Intrinsic Impurity-Band Stoner Ferromagnetism in ${\mathrm{C}}_{60}{\mathrm{H}}_n$

We identify Stoner ferromagnetism in fcc ${\mathrm{C}}_{60}{\mathrm{H}}_n$ ($n=\mathrm{odd}$) by using a local density approximation in the framework of the density functional theory. Hydrogen chemisorption on fullerenes creates quasilocalized $\pi$ electrons on the fullerene surface, overlapping of their wave functions giving rise to a narrow half filled impurity band in the fcc ${\mathrm{C}}_{60}{\mathrm{H}}_n$. The Stoner-type ferromagnetic exchange between the itinerant electrons leads to spin-split impurity bands. The magnetic moment per ${\mathrm{C}}_{60}{\mathrm{H}}_n$ molecule is $1{\mu }_B$ (for $n=\mathrm{odd}$) or 0 (for $n=\mathrm{even}$, including zero), only one of the hydrogens contributing to the spin-split states. Direct overlapping of the quasilocalized $\pi$-electron orbitals is essential for the ferromagnetism.

Figure 1

Electronic structure of the ${\mathrm{C}}_{60}{\mathrm{H}}_n$ molecules. The dashed (black) line in (a) corresponds to ${\mathrm{C}}_{60}$. The solid (black) and dotted (red) lines correspond to the spin-up and -down states, respectively.

Figure 2

Local density of states of the ${\mathrm{C}}_{60}{\mathrm{H}}_n$ molecules near the Fermi energy for (a) $n=1$, (b) $n=2$, (c) $n=3$, (d) $n=5$ and for (e) $({\mathrm{C}}_{60}\mathrm{H}{)}_2$. The small (red) and medium-size (blue) circles correspond to hydrogen and carbon, respectively, the large (green) circles corresponding to the isosurface of the local density of states.

Figure 3

Spin-degenerate (a) and spin-polarized (b) electronic structures of a fcc ${\mathrm{C}}_{60}\mathrm{H}$ crystal along the symmetry line in the fcc Brillouin zone. The dotted (red) and the solid (black) lines in (b) correspond to the spin-up and -down bands, respectively.