Ferrofluids — Superparamagnetic nanoparticles

The first ferrofluids were synthesized by F. Bitter in the early 1930s, with the idea of demonstrating Weiss domains. These domains, the existence of which had been postulated in 1907 by French physicist Pierre Weiss, constitute zones in which magnetization has a uniform direction and intensity. Direct experimental verification of this hypothesis was essential to ensure compatibility between the theory of magnetism (Weiss' molecular field theory) and the existence of a macroscopically demagnetized state. Bitter's idea was to demonstrate the domains in the same way as one demonstrates the magnetic field with iron filings. However, as the scale was much smaller, very small magnetic particles were needed . Improved by W.C. Ellmore , the solution obtained is not very stable.

In 1966, Papell mixed magnetite powder with kerosene and ground the mixture for 10 months in the presence of oleic acid. Rosensweig improved Papell's fluid and set up the company Ferrofluidics with R. Moskowitz. In 1980, R. Massart invented the surfactant-free ferrofluid: in this case, it is the surface ionic charge that ensures stability.

For a long time, ferrofluids were confined to the visualization of Weiss domains (Bitter figures) before arousing theoretical interest in the late 1940s. In particular, Néel studied iron particles and rocks containing small amounts of magnetite particles, which enabled him to introduce the notion of "relaxation". One of the main obstacles was to obtain dispersed particles of controlled size and shape. This was achieved in the late 1950s by rapid solidification of a mixture of iron and copper in the majority, leading to the formation of small iron precipitates in the copper matrix. It was while working on these samples that Bean and Livingstone identified the paramagnetic-like behavior of nanoparticles, defined the blocking temperature and coined the term "superparamagnetic".

Since their first industrial applications in the 1970s, ferrofluids have remained a niche product, with a very limited number of applications (loudspeakers and seals for turbomolecular pumps), but in recent years there has been strong industrial interest in applications of greater economic scope, such as active dampers, and above all a wide variety of applications in biology.