Magnetic properties of defects in low-dimensional transition-metal complexes

Abstract

The occurrence of chemical or structural defects in low-dimensional materials is quite a general phenomenon and can have drastic consequences on magnetic and transport properties. In a 1D magnetic chain of S= 1/2 spins which are coupled by antiferromagnetic interaction, a single-vacancy, misoriented chain member or impurity substitution cuts the chain into two segments by decoupling the spins on either side of the ‘‘defect.’’ The finite chains with odd-length segments behave essentially like Curie-type magnets and lead to electron paramagnetic resonance (EPR) spectra and divergencies in magnetic susceptibility at low temperatures. In the series of compounds we have chosen to study, viz., R+ Ni(mnt)−2, the d7 S= 1/2 platelike anions stack like poker chips along a chain and have strong antiferromagnetic interactions which can be varied easily by choosing different R+ cations. These are very good examples of 1D systems and have the great advantage of possessing a tunable interaction. We present here the static and dynamic magnetic properties investigated by dc magnetic susceptibility, EPR, and spin lattice relaxation time measurements. We have established that the so-called ‘‘impurity’’ is a structural defect. From a detailed study of the defect we have been able to learn about the various cooperative magnetic interactions such as the strong interaction in the chains and the weak interaction between the chain and the defect.