Magnetic, electric and thermal properties of cobalt ferrite nanoparticles

• CoxFe3 − xO4 nanograins 4–8 nm in size are a superparamagnetic assembly of monodisperse particles.
• Co1.8Fe1.2O4 presents superparamagnetism at ambient and very high anisotropy constant.
• CoxFe3 − xO4 nanograins have a normal spinel structure contrary to bulk cobalt ferrite.
• CoxFe3 − xO4 nanograins are stable up to 600 °C, thus suitable for high temperature applications.

The electric and magnetic properties as well as the thermal stability of CoxFe3 − xO4 nanopowders, (0.6 < x < 1.8) were investigated. These powders were synthezised using a one pot solvo-thermal route with acetylacetonates as precursors. The properties were linked to the size, morphology, composition of the particles and to the cation distribution. With the exception of x = 0.6, the powders are stable up to 600 °C. Whatever the composition, Co2+ has a strong tendency to occupy tetrahedral sites, contrary to what occurs in bulk ferrites. The nanopowders display a semi conducting behaviour. Between ambient and 500 °C, conduction occurs between Co2+ ↔ Fe3+ pairs, and intergrain conduction predominates. The conductivity is in the 10−7 Ω−1 cm−1 range. The CoxFe3 − xO4 nanopowders behave magnetically as a superparamagnetic assembly of single-domain particles. The magnetocrystalline anisotropy constant is significantly higher for these nanoparticles than for bulk ferrites. Co1.8Fe1.2O4 displays the lowest blocking temperature (200 K) and the highest anisotropy (K = 21 × 106 erg/cm3).

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