Nanocomposite Fe1-xO/Fe3O4, Fe/Fe1-xO thin films prepared by RF sputtering and revealed by magnetic coupling effects

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Abstract

Magnetic and semi-conducting nanocomposite iron oxide thin films have been prepared under bias polarization, by radio-frequency sputtering of a magnetite target. The nature of the phases obtained in the thin films depends on the bias power density. The increase in power density, from 0 to 110mW/cm2, allows the preparation of magnetite, magnetite/wustite and wustite/α-iron nanocomposites successively. Magnetic measurements at low temperature show exchange bias for two-phases films even though the minor phase is not detected by grazing angle X-ray diffraction. The exchange bias can reach very high values of about 4300 Oe. Electrical properties at room temperature are interpreted taking into account both the modifications of the film compactness, and the nature of the phases from which they are made.

Introduction

The preparation of iron oxide thin films can lead to devices with attractive optical, magnetic, and semi-conducting properties, which can be tailored by altering the preparation parameters. Among all the vacuum processes used for producing films, the sputtering process is one of the most popular. It allows preparation of film at moderate temperatures, making deposition possible on various substrates with high homogeneity and good uniformity. As a consequence, the sputtering technique is widely used in research laboratories as well as in industrial production units. But, the attractiveness of RF sputtering is also that it offers the possibility of preparing materials, especially oxides, which can be out-of-equilibrium or non-stoichiometric at room temperature (RT).

During sputtering of an oxide target, the layer grown on the substrate is submitted to continuous bombardment with high energy species from plasma and target, which can induce specific characteristics or properties [1], [2], [3]. In this study, we tuned bias sputtering conditions to obtain nanocomposite iron oxide from a magnetite target. With bias sputtering, the growing layer is submitted to a strong Ar+ bombardment and leads to the formation of reduced thin films by impoverishment of the oxygen content in the growing layer.

Because of its great sensitivity, magnetic measurement, is particularly suited for determining the presence of phases with different magnetic properties. After field cooling and below the Néel temperature (TNe´el), the hysteresis loop of materials made of both antiferromagnetic (AFM) and ferromagnetic (FM) or antiferromagnetic and ferrimagnetic (FI) phases, is for instance shifted along the negative field axis. This loop shift is known as exchange bias (He) [4], [5], [6], [7], [8]. The measurement of the exchange bias parameter is particularly suitable to study nanocomposites made of AFM/FM or AFM/FI phases which are often difficult to reveal by X-ray or electron diffraction. In this report, magnetic measurements are used to study the influence of substrate polarization on phase formation and the occurrence of nanocomposites in thin films made of iron and oxygen. The resulting changes in the semi-conducting properties of such films are also studied.

Section snippets

Experimental

Iron oxide thin films were prepared by RF magnetron sputtering using a Fe3O4 ceramic target containing 5% of FeO. The apparatus is an Alcatel SCM-400 equipped with an RF-generator (13.56 MHz), and a pumping system composed of a mechanical pump coupled with a diffusion pump. A residual vacuum of 10-5Pa was reached in the sputtering chamber before introducing the argon gas. The films were deposited on glass slides under pure argon gas flow and the working pressure was kept at a value of 0.5 Pa. The

Preparation

Nanocomposite iron oxide thin films were deposited with variable bias applied during film growth. Fig. 1 shows that the deposition rate was affected by the substrate bias. When the RF bias applied to the substrate was changed from 0 to 110W/cm2, the deposition rate decreased linearly from 5.1 to 1.8 nm/mn. The deposition rate is due to sputtering from the accelerated Ar+ ions at the surface of the growing film.

Fig. 2 shows XRD patterns of the biased and unbiased thin films. When no bias was

Conclusion

Bias sputtering is generally used to improve the microstructure of thin films prepared by sputtering. In this paper we showed that this easy-to-control process can generate strongly reducing conditions during deposition of thin films and lead to the preparation of different iron oxides nanocomposites. The nanocomposites (oxide–oxide or metal–oxide) obtained by applying a bias to the substrate were mainly revealed by magnetic coupling effects. For specific preparation conditions, a maximum value

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