Nonlinear Field-dependent Susceptibilities of C60 and Carbon Nanotubes

Abstract

We have found strongly nonlinear, field-dependent magnetic susceptibilities in a range of carbon-based materials such as C60, carbon nanotubes, diamond and graphite. This nonlinear behaviour is prominent at low fields and it vanishes at relatively high fields. We have interpreted this phenomenon in terms of the field dependence of the spectral energy separations of the van Vleck paramagnetic term of the susceptibility. Our results should have significant bearing on the interpretation of the magnetic properties of this class of materials.

EPR and magnetism of the nanostructured natural carbonaceous material shungite

“…Formally speaking, the nonzero value of v van Vleck should be expected for all low symmetry compounds due to a mixing of the excited states with the electronic ground state by a magnetic dipole moment (van Vleck 1927;Haddon 1995), if the separation of these states is not too large. The van Vleck paramagnetism was shown to cancel almost completely the large diamagnetism in fullerenes (Elser and Haddon 1987;Ruoff et al 1991;Ling et al 1999) and to suppress diamagnetism in carbon nanohorns (Bandow et al 2001). Its possible occurrence in carbon nanotubes was discussed by Ling et al (1999).…”

“…The van Vleck paramagnetism was shown to cancel almost completely the large diamagnetism in fullerenes (Elser and Haddon 1987;Ruoff et al 1991;Ling et al 1999) and to suppress diamagnetism in carbon nanohorns (Bandow et al 2001). Its possible occurrence in carbon nanotubes was discussed by Ling et al (1999).…”

“…Additional information allowing a judgment about the existence of van Vleck paramagnetism can be drawn from the field dependence of the magnetic susceptibility (Elser and Haddon 1987;Ling et al 1999), which was shown to vary with the magnetic field B according to the expression:…”

Unconventional Magnetism in Carbon Based Materials

“…Thus, it is possible that the true value of χ || is several orders of magnitude smaller or even has the opposite sign [9]. Some of the authors point to the nonlinear, field-dependent magnetic susceptibilities in a range of carbon-based materials such as C 60 , carbon nanotubes, diamond and graphite This nonlinear behaviour is prominent at low fields and it vanishes at relatively high fields [8,10,11]. The low-field dependence of the moment is paramagnetic, as shown in the Fig.…”

“…The origin of the low-field paramagnetic contribution is still unknown, and it cannot be due to the misalignment, since the c-axis susceptibility is strongly diamagnetic.. This behavior was attributed to the field dependence of the spectral energy separations of the van Vleck paramagnetic term of the susceptibility [10], to the Aharonov-Bohm effect (in the case of nanotubes) [11] or to the presence of impurities [8] In view of recent findings [9], we consider this behaviour as the intrinsic property of this class of materials. Understanding of the non-linear behaviour in low magnetic fields should have significant bearing on the interpretation of the magnetic properties of this class of materials.…”