ALTERNATING CHAINS WITH FERROMAGNETIC AND ANTIFERROMAGNETIC INTERACTIONS - THEORY AND MAGNETIC-PROPERTIES

In this paper we focus on the thermodynamical properties of the S = 1/ 2 Heisenberg chain with alternating antiferromagnetic and ferromagneti c exchange interactions, J(1) and J(2) In the first step, the magnetic and specific heat properties of this system have been calculated as f unction of alpha = J(2)/\J(1)\ from a general numerical procedure base d on closed spin chains of increasing length. A distinctive behavior c haracterized by a maximum in the chi T vs T plot is predicted when the ferromagnetic interaction is the dominant one (i.e., for alpha > 1). Conversely, antiferromagnetic-like behaviors are predicted when the an tiferromagnetic interaction dominates (i.e., for alpha < 1). With resp ect to the magnetic specific heat, two rounded maxima can be distingui shed in the C-p/R vs T plot for sufficiently different exchange values and dominant ferromagnetic exchange, which have been associated with the two kinds of exchange interactions. The magnetic susceptibility re sults derived from the model have been conveniently fitted to rational unified expressions. These expressions were used to describe the magn etic behaviors of two copper(II) complexes exhibiting alternating chai n structures: The compound Cu(TIM)CuCl4 (TIM = o-1,4,8,11-tetraazatetr adecane-1,3,8,10-tetraene), which shows dominant ferromagnetic exchang e (J(1)/k = -7.5 K; alpha = 3), and the compound [Cu(bpym)(OH)(2)(NO3) (2)].2H(2)O (bpym = 2,2' bipyrimidine), which exhibits dominant antife rromagnetic exchange (J(1)/k = -200 K; alpha = 0.75).