Magnetic resonance of the weak organic magnet TDAE-C60

Abstract

The charge transfer compound TDAE-C 60 (TDAE = tetrakis-dimethylamino-ethylene) is so far the only known C 60 based molecular ferromagnet. The ESR spectra of TDAE-C 60 single crystals demonstrate that freshly grown regularly shaped single crystals show at first an AFM (antiferromagnetic)-like behavior which changes upon annealing at room temperature in vacuum to a FM (ferromagnetic) one. It thus seems that annealing induces a change in the relative orientation of the distorted C 6 0 ions resulting in a change in the sign of the effective magnetic interactions between neighboring C 6 0 ions. A structural transition in the TDAE methyl protons at 250 K seems to be connected with this process. Here we review the structure, the ESR spectra, the proton NMR spectra and the UC NMR spectra of TDAE-C 60 .

Ferromagnetic Resonance in TDAE-C60

“…A nonlinear variation of the resonance frequency with resonance field characteristic of ferromagnetic resonance was observed via low field ESR in the 30 MHz-1.2 GHz range. The obtained results provide completely new insights on the magnetic order in this system and a better understanding of high field ESR experiments performed previously [5,6,8,13]. For H k a a zero magnetic field gap of 105 MHz has been detected below T C as well as a dip in the Larmor frequency versus magnetic field relation at H 31 G. For H k c the zero field gap is much smaller and the Larmor frequency increases linearly with increasing magnetic field.…”

“…Whereas muon spin rotation data [12] in powdered TDAE-C 60 indicate the presence of intrinsic long range magnetic order below T C , X-band and Q-band electron spin resonance (ESR) measurements [5,6,8] performed so far show only a single paramagneticlike ESR line near g 2 which slightly shifts and broadens below T C . Only a paramagneticlike ESR line near g 2 has also been found in high field ESR at 94 and 245 GHz [13] between 0 and 10 T. Here, as well as in the X and Q bands, no sign of nonlinear ferromagnetic or antiferromagnetic resonance behavior, characteristic of magnetically long range ordered states, could be detected below T C .…”

“…It represents a coherent precession of the entire magnetization or sublattice magnetization around the effective fields and is in contrast to normal paramagnetic resonance characterized by a nonlinear dependence of the resonance frequency on the external field. The number of modes and the field dependence of the resonance frequencies allow for a discrimination between ferromagnets, antiferromagnets, or spin canted antiferromagnets [11].Whereas muon spin rotation data [12] in powdered TDAE-C 60 indicate the presence of intrinsic long range magnetic order below T C , X-band and Q-band electron spin resonance (ESR) measurements [5,6,8] performed so far show only a single paramagneticlike ESR line near g 2 which slightly shifts and broadens below T C . Only a paramagneticlike ESR line near g 2 has also been found in high field ESR at 94 and 245 GHz [13] between 0 and 10 T. Here, as well as in the X and Q bands, no sign of nonlinear ferromagnetic or antiferromagnetic resonance behavior, characteristic of magnetically long range ordered states, could be detected below T C .…”

“…The magnetic transition temperature T C 16 K is exceptionally high for a purely organic ferromagnet consisting only of light elements such as carbon, hydrogen, oxygen, and nitrogen [2]. While the origin of superconductivity in A 3 C 60 compounds ͑A K, Rb, Cs͒ seems to be by now well understood [3,4], the nature of the magnetic transition in TDAE-C 60 still remains controversial [5]. Conflicting models such as itinerant ferromagnetism, superparamagnetism, spin glass behavior, and spin canted weak ferromagnetism have been proposed [1,[6][7][8] to account for the magnetic ordering below T C .…”

Indirect exchange interaction in fully metal-semiconductor separated single-walled carbon nanotubes revealed by electron spin resonance

“…Fluctuations along with structural phase transitions are well known to broaden ESR lines. [26,27] This broadening often follows a power law of the form a/(T − T c ) m with m of the order of 1. The fit depicted in the insert of Fig.…”