We report on the experimental observation of magnetic hysteresis up to room temperature in thiol-capped Au nanoparticles with 1.4 nm size. The coercive field ranges from 860 Oe at 5 K to 250 Oe at 300 K. It is estimated that the Au atoms exhibit a magnetic moment of $\mu =0.036{\mu }_B$. However, Au nanoparticles with similar size but stabilized by means of a surfactant, i.e., weak interaction between protective molecules and Au surface atoms, are diamagnetic, as bulk Au samples are. The apparent ferromagnetism is consequently associated with $5d$ localized holes generated through Au-S bonds. These holes give rise to localized magnetic moments that are frozen in due to the combination of the high spin-orbit coupling (1.5 eV) of gold and the symmetry reduction associated with two types of bonding: Au-Au and Au-S.

The influence of the magnetic properties of the amorphous matrix on the macroscopic behavior of soft nanocrystalline materials is analyzed. The random-anisotropy model is generalized to two magnetic phase systems. It is shown that recent observations dealing with a magnetic hardening at the first stage of nanocrystallization and a magnetic hardening at temperatures close to the matrix Curie temperature can be well understood with this generalization.

In this Letter, we experimentally show that the room temperature ferromagnetism in the Mn-Zn-O system recently observed is associated with the coexistence of ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ via a double-exchange mechanism. The presence of the ZnO around ${\mathrm{MnO}}_2$ modifies the kinetics of ${\mathrm{MnO}}_2\to {\mathrm{Mn}}_2{\mathrm{O}}_3$ reduction and favors the coexistence of both Mn oxidation states. The ferromagnetic phase is associated with the interface formed at the Zn diffusion front into Mn oxide, corroborated by preparing thin film multilayers that exhibit saturation magnetization 2 orders of magnitude higher than bulk samples.

Exchange interactions between ferromagnetic layers through paramagnetic spacers have been widely observed and studied in thin multilayers. In this paper we report an experimental analysis of the exchange correlation length in soft ferromagnetic nanocrystals. It has been observed that for an average distance between crystallites below 5 nm they are coupled by exchange interactions even above the Curie temperature of the amorphous matrix. The experiment indicates that exchange can propagate through noncrystalline paramagnetic layers. An estimation of the exchange strength has been made.