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Transport in ErAs:GaAs nanocomposites occurs via hopping of bound magnetic polarons between nanoparticles of magnetic semimetallic ErAs. A strong negative magnetoresistance, up to 3 orders of magnitude and strongly dependent upon ErAs particle size, is accompanied by a low field positive magnetoresistance. A model that features fluctuation controlled hopping captures this behavior.[S0031-9007 (98)08240-4] PACS numbers: 73.61. -r, 75.70.PaThe technological drive to produce magnetoelectronic materials for future magnetic field sensors and the integration of dense magnetic storage with high speed electronics requires physical understanding of transport in magnetoelectronic heterostructures. A large body of work has addressed the electronic and magnetic properties of dilute magnetic semiconductors (DMS) where localization of carriers to acceptors or donors is followed by the formation of bound magnetic polarons (BMPs) [1][2][3][4]. In marked contrast to those magnetic polarons, in this Letter, we explore nanocomposites of magnetic ErAs and GaAs in which the carriers are first strongly confined to paramagnetic, semimetallic ErAs particles embedded in GaAs.As the system develops paramagnetic susceptibility at low temperatures features associated with bound magnetic polarons emerge but the size of the BMP is that of the ErAs nanoparticle, which is controlled during molecular-beam epitaxy (MBE) growth. A striking positive magnetoresistance (MR) component precedes the anticipated strong negative MR, and it is shown to be a feature of fluctuation controlled hopping.In a DMS with a low concentration of acceptor or donor sites, carriers localized at impurities can lower their energy by inducing a spin polarization of the uniformly distributed magnetic moments encompassed by their wave function to form a BMP complex. The BMP increases the barrier for hopping transport between impurities. An external field will align all of the moments, reducing the carrier's affinity for one particular site, resulting in large negative MR. Time-resolved optical measurements have provided information about the dynamics of BMP formation [5], and the steady state effects of BMPs have been observed in transport and optical studies of Euchalcogenide alloys [1] and II-VI based DMSs [2][3][4]6]. Evidence for large polarons with sizes set by localization lengths much greater than impurity states has been found in the MBE grown III-V DMS InMnAs [7].ErAs has a small enough lattice mismatch with GaAs (11.8%) to permit epitaxial growth of ErAs layers as thick as 5 nm, ϳ20 monolayers (MLs), on a GaAs sub-strate [8]. However, the initial growth (#3 ML) is 3D. High-resolution transmission electron microscopy (TEM) demonstrated the presence of isolated ErAs islands 2-3 ML high [8]. The lateral size of the islands is a strong function of the growth temperature and we have been able to form islands in a size range of ϳ4 80 nm. By controlling the growth temperature and/or total ErAs deposition we can independently change both the island size and interisland spa...

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We analize electrical conductivity controlled by hopping of bound spin polarons in disordered solids with wide distributions of electron energies and polaron shifts (barriers). By means of percolation theory and Monte Carlo simulations we have shown that in such materials at low temperatures, when hopping occurs in the vicinity of the Fermi level, a hard polaron gap does not manifest itself in the transport properties. This happens because as temperature decreases the hopping polaron trades the decreasing electron and polaron barriers for increasing hopping distance.As a result, in the absence of the Coulomb correlation effects, in this variable-range variable-barrier hopping regime, the electrical resistivity, ρ, as a function of temperature, T , obeys a non-activation law: ln (ρ/ρ 0 ) = T /T p with p = 2/(d + 2), where d is the dimensionality of the system. It differs from the standard Mott law for which p = 1/(d + 1). Also, we studied the effects of upper and lower boundaries in the polaron shift distribution on hopping conduction, which may result in a partial re-entrance of the hard polaron gap. We discuss possible applications to the problem of giant negative magnetoresistance in dilute magnetic semiconductors and nanocomposites where for paramanetic materials p = 3/(d + 2).

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We studied electrical transport in magnetic semiconductors, which is determined by scattering of free carriers off localized magnetic moments. We calculated the scattering time and the mobility of the majority and minority-spin carriers with both the effects of thermal spin fluctuations and of spatial disorder of magnetic atoms taken into account. These are responsible for the magneticfield dependence of electrical resistivity. Namely, the application of the external magnetic field suppresses the thermodynamic spin fluctuations thus promoting negative magnetoresistance. Simultaneously, scattering off the built-in spatial fluctuations of the atomic spin concentrations may increase with the magnetic field. The latter effect is due to the growth of the magnitude of random local Zeeman splittings with the magnetic field. It promotes positive magnetoresistance. We discuss the role of the above effects on magnetoresistance of non-degenerate semiconductors where magnetic impurities are electrically active or neutral.