Magnetite/Nickel andMagnetite/Cobalt Multilayer Nanostructures Obtained by Pulsed Laser Deposition
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Magnetite/Nickel and Magnetite/Cobalt Multilayer Nanostructures Obtained by Pulsed Laser Deposition Monica Sorescu1, Agnieszka Grabias2 and Lucian Diamandescu1 1 Duquesne University, Department of Physics, Pittsburgh, PA, U.S.A 2 Institute for Electronic Materials Technology, Warsaw, Poland
ABSTRACT Nanostructured magnetite/T multilayers, with T = Ni, Co, Cr, have been prepared by pulsed laser deposition. The thickness of individual magnetite and metal layers takes values in the range of 5 - 40 nm with a total multilayer thickness of 100 -120 nm. X-ray diffraction has been used to study the phase characteristics as a function of thermal treatment up to 550 ºC. Small amounts of maghemite and hematite were identified together with prevailing magnetite phase after treatments at different temperatures. The mean grain size of magnetite phase increases with temperature from 12 nm at room temperature to 54 nm at 550 ºC. The thermal behavior of magnetite in multilayers in comparison with powder magnetite is discussed. INTRODUCTION In the last decade the study of magnetic thin films and multilayers has received considerable attention due to the possible applications in planar devices and high density recording [1-3]. In particular, magnetite multilayers are of great importance since the construction of a magnetite-based all-oxide spin valve was anticipated [4]. In the last few years, pulsed laser deposition (PLD) became a powerful method for preparing magnetic thin films and multilayers [5-7]. It is the aim of this paper to report on the preparation of magnetite/nickel, magnetite/cobalt and magnetite/chromium multilayers by PLD and to present the results of the X-ray diffraction (XRD) studies on the samples after annealing at different temperatures up to 550 °C. Generally, polycrystalline magnetite is quickly oxidized to maghemite (γ-Fe2O3) in air [8]. Magnetite crystals smaller than ~ 300 nm transform to maghemite at 200 to 250 °C, which in turn transform to hematite (α-Fe2O3) above 500 °C. If the magnetite particles are larger than 300 nm, hematite nuclei formed at lower temperatures can bypass the formation of maghemite. For temperatures greater than 500 °C, magnetite changes directly to hematite. In this report the thermal behavior of magnetite in multilayers is analyzed in comparison with powder magnetite. EXPERIMENTAL A Lambda Physik COMPEX 102 excimer laser working at a wavelength of 248 nm and a pulse width at half maximum of 8 ns was used to perform the pulsed laser deposition. The laser pulse energy was 450 mJ at a repetition rate of 10 Hz. For all samples the deposition was performed on Si (111) substrates using iron, nickel, cobalt and chromium targets in an oxidizing atmosphere. The multilayers obtained were magnetite/nickel, magnetite/cobalt and magnetite/chromium, with a total thickness of 100-120 nm. They were of the type: [Fe3O4/T]10,
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