Ferromagnetism induced by heavy-ion irradiation in fullerene films

Magnetic measurements using a superconducting quantum interference device and magnetic force microscopy were performed on fullerene films irradiated with $250\mathrm{keV}$ Ar and $92\mathrm{MeV}$ Si ions, to compare the effects of electronic excitation and collisional cascade on the magnetization. A ferromagnetic behavior increasing with ion fluence is observed. The magnetization is attributed to (i) the formation of an amorphous carbon network and (ii) the incorporation of oxygen in the irradiated films.

Figure 1

Magnetization measurement on $92\mathrm{MeV}$ Si ion irradiated ${\mathrm{C}}_{60}$ film at indicated fluences at $15\mathrm{K}$.

Figure 2

Magnetization measurement on $250\mathrm{keV}$ Ar ion irradiated ${\mathrm{C}}_{60}$ film at fluence of $5\times {10}^{16}\text{ions}∕{\mathrm{cm}}^2$ at 5, 50, 100, 200, and $300\mathrm{K}$ temperature. The enlarged part of the first quadrant is shown in inset for better clarity.

Figure 3

(Color online) Topographic (left) and magnetic force gradient (right) images of un-irradiated and ion irradiated fullerene films for (a) pristine, (b) $1\times {10}^{12}\text{ions}∕{\mathrm{cm}}^2$, and (c) $1\times {10}^{13}\text{ions}∕{\mathrm{cm}}^2$ fluence. The scanned area was of $1\times 1\mu {\mathrm{m}}^2$ and tip to sample distance was $20\mathrm{nm}$. The ordinate scale is $100\mathrm{nm}$ for topographic images and 2° for images of magnetic gradients.

Figure 4

Diagonal profile section of magnetic force gradient in the MFM images of irradiated fullerene film at the fluence of $1\times {10}^{13}\text{ions}∕{\mathrm{cm}}^2$.