Surface Effects on the Magnetic Behavior of Nanocrystalline Nickel Ferrites: The Effect of Surface Roughness and Dilutio
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Surface Effects on the Magnetic Behavior of Nanocrystalline Nickel Ferrites: The Effect of Surface Roughness and Dilution 1
H. Nathani1,2 and R.D.K. Misra1,2 Center for Structural and Functional Materials 2 Department of Chemical Engineering, University of Louisiana at Lafayette P.O. Box 44130, Lafayette, LA 70504-4130, USA and W.F. Egelhoff, Jr.3 3 Magnetic Materials Division, National Institute of Standards and Technology Gaithersburg, MD 20899, USA ABSTRACT The paper describes the surface roughness and dilution effects on the magnetic behavior of nanocrystalline nickel ferrites studied by SQUID magnetometer. Two different kinds of measurements were performed: (a) zero-field cooling (ZFC) and field cooling (FC) magnetization versus temperature and (b) magnetization as a function of the applied field. The analysis of magnetic measurements indicate that while the superparamagnetic behavior is retained by nanocrystalline ferrites of different surface roughness (0.8-1.8 nm) at 300K, the hysteresis loop at 2K becomes non-squared and the coercivity increases with increase in surface roughness. This behavior is discussed in terms of broken bonds and degree of surface spin disorder. In diluted dispersion systems containing 10-40% nickel ferrite in a polyethylene matrix, the interparticle attractions continue to be dominant even when the concentration of nickel ferrite is 10 wt.% in the diluted system. The general magnetic behavior of diluted dispersion system is similar to the undiluted system; however, coercivity, remanence, and saturation magnetization are altered. These changes in the magnetic data are ascribed to magnetization interactions that encourage flux closure configuration. INTRODUCTION Magnetic nanoparticles are a subject of considerable interest both from the viewpoint of probing their magnetic behavior (size and surface effects,1-4 quantum tunneling of magnetization4,5) and practical significance (high density magnetic recording media,8-10 giant magnetoresistive sensors,7 ferrofluids8-10). The magnetic behavior of nanoparticles have a marked dependence with decrease in particle size and the surface effects start to dominate.11-14 In nanoparticles with large surface/volume ratio, the surface-spin driven arrangements (spin disorder) may eventually modify the magnetic properties. This spin disorder is believed to be a consequence of lower coordination of the surface atoms, and broken exchange bonds that produce a spin-glass like state of spatially disordered (canted) spins in the surface cations with high anisotropy surface layer.3,5 In a system with single domain particles with large surface/volume ratio, the magnetic behavior is influenced by ‘surface roughness’ and is also ‘modified by interactions.’ While roughness may involve reduced coordination at the surface and consequently significant surface spin disorder, the support matrix system in which nanoparticles are dispersed influence interparticle dipole-dipole interactions. Interactions in the latter case can either be entirely
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