Spin dynamics in CoFe2O4 nanoparticles.

The bulk and local magnetisms of ∼10 nm diameter CoFe2O4 nanoparticles have been characterized from 5 to 425 K. Magnetometry results indicate that the nanoparticles have a blocking temperature around 380 K and they exhibit a temperature dependent cubic magnetocrystalline anisotropy similar to that of bulk CoFe2O4 with K(0)∼1.2×106 ergs/cm3. However, the local, atomic-level spin dynamics examined by Mössbauer spectroscopy indicates strongly that rather than exhibiting a typical static, blocked magnetism to superparamagnetism behavior, the CoFe2O4 nanoparticle moments become paramagnetic, with this transition percolating from the surface to the core of the nanoparticle with increasing temperature. [ABSTRACT FROM AUTHOR]