Permanent Magnetism, Magnetic Anisotropy, and Hysteresis of Thiol-Capped Gold Nanoparticles

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.

Figure 1

Transmission electron micrographs of Au-SR (a) and Au-NR (b) NPs. Their corresponding histograms, obtained from several micrographs, are plotted, respectively, in (c) and (d).

Figure 2

$\mathrm{A}\mathrm{u}\mathrm{\text{−}}{L}_3$-edge XANES spectra for the two different gold NPs compared to bulk gold. The insets show the modulus of the Fourier transform at the $\mathrm{A}\mathrm{u}\mathrm{\text{−}}{L}_3$ edge ($K$ weighted, $k$ space range of 3–12 Å without phase correction) for both samples and a schema of the proposed structure for Au-SR.

Figure 3

Magnetization curves of gold nanoparticles stabilized by means of a surfactant, Au-NR (a), and hysteresis loops corresponding to the gold thiol-capped NPs, Au-SR (b), at 5 and 300 K. For the Au-SR sample, the magnetization is given in emu per gram of gold.

Figure 4

UV-visible absorption spectra for the two studied gold nanoparticles. The strong surface plasmon resonance band around 550 nm, observed for the Au-NR sample, is absent in the case of Au-SR.