Static and dynamic magnetic properties of monodispersed Mn0.5Zn0.5Fe2O4 nanomagnetic particles

Static and dynamic magnetic properties of oleic acid/oleyamine coated Mn-Zn ferrite nanoparticles of diameter 82 angstrom are reported. The zero-field-cooled peak temperature decreases with increasing magnetic field and obeys the well known de Almeida-Thouless line. The zero-field-cooled magnetization data are simulated by assuming noninteracting magnetic particles with uniaxial anisotropy and lognormal particle size distribution. The relevant parameters give the values of particle diameter (D) 80 angstrom, standard deviation 0.3 in ln(D), and the anisotropy constant K to be 5.8 x 10(5) erg/cm(3). The observed higher value of standard deviation is due to the interparticle interaction. The complex magnetic susceptibility was measured as a function of temperature for frequencies ranging from 67 to 1800 Hz. The temperature at which the maximum in the ac-susceptibility curve is observed is well accounted by the Vogel-Fulcher law for both chi' and chi ''. The peak is also observed in a plot of chi ''/chi' versus temperature, which may mean the existence of magnetic aftereffect, and furthermore, it has an Arrhenius as well as Vogel-Fulcher law type dependence. An observed nonthermal activation type relaxation mechanism at 12 K is attributed to possible quantum tunneling effect in Mn-Zn ferrite nanoparticles. (C) 2010 American Institute of Physics. [doi:10.1063/1.3310807]