Hybrid thin films are prepared from 8 to 10 nm Fe3O4 nanoparticles and exfoliated montmorillonite clay by using layer-by-layer assembly on poly(diallyldimethylammonium bromide), PDDA. Distinct stratification of the Fe3O4/PDDA/clay films is obtained due to the sheetlike structure of the clay particles. This feature distinguishes these assemblies from their polyelectrolyte-polyelectrolyte analogues, where the layers of individual polyelectrolytes are strongly interdigitated. Being adsorbed on PDDA strictly parallel to the substrate surface, montmorillonite produces a dense layer of overlapping alumosilicate sheets, which virtually flawlessly separates one magnetite layer from another. The difference in magnetic properties between assemblies of various architectures is attributed to the insulation effect of clay layers inserted between magnetic layers. The montmorillonite sheets disrupt the electron exchange interactions between the magnetite nanoparticles in adjacent layers, thereby limiting the magnetization reversal to two dimensions. Some optical properties of Fe3O4/PDDA films are investigated as well. When they are deposited on thin plastic substrate, oscillations of optical density were observed in the red part of the UV-vis spectrum. This effect, which was never been observed for conventional, thick substrates such as glass slides, stems from the interference of the light beams passed through and reflected off of the assembled film.

Stratified Assemblies of Magnetite Nanoparticles and Montmorillonite Prepared by the Layer-by-Layer Assembly

The response of fine-particle magnetic oxides to applied magnetic fields is of primary importance in several areas of applied physics, such as magnetic separation, magnetic fluid devices, and magnetic recording. It is also of fundamental interest. In this article contemporary research on the important properties of magnetic oxide particles is reviewed, including both applied and basic work. The microscopic and macroscopic properties of ferrimagnetic and antiferromagnetic fine-particle systems are considered. Special attention is paid to the physics of magnetization reversal mechanisms, surface effects, and the intrinsic properties of fine-particle magnetic oxides.

http://iopscience.iop.org/article/10.1088/0953-8984/5/45/002/pdf

Fine-particle magnetic oxides

The basic magnetic characterization techniques for thin recording media are critically examined in relation to the requirements for modern high-storage-density materials. The applicability and usefulness of remanence curves for the characterization of thin-film media are discussed together with the recently developed Delta I(H) technique. The origins of the relation between these measurements and cooperative reversal are presented, together with the improved data showing the correlation between Delta I(H) and noise. This discussion is extended to include the time-dependent phenomena and in particular the activation volume, including the effects of intergranular coupling. Data are presented to show these effects in both single-layer alloy films and laminated systems, and the implications for ultimate recording density are discussed. >

Magnetic characterisation of thin film recording media

Coercivity enhancement in γ-Fe2O3 particles dispersed at low-volume fraction

The magnetic dipole-dipole interaction effect on the magnetic hysteresis at zero temperature in nanoparticles randomly dispersed within a plane

In an assembly of particles, as used in magnetic recording media, there are always magnetic interactions. The magnetic properties of the media are modified as a consequence of the effect of such interactions on the magnetic behavior of the particles. In this work an attempt is made to find an experimental correlation between the presence and strength of the interactions and the magnetization reversal of the particles. The reversal mode is investigated by rotational hysteresis measurements and by the evolution of the angular dependence of the coercivity in samples of iron oxides at different dilution degrees. The magnetic interactions are evaluated with measurements of the remanence curves. It is found that when the magnetic interactions among the particles increase, the parameters used to study the reversal mode evolve from the behavior typical of the incoherent magnetization processes to behaviors which can be considered as intermediate between coherent and incoherent reversal modes. This evolution can b...

G. Bottoni

Papers

Influence of the magnetic interactions on the reversal mode of magnetic recording particles