Magnetic Properties of ZnO:Ni Aerogel Nanopowders: Effect of Thermal Treatments
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Magnetic Properties of ZnO:Ni Aerogel Nanopowders: Effect of Thermal Treatments Lassaad El Mir1, H. Jurgen von Bardeleben2, M. Saadoun1, Aroussi Ben Mahmoud1, and JeanLouis Cantin2 1 Physique des matériaux et des Nanomatériaux Appliquée à l'Environnement, Faculté des Sciences de Gabès, Gabès, 6072, Tunisia 2 INSP, Université Paris 6 - UMR 7588 au CNRS, 140, rue de Lourmel, Paris, 75015, France
ABSTRACT We report the elaboration of Ni doped ZnO nanoparticles prepared by a sol-gel processing technique. In our approach the water for hydrolyse was slowly released by esterification of the metal acetate with methanol followed by a supercritical drying in ethyl alcohol. Doping concentrations between 5 and 25 at% have been investigated. In the as-prepared state the powders with an average particle size of 30nm present ferromagnetic properties; thermal annealing in the 500°C to 700°C temperature range in air or oxygen modifies the magnetic properties. We ascribe the observed ferromagnetism to the presence and transformation of Ni based secondary phases. INTRODUCTION The diluted magnetic semiconductor ZnO doped with 3d –transition metals (TM) such as Co, Mn or Ni has been extensively studied for its magnetic properties since early theoretical predictions of room temperature ferromagnetic (FM) phases [1,2]. As direct superexchange interaction between nn TM ions is expected to be antiferromagnetic, as has been experimentally confirmed for Co and Mn doping, carrier mediated exchange has been proposed to be at the origin of the FM behaviour. For recent reviews of the experimental results see [3-5]. Nonintentionally doped ZnO is generally found to be n-type conductive; shallow donor co-doping allows obtaining high free carrier concentrations in the 1020cm-3 range. On the contrary, high p-type doping is still an experimental challenge. Schematically, three types of samples have been elaborated: single crystal, epitaxial films, textured polycrystalline thin films and nanocrystalline powders with typical dimensions between a few nm up to 30nm. Most of the studies have been performed on textured thin films but more recently experimental studies on small ZnTMO particles have found increasing interest. It has been shown that the manipulation of the surface states of nm sized ZnO particles gives an additional parameter to influence the magnetic properties of these materials [6,7]. Above room temperature ferromagnetism in Ni doped ZnO nanopowders, which had been thermally annealed, has been reported by different authors [8-11], but the origin of the ferromagnetism has not been clarified. We report here the results of Ni doped aerogel powders prepared by a modified sol gel process. In particular we studied the effect of post synthesis thermal annealing which has to be applied to sol gel prepared nanocrystalline powders in order to achieve sufficient crystallization. Our results show a strong modification of the magnetic properties by such annealings related to the formation of extrinsic magnetic phases.
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