Measurement of the zero-field magnetic dipole moment of magnetizable colloidal silica spheres

Abstract

The magnetic properties of dispersions of magnetic silica microspheres have been investigated by measuring the magnetization curves and the complex magnetic susceptibility as a function of frequency and field amplitude. The silica spheres appear to have a net permanent magnetic dipole moment, even in zero field, which is increased significantly after a temporary exposure of the silica colloids to a saturating magnetic field. The magnetic properties of the microparticles in zero field are discussed in terms of the number and the orientations of the embedded nanoparticle dipoles along an easy axis of magnetization in the absence of an external field.

Complex magnetic susceptibility setup for spectroscopy in the extremely low-frequency range

“…This is unambiguous evidence that the colloidal particles have a permanent magnetic dipole moment. [8][9][10] By contrast, the particles of Fig. 7͑a͒ have a magnetic relaxation that is much faster than the rate at which the particles rotate by thermal motion, indicating that those particles do not have a permanent magnetic dipole moment.…”

“…1,54,55 The permanent magnetic dipole moment of the colloidal particles can be calculated from the low-frequency limit 0 of the magnetic susceptibility and the number concentration N / V of the particles: 0 = N 0 2 / ͑3Vk B T͒. [8][9][10] However, this assumes the weak field limit, meaning that the interaction of the dipoles with the magnetic field 0 H is negligible compared to the thermal energy k B T. The permanent magnetic dipole moments of the colloidal particles in Figs. 7͑b͒ and 7͑c͒ are relatively large, so that the weak field limit is already exceeded at fields below 1000 A / m. This is illustrated in Fig.…”

“…Examples of large magnetic colloidal particles are composite magnetic microspheres, [8][9][10] magnetic paints, 11 elongated iron ͑hydr͒oxide particles of hematite 12 or goethite, 13 and aggregates in destabilized ferrofluids. [14][15][16][17][18][19] For composite microspheres, measuring the complex magnetic susceptibility spectrum is a uniquely reliable way to determine the permanent magnetic dipole moment, since the frequency dependence indicates whether field-induced sample magnetization requires rotational motion of the microparticles.…”

“…[14][15][16][17][18][19] For composite microspheres, measuring the complex magnetic susceptibility spectrum is a uniquely reliable way to determine the permanent magnetic dipole moment, since the frequency dependence indicates whether field-induced sample magnetization requires rotational motion of the microparticles. [8][9][10] For ferrofluids, measuring the complex magnetic susceptibility is a sensitive way to detect aggregation, since the time scale at which aggregates respond to an alternating magnetic field can be orders of magnitude longer than for single particles. [14][15][16][17][18][19] The complex magnetic susceptibility is also directly relevant for biomedical applications of magnetic nanoparticles.…”

Biogenic and biomimetic magnetic nanosized assemblies

“…The existence of linear chains in ferrofluids in the absence of external magnetic field was predicted by a series of theoretic simulations prior to observations of experimental evidences [87][88][89][90]. In 1980, Chantrell et al investigated the effects of magnetostatic and repulsive interaction on the configuration of 150Å diameter cobalt particles [91].…”