The Origin of the Giant Hall Effect in Metal-Insulator Composites (original) (raw)
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Giant Hall effect in metal/insulator composite films
Vacuum, 2004
Metal/insulator composite materials have been fabricated by using co-sputtering at room temperature. Hall effect has been found to be greatly enhanced, when the metal volume fraction decreases from 1 to B0.5, in both magnetic and non-magnetic composite films. In non-magnetic Cu x -(SiO 2 ) 1Àx composite films, nearly three orders of magnitude enhancement in the Hall coefficient is observed. This large enhancement of the Hall coefficient is not only significantly larger than the prediction of the classical percolation theory, but also occurs at a metal concentration identified to be the quantum percolation threshold, which has been accounted for in the framework of the local quantum interference effect. The greatly enhanced extraordinary Hall effect has been observed in different magnetic composites; however, it decreases with increasing temperature. Recently, we have fabricated soft magnetic composite films by using sputtering, which show a nearly temperature-independent giant Hall effect from 5 to 300 K. r
Giant Hall effect in superparamagnetic granular films
Journal of Magnetism and Magnetic Materials, 2003
A comprehensive review of the giant Hall effect (GHE) is presented, with emphasis on novel experimental data obtained in Ni-SiO 2 and Co-SiO 2 films prepared by co-sputtering. GHE is observed close to and on both sides of the metal-insulator transition. From the point of view of microscopic conduction mechanisms, this means a crossover from metallic conductivity with weak localization to tunneling, or hopping, between separate granules across insulating barriers. Magnetic percolation is also interrupted at this concentration of metal, leading to superparamagnetic behavior of the composite and blocking phenomena. Temperature dependencies of magnetization and extraordinary Hall coefficient in the composites near the critical concentration are compared. In single phase magnetic metals and alloys, the extraordinary Hall is believed to be directly proportional to the total magnetization, due to side jumps or skew scattering. In a metal-insulator composite, only those electrons traveling in conduction critical paths can contribute to the Hall signal, thus only magnetization of the material belonging to these paths is important in the Hall measurements. Comparison with the magnetic results leads to new possibilities in understanding both the electronic and magnetic properties of granular nanocomposites.
An Experimental Study on the Hall Insulators
We have studied the Hall insulators (HIs) in a gated two-dimensional GaAs electron system containing self-assembled InAs quantum dots. It is shown that the semicircle law can become invalid in the quantum Hall (QH) liquid, so that the quantized Hall plateau is absent at the insulator-quantum Hall (I-QH) transition. The appearance and breakdown of the semicircle law in the insulating phase can both be observed when the QH liquid is destroyed by disorder. From our study, the quantized HI is not necessarily accompanied by the I-QH transition.
Experimental relations for the hall effect near the metal insulator transition
A quantitative analysis of the Hall coefficient of liquid mercury at low (11.0-8.5 g densities indicates that the conductivity in the strong scattering metallic region can be fitted by the relation 0=2600 ge ohm-' cm-' where the pseudogap depth parameter, g, is given in terms of the ratio of the experimental Hall coefficient to its free-electron value.
Physical Review B, 2014
Universality of the extraordinary Hall effect scaling was tested in granular threedimensional Ni-SiO2 films across the metal-insulator transition. Three types of magnetotransport behavior have been identified: metallic, weakly insulating and strongly insulating. Scaling between both the ordinary and extraordinary Hall effects and material's resistivity is absent in the weakly insulating range characterized by logarithmic temperature dependence of conductivity. The results provide compelling experimental confirmation to recent models of granular metals predicting transition from logarithmic to exponential conductivity temperature dependence when inter-granular conductance drops below the quantum conductance value and loss of Hall effect scaling when inter-granular conductance is higher than the quantum one. The effect was found at high temperatures and reflects the granular structure of material rather than low temperature quantum corrections.
Hall transport in granular metals
Physical Review B, 2008
We present a theory of Hall effect in granular systems at large tunneling conductance gT ≫ 1. Hall transport is essentially determined by the intragrain electron dynamics, which, as we find using the Kubo formula and diagrammatic technique, can be described by nonzero diffusion modes inside the grains. We show that in the absence of Coulomb interaction the Hall resistivity ρxy depends neither on the tunneling conductance nor on the intragrain disorder and is given by the classical formula ρxy = H/(n * ec), where n * differs from the carrier density n inside the grains by a numerical coefficient determined by the shape of the grains and type of granular lattice. Further, we study the effects of Coulomb interactions by calculating first-order in 1/gT corrections and find that (i) in a wide range of temperatures T Γ exceeding the tunneling escape rate Γ, the Hall resistivity ρxy and conductivity σxy acquire logarithmic in T corrections, which are of local origin and absent in homogeneously disordered metals; (ii) large-scale "Altshuler-Aronov" correction to σxy, relevant at T ≪ Γ, vanishes in agreement with the theory of homogeneously disordered metals.
Hall Effect in Composite Media: A Replica Approach
Europhysics Letters (EPL), 1994
In this letter we use the replica trick together with a variational method in order to compute the effective conductivity tensor of a disordered binary composite in a static magnetic field. When only one of the two components percolates, the effective magnetoresistivity pg)
Physica B: Condensed Matter, 2009
We present Hall effect measurements of two-leg ladder Sr 14Àx Ca x Cu 24 O 41 (0pxp11:5). In these composite materials charge and spin ordering occur within chain and ladder subsystems. They have been investigated intensively during the last years because of their fascinating physical properties, especially after the discovery of superconductivity under pressure (for x410). They are the first superconducting copper oxides (superconductivity occurs under pressure for x410) without the CuO 2 planes, characteristic for high-temperature superconductors. The question of charge dynamics and hole distribution between the ladders and chains is still open. Our Hall effect results show typical semiconducting behavior for xo10. The x ¼ 11:5 compound shows metallic behavior and our Hall effect results match qualitatively the behavior found in high-T c cuprates. We have determined the effective number of carriers and its dependence on composition (x) and compared it to the number of holes in the ladders obtained by other experimental techniques at room temperature.
On the nature of the Hall insulator
Solid State Communications, 1997
We have conducted an experimental study of the linear transport properties of the magnetic-field induced insulating phase which terminates the quantum Hall (QH) series in two dimensional electron systems. We found that a direct and simple relation exists between measurements of the longitudinal resistivity, ρxx, in this insulating phase and in the neighboring QH phase. In addition, we find that the Hall resistivity, ρxy, can be quantized in the insulating phase. Our results indicate that a close relation exists between the conduction mechanism in the insulator and in the QH liquid. 73.40.Hm, 72.30.+q, 75.40.Gb