Magnetic properties of superparamagnetic lithium ferrite nanoparticles

Magnetic properties of lithium ferrite nanoparticles of size in the range of 4-50 nm, synthesized by a low-temperature method, have been evaluated. A broad maximum at similar to 220 K in the temperature variation of the zero-field-cooled magnetization as well as the ac susceptibility and divergence of the zero-field-cooled and field-cooled magnetizations below this temperature indicate the superparamagnetic behavior of the lithium ferrite particles of size similar to 4 nm. On the other hand, at high temperatures, these particles show a cusp immediately below the Curie temperature of bulk lithium ferrite (895 K). This anomalous magnetic behavior of the lithium ferrite nanoparticles, similar to that arising from the Hopkinson effect for bulk materials, is probed in detail and is explained in terms of the cumulative effect of the temperature variation of the anisotropy and particle size growth during the measurements at high temperatures. (c) 2005 American Institute of Physics.