Elementary excitations in antiferromagnetic Heisenberg spin segments.

We report on ac susceptibility, low-temperature magnetization, and specific heat measurements on molecularcompounds, shortly named Cr62, Cr72, Cr8Cd, and NiCr62Zn, that comprise different variants of spinarrays. These systems constitute real examples of collections of identical antiferromagnetic Heisenberg spinsegments. We indeed show that this picture, with dominant exchange term in the spin Hamiltonian J/kBranging from 13 to 16 K in all compounds and weak anisotropy term, fits well the measured physical properties.The character of energy spectra and the low-lying magnetic excitations are discussed accordingly. Thedirect comparison of experimental results and of the energy spectra of these variants with those of similarcyclic spin systems evidences effects associated with i the breaking the cyclic boundary conditions and iiodd and even nuclearities of spin segments.