Intergranular Tunneling Research Articles

Comment on “A large low-field magnetoresistance of CrO2∕(CrO2∕Cr2O3) powder compact with two coercivities” [J. Appl. Phys. 97, 073907 (2005)

In a recent paper, Wang et al. [J. Appl. Phys. 97, 073907 (2005)] have studied the intergranular tunneling magnetoresistance (TMR) in CrO2∕(CrO2∕Cr2O3) powder compacts. However, the mistakes in their equations for TMR lead to the inconsistency between their model and experimental data. In this Comment, we have presented a proper equation for TMR, as well as explanations for the temperature dependence of TMR in half-metallic CrO2 composites.

Magnetic and transport properties of double distorted perovskites CaCuMn 6O 12 and CaCu 2Mn 5O 12

Magnetic and transport properties of double distorted perovskites CaCuMn 6O 12 and CaCu 2Mn 5O 12 are studied in a range 2–300 K. The leading role in magnetism of these compounds belongs to antiferromagnetic exchange interaction of Cu 2+ in square coordination with Mn 3+/Mn 4+ in octahedral coordination. The values of saturation magnetization indicate that Mn 3+ ions in square coordination are coupled ferromagnetically with Mn 3+/Mn 4+ in octahedral coordination. The colossal magnetoresistance in the pellet samples is due assumingly to intergranular spin-polarized tunneling of current carriers.

Percolative magnetotransport and enhanced intergranular magnetoresistance in a correlated resistor network

Based on phase separation and intergranular tunneling, a correlated resistor network model is proposed to simulate the percolative magnetotransport in some spontaneously phase-separated perovskite oxides and their artificial granular composites. For the inclusion of spin-dependent transport mechanism in a network with correlated structure, a directed tunneling percolation scenario is established for the magnetotransport in these oxides, instead of the previous two-component metal-to-insulator percolative description in a random resistor network. Within this framework, the concentration dependence of intergranular magnetoresistance in these mixed-phase materials is well explained and the mechanism to the largest magnetoresistive effects is revealed.

Intergranular Tunnelling and Field-Induced Percolation Fluctuationof Granular Composites(La1−zAgzMnO3)/(MnO2/Mn2O3)

We investigate the giant magnetoresistance of composites(La1−zAgzMnO3)1−y(MnO2/Mn2O3)y.The magnetoresistive property of the composites shows thecharacteristics of intergranular tunnelling. A conductivity leap hasbeen observed around y = 0.7. The composite(La0.926Ag0.074MnO3)0.698(MnO2/Mn2O3)0.302has been found to show the greatest magnetoresistance in the samples.Its room-temperature MR ratio reaches 24% under a field of 1.8 T. Thesephenomena suggest a percolation transformation and a kind offield-induced fluctuation in percolation in the samples investigated.

Intergranular tunneling magnetoresistance of mechanically alloyed (Cr-M)O/sub 2/ powder compacts

The (Cr-M)O/sub 2/ (M=V, Mn, Fe, Co) powders were synthesized by the mechanochemical method and the magnetoresistance (MR) effect of their powder compacts was investigated as a function of the type and content of the dopant. The findings are summarized as follows. 1) Substituting limit of M for Cr in CrO/sub 2/ was 30 at% for V, Mn, and Co, and 5 at% for Fe. 2) Doping with V effectively decreases the T/sub C/, while Fe significantly elevates the same (420 K with 5 at% Fe). Furthermore, the changes observed in the cases of Co and Mn was marginal. 3) MR ratio was significantly enhanced with the doping of 5 at% of Fe, however, decreased in the cases of V and Co doping. 4) The reason for the MR enhancement in (Cr-Fe)O/sub 2/ powder compacts is due to the change in the barrier characteristics.

Electronic properties of field aligned CrO2 powders

We have investigated in detail the electronic transport properties of half metallic CrO2 powder-based devices, fabricated using optical lithography and field alignment technique. A transition in the conduction mechanism from spin-dependent intergranular tunneling to inelastic hopping was observed at 215K. This transition temperature shifts to 230K in the presence of 10kOe field cooling due to reduction of the spin-independent hopping conductance channel. I–V characteristics exhibit strong temperature dependence and are non-linear even at room temperature. Our experimental results are in good agreement with a simple theoretical model. A novel ‘double switching’ phenomenon was observed in the I–V curves below the transition temperature.

