Nanocarbon Ferromagnets

As discussed in Chapter 5, the presence of iron or cobalt particles is necessary for the nucleation and growth of carbon nanotubes fabricated by pyrolysis. In the preparation of aligned carbon nanotubes by pyrolyzing iron II phthalacyanide (FePc) it has been demonstrated that nanotube growth involves two iron nanoparticles. A small iron particle serves as a nucleus for the tube to grow on, and at the other end of the tube larger iron particles enhance the growth. Aligned nanotubes are prepared on quartz glass by pyrolysis of FePc in an argon-hydrogen atmosphere.

Figure 7.11 shows a scanning electron microscope (SEM) image of iron particles on the tips of aligned nanotubes, and Fig. 7.12 shows the magnetization curve at 5 K and 320 K for the magnetic field parallel to the tubes showing that the hystersis is larger at 5 K. Figures 7.13 and 7.14 show plots of die temperature dependence of the coercive field HQ, and ratio of remnant magnetization Mr to saturation magnetization Af„. We see that the coercive field increases by more than a factor of 3 when the temperature is reduced from room temperature (300 K) to liquid helium temperature

0 1000 2000 3000 4000 5000 NUMBER OF Fe ATOMS PER MOLECULE

Figure 7.10. Resonance frequency of ferritin as a function of the number of atoms in the cavity of the molecule. [Adapted from D. D. Awschalom and D. P. DiVincenzo, Phys. Today (April 1995).]

0 1000 2000 3000 4000 5000 NUMBER OF Fe ATOMS PER MOLECULE

Figure 7.10. Resonance frequency of ferritin as a function of the number of atoms in the cavity of the molecule. [Adapted from D. D. Awschalom and D. P. DiVincenzo, Phys. Today (April 1995).]

Figure 7.11. Scanning electron microscope image of iron particles (light spots) on the tips of aligned carbon nanotubes. [With permission from Z. Zhang et al., J. Magn. Magn. Mater. 231, L9 (2001).]

Rgur« 7.12. Magnetization curve hystersis loops for iron particles on the tips of aligned nanotubes at the temperatures of 5 and 320 K, for a magnetic field H applied parallel to the tubes. An oersted corresponds to 10~4T. [Adapted from Z. Zhang et al., J. Magrt. Magn. Mater. 231, L9 (2001).]

(4 K). These iron particles at the tips of aligned nanotubes could be the basis of high-density magnetic storage devices. The walls of the tubes can provide nonmagnetic separation between the iron nanoparticles, ensuring that the interaction between neighboring nanoparticles is not too strong. If the interaction is too strong, the fields required to flip the orientation would be too large.

Figure 7.13. Plot of coercive field H. versus temperature T for iron particles on the tips of aligned nanotubes. A kilooersted corresponds to 0.1 T. [Adapted from Z. Zhang et al., J. Magn. Magn- Mater. 231, L9 (2001).]
0 0

Post a comment