Fluctuation Controlled Hopping of Bound Magnetic Polarons in ErAs:GaAs Nanocomposites

Abstract

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...

Variable-range hopping of spin polarons: Magnetoresistance in a modified Mott regime

“…Bound spin (magnetic) polarons, in contrast to their lattice counterparts, possess polaron shifts that, as well as their distribution, can be tuned by applied magnetic field thus resulting in giant or even colossal magnetoresistance [10,11,14,16]. Externally controlled switching off and on of the magneto-polaron effect also allows one to better identify the hopping mechanism in magnetic materials.…”

“…In heavily doped and compensated samples [2] of GaAs : Mn, the former can be determined by the total number of magnetic impurities in the localization volume while the latter may depend on the electrically active ones. For ErAs nanoislands embedded in GaAs matrix [16] , the distribution of polaron shifts follows the distribution of the islands' volumes. Meanwhile, the "bare" electron spectrum of the islands is apparently determined by the distribution of the islands sizes and shapes.…”

“…Recent technological developments in producing novel dilute magnetic semiconductors, especially GaAs : Mn, and magnetic nanostructures, such as GaAs/ErAs, made it possible to observe electrical conductivity due to hopping via magnetic impurities [5] and nano-islands [16] thus revealing a rich interplay of magnetic and electrical transport properties. For instance, Mn atoms play a dual role as acceptor sites and magnetic ions in GaAs resulting in the very unusual magnetic properties of this material [5,17].…”

Time-resolved magnetophotoluminescence studies of magnetic polaron dynamics in type-II quantum dots

“…12, 26-28, 36, 37, 40 Despite entirely different growth procedures, in both classes of QDs the MP formation is associated with the observed magnetic ordering. 12, 20-22, 30, 37 Several interesting effects have been attributed to MPs in nanostructures, such as the long "spin memory" times in (Cd,Mn)Te QDs 30 , giant magnetoresistance in ErAs:GaAs nanocomposites, 45 and roomtemperature ferromagnetic ordering in MnGe QDs. 21 The temporal evolution of the MP (Fig.…”

Size effects on the electronic structure of ErSb nanoparticles embedded in the GaSb(001) surface

“…One promising nanocomposite system consists of rare-earth monopnictide (RE-V) nanostructures embedded within a III-V semiconducting matrix. This system has already shown a range of functionalities, such as enhanced interband tunneling across p-n junctions [1,2], spin-dependent resonant tunneling [3,4], fast electron-hole recombination for THz devices [5], both electrical doping and phonon scattering for thermoelectrics [6,7], and giant magnetoresistance [8,9].…”