Magnetoresistance of polycrystalline Fe3O4 films prepared by reactive sputtering at room temperature

The magnetic, structural, and transport properties of single-layer magnetite (Fe3O4) films prepared by reactive sputtering were investigated. Magnetoresistance (MR) was measured at various thicknesses and temperatures. The increase in MR with thickness is related to grain crystallinity and size, as confirmed by transmission electron microscopy. MR arises from intergranular tunneling, which is supported by the temperature dependence of resistivity (logρ∼T−1∕2). Field-dependent MR correlates with the M curve. Magnetoresistance versus magnetization curves clearly show that the MR effects come from the surface spin arrangement near the grain boundaries. The dependence of MR on the magnetic field observed in polycrystalline Fe3O4 films can be attributed to a surface magnetization near the grain boundary, which will be discussed.

A large low-field tunneling magnetoresistance of CrO2/(CrO2∕Cr2O3) powder compact with two coercivities

Two channels of the conductance G exist in cold-pressed powder compacts of CrO2/(CrO2∕Cr2O3), where two types of granules have different coercivities. One of the channels is the spin-dependent intergranular tunneling conductance, and the other one is the spin-independent higher-order inelastic hopping conductance. The conductance is mainly related with the spin-dependent tunneling channel at low temperature. A large low field tunneling magnetoresistance (TMR) of 30.4% is achieved in 950 Oe field at 2 K. The reason is the relative orientation of the magnetization that tends to be antiparallel between the two coercivities. Field dependence of the tunneling magnetoresistance shows two better-separated peaks than that of CrO2 powder compact. The results suggest that the introduction of CrO2/Cr2O3 improves low field sensitivity, TMR, and switching characteristics.

Formation of an interface layer in thermionic oxide cathodes for CRT applications

Scanning electron microscopic techniques were employed to study the surface morphological changes of oxide cathodes and nickel caps as a result of cathode activation extending over periods of 1–12 h. Elemental analysis of barium, strontium, tungsten, magnesium and aluminium was performed using energy dispersion x-ray spectroscopy. An abrupt change was observed after activation longer than 3 h. Conduction through well activated cathode assemblies was found to be due to intergranular electron tunnelling at low temperatures (T ⩽ 500 K), while trapping and detrapping at grain boundaries becomes the dominant mechanism at high temperatures (T ⩾ 500 K). The contribution of the interfacial layer to conductivity was found to be significant for cathodes activated for smaller periods.

Suppression of superconductivity in granular metals.

We investigate the suppression of the superconducting transition temperature due to Coulomb repulsion in granular metallic systems at large tunneling conductance between the grains, g(T)>>1. We find the correction to the superconducting transition temperature for 3D granular samples and films. We demonstrate that, depending on the parameters of superconducting grains, the corresponding granular samples can be divided into two groups: (i). the granular samples that belong to the first group may have only insulating or superconducting states at zero temperature depending on the bare intergranular tunneling conductance g(T), while (ii). the granular samples that belong to the second group in addition have an intermediate metallic phase where superconductivity is suppressed while the effects of the Coulomb blockade are not yet strong.

Magnetotransport properties of Co- and Ni-doped manganites: from conductive to insulating behavior

The study of the effect of the progressive substitution of Mn by a transition metal, up to a 30% of Co or Ni, are presented. We have measured a progressive decrease, respect to the undoped La 0.7 Pb 0.3 MnO 3 composition, both in the low-temperature spontaneous magnetization and Curie temperature . A detailed study of the magnetostransport properties of the samples reveals a transition from conductive to insulating behavior, with an intermediate state in which not only the intergranular tunneling mechanism, but also the Coulomb blockade plays a crucial role.

