Synthesis of Ni ferrite and Co ferrite rodlike particles by superposition of a constant magnetic field
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e report the fabrication of micron-sized rodlike particles of nonstoichiometric Co and Ni ferrites by aging coprecipitated Fe(OH)2 and M(OH)2—where M is either Ni or Co—at 90 °C in the presence of an external magnetic field (B ≈ 405 mT). Potassium nitrate was used as a mild oxidant. Resultant particles were analyzed by means of electron microscopy, x-ray powder diffraction (XRD), magnetometry, energy dispersive x-ray (EDX) spectrometry, and atomic absorption spectroscopy. Rodlike particles of both types of ferrite exhibited a relatively uniform thickness, an average aspect ratio close to 10, and have a spinel crystalline structure. EDX spectrometry and atomic absorption spectroscopy confirmed the incorporation of Ni2+ and Co2+ in the respective ferrite particles. The incorporation of Co2+ led to non-negligible remanence and coercivity. The incorporation of Ni2+ led to a lower saturation magnetization, whereas the remanence and coercivity of the Ni ferrite were very low, still typical of a soft ferrimagnetic material. The mechanism of formation of the rodlike particles was investigated by the time-dependent observation of growing Ni ferrite rods.
I. INTRODUCTION
Elongated particles exhibit an anisotropic friction coefficient. Furthermore, if the elongated particles are magnetizable, their shape also leads to an additional contribution to the magnetic anisotropy. This additional magnetic anisotropy can be desirable for some applications, such as magnetic recording or the fabrication of permanent magnets. It also allows, in colloidal suspensions where particles are free to rotate, for control of their orientation, since the elongated particles tend to align their axis along the direction of an external magnetic field. The conjunction of both anisotropies that elongated magnetic particles exhibit is expected to allow for the preparation of magnetorheological fluids with an enhanced response to external magnetic fields. Not many theoretical calculations or experiments dedicated to the understanding of the mechanical properties of suspensions of elongated particles can be found in the literature. Examples of the former are the model put forward by Shine and Armstrong1 to describe the rotation of a suspended axisymmetric ellipsoid subjected to a magnetic field and the model proposed by Satoh and Sakuda2 to describe the rheological behavior of suspensions of spheroidal hematite particles.
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0218 1764 J. Mater. Res., Vol. 23, No. 6, Jun 2008 http://journals.cambridge.org Downloaded: 11 Mar 2015
However, to date most experimental rheological studies on elongated particles that we found in the literature focused on nonmagnetic particles. In particular, rheological investigations have been carried out on organic rodlike macromolecules such as the tobacco mosaic virus,3 the FD-virus,4 and poly-␥-benzyl-L-glutamate.5 Inorganic rodlike particles have also been investigated: e.g., silica rods were used by Wierenga and Philipse6 as model col
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