Tunneling magnetoresistance inCo−ZrO2granular thin films

Abstract

Granular films composed of well defined nanometric Co particles embedded in an insulating ZrO 2 matrix were prepared by pulsed laser depositon in a wide range of Co volume concentrations ͑0.15Ͻ x Ͻ 0.43͒. High-resolution transmission electron microscopy ͑TEM͒ showed very sharp interfaces between the crystalline particles and the amorphous matrix. Narrow particle size distributions were determined from TEM and by fitting the low-field magnetic susceptibility and isothermal magnetization in the paramagnetic regime to a distribution of Langevin functions. The magnetic particle size varies little for Co volume concentrations x Ͻ 0.32 and increases as the percolation limit is approached. The tunneling magnetoresistance ͑TMR͒ was successfully reproduced using the Inoue-Maekawa model. The maximum value of TMR was temperatureindependent within 50-300 K, and largely increased at low T, suggesting the occurrence of higher-order tunneling processes. Consequently, the tunneling conductance and TMR in clean granular metals are dominated by the Coulomb gap and the inherent particle size distribution.

ac conductance in granular insulatingCo-ZrO2thin films: A universal response

“…6͒, is found to follow an Arrhenius law, 11,12 ͑x , T͒ = 0 exp͑W / k B T͒, being the value of the activation energy W comparable to that of the parameter B in the temperature dependence of the dc resistivity for the three sample studied in this work ͑see the inset to Fig. 6͒.…”

“…The particle-size distribution was also obtained by fitting the low-field magnetic susceptibility ͑zero-field cooling͒ and high-temperature isothermal magnetization curves M͑H͒ to a distribution of Langevin functions. 21 Figure 1 shows bright field TEM images of Co-ZrO 2 thin films for various Co contents. For the sake of simplicity, three samples in the dielectric regime are presented.…”

“…The particles have very welldefined interfaces with the matrix. 12,20 For Co concentration x Ͻ 0.31, the particles have mostly spherical shape ͓Figs. 1͑a͒ and 1͑b͔͒; while for x Ն 0.31, the neighboring particles start to coalesce, giving rise to larger nonspherical particles ͓Fig.…”

“…When particles are reduced to nanometer sizes, their properties differ considerably from the corresponding bulk counterpart, giving rise to new phenomena which result from finite-size, surface, and proximity effects, and interparticle interactions, leading to a variety of enhanced properties. [2][3][4][5][6][7][8][9][10][11][12] While dc transport properties of insulating granular thin films have been widely studied since 1970s, 4,5,7-10 much less attention has been paid to the ac response of these systems, for which many effects are not yet completely understood. In particular, we recently showed that the ac electrical properties of Co nanoparticles embedded into a dielectric ZrO 2 matrix display a complex lowfrequency absorption phenomenon 11,12 in the dielectric regime below the percolation threshold for Co that mimics the so-called universal response 13 of disordered dielectric materials, which remains a topic of very active experimental and theoretical researches.…”

“…[2][3][4][5][6][7][8][9][10][11][12] While dc transport properties of insulating granular thin films have been widely studied since 1970s, 4,5,7-10 much less attention has been paid to the ac response of these systems, for which many effects are not yet completely understood. In particular, we recently showed that the ac electrical properties of Co nanoparticles embedded into a dielectric ZrO 2 matrix display a complex lowfrequency absorption phenomenon 11,12 in the dielectric regime below the percolation threshold for Co that mimics the so-called universal response 13 of disordered dielectric materials, which remains a topic of very active experimental and theoretical researches. [14][15][16][17] In particular, the ac conductivity and permittivity are characterized in disordered dielectric materials by the existence of a transition above a critical value of the angular frequency from a low-frequency dc plateau to a dispersive high-frequency region, where both the conductivity and the permittivity show an anomalous fractional power-law dependence on the angular frequency.…”