EXCHANGE TRANSFER IN HIGH-NUCLEARITY MIXED-VALENCE MAGNETIC CLUSTERS - THEORETICAL APPROACH AND EXPECTED MANIFESTATIONS

We report here a general solution of the exchange transfer problem in the high-nuclearity mixed valence clusters containing arbitrary number of itinerant electrons. The concept of two kinds of exchange transfer , namely kinetic and potential, is introduced by analogy with basic An derson's mechanisms of the magnetic exchange. The kinetic exchange tra nsfer is treated as a second order transfer process between two centre s through the excited state of a third centre. The potential exchange transfer is also considered as a three-centre interaction but in this case only the ground states of the constituent ions are involved. The actual parameters of the exchange transfer are expected to be of the s ame order of magnitude as those for the magnetic exchange. An efficien t computational approach is proposed making it possible to build up th e matrices of all kinds of exchange transfer interactions in a closed analytical form, expressing the matrix elements in terms of the releva nt spin quantum numbers. As distinguished from the magnetic exchange t he kinetic and potential contributions to the exchange transfer act in dependently and their parameters cannot be, in general, combined to gi ve a joint parameter of the exchange transfer. The magnetic manifestat ions of the exchange transfer are considered for selected trimeric sys tems with different topologies (linear and trigonal) and different ele ctronic shells involving either migrating electrons or holes. The magn etic nature of the ground state is found to depend on the topology of the system and on the sign of the double exchange parameter. (C) 1998 Elsevier Science B.V.