Mechanism of ferromagnetism in TDAE-C60: ESR study under pressure

The low-frequency electron spin resonance (ESR) under hydrostatic pressure has been studied in the soft-ferromagnet TDAE-C-60, where TDAE is tetrakis-dimethylamino-ethylene. A demagnetising field causes ESR line shift proportional to the spontaneous magnetisation. The pressure dependence of T-C deduced from the ESR line shift, is found to follow a relationship of T-C(P) approximate to T-C(0)[1 - (P/PC)(2)], where T-C approximate to 16 K and P-C approximate to 9 kbar. The antiferro- orbital-ordering model of Jahn-Teller distorted C, is investigated as a possible mechanism for the ferromagnetism, With the realistic magnitudes of parameters a prediction of this model is in good quantitative agreement with the data. The pressure above 10 kbar promotes an irreversible phase change ; a reduced lattice constant along c-axis, a reduced ESR line width, an increased number of spins and an increased g-shift. From the c-lattice constant of 9.17 +/-0.07 Angstrom we concluded that a new beta -phase induced by pressure is a [2+2] cycloadditive 1D polymer phase, where the spin revives on TDAE(+), missing in the ferromagnetic alpha -phase.