Ferromagnetism of Pt nanoparticles induced by surface chemisorption

The magnetic properties of different-sized Pt nanoparticles coated by alkanethiol molecules with different carbon chain lengths (1-octadecanethiol, 1-dodecanthiol, and 1-octanethiol) were systematically studied by means of magnetic measurement, x-ray magnetic circular dichroism (XMCD), x-ray absorption near-edge structure (XANES), and electron spin resonance (ESR). Furthermore, azobenzene-derivatized thiol coating of Pt nanoparticles was performed to detect the effect on magnetic characteristics of the geometrical transformation between cis and trans states induced by photoisomerization. All samples showed the ferromagnetism inherent in Pt. In all of the prepared particles Curie temperatures above 300 K were observed. The largest magnetization at 5 K was observed for the longest chain length of alkanethiol and the smallest particle size. The coercive force at 5 K increased as the chain length increased and the particle size decreased. The geometrical transformation of coated molecules barely affected the magnetization and coercive force. The increase in coercive force was accompanied by the enhanced contribution of the orbital magnetic moment. The origins of ferromagnetism and magnetic anisotropy in Pt nanoparticles are discussed in terms of the contribution from the coating molecules and downsizing effects. The appearance of ferromagnetism is mainly interpreted based on two mechanisms, i.e., the electronic band magnetism based on the Stoner criterion for ferromagnetism and the orbital ferromagnetism due to electrons captured in the atomiclike orbital on the particle surface. The chain-length and particle-size-dependent magnetic anisotropy is explained in terms of the change in angular momentum depending on the adsorption condition, especially the coverage ratio, of alkanethiol on the particle surface. This is consistent with the existence of orbital ferromagnetism in the Pt nanoparticle.