Surface effects on magnetic properties of superparamagnetic magnetite nanoparticles

• Published in: Physica status solidi. A, Applications and materials science Vol. 203; no. 7; pp. 1595 - 1601
• Main Authors: Köseoğlu, Y., Kavas, H., Aktaş, and B.
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Berlin WILEY-VCH Verlag 01.05.2006; WILEY‐VCH Verlag; Wiley-VCH
• Subjects: 75.50.Bb; 75.50.Tt; 75.50.Vv; 75.75.+a; 76.30.Da; 76.50.+g; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Diamagnetism, paramagnetism and superparamagnetism; Exact sciences and technology; Magnetic properties and materials; Magnetic properties of nanostructures; Physics; Fluctuations; Line shape; Line widths; Magnetic moments; Superparamagnetism; Magnetic susceptibility; g-factor; Nanoparticles; Magnetite; Domain structure; Temperature effects; Magnetic anisotropy; Magnetic domains; Iron oxides; Resonance technique
• ISSN: 1862-6300; 1862-6319
• DOI: 10.1002/pssa.200563104

Superparamagnetic nanoparticles of magnetite (Fe3O4) 2 nm in size were produced by a co‐precipitation method. Superparamagnetic resonance (SPR) spectra at room temperature show a broad line with a Landé g ‐factor, g eff ≈ 2. It was observed that, as the temperature decreased to 24 K, the apparent resonance field decreases while the line width considerably increases. We used a theoretical formalism based on a distribution of diameters or volumes of the nanoparticles. The nanoparticles behave as single magnetic domains with random orientations of magnetic moments which are subject to thermal fluctuations. A Landau–Lifshitz line shape function presents adequate results which are in good agreement with the experimental ones. A single set of parameters provides good fits to the spectra recorded at different temperatures. At high T the SPR line shape is governed by the core anisotropy and the thermal fluctuations. By decreasing the temperature, the magnetic susceptibility of shell spins increases. As a result of this, the surface spins produce an effective field on the core leading to a decrease of resonance field, B r. Also, the effective anisotropy increases as the shell spins begin to order. So, the results are interpreted by a simple model, in which each single‐domain nanoparticle is considered as a core–shell system, with magnetocrystalline anisotropy on the core and surface anisotropy on the shell. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)