Magnetic properties of transition-metal nanoclusters on biological substrates

Compared to their bulk counterparts, micro- and nanogranular materials can reveal a diverse appearance of physical properties. In particular, their magnetic and superconducting behaviour can be drastically changed. Recently we have started to investigate transition metal clusters with a well defined grain size of 1 nm. These metal nanoclusters have been deposed on a regular square lattice of a biological template. In more detail, this template is a purified self-assembling paracrystalline surface layer (S-layer) of the bacillus sphaericus JG-A12 which exhibits square symmetry and is composed of identical protein monomers. The nucleation sites of the nanoclusters are most probably the pores of the S-layers. The S-layer proteins are capable of selective and reversible binding of high amounts of metals, making the S-layer also interesting for technological applications. The transition metal nanoclusters were investigated using EXAFS-spectroscopy and SQUID-magnetometry. First data of their magnetization at 0 < B < 7T and 1.8 K < T < 350 K reveal interesting magnetic properties. The Stoner enhancement factor of the d conduction-electron susceptibility in the Pd and Pt nano clusters is dramatically reduced compared to the one of bulk transition metals. In the case of granular Pt, the weakened magnetism of the 5d electrons is considered to play a crucial role for the occurrence of superconductivity by adjusting the balance between electron-phonon interaction and competing magnetic interactions observed for micron [1] and submicron [2] grain size powders. As the superconducting transition temperature has been observed to increase clearly with decreasing Pt grain size, we focus our interest also on the search of superconductivity in the transition metal nanoclusters.