Magnetism of Au Nanoparticles on \textit{Sulfolubus Acidocaldarius} S-Layer

Au nanoparticles (NP) with diameters of a few nm have been synthesized on a protein S-layer of \textit{Sulfolobus Acidocaldarius} bacteria. SQUID magnetization (1.8 K $<$ T $<$ 300 K and 0 $<$ B $<$ 7 T) shows superparamagnetic behavior at low-T. Its origin lays at the Au NP's, as has been proven by Au L$_{2,3-}$edge XMCD spectroscopy, performed in the range 2.2$<$T$<$20 K and up to B$_{app}$=17 T. XMCD analysis yields a total magnetic moment per Au atom $\mu _{Au}$= 0.050(1) $\mu _{B}$, a particle average moment m$_{part }$= 2.3 $\mu _{B}$, Au orbital to spin moment ratio of m$_{L}$/m$_{S }$= 0.29, and Curie-like superparamagnetism. Au-S bonds are detected by S K-edge XAS measurements. Besides, EXAFS at the Au L$_{3}$-edge shows that the Au NP internal structure is fcc, and Au-S bonds are located at the particle surface. An increase of the hole charge carrier density in the Au 5d band due to electron transfer with the S-layer explains the Au magnetism. The observed magnetic moment per Au atom is 25 times larger than those previously found by XMCD in Au-thiol capped NPs.