Investigation on the intergranular magnetoresistance of polycrystalline Sr 2FeMoO 6

In this paper, the temperature dependence of the magnetoresistance (MR) of Sr 2FeMoO 6 powder compacts, whose interfacial state was modulated by mechanical ball milling, has been investigated. At low temperatures, the MR is enhanced by the improvement of the spin-dependent intergranular tunneling across insulating tunneling barriers at grain boundaries, which are mostly the SrMoO 4 impurity phase introduced by ball milling. This tunneling conductance is found to be proportional to exp[−( Δ/ T) 1/2], and the MR at low temperatures is mainly determined by variation of Δ under magnetic field. However, at high temperatures, the MR decreases rapidly since the inelastic spin-independent hopping that follows the power law T r dominates the conductance. The enhancement of this inelastic hopping, which is associated with the defects induced by mechanical ball milling, makes the MR show strong temperature dependence with the increase in milling time.

Enhancement of Magnetoresistance in GranularCrO2/Polystyrene Composites

We present a study of magnetotransport in CrO2/polystyrene (PS)composites over a range of polystyrene concentration (0-30 wt.%). Inthe experiment, an obvious enhancement in magnetoresistance (MR) isobserved at 77 K and at room temperature as the half-metallicCrO2 particles are encapsulated with a thin layer of insulatingpolystyrene. The enhanced MR can be interpreted in terms ofspin-dependent intergranular tunnelling with 4-nm-thick PS barrier.Moreover, it is found that the novel PS barrier contributes toroom-temperature MR more significantly than that at 77 K. Temperaturedependence of resistance is good agreement with ~T-1/4 inthe temperature range from 77 to 298 K.

Magnetotransport study of granular chromium dioxide thin films prepared by the chemical vapor deposition technique

The temperature-dependent magnetotransport properties of granular chromium dioxide (CrO 2) films with grain size about 1 μm grown by chemical vapor deposition on Si(0 0 1) substrates has been studied over a temperature range from 2 to 300 K. The magnetoresistance (MR) of about 20% at T=2 K decreases very fast with increasing temperature as a result of the rapid increase of the spin-independent hopping channel, whilst the temperature-dependent MR is almost the same for samples after 100 days annealing in air at room temperature, and its electrical resistivity at room temperature increases from roughly 50 to >75 μΩ m. The contribution to the conductance at T<6 K can be attributed to the intergranular tunneling and that at 6 K<T<25 K is the second order inelastic hopping, which is amenable to σ∼ T 1.33. It becomes a poor metal at temperatures >160 K and its resistance follows R∼T 2 exp(−Δ/T) . We have shown experimentally that the room temperature annealing results in the decomposition of CrO 2 into Cr 2O 3 mainly near the surface of the films.

High-temperature conduction of granular metals

In theoretical and experimental studies of conductivity associated with intergranular tunneling of electrons in nanocomposites (granular metals), only the range of relatively low temperatures was traditionally investigated, in which only electron transitions involving singly-charged metal granules are significant. In this mode, the temperature dependence of conductivity is exponential. However, experiments show that the type of conduction of nanocomposites at high temperatures changes significantly. In the model proposed in this article, the features of conduction of granular metals at high temperatures are attributed to multiply charged granules with a large spread of the size. The conclusions of the model are in good agreement with the experimental results.

Low-temperature resistivity minima in single-crystalline and ceramic La0.8Sr0.2MnO3: Mesoscopic transport and intergranular tunneling

A slight minimum in the zero-field resistivity of a single crystalline La0.8Sr0.2MnO3 and shallow one for ceramic La0.8Sr0.2MnO3 samples were observed at T∼4 K and at T∼25–30 K, respectively. The minimum for the ceramic shifts towards lower temperatures, flattens with increasing magnetic fields (H), and vanishes at some critical H. The above effects are accompanied by an appreciable negative magnetoresistance (MR). On the other hand, the minimum for the single-crystalline sample is almost field independent and the MR in the relevant temperature range is very small. Two different mechanisms were found to account for the results observed in the single crystal and the polycrystalline samples: (i) mesoscopic corrections to the bulk resistivity that include Coulomb interaction and weak localization and (ii) intergranular tunneling. The resistivity of the large-grain ceramic sample comprises both types of behavior. The minimum of the bulk contribution becomes clearly seen under H, which suppresses the ceramic-type minimum.

