Magnetic particle size effect

When the catalytic properties of supported clusters are measured by standard electrochemical methods such as cyclic voltammetry or oxidation transient measurement, only the average properties of the entire distribution of active particles on the electrode surface can be measured. A range of properties of supported nanoparticles, e.g., their geometric structure, their electronic and magnetic properties, as well as their catalytic activity, depends on the size of the particles. Geometric as well as electronic effects have been used to explain particle-size effects in electrocatalysis. 

Sharrock MP (1990) Time-dependent magnetic phenomena and particle-size effects in recording media. IEEE Trans Magn 26 193-197... 

Sanchez, RD., Rivas, J., Vaqueiro, R, Lopez-Quintela, M.A., Caeiio, D. Particle size effects on magnetic properties of yttrium iron garnets prepared by a sol-gel method. J. Magn. Magn. Mater. 247,92-98 (2002)... 

Solids separation based on density loses its effectiveness as the particle size decreases. For particles below 100 microns, separation methods make use of differences in the magnetic susceptibility (magnetic separation), elec trical conductivity (electrostatic separation), and in the surface wettability (flotation and selec tive flocculation). Treatment of ultrafine solids, say smaller than 10 microns can also be achieved by utilizing differences in dielectric and electrophoretic properties of the particles. 

The release of adriamycin from BSA microspheres both with and without magnetic particles present as magnetite was investigated (137). While the presence of drug and/or magnetite had no effect on the size of the hydrated or unhydrated microspheres, the stabilization temperature affected the size of hydrated microspheres. 

Mossbauer spectroscopy has been extensively used for studies of nanostructured materials and several reviews on magnetic nanoparticles have been published, see e.g. [6-8, 46 8]. The magnetic properties of nanoparticles may differ from those of bulk materials for several reasons. The most dramatic effect of a small particle size is that the magnetization direction is not stable at finite temperatures, but fluctuates. 

Composites containing nanometer-sized metal particles of a controllable and uniform size in an insulating ceramic matrix are very interesting materials for use as heterogeneous catalysts and for magnetic and electronic applications. They show quantum size effects, particularly the size-induced metal-insulator transition (SIMIT) [1],... 

The height of the energy barrier between the forward and reverse states is the product of the particle volume, V, and the anisotropy constant Kefr (which is, to some extent, a function of particle size). Superparamagnetic relaxation occurs when the thermal energy of the particles exceeds the activation energy barrier between the spin states and so allows rapid, spontaneous fluctuations between these states. The effect of these spin reversals is that the observed magnetic field is reduced or even absent. 

---