the HEMT structure can be greatly enhanced simply by increasing the driving current. At T=80K and B=6T, the MR value can be increased from ∼150% to 4000%, almost a 30-fold increase when a large current of 40μA is applied. Such results are due to electron heating effects and our data lay the foundation for practical magnetic device applications which can be readily combined with high-speed electronics, high-frequency amplifiers, and radar using the mature GaAs-based HEMT technology.

Huge positive magnetoresistance in a gated AlGaAs∕GaAs high electron mobility transistor structure at high temperatures

Magnetoresistivity measurements on a gated AlGaAs∕GaAs high electron mobility transistor (HEMT) structure were performed at high temperatures T. By changing the applied gate voltage Vg, we can investigate the observed huge positive magnetoresistance (PMR) at different effective disorder and density inhomogeneity within the same HEMT structure. The observed PMR value increases with increasing disorder in the depletion mode (Vg⩽0). Moreover, the PMR value is not limited by the quality of the HEMT structure at T=80K. Such results pave the way for low-cost, high-throughput GaAs-based HEMT fabrication for future magnetic sensing and recording devices fully compatible with the mature HEMT technology.

Quantum-critical region of the disorder-driven superconductor–insulator transition

We investigate low temperature transport properties of thin TiN superconducting films, differing by the degree of disorder. At zero magnetic field we find an extremely sharp separation between the superconducting- and insulating phases, indicating a direct superconductor–insulator transition without an intermediate metallic phase. We show that in the critical region of the transition a peculiar highly inhomogeneous insulating state with superconducting correlations forms. The insulating films exhibit thermally activated conductivity and huge positive magnetoresistance at low magnetic fields. A sharp depinning transition at some voltage V T is observed in the I– V curves at very low temperatures. We propose a percolation type of depinning with the threshold voltage determined by the Coulomb blockade energy for the Cooper pairs between neighboring self-induced superconducting islands, with V T being the total voltage along the first conduction path. The observed hysteretic behavior of the threshold and steps on the d I/d V vs. V curves support this percolation picture of the depinning transition.

Huge positive magnetoresistance of GaAs∕AlGaAs high electron mobility transistor structures at high temperatures

The authors have performed magnetoresistivity measurements ρxx(B) on GaAs∕AlGaAs high electron mobility transistor (HEMT) structures at high temperatures T. These HEMT structures show huge positive magnetoresistance (MR). For B=±6T, the MR values are >1300% and >200% at T=20 and 80K, respectively. Since a GaAs-based HEMT structure is not susceptible to ferromagnetic noise which appears to represent a fundamental challenge to the scalability of magnetic MR devices to ultrahigh area densities, the experimental results pave the way for the integration of scalable nonmagnetic MR devices with the mature HEMT technology using the same material system.

Huge positive magnetoresistance in an InN film

The authors report on magnetoresistance measurements on an InN film grown by molecular beam epitaxy. The application of a low magnetic field (B≈0.15T) perpendicular to their InN film causes striking huge positive magnetoresistance (PMR) of up to ∼106%. This PMR shows a strong temperature dependence and therefore their device can also be used as a temperature sensor. The authors’ experimental data pave the way for integrating magnetic sensors with optoelectronic devices using the same nonmagnetic material system which is not susceptible to ferromagnetic noise.

Magnetic-Field-Sensing Materials Composed of Metal–Semiconductor Hybrid Nanostructures

A huge positive magnetoresistance effect, about 4 orders of magnitude at room temperature, was discovered in metal-semiconductor hybrid nanostructures. The hybrid material consisting of metallic nanostructures that are fabricated on a GaAs substrate by ultra-high vacuum deposition method exhibits magnetic field-sensitive current-voltage characteristics. When a constant voltage above the threshold value, is applied to the film, a very steep change in the current, which we term magnetoresistive switch, is driven by the huge magnetoresistance effect under a relatively low magnetic field at room temperature. The magnetoresistance effect is very sensitive to the nanoscale morphology of the hybrid film; in other words, one can control the magnetoresistance function at will by modifying the nanostructure of this material. The origin of the magnetoresistive switch effect and its possible application as a magnetic field sensor is discussed.

