Magnetism of the endohedral metallofullerenes $M@{\mathrm{C}}_{82}$ $(M=\mathrm{Gd},\mathrm{Dy})$ and the corresponding nanoscale peapods: Synchrotron soft x-ray magnetic circular dichroism and density-functional theory calculations

Synchrotron soft x-ray magnetic circular dichroism (SXMCD) spectroscopy at the Gd and Dy $M_5$ edges is reported on endohedral metallofullerenes ($M@{\mathrm{C}}_{82}$, $M=\mathrm{Gd}$ and Dy) and the corresponding nanopeapods [$\left(M@{\mathrm{C}}_{82}\right)@\mathrm{SWNT}$, SWNT represents single wall carbon nanotube] in a temperature range between 10 and $40\mathrm{K}$. The magnetic moment has also been determined by theoretical calculations, which are based on the Hartree-Fock approximation with relativistic corrections. Because of the element-specific measurement of SXMCD, magnetization processes of Gd and Dy ions of nanopeapods have been selectively observed. The temperature dependence of magnetic moments of the metallofullerenes and nanopeapods follows the Curie-Weiss law with a small Weiss temperature, indicating that the magnetic interaction between encapsulated rare-earth metal atoms is relatively weak. Although the observed differences in Curie constants and Weiss temperatures between $\mathrm{Gd}@{\mathrm{C}}_{82}$ and $\left(\mathrm{Gd}@{\mathrm{C}}_{82}\right)@\mathrm{SWNT}$ are small, those of $\mathrm{Dy}@{\mathrm{C}}_{82}$ and $\left(\mathrm{Dy}@{\mathrm{C}}_{82}\right)@\mathrm{SWNT}$ are significant. This observation is consistently explained by charge transfer-induced crystal-field effects.

Figure 1

A HRTEM image of $\left(\mathrm{Gd}@{\mathrm{C}}_{82}\right)@\mathrm{SWNT}$. The scale bar is approximately $2\mathrm{nm}$.

Figure 2

(Color online) Optimized structure of $\mathrm{Dy}@{\mathrm{C}}_{82}$ together with spin density distribution that is shown as contour of equidensity $(1.3e∕{\mathrm{\AA }}^3)$. The isodensity surface is defined by difference between $\alpha$ and $\beta$ spin where red and blue surfaces represent a plus and minus spin density region, respectively.

Figure 3

(Color online) Theoretical and observed isotropic XAS and SXMCD spectra of $\mathrm{Dy}@{\mathrm{C}}_{82}$, $\left(\mathrm{Dy}@{\mathrm{C}}_{82}\right)@\mathrm{SWNT}$, $\mathrm{Gd}@{\mathrm{C}}_{82}$, and $\left(\mathrm{Gd}@{\mathrm{C}}_{82}\right)@\mathrm{SWNT}$. [(a) and (b)] Theoretical XAS spectra (solid line), and observed XAS spectra of metallofullerenes (upper solid circle) and nanopeapods (lower solid circle). [(c) and (d)] Theoretical SXMCD spectra (dotted line), and observed SXMCD spectra of metallofullerenes (upper open circle) and nanopeapods (lower open circle).

Figure 4

(Color online) Plots of reciprocal magnetization in external magnetic field $B=1.9T$. (a) Plot of $\mathrm{Dy}@{\mathrm{C}}_{82}$ and $\left(\mathrm{Dy}@{\mathrm{C}}_{82}\right)@\mathrm{SWNT}$, and (b) plot of $\mathrm{Gd}@{\mathrm{C}}_{82}$ and $\left(\mathrm{Gd}@{\mathrm{C}}_{82}\right)@\mathrm{SWNT}$. The dotted line represents fitting results by the Curie-Weiss law.