Intergranular Tunneling Research Articles

Effects of sintering temperature on physical properties of nanocrystalline La0.85Na0.15MnO3ceramics

Nanocrystalline La 0.85 Na 0.15 MnO 3 ceramics were prepared via a novel method based on chemical solution combustion. Effects of sintering temperature on electrical transport and magnetic properties were systematically investigated. It was found that the resistivity increases, the magnetization decreases, and the metal-insulator transition temperature shifts toward low temperatures with the decrease of the sintering temperature. In addition, the magnetization of the samples sintered at low temperatures exhibits a broadened paramagnetic-to-ferromagnetic transition temperature. The low field magnetoresistance evidently increases with decreasing the sintering temperature due to the spin-polarized intergranular tunneling.

Synthesis, characterization and magnetoresistance properties study of magnetite thin films by electroless plating in aqueous solution

Polycrystalline half-metallic Fe 3O 4 films with 1 μm in thickness were synthesized on glass substrates directly by electroless plating in aqueous solution at 90 °C without heat treatment. The films have single pure spinal phase structure and the well-crystallized columnar grains grow perpendicularly to the substrates, as revealed by XRD, XPS and SEM. At room temperature, the films exhibit negative magnetoresistance ( MR) ratio of about −5.1%, which is ascribed to intergranular tunneling of spin polarized electrons of Fe 3O 4. The resistivity R of the films with 1 μm in thickness at room temperature is about 5.2 × 10 −1 Ω cm. The cation distribution and the arrangement of the magnetic moments of the plated Fe 3O 4 ferrite thin films are different from that of the bulk materials, which is likely to be one of the reasons for the modification of R and MR properties.

Magnetic and Magnetoelectric Properties of Self-Assembled Fe2.5Mn0.5O4 Nanocrystals

We report magnetoresistance of -40%, corresponding to 80% spin polarization, at magnetic field of 0.5 T and 200 K for oleic acid-coated Fe(2.5)Mn(0.5)O(4) nanocrystals (FMO NCs) self-assembled on a SiO(2)/Si substrate by drop casting fabrication. The FMO NCs exhibited spin glass transition around 150 K and nonlinear current-voltage (I-V) characteristics. Fowler-Nordheim plot of the I-V characteristics indicated that electrons tunnel directly barriers between the FMO NCs. Transmission electron microscopy revealed that the FMO NCs are elongated hexagon in shape with size of ∼15 × 20 nm. The FMO NCs self-assembled in two-dimension hexagonal networks of collinear ferromagnetic moments. The [111] easy magnetization axis of each FMO NC was parallel to each other in the hexagonal arrays. Geometrically frustrated lattice of collinear ferromagnetic moments supports both a low and a high intergranular tunneling conductance for the self-assembled FMO NCs without and with magnetic fields, respectively.

Variable-range-hopping conductivity in polygrain samples of icosahedral Al–Re–Pd

We further develop a model, based on an analogy with granular electronic conductors, which explains the nature of the variable-range-hopping conductivity in polygrain samples of icosahedral Al–Re–Pd with resistivity ratios of in an excess of ∼13. The electrical conductivity of the model is determined by intergranular tunnelling and electrostatic energy. In polygrain icosahedral Al–Re–Pd, pores and precipitates of secondary phases create the barriers. Depending on the barrier height and electrostatic disorder, either an insulating variable-range-hopping regime of an Efros–Shklovskii or Mott type with elastic co-tunnelling or a metallic regime typical of homogeneously disordered conductors is possible.

Percolation effects in the conductivity and magnetoresistance of compacted chromium dioxide powder

The resistive and magnetoresistive properties of a submicron (120 nm) compacted CrO2 powder with a Curie temperature of ∼385 K were investigated in the temperature range of 5–430 K for magnetic fields of up to 1.6 T. The specimen revealed a nonmetallic temperature dependence of resistance and high negative magnetoresistance (MR) (20%) in a low-temperature range. The MR magnitude rapidly decreased with an increase in temperature and was less than 0.3% for T > 200 K. Such MR behavior is shown to be characteristic for granular systems with spin-dependent intergranular tunneling. Some features of MR behavior in a low-temperature range (lower than 40 K) can be attributed to the percolation nature of the tunnel conductivity of such a granular system in the presence of a limited number of dominant conducting current paths.

