Low Temperature Susceptibility Measurements on the Ultrafine Fe₃O₄ Powders

Abstract

Magnetic behavior of the ferrite powders in the nanometer grain size is of increasing fundamental and applied interest in materials research. Central to this activity are the development and application of effective experimental techniques for performing characterization. The magnetic susceptibility χ of the soft ferrites decisively depends on their granular structure and the measurement of it is one of the most powerful methods to estimate the relations between the magnetic behavior of the materials and the particle size. An extreme example of the effect of grain size on the susceptibility spectra χ=f(D_m) of a spinel ferrite was presented for the first time by Globus [1]. The grain size dependence of permeability in the kHz and MHz frequency range was described by Globus as a domain wall permeability effect. Such large effects result in drastic decreases of the permeability values at low temperatures and increase of the domain wall relaxation frequency was described in work [2]. In the single domain particle when the domain wall not exist this behavior could be related with the divergence from stoichiometry or inhomogenity in the grain size. In the work we investigated the temperature dependence of susceptibility in dynamic regime at the range of frequency from 3 to 50 kHz and for different sorts of magnetite at point of view the grain size and phase inhomgeanity in nanoscale.