Demagnetization Treatment of Remanent Composite Microspheres Studied by Alternating Current Susceptibility Measurements

Abstract

The magnetic remanence of silica microspheres with a low concentration of embedded cobalt ferrite nanoparticles is studied after demagnetization and remagnetization treatments. When the microspheres are dispersed in a liquid, alternating current (AC) magnetic susceptibility spectra reveal a constant characteristic frequency, corresponding to the rotational diffusion of the microparticles; this depends only on particle size and liquid viscosity, making the particles suitable as a rheological probe and indicating that interactions between the microspheres are weak. On the macroscopic scale, a sample with the dry microparticles is magnetically remanent after treatment in a saturating field, and after a demagnetization treatment, the remanence goes down to zero. The AC susceptibility of a liquid dispersion, however, characterizes the remanence on the scale of the individual microparticles, which does not become zero after demagnetization. The reason is that an individual microparticle contains only a relatively small number of magnetic units, so that even if they can be reoriented magnetically at random, the average vector sum of the nanoparticle dipoles is not negligible on the scale of the microparticle. In contrast, on the macroscopic scale, the demagnetization procedure randomizes the orientations of a macroscopic number of magnetic units, resulting in a remanent magnetization that is negligible compared to the saturation magnetization of the entire sample.

Graphene oxide-Fe3O4 nanoparticle composite with high transverse proton relaxivity value for magnetic resonance imaging

“…The observed decrease in the magnetic moment value of the nanoparticles with decrease in average particle size is also supported by the investigations reported in the literature. 34,35 …”

Swelling Enhanced Remanent Magnetization of Hydrogels Cross-Linked with Magnetic Nanoparticles

“…The magnetic contact interaction of our nanoparticles has a magnitude of 6 k B T , sufficient for the presence of small dipolar structures . Whereas the orientation of the magnetic easy axis of the nanocrystals in the hydrogel is most probably random, the z component of the nanoparticle dipoles is positive after the magnetization treatment . When the gel is stretched in the z direction without changing in the other directions, elongated dipolar structures will align in the swelling direction (Figure a).…”

Magnetic Reprogramming of Self‐Assembled Hard‐Magnetic Cilia

“…Demagnetization with a damped oscillating magnetic field, known as alternating current (AC) or alternating field (AF) demagnetization, facilitates reprogramming by significantly reducing the magnetic moment along the initial magnetization direction. AC demagnetization is especially useful for structures that cannot be demagnetized through thermal techniques, such as magnetic composites, [24] recording media, [25,26] and geomagnets. [27,28] Building on previous studies of NdFeB magnetic cilia, [19][20][21][22][23][29][30][31] this work combines experiment and theory to investigate and improve the process of magnetically reprogramming NdFeB magnetic cilia and their altered actuation behaviors.…”