CVD synthesis of polycrystalline magnetite thin films: structural, magnetic and magnetotransport properties

Magnetite (Fe3O4) is predicted to be half metallic at room temperature (RT) and it shows the highest Curie temperature among oxides. The use of Fe3O4 thin films is therefore promising for spintronic devices such as magnetic tunnel junctions (MTJs) and magnetoresistive sensors. The structural, magnetic and magnetotransport properties of magnetite are reported to be strongly dependent on the growth conditions. We have developed a very simple deposition chamber for growing thin magnetite films via a chemical vapour deposition (CVD) process based on the Fe3(CO)12 carbonyl precursor. The structural, morphological, and magnetic properties of the as deposited Fe3O4 films have been investigated by means of time of flight secondary ion mass spectrometry, grazing incidence x-ray diffraction, x-ray reflectivity, atomic force microscopy, conversion electron Mössbauer spectroscopy and superconducting quantum interference device magnetometry. Magnetotransport measurements show magnetoresistance up to −2.4% at RT at the maximum applied field of 1.1 T. Resistivity measurements in the 100–300 K temperature range reveal that the magnetotransport properties of the Fe3O4 films are governed by inter-granular tunnelling of the spin-polarized electrons. The spin polarization is estimated to be around −16%. A possible route for increasing the spin-polarized performances of our magnetite films is proposed. We have also deposited Fe3O4/MgO/Co stacks by using a combined CVD and atomic layer-deposition process. The trilayer's hysteresis curve evidences the presence of two distinct switching fields making it promising for magnetite-based MTJ applications.

Crossover Behavior of Variable Range Hopping in Bi-Doped C60

Abstract: Mott variable-range hopping (VRH) is characterized by an exp [1/T1/4] behavior in the temperature dependence of resistivity and is generally observed near the metal–insulator transition as electron screening increases and the Coulomb gap disappears near the metallic phase, while Efros–Shklovskii VRH, characterized by exp [1/T1/2], is found deeper in the insulating region. We have investigated the transport properties of Bi-doped fullerene C60. Samples were prepared via solid state reaction in a sealed quartz tube near 600°C. The resistivity data can be fit with a single function exp [1/Tυ] (υ=1/4 or 1/2, depending on the Bi concentration) over the entire temperature range below 300 K and over 5–6 orders of magnitude in resistivity. υ changes from 1/4 to 1/2 as the Bi concentration increases, suggesting a crossover from Mott VRH to intergranular tunneling at higher Bi concentration. The thermoelectric Seebeck coefficient was also measured and is about 20 μV/K at room temperature. It decreases with decreasing temperature. The thermal conductivity of the doped samples is extremely low.

Spin-dependent intergranular transport in highly spin-polarized Co1−xFexS2 thin films

Magnetic and magnetotransport measurements on polycrystalline thin film Co1−xFexS2, a system that exhibits tunable conduction electron spin polarization in bulk, are reported. The films exhibit a low field hysteretic magnetoresistance (MR) due to spin-dependent intergranular tunneling, thus providing a simple probe of the spin polarization. The MR increases rapidly with x, saturating as the magnetization reaches 1.0μB/Co, demonstrating that high, doping tunable, spin polarization can be readily achieved in thin films synthesized by simple methods. Quantitative estimates yield spin polarizations as high as 90%.

Enhanced tunneling magnetoresistance of Fe3O4 in a Fe3O4-hexabromobenzene (C6Br6) composite system

Magnetotransport of Fe3O4-hexabromobenzene (C6Br6) composite has been studied. Powders of C6Br6 and Fe2O3 nanoparticles were mixed together. They were annealed in hydrogen flow. There was a phase transformation from Fe2O3 to Fe3O4 after annealing. Giant negative magnetoresistance (MR) was observed at room temperature and the MR ratio is about 13.4% in an applied field of 5 T. The maximum MR ratio is 21.5% at 130 K. The temperature dependence of the resistivity exhibits characteristics of intergranular tunneling in the samples. The enhancement of the MR ratio is attributed to the fact that the C6Br6 can act as barrier material and, more importantly, can prevent the oxidation of the surface of Fe3O4, which is believed to alter the half-metallic state at the surface.

