Effect of dipolar and exchange interactions on magnetic blocking of maghemite nanoparticles

• Published in: Journal of magnetism and magnetic materials Vol. 323; no. 15; pp. 1998 - 2004
• Main Authors: Nadeem, K., Krenn, H., Traussnig, T., Würschum, R., Szabó, D.V., Letofsky-Papst, I.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.08.2011; Elsevier
• Subjects: Compacting; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Dipolar; Exact sciences and technology; Exchange; Fittings; Freezing; Interparticle interaction; Law; Magnetic permeability; Magnetic properties and materials; Magnetic properties of nanostructures; Magnetism; Nanoparticle; Nanoparticles; Physics; Retarding; Exchange; Interparticle interaction; Dipolar; Nanoparticle; Spin glasses; Iron oxide; Microwave radiation; Superspin glass; Magnetic nanomaterial; Critical slowing down; Dipole interactions; Spin freezing; Magnetic susceptibility; Nanoparticles; Magnetic particles; Particle interactions; Coercive force; Scaling laws; Arrhenius equation; Exchange interactions
• ISSN: 0304-8853
• DOI: 10.1016/j.jmmm.2011.02.041

Magnetic interparticle interactions compete with the magnetic blocking of ultrafine magnetic nanoparticles. We have prepared maghemite (γ-Fe 2O 3) nanoparticles by microwave plasma synthesis as a loose powder and in compacted form. In ZFC/FC measurements, blocking temperature of the compacted sample C is larger than that of the powder sample P. The frequency dependence of AC susceptibility of the sample C shows a large shift of blocking temperature with increasing frequency. Vogel–Fulcher law gives a large value of T 0 for the sample C. To get evidence of a possible spin-glass freezing in both samples, scaling law fitting is applied to the AC susceptibility data. The value of the exponent ( zv) of the critical slowing down dynamics fits to the spin-glass regime for both samples. For the sample P, spin-glass freezing occurs on the surface of individual nanoparticles, while in the sample C surface spin-glass freezing is concomitant with a superspin-glass formation as a consequence of coupling between particles. The sample C also shows an enhancement of coercivity due to dipolar interactions among the nanoparticles. Exchange interactions are attributed only to touching nanoparticles across their interfaces. All these measurements indicate the presence of strong interparticle dipolar interactions in the compacted sample C. ►We have prepared maghemite (4 nm) nanoparticles with an average particle size of 4 nm by microwave plasma synthesis. ► The particles are highly mono-disperse in diameter as evidenced by TEM and magnetic measurements. ► Due to the narrow size distribution, our prepared maghemite nanoparticles are good model substances for a reliable fit of experimental data to numerical simulations. ► We have studied the effect of interparticle interactions on magnetic blocking of the nanoparticles by using SQUID-magnetometry.