Probing edge magnetization in antiferromagnetic spin segments

Antiferromagnetic spin segments are quantum objects with discrete spectrum of excitations. Here we report the characterization of two molecular derivatives, namely, Cr6 InNi and Cr7 InNi, which are odd- and even-membered spin cyclic systems magnetically broken by an In3+ ion. Analysis of low-temperature specific heat, susceptibility, and magnetization allows us to determine the microscopic parameters of the spin Hamiltonian and the pattern of the spin excitations. It turns out that the exchange coupling and the anisotropy constants in Cr6 InNi and Cr7 InNi are quite close to those in Cr7 Ni-piv. X-ray absorption experiments evidence that the oxidation state of Cr3+ and Ni2+ and their local symmetries are essentially the same in the Cr6 InNi, Cr7 InNi, and Cr7 Ni-piv clusters. Thus the main difference resides in the number and topology of spins in the different derivatives. Based on these grounds, we show how the spin of Ni2+ can be used as a marker to probe the local magnetization of spin segments by directly comparing the sign and the intensity of the dichroic signal measured in the three derivatives. Quantitative analysis of x-ray absorption spectra allows the evaluation of the spin and orbital magnetic moment in the different derivatives, whose dependence on the magnetic field fits well the results of the microscopic spin-Hamiltonian calculations. © 2009 The American Physical Society