THERMODYNAMICS OF ALTERNATING SPIN CHAINS WITH COMPETING NEAREST-NEIGHBOR AND NEXT-NEAREST-NEIGHBOR INTERACTIONS - ISING-MODEL

The thermodynamical properties of an alternating spin (Ss) one-dimensi onal (ID) Ising model with competing nearest- and next-nearest-neighbo r interactions are exactly calculated using a transfer-matrix techniqu e. In contrast to the case S = s = 1/2, previously investigated by Har ada, the alternation of different spins (S not-equal s) along the chai n is found to give rise to two-peaked static structure factors, signal ing the coexistence of different short-range-order configurations. The relevance of our calculations with regard to recent experimental data by Gatteschi et al. in quasi-1D molecular magnetic materials, R (hfac )3 NITEt (R = Gd, Th, Dy, Ho, Er, . . .), is discussed; hfac is hexafl uoro-acetylacetonate and NlTEt is ramethyl-4,5-dihydro-1H-imidazolyl-1 -oxyl-3-oxide.