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Abstract
Magnetic susceptibility and magnetic excitation of antiferromagnetic and ferromagnetic (AF-F) alternating chains are studied by the pair dynamical correlated-effective-field approximation. The static magnetic susceptibility x(T) vanishes at T = 0 K reflecting the existence of a gap between the singlet ground state and the excited states. At finite temperatures x(T) shows a broad maximum around the temperature corresponding to the exchange coupling. The calculated x(T) agrees fairly well with the results of exact diagonalization method for finite spins. The magnetic excitation exhibits a minimum value at q = ± π/2. This makes a striking contrast with the AF alternating chains in which the magnetic excitation has a minimum at q= 0 and q = π. The magnetic susceptibility of real S=½ organic AF-F chains, Cu(TIM)CuCl4 and [Cu(bpym)(OH)2]-2H2O, are analyzed by our method and inelastic neutron scattering spectra for these systems are calculated.