Dominant dipolar interaction and glassy magnetic behaviour in polymer-coated magnetite nanoparticles.

The static and dynamic properties of magnetization have been investigated for polymer-coated magnetite nanoparticles with sizes from 5 to 15 nm. The analysis of the temperature dependence of zero-field-cooled magnetization indicates that the effective anisotropy constant is found to increase with the decrease of particle size, which is ascribed to the increase of surface anisotropy. The relaxation of the remanent magnetization clearly shows the signature of dominant dipolar interparticle interaction. The dynamics of magnetization also indicates the signature of glassy magnetic behaviour. The memory effect in the temperature dependence of field-cooled magnetization is noticed, which is inconsistent with the glassy magnetic behaviour.