Magnetization experiments on frozen ferrofluids

Abstract

Ensembles consisting of Co nanoparticles fixed in space by freezing ferrofluids are investigated. In frozen ferrofluids a well defined low temperature magnetic state of the nanoparticle system can be achieved. We distinguish between textured samples (preferred orientation of easy axes) and non-textured with randomly oriented easy axes. Considering the magnetization curves of such systems bears in principle the possibility to distinguish between influences arising from particle anisotropy and particle-particle interaction. Our intention is to present new experimental strategies aiming to separate these influences from magnetization measurements versus temperature. In another approach we analysed the susceptibility derived from magnetization measurements and determined the mean magnetic moment of the particles and an additional field which may be caused by the anisotropy of the particles or the interaction between them. Transmission electron microscopy was used for the direct measurement of the particles' sizes and their size distribution.

Magnetization measurements on frozen ferrofluids: an attempt to separate interaction and anisotropy influences

“…always holds, as shown by Michele et al [20,21]. This fact is used here to define first a normalized temperature dependent difference (T ), which is non-zero when interaction is present.…”

“…In a recent paper, Michele et al [20] summarized many experimental ways to prepare well defined low-temperature magnetic states of a nanoparticle system. From that, three processes are important for this contribution: zero field cooling (ZFC), positive high field cooling (PHFC) and negative high field cooling (NHFC).…”

“…Instead of the very popular consideration of the magnetization versus applied external magnetic field (hysteresis measurements) we prefer the consideration of the magnetization versus temperature, applying a constant magnetic field. As has been recently reported [20], by considering magnetization measurements after well defined processes of reaching the low-temperature magnetic state it is possible to apply new strategies in performing magnetization experiments when measuring the magnetization versus temperature. Using these strategies we discovered that the SW model bears a generally valid mean value rule which allows one to define a deviation resulting from the difference in the behaviour of a real particle system in comparison with the ideal SW particle system.…”

Modification of nanocrystalline magnetite by milling

“…M s is the bulk magnetization. Taking this into account, the parameter α which is used in the description of spectra is distributed by the lognormal law as is the particle volume V [16]. For description of the contribution of ferrimagnetic magnetite in Mössbauer spectra the sets of sextets are used as a function of α.…”

Initial magnetization analysis of iron cluster assemblies

“…The position of these minima can not be calculated analytically in the general case. Pfeiffer [4] found a numerical approach for these minima. To avoid these approaches we propose the following model including both possible directions of the magnetization of a Stoner-Wohlfarth particle: The complete magnetization function includes an additional component for a (small) background signal bgrnd(B) which may be related to the dia-or para-magnetic signal of the sample holder:…”