Electric transport behaviour of sodium-substituted perovskitesLa1−xNaxMnO3 (for x = 0.1 and 0.2) and the effect of magnetic fields

Sodium-substituted perovskitesLa0.9Na0.1MnO3 and La0.8Na0.2MnO3, which crystallize in pseudo-cubic symmetry, and are respectively a disordered and an orderedferromagnet, have been studied for their electrical behaviour. The two samples show large negativemagnetoresistance. The lesser sodium-containing system shows a metal–insulator transition at∼210 K, which is well below the temperature where it completely goes to the paramagnetic state.In the higher sodium-containing sample both the magnetic and electrical transitions wouldfall only above 300 K. In the conducting state in both samples, resistivity hascontributions from scattering from grain boundaries, electron–electron scattering andelectron–magnon (spin wave) scattering. However, in the insulating (semiconducting) stateeven the lesser sodium-containing system (in which the insulating state has beenobserved in the temperature range of study) follows small polaron hopping under anadiabatic approximation and the estimated activation energy values at differentfields are also identical to those reported in magnetically ordered perovskites.La0.9Na0.1MnO3 shows an upturn in resistivity with decreasing temperature at∼30 K which is attributed to spin-polarized intergranular tunnelling.

Role of oxide barrier in intergranular tunnel junctions: An enhanced magnetoresistance in SiO2 and ZnO coated Fe3O4 nanoparticle compacts

A systematic study of oxide coated magnetic nanoparticles is carried out to understand and evaluate the effect of oxide tunnel barrier on the magnetoresistance (MR) of magnetite core-shell nanocompacts. SiO2 and ZnO coated Fe3O4 nanoparticles with a shell thickness of ∼2nm have been synthesized and characterized. Magnetic measurements on oxide coated Fe3O4 nanoparticles reveal blocking behavior and superparamagnetism below ∼200K. The magnitude of electrical resistivity at room temperature of SiO2 and ZnO coated Fe3O4 compacts is found to be two to five orders higher than that of pure nanocrystalline Fe3O4 compacts due to resulting insulating oxide layer between magnetite domains. For these nanocompacts, large low field magnetoresistances of −11.5% and −5.8% are achieved in an applied magnetic field of 1T at 100K that could have origins of spin-dependent tunnel MR across the nanometric granules of Fe3O4 safely embedded in insulating oxide matrix.

Large low field magnetoresistance in ultrathin nanocrystalline magnetite Fe3O4 films at room temperature

Ultrathin (15nm) Fe3O4 nanocrystalline films with (111) spinel texture have been prepared by rapid annealing of amorphous ion oxide films. Large low field magnetoresistance (LFMR), with the values of about −6.3% at 300K and −10% at 200K under a field of 0.5T, has been observed in the films. The LFMR is mainly attributed to the boundary tunneling of high spin-polarized electrons in Fe3O4 grains of the films and nearly follows a simple relationship between MR and polarization for intergranular tunneling. The fabricating method here seems to be a good approach to prepare high quality Fe3O4 nanocrystalline films with a large LFMR at room temperature.

Magnetoresistance in Sr2FeMoO6:x glass composites

The effects of interfacial states on the temperature dependence of the magnetoresistance (MR) of Sr2FeMoO6-glass composites have been studied. X-ray diffraction analyses show that the glass is most likely located at the grain boundary without causing a change of the crystal structure of Sr2FeMoO6. The variation of the resistance with temperature and magnetic field indicates that the added glass layer has profound influence on the MR properties. At low temperature, the MR in low fields is enhanced notably because the insulating barrier for the intergranular tunneling is improved by adding the glass layer at the grain surface. However, at high temperature, the MR decreases rapidly with the increase of temperature due to, in addition to the enhancement of spin-independent hopping of electrons through the localized states, the fast decay of spin polarization at the surfaces of the grains. This decay is induced by the separation of the ferromagnetic grains with the nonmagnetic glass layer at the grain boundaries.

Enhanced spin-dependent tunneling magnetoresistance in magnetite films coated by polystyrene

Hematite films were deposited by magnetron sputtering. A phase transformation from hematite to magnetite occurred when polystyrene (PS) coated hematite films were annealed above 200°C in hydrogen flow. Giant negative magnetoresistance (MR) was observed with the best MR ratio of over 8% (at room temperature and in a field of 5.5T) found in samples annealed at 230°C. The temperature dependence of the resistivity is characteristic of intergranular tunneling. After the PS layer was removed and the films annealed again at 230°C in hydrogen flow, the resistivity increased by about one order of magnitude and the MR ratio decreased to 4.3%. These data show that PS coating layer can protect magnetite films from oxidation and enhance interganular spin-dependent tunneling magnetoresistance.

