Structural and Magnetic Properties of Nanogranular BaTiO3-CoFe2O4 Thin Films Deposited by Laser Ablation on Si/Pt Substr
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0966-T03-24
Structural and Magnetic Properties of Nanogranular BaTiO3-CoFe2O4 Thin Films Deposited by Laser Ablation on Si/Pt Substrates José Barbosa1, Bernardo Almeida1, Jorge A. Mendes1, Anabela G. Rolo1, João P. Araújo2, and João B. Sousa2 1 Departamento de Física, Universidade do Minho, Campus de Gualtar, Braga, 4710-057 Braga, Portugal 2 Dep. de Física and IFIMUP, Universidade do Porto, Rua Campo Alegre, 687, Porto, 4169-007 Porto, Portugal
ABSTRACT Thin film nanogranular composites of cobalt ferrite (CoFe2O4) dispersed in a barium titanate (BaTiO3) matrix were deposited by laser ablation with different cobalt ferrite concentrations (x). The films were polycrystalline and composed by a mixture of tetragonalBaTiO3 and CoFe2O4 with the cubic spinnel structure. A slight (111) barium titanate phase orientation and (311) CoFe2O4 phase orientation was observed. As the concentration of the cobalt ferrite increased, the grain size of the BaTiO3 phase decreased, from 91nm to 30nm, up to 50% CoFe2O4 concentration, beyond which the BaTiO3 grain size take values in the range 3035nm. On the other hand the cobalt ferrite grain size did not show a clear trend with increasing cobalt ferrite concentration, fluctuating in the range 25nm to 30nm. The lattice parameter of the CoFe2O4 phase increased with increasing x. However, it was always smaller than the bulk value indicating that, in the films, the cobalt ferrite was under compressive stress that was progressively relaxed with increasing CoFe2O4 concentration. The magnetic measurements showed a decrease of coercive field with increasing x, which was attributed to the relaxation of the stress in the films and to the increase of particle agglomeration in bigger polycrystalline clusters with increasing cobalt ferrite concentration. INTRODUCTION Recently, nanostructured multiferroic composites formed by the combination of a piezoelectric ceramic and a magnetostrictive material, such as in the BaTiO3-CoFe2O4 system, have been attracting much scientific and technological interest [1]. In these systems, the elastic interactions between the phases provides the coupling mechanism inducing a magnetoelectric behavior [1,3]. As a result, the properties and performance of these nanostructures depend critically on the phase morphology and internal stress distribution, which, in turn, are determined by the elastic phase/phase and phase/substrate interactions. In order to address this problem thin film nanocomposites of cobalt ferrite (CoFe2O4) dispersed in a barium titanate (BaTiO3) matrix were deposited with different cobalt ferrite concentrations, as well as pure barium titanate and cobalt ferrite films (end members). CoFe2O4 has a cubic inverse spinnel structure [4] in which the octahedral B sites are occupied by eight Co2+ and eight Fe3+ cations, while the tetrahedral A sites are occupied by the
remaining eight Fe3+. It presents a high magnetocrystalline anisotropy and magnetostriction [5], making it suitable for application in magnetoelectric composite thin films. BaTiO3 is a well s
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