High temperature magnetic behaviour of a manganese(II)binitroxide alternate ferrimagnetic linear chain

Abstract

Atheoretical approach of the magnetic behaviour of a new quasi-unidimensional metal nitroxide complex whose structure is an alternate linear chain of Mn(II) ions (S = 5/2) with free radical ligands (s = 1/2) is performed. The striking feature of this compound is that each Mn ion is interacting not only with two neighbour nitroxide radicals within the chain, but also with another pending radical outside the chain. For the high temperature region we develop a model of an infinite alternate linear chain of classical Mn spins and quantum nitroxide spins. It is shown that the appropriate S value for the classical spin is S = [S(S + 1)]^1/2 = (35/4)^1/2 instead of s = 5/2. Through the evaluation of the various correlation functions between the various spins, using linear response theory, it is possible to obtain an analytical expression of the magnetic susceptibility. This leads to the determination of the exchange integrals J ₁ = -49cm^-1 and J ₂ = -82cm^-1 for the Mn-nitroxide couplings within the chain and with the pendant nitroxide, respectively, and to a good fit with experiment for 50K < T < 400K.