Spin-transfer mechanism of ferromagnetism in polymerized fullerenes: Ab initio calculations

A mechanism of the high-temperature ferromagnetism in polymerized fullerenes is suggested. It is assumed that some of the ${\mathrm{C}}_{60}$ molecules in the crystal become magnetically active due to spin and charge transfer from the paramagnetic impurities (atoms or groups), such as hydrogen, fluorine, hydroxyl group OH, amino group $\mathrm{N}{\mathrm{H}}_2$, or methyl group $\mathrm{C}{\mathrm{H}}_3$, dispersed in the fullerene matrix. The exchange interaction between the spins localized on the magnetically active fullerenes is evaluated using ab initio calculations. The nearest neighbor and next nearest neighbor exchange interaction is found to be in the range $0.1–0.3\mathrm{eV}$, that is, high enough to account for the room temperature ferromagnetism.

Figure 1

The charge (a) and spin (b) density spatial distribution (Milliken atomic charges and spins) of the doped fullerenes ${\mathrm{C}}_{60}\mathrm{H}$ and ${\mathrm{C}}_{60}\mathrm{F}$. Black (gray) corresponds to positive (negative) charge or up (down) spins. The volume of the spheres is proportional to the absolute value of the corresponding variable on a given atom.

Figure 2

The charge (a) and spin (b) density spatial distribution (Milliken atomic charges and spins) of the doped CA dimers ${\left({\mathrm{C}}_{60}\mathrm{H}\right)}_2$, ${\left({\mathrm{C}}_{60}\mathrm{F}\right)}_2$, ${\left({\mathrm{C}}_{60}\right)}_2\mathrm{H}\mathrm{F}$. Black (gray) corresponds to positive (negative) charge or up (down) spins. The volume of the spheres is proportional to the absolute value of the corresponding variable on a given atom.

Figure 3

The charge (a) and spin (b) distribution in the triplet spin state of the doped CA tetramer ${\left({\mathrm{C}}_{60}\right)}_4{\mathrm{H}}_2$. Black (gray) corresponds to positive (negative) charge or up (down) spins. The volume of the spheres is proportional to the absolute value of the corresponding variable on a given atom.