Direct Observation of Ferromagnetic Spin Polarization in Gold Nanoparticles

We report the first direct observation of ferromagnetic spin polarization of Au nanoparticles with a mean diameter of 1.9 nm using x-ray magnetic circular dichroism (XMCD). Owing to the element selectivity of XMCD, only the gold magnetization is explored. Magnetization of gold atoms as estimated by XMCD shows a good agreement with results obtained by conventional magnetometry. This evidences intrinsic spin polarization in nanosized gold.

Figure 1

(a) Schematic of PAAHC-Au nanoparticles. (b) TEM image of Au nanoparticles protected by PAAHC. Mean diameter of the sample is 1.9 nm. Scale bar is shown in the image. (c) Size distribution of PAAHC-Au nanoparticles. The solid line represents the fitting curve assuming the log-normal function $f(r)=\frac{1}{\sqrt{2\pi }r\sigma }\exp (-\frac{(\ln r-r_0{)}^2}{2{\sigma }^2})$ where $r$ is the particle diameter. The fitting results of parameters are the mean particle diameter $r_0=1.9\mathrm{n}\mathrm{m}$ and the standard deviation $\sigma =0.2\mathrm{n}\mathrm{m}$.

Figure 2

XMCD and XAS spectra at the Au $L_3$- and $L_2$-edge with an applied magnetic field of 10 T. The XMCD and XAS spectra include a same scale factor so that the height of the edge jumps of the XAS spectra (solid lines) are unity. Solid lines with closed circle symbols represent an XMCD spectrum with an applied magnetic field of 10 T, while the dotted line indicates an XMCD spectrum in a magnetic field applied in the opposite direction, implying the observed signal is not an artificial effect.

Figure 3

(a) ESM of PAAHC-Au as a function of the applied magnetic field and magnetization process obtained by SQUID magnetometer. The integral of the peak intensity yields similar results. Dotted line is the fit to the data assuming a Langevin function plus a linear field-dependent term. (b) Temperature dependence of the XMCD peak intensity at 10 T recorded at the Au $L_3$-edge and temperature variation of the magnetization measured by SQUID magnetometer at 7 T. Dotted line is the fit to the data assuming $\chi (T)=N\mu L(x)/H+{\chi }_{\mathrm{P}\mathrm{a}\mathrm{u}\mathrm{l}\mathrm{i}}$. Solid lines are guides for the eye.