Marco Aurelio Silveira Boff

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.

Turn on of new electronic paths in Fe-SiO2 granular thin film

The electrical properties of Fe-SiO2 have been studied in the low-field regime (eΔV ≪ kBT), varying the injected current and the bias potential. Superparamagnetism and a resistance drop of 4400 Ω (for a voltage variation of 15 V) were observed at room temperature. This resistance drop increased at lower temperatures. The electrical properties were described with the “Mott variable range hopping” model explaining the behavior of the electrical resistance and the electronic localization length as due to the activation of new electronic paths between more distant grains. This non-ohmic resistance at room temperature can be important for properties dependent of electrical current (magnetoresistance, Hall effect, and magnetoimpedance).

Change of the electrical properties in Fe-Al2O3 granular films

A systematic study of the electrical resistance as a function of the temperature was performed in Fe-Al2O3 granular thin films. Our findings revealed a nonlinear dependence of the current versus voltage in the low field regime at low temperature. The variable range hopping mechanism is the best description of the behavior of our samples. A change of the electronic properties can be observed depending on the direct current applied to the sample’s plane, and is related to different localization lengths.

A simple formulation for magnetoresistance in metal-insulator granular films with increased current

We studied the tunnel magnetoresistance in metal/insulator granular films when the applied current is varied. The tunnel magnetoresistance shows a strong modification related to a non-Ohmic behaviour of theses materials. It was verified that spin-dependent tunnelling is the main mechanism for magnetoresistance at low applied current. However, when the current is high, another mechanism gets to be important: it is independent of the magnetization and is associated to variable range hopping between metallic grains. In this work, we propose a simple modification of Inoue and Maekawas model for tunnelling magnetoresistance in granulars, rewriting the expression for resistance as a function of magnetic field and temperature, also taking into account the two different contributions.

Non-ohmic behavior of metal-insulator granular thin films in low-field regime (eΔV ≪ kBT)

Non-ohmic behavior is not expected in metal–insulator granular systems in a low-field regime. There is no model to explain this behavior, even though it has been reported in several metal-insulator granular thin films (Fe-Al2O3, Co-Al2O3, and Ti-SiO2). In this paper, we show additional experimental results of Fe-SiO2 granular films and propose an explanation for the electrical properties of all above mentioned systems, based on Mott variable range hopping. The experimental results show that the localization length increases and the electrical resistance decreases with the increase of electrical potential or current. The non-ohmic behavior of the resistance and the increase of the localization length with increasing current are explained by the activation of new pathways for electrons in granular thin films that contain variable grain sizes and/or have different distances between grains.