Spin-dependent transport in cluster-assembled nanostructures: influence of cluster size and matrix material

Abstract

Abstract. Spin-dependent transport in granular metallic nanostructures has been investigated by means of a thermoelectric measurement. Cobalt clusters of well-defined size ( n = 15 − 600) embedded in copper and silver matrices show magnetic field responses of up to several hundred percent at low temperature. The experimental observations are identified as due to spin mixing. The influence of cluster size and matrix are discussed.

Exchange bias and negative magnetoresistance in [Co/Bi/Co]/IrMn thin films]

“…So far it was observed that in crossed-line [CoFe/Bi/Co] trilayers [6][7][8][9][10] the magnetic coupling between the two ferromagnetic layers oscillates as a function of Bi spacer thickness between ferromagnetic and antiferromagnetic couplings whereas [Co/Bi] n thin films may exhibit [10,11] a change of polarity in anomalous Hall effect (AHE) loops. In addition, preliminary DC-magnetoresistance measurements [11] in line structures of [Co(1 nm)/Bi(2.5 nm)] 15 stripes provide evidence for a positive MR effect of 80% at 10 K, which is reminiscent of the magnetothermogalvanic voltage (MTGV) signal [12,13].…”

“…To extract temperature and spin-dependent effects from the dominant part of the resistivity, a specific thermoelectric measurement protocol uses [12] a chopped laser beam that irradiates a film of nanogranular Co embedded in a Cu or Ag matrix, inducing a temperature oscillation of about 1 K at the center of the film (without to induce a net temperature gradient at 14 K). It seems that the MTGV signal is not directly correlated to spin-dependent transport properties like the giant magnetoresistance (GMR) effect observed [15] in films of nanogranular Co embedded in a Cu or Ag matrix. Thus, the detection of the MTGV signal is independent [15] of the nonmagnetic metallic matrix because it detects spin-mixing asymmetries from the local spin polarization of the conduction electrons that precess about the exchange field as they traverse the magnetic nanoparticles.…”

“…It seems that the MTGV signal is not directly correlated to spin-dependent transport properties like the giant magnetoresistance (GMR) effect observed [15] in films of nanogranular Co embedded in a Cu or Ag matrix. Thus, the detection of the MTGV signal is independent [15] of the nonmagnetic metallic matrix because it detects spin-mixing asymmetries from the local spin polarization of the conduction electrons that precess about the exchange field as they traverse the magnetic nanoparticles. In polycrystalline [Co(1 nm)/Bi(2.5 nm)] 15 stripes the large positive MR effect observed [11] at 10 K may arise from temperature gradients in Co/Bi interfaces or grain boundaries.…”

“…Thus, the detection of the MTGV signal is independent [15] of the nonmagnetic metallic matrix because it detects spin-mixing asymmetries from the local spin polarization of the conduction electrons that precess about the exchange field as they traverse the magnetic nanoparticles. In polycrystalline [Co(1 nm)/Bi(2.5 nm)] 15 stripes the large positive MR effect observed [11] at 10 K may arise from temperature gradients in Co/Bi interfaces or grain boundaries. Such gradients can be created by the DC current during the MR measurement, and can be due to contributions from the considerable Magneto-Peltier effect, that is related [16] via the Kelvin relation p ij (B) ¼ Ta ji (ÀB) with the very high values [17] of the Magneto-Seebeck and Nerst coefficients a ji of Bi.…”

“…Thus, it can be argued that the large positive MR effect [7] at 10 K can be induced: (i) from an MTGV signal due to the derivative of the resistivity with respect to temperature that arises from contributions of the Magneto-Peltier effect locally in Co/Bi junctions and, (ii) by the Nerst-Ettigshausen effect in Bi. Currently, our experimental facilities are not adequate to resolve from these [Co(1 nm)/Bi(2.5 nm)] 15 line structures: (a) the pure MTGV signal by using the specific [12] measurement protocol with a chopped laser beam, and (b) the equivocal [19] contributions from the thermomagnetic effects observed in Bi. Due to these experimental difficulties, at present, it remains to examine whether thin films, with similar microstructure as the line structures of [Co(1 nm)/Bi(2.5 nm)] 15 , exhibit the GMR effect.…”