Size dependence of the magnetic properties of cobalt oxide nanoparticles mineralized in protein cages
Resnick, Damon Aaron
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A major question in the physics of magnetic nanoparticles is how the size affects the magnetic properties in magnetic nanoparticle systems. In particular, the magnetic properties can be affected by finite-size effects or surface effects. It is this author's belief that surface effects and not finite-size effects play the dominate role. This study is a specific example of how to try to answer this question by looking at different sizes of Co 3O 4 nanoparticles. In order to answer this question as well as better understand this system, different antiferromagnetic Co 3O 4 nanoparticles of 4.35 nm and 6.3 nm in diameter were synthesized. These particles were determined to be relatively uniform and monodispersed. In this study, Transmission Electron Microscopy (TEM), electron diffraction (ED) and X-ray Absorption Spectroscopy (XAS) were used to study the physical and electronic structure of the nanoparticles. Alternating Current Magnetic Susceptibility (ACMS) was used to measure the magnetic anisotropy energy density of the different size nanoparticles. It was found that the anisotropy energy density increases with decreasing size, from 1.09 x10 4 J/m 3 for the 6.3 nm particles to 7.53x10 4 J/m 3 for the 4.35 nm particles, consistent with the importance of surface anisotropy with decreasing particle size. Vibrating Sample Magnetometry (VSM) was used to measure the Neel temperature and coercive field of the different particles. It was found that the Neel temperature decreases with decreasing size, from 40 K to 15 K, consistent with a simple surface approximation of the finite-size scaling theory, while the coercive field increased with decreasing particle size consistent with a surface model. The main conclusion of this work is that surface effects and not finite-size effects play a major role in the change of the magnetic properties with size in Co 3O 4 nanoparticles. The evidence also suggests that the increase in the anisotropy energy density is due to the creation of a surface anisotropy component normal to the surface of the particles.