Ferenc Fülöp

Magnetic Fullerenes inside Single-Wall Carbon Nanotubes

C(59)N magnetic fullerenes were formed inside single-wall carbon nanotubes by vacuum annealing functionalized C(59)N molecules encapsulated inside the tubes. A hindered, anisotropic rotation of C(59)N was deduced from the temperature dependence of the electron spin resonance spectra near room temperature. Shortening of the spin-lattice relaxation time T(1) of C(59)N indicates a reversible charge transfer toward the host nanotubes above approximately 350 K. Bound C(59)N-C(60) heterodimers are formed at lower temperatures when C(60) is coencapsulated with the functionalized C(59)N. In the 10-300 K range, T(1) of the heterodimer shows a relaxation dominated by the conduction electrons on the nanotubes.

Azafullerene C59N, a stable free radical substituent in crystalline C60

Abstract Solid solutions of C 59 N azafullerene in C 60 with concentrations of 10 −5 to 10 −4 were produced in large quantities in an electric gas discharge tube. C 59 N is a stable monomeric substituent molecule in crystalline C 60 . The isotropic 14 N and 13 C hyperfine coupling constants measured by electron spin resonance (ESR) are characteristic of the extent of delocalization of the charge over the cage and are a sensitive test of electronic structure calculations. The C 59 N reorientational activation energy measured below the face centered cubic (fcc) to simple cubic (sc) transition is 2300 K. This value is similar to that of the matrix C 60 molecules, indicating that C 59 N–C 60 intermolecular interactions are weak.

Spin diffusion and magnetic eigenoscillations confined to single molecular layers in the organic conductors κ-(BEDT-TTF)2Cu[N(CN)2]X (X=Cl,Br)

The layered organic compounds, kappa-(BEDT-TTF)2Cu[N(CN)2]X X=Cl, Br) are metals at ambient temperatures. At low temperatures, the Cl compound is a weakly ferromagnetic Mott insulator while the isostructural Br compound is a superconductor. We find by conduction electron spin resonance and antiferromagnetic resonance (AFMR) an extreme anisotropy of spin transport and magnetic interactions in these materials. In the metallic state spin diffusion is confined to single molecular layers within the spin lifetime of 10(-9) s. Electrons diffuse several hundreds of nm without interlayer hopping. In the magnetically ordered insulating phase of the Cl compound we observe and calculate the four AFMR modes of the weakly coupled single molecular layers. The interplane exchange field is comparable or less than the typically 1 mT dipolar field and almost 10(6) times less than the intralayer exchange field.

Pressure and temperature dependence of interlayer spin diffusion and electrical conductivity in the layered organic conductors kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]X (X = Cl, Br)

A high frequency (111.2-420 GHz) electron spin resonance study of the inter-layer (perpendicular) spin diffusion as a function of pressure and temperature is presented in the conducting phases of the layered organic compounds, {\kappa}-(BEDT-TTF)2-Cu[N(CN)2]X ({\kappa}-ET2-X), X=Cl or Br. The resolved ESR lines of adjacent layers at high temperatures and high frequencies allows for the determination of the inter-layer cross spin relaxation time, Tx and the intrinsic spin relaxation time, T2 of single layers. In the bad metal phase spin diffusion is two-dimensional, i.e. spins are not hopping to adjacent layers within T2. Tx is proportional to the perpendicular resistivity at least approximately, as predicted in models where spin and charge excitations are tied together. In {\kappa}-ET2-Cl, at zero pressure Tx increases as the bad metal-insulator transition is approached. On the other hand, Tx decreases as the normal metal and superconducting phases are approached with increasing pressure and/or decreasing temperature.

Encapsulating C59N azafullerenes inside single-wall carbon nanotubes

Filling of single-wall carbon nanotubes with C 59 N azafullerene derivatives is reported from toluene solvent at ambient temperature. The filling is characterized by high resolution transmission electron microscopy and Raman spectroscopy. The tube-azafullerene interaction is similar to the tube-C 60 interaction. The encapsulated C 59 N monomer radical is observed using electron spin resonance spectroscopy after vacuum annealing of the azafullerene derivatives.

Production of C59N:C60 solid solution

We describe a simple way to produce large quantities of solid solutions of monomer C59N in pure C60 using an electric gas discharge tube. Typical concentrations are 10−5 to 10−4 C59N with respect to C60. The 14N and several 13C hyperfine constants were measured by ESR. These are a sensitive test for electronic structure calculations of the monomer. As the temperature is raised towards the sc to fcc structural transition at 261 K, the ESR spectrum motionally narrows and the activation energy for reorientation is measured. The rotational dynamics of the C59N monomer between 130 and 600 K parallels that of C60 in the bulk thus interactions between C59N and C60 are surprisingly weak.

Electron spin resonance of N@C606− in the fulleride salt Rb6C60

The use of N@C60 as a spin probe is demonstrated in the ionic fulleride salt Rb6C60. The salt contains a few ppm of N@C60 and was synthesised using a low temperature reaction. The electron spin resonance of N@C606− of this compound at 225 GHz (g=2 at 8.1 T) shows a small diamagnetic shift of 88±15 ppm with respect to pure N@C60.