MICROWAVE MAGNETIC-RESONANCE IN THE ONE-DIMENSIONAL FERROMAGNET FETAC

Magnetic resonance spectra at 12-36 GHz are reported for Fe2+(3d6, S=2 ) in the 1D Ising ferromagnet [(CH3) 3NH]FeCl3 . 2H2O, known as FeTAC, between 2.5 and 13 K. The main signal was identified as a spin-cluste r resonance (SCR) because of its characteristic temperature dependence below 6 K and the linear dependence of the resonance field on the mic rowave frequency for H parallel to the easy axis at 4.2 K. The tempera ture dependence of the line intensity was fitted to the function exp[- (2J+hnu)/k(B)T], with 2J/k(B)=(41 +/- 1) K, and the linear field depen dence was explained by the value g(z)=8.26+/-0.05 in the S=1/2 represe ntation and a demagnetization field of several hundred gauss, which de pended on the sample shape. A frequency-dependent satellite structure observed on the low field side of the main line, which appeared as a b road line below 15 GHz, is tentatively associated with energy shifts o f the lower-lying spin-cluster states produced by the magnetic dipole interaction and other terms in the Hamiltonian.