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.

Evidence for intergranular tunnelling in polyaniline passivatedα-Fe nanoparticles

Nanoparticles are of immense importance both from the fundamental and applicationpoints of view. They exhibit quantum size effects which are manifested in their improvedmagnetic and electric properties. Mechanical attrition by high energy ball milling(HEBM) is a top down process for producing fine particles. However, finenessis associated with high surface area and hence is prone to oxidation which hasa detrimental effect on the useful properties of these materials. Passivation ofnanoparticles is known to inhibit surface oxidation. At the same time, coating polymerfilm on inorganic materials modifies the surface properties drastically. In thiswork a modified set-up consisting of an RF plasma polymerization technique isemployed to coat a thin layer of a polymer film on Fe nanoparticles producedby HEBM. Ball-milled particles having different particle size ranges are coatedwith polyaniline. Their electrical properties are investigated by measuring the dcconductivity in the temperature range 10–300 K. The low temperature dc conductivity(I–V) exhibited nonlinearity. This nonlinearity observed is explained on the basis of the criticalpath model. There is clear-cut evidence for the occurrence of intergranular tunnelling. Theresults are presented here in this paper.

Negative magnetoresistance, negative electroresistance, and metallic behavior on the insulating side of the two-dimensional superconductor-insulator transition in granular Pb films

Granular Pb thin films on the insulating side of the two-dimensional superconductor-insulator transition are observed to exhibit a large negative magnetoresistance and electroresistance (change in resistance with electric field) at low temperatures. At high measurement voltages and low temperatures, the film resistances become temperature independent creating a ``metallic'' state. These phenomena are explained as manifestations of transport due to intergranular quasiparticle tunneling. This explanation might also provide insights into the similar behavior observed in other superconductors.

Enhanced tunneling magnetoresistance and high-spin polarization at room temperature in a polystyrene-coatedFe3O4granular system

Polystyrene-coated ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ nanoparticles through surface engineering exhibit an intergranular tunneling magnetoresistance (MR) ratio of 22.8% at room temperature and a maximum MR of 40.9% at $110\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The drastic enhancement of the MR ratio clearly suggests that there is high degree of spin polarization even at room temperature for half metallic ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$. The estimated spin polarization $P$ is about 54% and 83% at room temperature and $110\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, respectively.

Dc transport properties and resistance fluctuation processes inSr2FeMoO6polycrystalline thin films

We performed transport measurements and voltage noise analysis on ${\mathrm{Sr}}_{2}\mathrm{Fe}\mathrm{Mo}{\mathrm{O}}_{6}$ thin films, in zero applied magnetic field as a function of temperature $T$, down to $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The samples, grown by pulsed laser deposition on $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ substrates, showed a negative resistivity thermal coefficient and, at $T<50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the observed temperature dependence of resistivity and the $I\text{\ensuremath{-}}V$ curves were correctly described by the fluctuation-induced tunneling model which assumes the presence of intergranular tunneling of charge carriers. Moreover, the analysis of the low temperature resistance fluctuation processes, pointed out the presence of a voltage noise component independent of the frequency within the whole investigated bandwidth with a spectral density directly proportional to the square of the bias current. We interpreted this unusual behavior as a possible consequence of tunneling intergranular processes.

Influence of the compacting pressure on the transport properties of CrO2 at low temperatures

The influence of pressure on the magnetotransport properties of CrO2 samples at low temperatures has been studied in this paper. The X-ray diffraction patterns of the samples show that the relative intensity of (110) peak increases with increasing pressure, which implies that pressure has the orientation effect on the CrO2 needle-shaped nanoparticles. At low temperatures, the resistance and magnetoresistance of CrO2 samples decrease with the increase of pressure. The experimental results suggest that at low temperatures, the conductance of the sample shows the typical characteristic of intergranular tunneling. Δ decreases with increasing pressure and magnetic field; and furthermore, the range of change of Δ with the magnetic field is relatied to the compacting pressure. The higher the compacting pressure, the smaller the variation of Δ with the magnetic field. These changes can be attributed to the adjustment of pressure on the intergranular tunnel barrier.