Ferromagnetic Resonance in TDAE-C60

Abstract

A nonlinear variation of the electron resonance frequency with resonance field characteristic of ferromagnetic resonance has been observed in a TDAE-C 60 in the radio-frequency region. The results not only definitely prove the existence of long range order below T C but also show that well annealed TDAE-C 60 behaves as an easy axis three dimensional Heisenberg ferromagnet with an exceptionally small anisotropy field H K 29 G. The easy axis coincides with the crystal c direction which is the direction of closest approach of the C 60 2 ions. [S0031-9007 (98)05369-1] PACS numbers: 75.50.Dd, 76.50. + gThe magnetic properties of the charge transfer compound TDAE-C 60 , where TDAE stands for tetrakisdimethylamino-ethylene, represent one of the most challenging physical phenomena discovered in the field of fullerenes [1]. The origin of the magnetic behavior has been shown to be due to an unpaired electronic spin localized on the C 60 2 ion. 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 [1,6-8] to account for the magnetic ordering below T C . The basic question is whether we deal with a magnetic field induced ordering of superparamagnetic clusters or with an intrinsic long range ordered state, and if the latter is true, what is the nature of this state. Whereas detailed conductivity measurements [9,10] have ruled out itinerant ferromagnetism, no experiment allowing for a definite discrimination between the various possible models has been carried out so far.Electron and muon spin resonance in magnetically ordered phases represents one of the most direct ways to determine the microscopic nature of the magnetic ground state. In view of the presence of strong exchange fields electron spin resonance in ferromagnetic and antiferromagnetic systems, in particular, is completely different from ordinary paramagnetic spin resonance. 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 shift...

Macroscopic magnetic measurements on the nonferromagnetic modification of TDAE-C60

“…1-3 The usual modification ␣-TDAE-C 60 exhibits a ferromagnetic transition at 16 K. 4 The magnetic ground state of this modification can be described in terms of the Heisenberg model with a very low anisotropy. 5 A residual orientational disorder of nearly spherical C 60 molecules, which is always present in the real samples, gives this system some characteristics of a spin glass. [6][7][8][9] On the contrary, the newly discovered ␣Ј-TDAE-C 60 modification shows no ferromagnetic transition down to 1.7 K. 3 Interestingly, both modifications crystallize in the same C2/c space group with the same parameters of the unit crystal cell.…”

Spin-orbit effects in heavy-atom organic radical ferromagnets

“…27 This technique has been employed in the study of various organic materials including charge-transfer salts, 28,29 doped fullerenes, 30 and radicals. 31,32 In a typical single crystal experiment, microwave radiation is applied to a sample at a set frequency, and the energies of magnetic excitations are tuned using an external field H ext that can be varied in magnitude and direction with respect to the crystal.…”

BeltlikeC60−Electron Spin Density Distribution in the Organic Ferromagnet TDAE-C60<

“…Here we report on the first direct observation of a JahnTeller induced beltlike redistribution of the unpaired electron spin density on the C 2 60 ions in any fullerene exhibiting superconducting [7] or ferromagnetic [3,8,9] transitions. The observation of this effect in TDAE 1 -C 2 60 is made possible by the changes in the Fermi contact electron-nuclear coupling shifts of the 13 C NMR lines which accompany the Jahn-Teller distortion and act as a magnifying glass, making the minute Jahn-Teller distortions and the resulting redistribution of the unpaired electron density visible in the 13 C NMR spectra.…”