Effects of grain boundary width and crystallite size on conductivity and magnetic properties of magnetite nanoparticles

• Published in: Journal of nanoparticle research : an interdisciplinary forum for nanoscale science and technology 2014-07, Vol.16 (7), p.1-12, Article 2482
• Main Authors: Lopez Maldonado, K. L., de la Presa, P., de la Rubia, M. A., Crespo, P., de Frutos, J., Hernando, A., Matutes Aquino, J. A., Elizalde Galindo, J. T.
• Format: Article
• Language: English
• Subjects: Characterization and Evaluation of Materials; Charging; Chemistry and Materials Science; Coercive force; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Conductivity; Cross-disciplinary physics: materials science; rheology; Crystallites; Electronic structure and electrical properties of surfaces, interfaces, thin films and low-dimensional structures; Electronic transport in multilayers, nanoscale materials and structures; Exact sciences and technology; Grain boundaries; Inorganic Chemistry; Lasers; Low-dimensional structures (superlattices, quantum well structures, multilayers): structure, and nonelectronic properties; Magnetic properties; Magnetic properties and materials; Magnetic properties of nanostructures; Magnetite; Materials Science; Mathematical analysis; Nanocrystalline materials; Nanoparticles; Nanoscale materials and structures: fabrication and characterization; Nanotechnology; Optical Devices; Optics; Photonics; Physical Chemistry; Physics; Research Paper; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties)
• ISSN: 1388-0764; 1572-896X
• DOI: 10.1007/s11051-014-2482-3

The structural, electrical, and magnetic properties of magnetite nanoparticles, with crystallite sizes 30, 40, and 50 nm, are studied. These crystallite sizes correspond to average particle sizes of 33, 87, and 90 nm, respectively, as determined by TEM. By HRTEM images, it is observed that grain boundary widths decrease as crystallite size increases. Electrical and microstructural properties are correlated based on the theoretical definition of charging energy. Conduction phenomena are investigated as a function of grain boundaries widths, which in turn depend on crystallite size: the calculations suggest that charging energy has a strong dependence on crystallite size. By zero-field-cooling and susceptibility measurements, it is observed that Verwey transition is crystallite size dependent, with values ranging from 85 to 95 K. In addition, a kink at the out-phase susceptibility curves at 35 K, and a strong change in coercivity is associated to a spin-glass transition, which is independent of crystallite size but frequency dependent. The activation energy associated to this transition is calculated to be around 6–7 meV. Finally, magnetic saturation and coercivity are found to be not significantly affected by crystallite size, with saturation values close to fine powders values. A detailed knowledge on the effects of grain boundary width and crystallite size on conductivity and magnetic properties is relevant for optimization of materials that can be used in magnetoresistive devices.