Structural and electrical manifestation of ageing in thin-film Fe-Ta-O nanocomposite prepared by plasma jet technique

Electrical transport in the Fe-Ta-O nanocomposite prepared by a plasma jet technique was studied in a 4-300 K temperature range. Since the mean size of the Fe nanoparticles embedded in tantalum oxide is about 4 nm, the Coulomb blockade of tunneling is revealed in differentiated I-V curves below about 180 K, where the charging energy prevails the thermal energy. Below about 13 K the I-V curves are hysteretic, which implies the presence of charging traps in the insulating matrix. Electron diffraction reveals the presence of an additional oxide in the matrix - Fe2O3 regardless of much higher oxygen affinity to Ta than Fe. The occurrence of Fe2O3 is unavoidable due to the technology used. The presence of the traps in the tunneling barrier, which could behave as spin-flip centres, is considered the main reason for the low tunneling magnetoresistivity (TMR = -1.5% at 77 K in the field of 0.6 T). The chemical instability of the nanocomposite film is inferred from significant changes found in tunneling characteristics that were measured several times in the period of four months. This ageing effect is clearly manifested also in an altered nanoparticle size distribution. The measures for the quality improvement of the insulating matrix are proposed. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.