Numerical simulation of field-cooled and zero field-cooled processes for assembly of superparamagnetic nanoparticles with uniaxial anisotropy

The results of the numerical simulation of field-cooled and zero field-cooled (ZFC) experiments in a dilute assembly of superparamagnetic nanoparticles with uniaxial anisotropy are presented. The numerical simulation uses a solution of the kinetic rate equations for population numbers of the potential wells. The particle relaxation times are rigorously obtained from the corresponding Fokker-Planck equation. For an assembly of particles with a single diameter a monotonic decrease in a blocking temperature as a function of the applied magnetic field is found, the blocking temperatures of aligned and randomly oriented assemblies being close. For an assembly with lognormal volume distribution the location of the maximum at the ZFC assembly magnetization differs considerably from the blocking temperature of particles with the average diameter. Equating of both quantities may lead to a considerable overestimation of the particle effective anisotropy constant. The effective blocking temperature of the assembly with lognormal volume distribution may experience nonmonotonic magnetic field dependence, as the particles of large diameters begin to contribute to the assembly magnetization with the increase in the applied field. A prominent dependence of the ZFC assembly magnetization maximum on the width of the lognormal distribution is also revealed. (c) 2011 American Institute of Physics. [doi: 10.1063/1.3536632]