Magnetization ofPt13clusters supported in a NaY zeolite: A XANES and XMCD study

Abstract

X-ray absorption and x-ray magnetic circular dichroism (XMCD) spectra have been recorded at the Pt ${L}_{2,3}$ edges of ${\text{Pt}}_{13}$ and hydrogenated ${\text{Pt}}_{13}{\text{H}}_{m}$ clusters dispersed in NaY zeolite. A Pt foil was also measured as reference for comparison with bulk Pt. From the x-ray absorption near edge structure measurements the average number of holes ${n}_{h}$ in the $5d$ band of these systems is found. In the ${\text{Pt}}_{13}{\text{H}}_{m}$ sample the Pt-H antibonding state increases ${n}_{h}$ with respect to the ${\text{Pt}}_{13}$ sample. It gives rise to a shape resonance with energy ${E}_{\text{res}}=2.3\text{ }\text{eV}$. The average orbital and spin moments ${m}_{L}$ and ${m}_{S}$ for the Pt atom have been determined from the XMCD measurements. The total magnetic-moment field dependence XMCD($H$) was determined and scaled to the absolute value of the average moment per Pt atom. The fit of XMCD($H$) to a Langevin function yields a magnetic moment per cluster of $\ensuremath{\mu}=3.7\ifmmode\pm\else\textpm\fi{}0.4$ and $3.0\ifmmode\pm\else\textpm\fi{}0.4\text{ }{\ensuremath{\mu}}_{\text{B}}$, for ${\text{Pt}}_{13}$ and ${\text{Pt}}_{13}{\text{H}}_{m}$, respectively. It is shown that nonmagnetic clusters coexist with those giving magnetic signal. The amount of Pt constituting the magnetic ${\text{Pt}}_{13}$ clusters is 15\char21{}20% of the total Pt content. The XMCD data are compared with superconducting quantum interference device magnetization measurements. The moment found indicates that the cluster spin of ${\text{Pt}}_{13}$ and ${\text{Pt}}_{13}{\text{H}}_{m}$ is in the range $S\ensuremath{\approx}2\char21{}3$. They are compared to predictions calculated by density-functional methods.