Huge magnetoresistance effect of highly oriented pyrolytic graphite

Graphite is a quasi-two-dimensional semimetal. However, for usual graphite the magnetoresistance is not so high due to its small crystal size and no preferred orientation. Huge positive magnetoresistance up to 85300% at 4.2 K and 4950% at 300 K under 8.15 T magnetic field was found in highly oriented pyrolytic graphite. The mechanism of huge positive magnetoresistance is not only due to ordinary magnetoresistance but also due to magnetic-field-driven semimetal-insulator transition.

Phonons and electron-phonon scattering in carbon nanotubes

Electron-phonon scattering is studied within an effective-mass theory. A continuum model for acoustic phonons is introduced and electron-phonon interaction due to modification of band structure is derived as well as a normal deformation potential. In a metallic nanotube, the deformation potential does not participate in electron scattering and a metallic nanotube becomes nearly a one-dimensional ballistic conductor even at room temperature. A resistivity determined by small band-structure interaction depends on the chirality at low temperatures. A magnetic field perpendicular to the axis induces electron scattering by the deformation potential, giving rise to huge positive magnetoresistance.

Metal-nanocluster equipped GaAs surfaces designed for high-sensitive magnetic field sensors

A huge positive magnetoresistance (MR) effect, more than 10 5% at room temperature (RT), has been discovered in metal-nanocluster equipped GaAs surfaces. Granular film consisting of nanoscale MnSb metal-clusters that are grown on a sulfur-terminated GaAs(0 0 1) substrate by molecular-beam epitaxy exhibits magnetic-field-sensitive current–voltage characteristics. When a constant voltage, above the threshold value, is applied to the film, very steep change in the current, which we term magnetoresistive switch, is driven by the huge MR effect under a relatively low magnetic field at RT. The origin of the magnetoresistive switch effect is discussed in terms of a novel concept of magnetic-field-sensitive avalanche breakdown. These properties strongly depend on the morphological condition of metal-nanoclusters, and seem to depend on electronic properties of the GaAs surface.

Magnetoresistive switch effect in metal/semiconductor hybrid granular films: extremely huge magnetoresistance effect at room temperature

Hybrid granular films, consisting of nanoscale metal clusters that are grown on a GaAs substrate by molecular beam epitaxy, exhibit magnetic-field-sensitive current–voltage (I–V) characteristics at room temperature. When a constant voltage, above the threshold value, is applied to the film, the I–V curve becomes nonlinear and a breeding-like current increase appears. The abrupt increase disappears with the application of magnetic fields, which we call the magnetoresistive switch. The switch yields the huge positive magnetoresistance effect, for example 320 000% at 2 kOe. The origin of the magnetoresistive switch effect is discussed in terms of a magnetic-field dependence of an avalanche breakdown.

Magnetoresistive switch effect in metal/semiconductor hybrid granular films—extra huge magnetoresistance effect at room temperature

A granular film consisting of nanoscale metal clusters that are grown on a GaAs substrate by molecular-beam epitaxy exhibits magnetic-field-sensitive current–voltage characteristics at room temperature. When a constant voltage, above the threshold value, is applied to the film, a steep current-increase appears. The abrupt increase disappears on applying magnetic fields, which we term magnetoresistive switch. The switch yields a huge positive magnetoresistance effect, 320 000% at 2000 Oe.

Commensurability effects in two-dimensional electron gases with periodically arranged Ni and NiFe nanopillars

We have investigated the magnetoresistance of high mobility two-dimensional electron gases (2DEG) under the influence of periodically arranged ferromagnetic nanopillars. A huge positive magnetoresistance and commensurability oscillations (COs) indicate that the 2DEG is subjected to a surprisingly strong modulation. Angular-dependent magnetoresistance measurements as well as the phase of the COs suggest that the modulation is dominated by electrostatic rather than by magnetic contributions.

Magnetoresistive switch effect in MnSb granular films grown on sulfur-passivated GaAs: more-than 10 000% magnetoresistance effect at room-temperature

A huge positive magnetoresistance effect, more than 10 000% at room temperature, has been discovered in MnSb granular films. Granular films consisting of nanoscale MnSb dots were fabricated on a sulfur-passivated GaAs (0 0 1) substrate by molecular-beam epitaxy, then covered with an Sb thin layer. The MnSb granular films exhibit a strong in-plane anisotropy of the magnetic-field-sensitive current–voltage characteristics. When a constant voltage, above the threshold value, is applied in the [1 1 0] direction of the GaAs (0 0 1) surface, a steep change in the current, which we term magnetoresistive switch (MRS), is driven by the huge magnetoresistance effect under a relatively low magnetic field (less than about 0.2 T ). On the other hand, less than 1% magnetoresistance effect was observed when the voltage was applied in the [1 1 ̄ 0] direction of the GaAs surface. The origin of the anisotropy is discussed in terms of the microscopic structural anisotropy at the heterointerface.

