Structural properties of cobalt ferrite thin films deposited by pulsed laser deposition: Effect of the reactive atmosphere
Introduction
Numerous studies are devoted to magnetoresistive devices for their uses in various applications such as magnetic recording. The relative control of the magnetizations of the layers in these devices necessitates to pin the magnetization of one of the layers. This is possible by magnetically coupling this layer to a hard one. Cobalt ferrite thin films are receiving increasing interest for their potential use as hard pinning layer thanks to their high coercivity, high Curie temperature around 800 K [1], high corrosion resistance and good mechanical stability [2], [3]. Some works address the problem of exchange coupling between cobalt ferrite and other magnetic layers [4], [5], [6], [7], [8]. However, elaboration processes at low temperatures have to be developed to involve these films in electronic devices and avoid chemical reactions with the other components.
Thin cobalt ferrite films have been elaborated in the past by various methods, the most used being pulsed laser deposition (PLD) from a CoFe2O4 target. The films obtained by Guyot et al. [9] on glass, quartz or MgO at 450 °C were amorphous. The crystallization occurred by annealing at 600 °C. Epitaxially crystallised cobalt ferrite thin films have been obtained at low elaboration temperatures only when deposited on costly single crystal substrates like MgO [10], [11] (the authors then worked at temperatures between 200 and 800 °C) or CoCr2O4 buffered SrTiO3 and MgAl2O4 substrates [12], [13], [14] (the authors then worked at 400 °C).
However, mass-produced spin electronics devices require inexpensive substrates. Silicon substrates fulfil these conditions and, furthermore, have a good flatness. But the large lattice mismatch (29%) with CoFe2O4 makes it difficult to grow epitaxial films. Kennedy et al. [15] obtained (111) textured Fe3O4 films by PLD of a metallic target (Fe) on Si (100) substrates at 450 °C in a molecular oxygen atmosphere. However, magnetization values of 650–800 kA/m (650–800 emu/cm3) were measured, which are too high to be attributed to ferrite, and lead the authors to consider the formation of metallic Fe-rich regions. Matsushita et al. [16] obtained a strong (111) orientation of CoFe2O4 on silicon substrates buffered with ZnO underlayers at low temperature (80 °C) but they, as well as Ding et al. [17], obtained polycrystalline cobalt ferrite when depositing directly on Si substrates at high temperature (600 °C). Until now, highly (111) oriented cobalt ferrite thin films by PLD on Si substrates have only been obtained by Terzzoli et al. [18] at high temperature (700 °C) using a CoFe2O4 oxide target, but the surface roughness was important (around 8 nm).
We have recently shown that it was possible to get polycrystalline CoFe2O4 films by pulsed laser deposition between 200 °C and 400 °C from a metallic target in a reactive atmosphere and on silicon substrates [19]. This paper is devoted to the determination of the optimal deposition conditions to obtain highly (111) oriented films on Si (100) substrates at low temperature (300 °C). The effect of the nature of the reactive atmosphere on the structural properties of the films is discussed.
Section snippets
Experimental procedure
The CoFe2O4 films were grown by PLD technique using a nanosecond KrF excimer laser (wavelength 248 nm, pulse length 20 ns, repetition rate 10 Hz and maximum energy 32 mJ/pulse). The laser fluence was 3 J/cm2. The laser beam moved in two directions of the fixed CoFe2 target plane in order to homogeneously ablate it. The metallic CoFe2 target was obtained from the fusion and solidification in the shape of a disk of a stoichiometric mixture of metallic Co and Fe. Its density was 8060 kg/m3. The
Results
The XRD patterns measured in the θ–2θ mode for cobalt ferrite films deposited at various (O2/N2, 20:80) and O2 pressures are displayed in Fig. 1, Fig. 2, respectively.
The intense (111), (222) and (333) peaks of the spinel structure in Fig. 1 indicate a strong preferential orientation with the 〈111〉 directions of the crystallites perpendicular to the film plane, for the films deposited in O2/N2 at pressures lower than 10 Pa. This preferential 〈111〉 orientation had not been observed on CoFe2O4
Discussion
Whatever the reactive gas used for the deposition, the same general texture trend is observed for the films. A preferred orientation with 〈111〉 directions of the spinel aligned with the films' normal occurs for low deposition pressures (= 0.5 Pa). This preferred orientation is improved when O2/N2 pressure increases and disappears when the O2/N2 pressure reaches 10 Pa, contrary to what happens in oxygen gas for which more randomly oriented films are observed. The best 〈111〉 preferred orientation
Conclusion
CoFe2O4 films with a strong (111) preferred orientation were elaborated on Si (100) at 300 °C by pulsed laser deposition of a CoFe2 metallic target in different reactive atmospheres. The best results in terms of crystallinity and degree of orientation were obtained with the O2/N2 (20:80) reactive mixture at 5 Pa.
The low surface roughness and good magnetic properties of the thus elaborated films allow to consider their use in spin electronic devices.
Acknowledgements
DC greatly acknowledges the Région Basse-Normandie for partial financing of the X-ray three-circles diffractometer used in this study.
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