The exchange bias (EB) effect has been studied in Ni/NiO nanogranular samples obtained by an original method which combines mechanical milling and hydrogen partial reduction of NiO. In this procedure, precursor NiO powders are ball-milled to reduce the grain size to the nanometric scale; subsequently, the milled powder is subjected to high temperature treatments in H2, inducing the reduction to metallic Ni. The prepared samples typically consist of Ni nanoparticles dispersed in a nanocrystalline NiO matrix, as observed by electron microscopy (TEM). The advantage of this method is that by varying the temperature and duration of the H2-treatment, we can well control the amount of Ni that forms. Moreover, we can predetermine, to some extent, the structural features of the NiO phase, by varying the milling parameters (milling time, ball-to-powder weight ratio). This presentation reports about the EB properties of a series of Ni/NiO samples obtained by annealing in H2, at different temperatures (200 < T <300 °C), 20 hours-milled NiO powder. The exchange field closely depends on the Ni amount, being maximum (600 Oe) in the sample annealed at 250 °C, with 15 % Ni. In all the samples, EB vanishes at ~200 K, irrespective of the Ni content. The structural features of the samples have been investigated by X-ray diffraction, electron microscopy and X-ray Absorption Fine Structure Spectroscopy and the low-temperature magneto-thermal behaviour has been thoroughly analysed. The results allow us to propose an EB mechanism based on the existence of a structurally and magnetically disordered NiO component, which mediates the exchange interaction between the ferromagnetic Ni nanoparticles and the NiO nanocrystalline matrix.

Magnetic disorder and exchange bias in nanogranular metal / metal-oxide systems

DEL BIANCO, Lucia;TAMISARI, Melissa;
2007

Abstract

The exchange bias (EB) effect has been studied in Ni/NiO nanogranular samples obtained by an original method which combines mechanical milling and hydrogen partial reduction of NiO. In this procedure, precursor NiO powders are ball-milled to reduce the grain size to the nanometric scale; subsequently, the milled powder is subjected to high temperature treatments in H2, inducing the reduction to metallic Ni. The prepared samples typically consist of Ni nanoparticles dispersed in a nanocrystalline NiO matrix, as observed by electron microscopy (TEM). The advantage of this method is that by varying the temperature and duration of the H2-treatment, we can well control the amount of Ni that forms. Moreover, we can predetermine, to some extent, the structural features of the NiO phase, by varying the milling parameters (milling time, ball-to-powder weight ratio). This presentation reports about the EB properties of a series of Ni/NiO samples obtained by annealing in H2, at different temperatures (200 < T <300 °C), 20 hours-milled NiO powder. The exchange field closely depends on the Ni amount, being maximum (600 Oe) in the sample annealed at 250 °C, with 15 % Ni. In all the samples, EB vanishes at ~200 K, irrespective of the Ni content. The structural features of the samples have been investigated by X-ray diffraction, electron microscopy and X-ray Absorption Fine Structure Spectroscopy and the low-temperature magneto-thermal behaviour has been thoroughly analysed. The results allow us to propose an EB mechanism based on the existence of a structurally and magnetically disordered NiO component, which mediates the exchange interaction between the ferromagnetic Ni nanoparticles and the NiO nanocrystalline matrix.
exchange bias; nanogranular systems
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11392/1392256
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