Low field intergranular tunneling effect in CrO2 nanoparticles and characterization of the barriers

The magnetoresistance (MR) and microstructures of half-metallic CrO2 nanoparticle systems were studied. Using field alignment, the needle-shaped CrO2 single domain particles were aligned in the same orientation. The MR of this structure showed a magnetic junction-like behavior with two well-separated peaks in the MR at coercivity fields and the MR of the aligned CrO2 particles reached &amp;gt;41% at a relatively low field of about 1000 Oe. The magnetotransport mechanism was analyzed in terms of spin dependent tunneling between CrO2 nanoparticles. Using transmission electron microscopy, x-ray diffraction, and x-ray photoelectron spectroscopy techniques, the intergranular tunneling barrier was characterized to be a very thin Cr2O3 interface layer between the CrO2 particles. Temperature dependence of MR and conductivity in cold-pressed CrO2 nanopowders were studied. The MR significantly decreased with increasing temperature and the spin independent hopping conduction is suggested to be responsible for the suppression of MR at high temperature.

Temperature dependence of the conductance and magnetoresistance ofCrO2powder compacts

${\mathrm{CrO}}_{2}$ cold-pressed powder compacts show different electronic transport property than the single crystals. We have analyzed the temperature dependence of the conductance and magnetoresistance (MR) of cold-pressed compacts of ${\mathrm{CrO}}_{2}$ powders. Our results show that there exist two channels of the conductance $G.$ One is the inter-granular spin-dependent tunneling conductance ${(G}_{\mathrm{SDT}}),$ which is found proportional to $\mathrm{exp}(\ensuremath{-}{1/T}^{1/2}).$ The other is a spin-independent channel ${(G}_{\mathrm{SI}})$ due to the higher-order inelastic hopping through the localized states due to imperfections in the barrier. ${G}_{\mathrm{SI}}$ is found to follow the power law ${T}^{\ensuremath{\gamma}}$ and \ensuremath{\gamma} is related to the number of localized states in the barrier. At low temperatures the conductance is determined essentially by the SDT channel. As the temperature increases, the hopping channel becomes dominant. This spin-independent inelastic hopping conductance is one of the main reasons for the rapid decrease of MR with increasing temperature.

Junction-like magnetoresistance of intergranular tunneling in field-aligned chromium dioxide powders

The magnetic and magnetotransport properties of field-aligned half-metallic ${\mathrm{CrO}}_{2}$ powders have been studied. Needle-shaped nanoparticles of ${\mathrm{CrO}}_{2}$ have been aligned in a strong magnetic field. The aligned powder sample shows a strong anisotropy along the alignment direction. The conduction mechanism of the aligned ${\mathrm{CrO}}_{2}$ powder sample has been examined and is consistent with the intergranular spin-dependent tunneling. Negative tunneling magnetoresistance of about 41% is achieved in a small field in the vicinity of the coercive field at 5 K. The magnetoresistance (MR) versus field curve shows two well-separated narrow peaks at the coercive fields and resembles that of a magnetic tunnel junction. This junctionlike MR results from the narrow switching field distribution of the aligned powders. Our results suggest that the aligned magnetic ${\mathrm{CrO}}_{2}$ particles may find novel applications in spin-transport structures and devices.

Characterization of the natural barriers of intergranular tunnel junctions: Cr2O3 surface layers on CrO2 nanoparticles

Cold-pressed powder compacts of CrO2 show large negative magnetoresistance (MR) due to intergranular tunneling. Powder compacts made from needle-shaped nanoparticles exhibit MR of about 28% at 5 K. Temperature dependence of the resistivity indicates that the Coulomb blockade intergranular tunneling is responsible for the conductance at low temperature. In this letter we report direct observation and characterization of the microstructure of the intergranular tunnel barriers, using transmission electron microscopy, x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS). A very thin native oxide layer with a thickness of 1–3 nm on the surface of CrO2 powders has been observed. The composition and crystal structure of this surface layer has been determined to be Cr2O3 by XPS and XRD. The dense and uniform Cr2O3 surface layers play an ideal role of tunnel barriers in the CrO2 powder compacts.