Gold nanoparticles currently elicit an intense and very broad research activity because of their peculiar properties. Be it in catalysis, optics, electronics, sensing or theranostics, new applications are found daily for these materials. Approximately a decade ago a report was published with magnetometry data showing that gold nanoparticles, most surprisingly, could also be magnetic, with features that the usual rules of magnetism were unable to explain. Many ensuing experimental papers confirmed this observation, although the reported magnetic behaviours showed a great variability, for unclear reasons. In this review, most of the experimental facts pertaining to "magnetic gold" are summarized. The various theories put forth for explaining this unexpected magnetism are presented and discussed. We show that despite much effort, a satisfying explanation is still lacking and that the field of hypotheses should perhaps be widened.

This paper is dedicated to Marc Drillon, on the occasion of his 60th birthday.The much-talked-about "nanotechnology revolution" shall probably not occur without the prior emergence of reliable self-assembling techniques. It is quite clear today that in spite of incredible progress, top-down technologies and lithographic techniques are getting close to the end of their evolutionary course. With the promise of lower cost and high throughput, self-assembling techniques stand unrivaled, the mere existence of Nature's billions of self-assembled living creatures being an obvious proof of their ultimate efficiency. Moreover, as space is no longer a problem in nanoscale devices, such systems can have an inherent potentially high redundancy making them much less sensitive to damage or fabrication errors than the present ones.In the current literature on nanodevices, the term nanoparticles (NPs) can actually convey many different meanings. In terms of size, first, since the "nanoworld" spans several orders of magnitude, and in terms of functionality, second, as the function may be that of a single particle (e.g., quantum dots) or one brought about by the periodic structuring of space via the self-organization of hundreds of particles (e.g

Self-assembly of nanoparticles (NPs) into tailored structures is a promising strategy for the production and design of materials with new functions. In this work, 2D arrays of iron oxide NPs with interparticle distances tuned by grafting fatty acids and dendritic molecules at the NPs surface have been obtained over large areas with high density using the Langmuir-Blodgett technique. The anchoring agent of molecules and the Janus structure of NPs are shown to be key parameters driving the deposition. Finally the influence of interparticle distance on the collective magnetic properties in powders and in monolayers is clearly demonstrated by DC and AC SQUID measurements. The blocking temperature T(B) increases as the interparticle distance decreases, which is consistent with the fact that dipolar interactions are responsible for this increase. Dipolar interactions are found to be stronger for particles assembled in thin films compared to powdered samples and may be described by using the Vogel Fulcher model.