Magnetoresistance properties of quasi-two-dimensional charge-density wave compounds AMo6O17 (A=Na, K, and Tl)

Temperature dependence of resistivity in quasi-two-dimensional purple bronzes AMo6O17 (A=Na, K, and Tl) has been comparatively studied between 2 and 300 K at different magnetic fields. Huge positive magnetoresistance (MR) behavior below the Peierls transition temperature TP∼110 K was found in both K/Tl purple bronzes for H∥c, but only a weak one in the Na purple bronze. At lower temperature TM∼16 K, a second anomaly of the resistivity, which moved to higher temperatures under a magnetic field, was also observed in both K/Tl compounds, with no corresponding anomaly in the Na purple bronze. For H⊥c, the MR effect becomes much weaker in all three compounds. These results suggest that the second anomaly is possibly related to the second Peierls instability, and the huge MR effect results from the large modification of the third FS along the [100] direction induced by the magnetic field, in which the third FS is imperfectly nested in K/Tl purple bronzes and absent in the Na purple bronze.

Magnetotransport and theShubnikov-de Haas effect in quasi-two-dimensional purple bronzeTlMo6O17

Magnetotransport properties of quasi-two-dimensional thallium purplebronze were measured at different magnetic fields for H∥c. Huge positive magnetoresistance below thePeierls transition TP and Shubnikov-de Haas oscillation in thehigh-field range were found. In the charge-density-wave (CDW) statebelow TP, nonlinearity of the voltage-current characteristicsdue to the depinning of a CDW was also observed at 2 K and 14 T.These results supported the suggestion that the CDW gap openingbelow TP still leaves small electron and/or hole pockets, but anapplied magnetic field can remove these pockets to a great extent,thus leading to an enhanced gap.

Room-temperature thousandfold magnetoresistance change in MnSb granular films: Magnetoresistive switch effect

A huge positive magnetoresistance effect has been discovered in MnSb granular films. Granular film consisting of nanoscale MnSb dots that are grown on a sulfur-passivated GaAs (001) substrate by molecular-beam epitaxy, then covered with an Sb thin layer, exhibits magnetic-field-sensitive current–voltage characteristics. When a constant voltage, above the threshold value, is applied to the film, more than 1000% change in the current, which we term magnetoresistive switch, is driven by the magnetoresistance effect under a relatively low magnetic field (less than 0.5 T) at room temperature.

ＭｎＳｂグラニュラー構造における室温超巨大磁気抵抗効果

A huge positive magnetoresistance effect in a relatively low magnetic field (less than 0.5 T) at room temperature was discovered in MnSb granular films. A granular film consisting of nano-scale MnSb clusters was grown on a sulfur-passivated GaAs(001) substrate by molecular beam epitaxy, then covered with an Sb thin layer. The granular film shows magnetic-field-sensitive current-voltage characteristics. When a constant voltage, above the threshold value, is applied to the film, a more than 1000 percent change in the current, which we term a magnetoresistive switch, is driven by the magnetoresistance effect. A possible mechanism for the magnetoresistive switch effect is discussed.

Impurity Scattering in Carbon Nanotubes – Absence of Back Scattering –

The effective potential of an impurity in a k · p scheme is derived in two-dimensional graphite sheet. When the potential range is smaller than the lattice constant, it has an off-diagonal matrix element between K and K ′ points comparable to the diagonal element. With the increase of the range, this off-diagonal element decreases rapidly and the diagonal element for envelopes at A and B sites becomes identical. The crossover between these two regimes occurs around the range smaller than the lattice constant. In the latter regime, back scattering between states with + k and - k vanishes identically for the bands crossing the Fermi level in the absence of a magnetic field, leading to an extremely large conductivity. The absence of the back scattering disappears in magnetic fields, giving rise to a huge positive magnetoresistance.