W.H. Flores

Bias dependence of magnetoresistance in Fe–Al2O3 granular thin films

This paper reports on the magnetotransport behavior of Fe–Al2O3 granular thin films when the injected dc current is varied. The electric resistance as a function of temperature, magnetoresistance, and the current vs applied bias potential measurements were used to characterize the samples. It was found that the transport mechanism which best describes the electronic properties of these samples is variable range hopping. Non-Ohmic behavior was observed and is claimed as responsible for the great modification of the electronic characteristics of the system as a function of the applied bias potential. Inversion of the tunneling magnetoresistance is observed for applied bias potential greater than 3 V. Such inverted magnetoresistance comes from the activation of low resistivity tunneling paths that are promoted by increasing the bias potential. An expression is proposed to describe the magnetoresistance behavior.

Structural modifications in FexCo1−x∕Cu multilayers induced by ion irradiation

The structural evolution of Fe30Co70∕Cu multilayers under ion irradiation is investigated in detail using x-ray techniques. The samples were irradiated with two different ions, 50keV of He+ and 600keV of Kr+, at room temperature. No substantial changes were observed after He+ irradiation; the He+ ions promote some disorder in the FeCo layers, an increase of the Cu(111) texture, and grain size. After Kr+ irradiation a structural phase transition from bcc to fcc occurs in the FeCo layers. A very pronounced increase of the Cu(111) texture and grain size is also observed. According to the equilibrium phase diagrams such fcc phase is not expected for the FeCo alloy at the composition of Fe30Co70. This fcc phase is imposed by the Cu fcc structure of the adjacent layers, which induce the regrowth of the FeCo layers structure from bcc to fcc during the relaxation period of the atomic collision cascades. Also, after the Kr+ irradiation a multilayer structure still persists, as showed by the x-ray reflectivity.

Magnetoresistance dependence on Co composition in discontinuous CoxFe1−x/Ag multilayers

Abstract We report on the structural evolution and magnetic behavior of Co x Fe 1 −x /Ag discontinuous multilayers with x =0, 0.3, 0.7 and 1. The structural characterization was performed using X-ray absorption spectroscopy (XAS). It is found from XAS measurements that there is a better ordered structure around Co than Fe absorbing atoms. For CoFe alloys, we conclude that Co atoms are within the core of the magnetic particles, while Fe atoms preferentially occupy the interfacial positions near Ag. The magnetoresistance increases when Co is added to Fe.