Spin dynamics in CoFe_2O_4 nanoparticles

摘要

The bulk and local magnetisms of ～10 nm diameter CoFe_2O_4 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 CoFe_2O_4 with K(0)～1.2× 10~6 ergs/cm~3. However, the local, atomic-level spin dynamics examined by Mossbauer spectroscopy indicates strongly that rather than exhibiting a typical static, blocked magnetism to superparamagnetism behavior, the CoFe_2O_4 nanoparticle moments become paramagnetic, with this transition percolating from the surface to the core of the nanoparticle with increasing temperature.