Microwave Conductivity of the Soft Ferromagnet (TDAE)-C60

Fullerenes and carbon derivatives: From insulators to superconductors

“…The magnetic susceptibility of the fullerene charge-transf er (CT) complex TDAE± C60, shown in ® gure 58, exhibits a strong increase below 16 K [187,189]. This was claimed to be the sign that TDAE± C60 undergoes a`soft' ferromagnetic spin-glass'-or`superparamagnetic'-type ordering below Tc, where the magnetization against magnetic ® eld curves becomes S shaped and strongly nonlinear [187].…”

“…Typical ac susceptibility curve in TDAE± C60 at 10 kHz ac ® eld frequency in arbitrary units. The ferromagnetic ground state below 16 K clearly emerges [189].…”

Solid State and Magnetic Properties of Pure, Intercalated and Superconducting Fullerenes

“…Moreover, the strong narrowing of the ESR line with decreasing temperature together with the observation of relatively high ambient temperature conductivity on a pressed pallet (10 −2 Ω cm −1 ) have led to the speculation that TDAE-C 60 might be metallic and therefore an itinerant ferromagnet [93]. Very recent measurements [96,113] of the microwave conductivity and optical experiments indicate a clear nonmetallic temperature dependence of the conductivity with a value of the order of 10 −4 Ω cm −1 at 300 K. These authors placed TDAE-C 60 into the class of semiconducting systems without long-range structural order, where electron localisation eventually leads to a hopping process for electrical transport [96,113]. Fig.…”

“…In the 1:1 stoichiometry each TDAE molecule donates one electron to the band formed by the lowest unoccupied t 1u molecular orbit (LUMO) of the C 60 units. The crystal is monoclinic [95], with a shorter C 60 -C 60 distance along the c axis, and displays 'activated' DC transport properties characteristic of large gap semiconductors [96,97], or rather a Mott-Hubbard insulator [98].…”

Magnetic resonance of the weak organic magnet TDAE-C60

“…X-ray measurements on powdered TDAE-C 60 showed that TDAE-C 60 possesses a c-centered monoclinic unit cell and suggested the possible existence of an anisotropic lowdimensional band structure with metallic quasi I-dimensional C 60 chains along the c-axis. Microwave conductivity [5] and optical conductivity [6] measurements however showed that TDAE-C 60 is a semiconductor. l3C NMR measurements [7,8] and ESR g-values [9][10][11][12][13] seem to show that the unpaired electron is mainly localized at the C 60 ion as the g-value is close to the one observed for the electrochemically prepared C 60 ion.…”