Size Dependent Thermopower in Mesoscopic AuFe Wires

Abstract

We have combined electron heating experiments and noise thermometry to perform quantitative measurements of the thermopower in mesoscopic samples. This new measuring technique allows us to detect finite size effects in the thermopower of narrow AuFe wires with an Fe concentration ranging from 50 to 3000 ppm. The size effects emerge when reducing the width of the wires below Ӎ300 nm and may be related to a spin-orbit induced magnetic anisotropy close to the wire surface.[ S0031-9007(98) The scattering of conduction electrons at transition metal impurity spins (e.g., Fe, Cr, and Mn) substantially alters the low temperature properties of noble metals (e.g., Au, Ag, and Cu) [1]. For small concentrations (ϳ100 ppm) of the magnetic dopant the Kondo effect gives rise to a logarithmic increase of the resistivity r͑T ͒. Below the Kondo temperature T K a compensation cloud of conduction electrons is formed around the impurity spins which gradually cancels their magnetic moment. At higher concentrations (ϳ1 at. %) the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the randomly distributed impurity spins competes with the Kondo effect and causes a freezing of the impurity spins into a disordered configuration called a spin glass. This is reflected by the appearance of a typical broad maximum in r͑T ͒ since the spin scattering rate again decreases at the lowest temperatures due to the freezing process.Recent experimental work has addressed the existence of intrinsic length scales for both the Kondo effect and the spin glass freezing process [2][3][4][5][6]. The results have so far been controversial: While some groups [2,3] report a pronounced depression of the Kondo slope with decreasing film thickness and wire width, other authors claim that the observed size effects are small and can moreover be explained quantitatively in terms of disorder enhanced electron-electron interaction effects [5,6]. It was also noted that structural disorder may have an important influence on the spin dependent part of r͑T ͒ for Kondo alloys [7] as well as for more concentrated spin glass alloys where a damping of the RKKY interaction occurs [6,8].Theoretical calculations have shown that in the single impurity limit spin-orbit interactions can induce a size dependent magnetic anisotropy which results in a size dependent resistivity [9]. For more disordered samples, an interplay between weak localization and the Kondo effect has been predicted which can account for both a disorder effect and a size effect [10]. In order to address the open questions related to the size effects,

Thermal spin injection and interface insensitivity in permalloy/aluminum metallic nonlocal spin valves

“…Though thermopower is often assumed to be independent of geometry, this is only strictly true in the case where thermal gradient is simply aligned with the sample and in the regime where size effects cannot play a role. Nanoscale metal features are not always in this simple limit [50,51], so our model could be improved using actual measurements of Seebeck effects in nanowires of the same dimension as used in the NLSV.…”

Size-dependent thermopower in nanocrystalline nickel

“…To elucidate these intriguing physics, extensive, experimental, and theoretical works on variety of thin films and nanowires or nanoparticles, including single nanowire and compacted nanoparticles, have been carried out. [1][2][3][4][5][6][7] Theoretical predictions have opened up several related aspects in terms of size dependence of scattering mechanism of electrons and phonons relevant to thermoelectric properties of nanoscale metals. 8,9 While their experimental verifications are reported scarcely for single wires limited to above 100 nm, 3 and for 30 nm wire, 1 evolution of transport process with change of size below 50 nm is unavailable despite the vital importance for searching more efficient thermoelectric nanostructures.…”

“…[1][2][3][4][5][6][7] Theoretical predictions have opened up several related aspects in terms of size dependence of scattering mechanism of electrons and phonons relevant to thermoelectric properties of nanoscale metals. 8,9 While their experimental verifications are reported scarcely for single wires limited to above 100 nm, 3 and for 30 nm wire, 1 evolution of transport process with change of size below 50 nm is unavailable despite the vital importance for searching more efficient thermoelectric nanostructures. [10][11][12] Fascinating aspects of nanostructures in this regard are spatial confinement of carriers and corresponding change in carrier density of states.…”

Scaling of1∕fnoise in tunable break junctions

“…1 / f noise has intensively been studied for bulk and thin-film conductors, [9][10][11][12][13][14][15][16][17][18] in particular as a diagnostic tool for the technologically relevant electromigration ͑EM͒ mechanism. [19][20][21][22] Noise at low and high frequencies has also been explored in small constrictions, [23][24][25][26] nanoelectronic devices, 27,28 quantum point contacts, 29 submicron interconnects, 30,31 quantum coherent, quasiballistic and ballistic nanowires, [32][33][34][35][36][37][38][39] and tunneling contacts. 40,41 The power-spectral density of resistance fluctuations S R can phenomenologically be described by Hooge's law, 9,12 …”