Electric Field Controllable Magnetic Spin Communication in Partially Localized Mixed-Valence Molecules: A Tutorial Review

This article is a short overview of the problem of controllable magnetic spin communication as an important ingredient of spintronics and molecular electronics. We discuss the problem of communication of the two localized spins mediated by the mobile electron which is shared between two non-magnetic sites coupled to the mobile spins. The model system which is assumed to mimic the key features of spin communication has a tetrameric linear structure with two terminal spins connected via the central mixed-valence (MV) dimeric unit. We propose a “toy” model which is expected to describe the exchange coupling between the localized spins mediated by the itinerant electron in complex systems such as reduced polyoxometalates hosting metal ions. The proposed model is parametric and intentionally simplified in order to give a qualitative insight on a wide class of the systems bearing in mind the main physical phenomena and at the same time avoiding analysis of their specific details. The model takes into account the following interactions peculiar for complex systems: electron transfer in the MV moiety, magnetic exchange between the localized spins and the delocalized electrons, coupling of delocalized electron with molecular vibrations and interaction of the system with an external electric field. The aim of the model is to describe in a parametric way a series of compounds with partially delocalized electrons with minimal number of the fitting parameters that reflect the main physical features of complex systems. To make the conclusions imaginative we discuss the limiting cases in details. In the case of relatively strong exchange coupling the combined action of the named interactions is shown to give rise to a specific kind of double exchange coupling termed here “external core” double exchange. In the opposite case of relatively strong electron transfer we arrive to an effective indirect exchange. A possibility to control the coupling between the localized spins by the external electric field acting on the mobile electron is discussed. It is demonstrated that the effect of the electric field on the strength of spin communication is amplified by the vibronic coupling so that this coupling decreases the value of the field required for a tangible control of the strength of spin communication. In the presentation of the problem we sought to emphasize the main concepts in a qualitative way cleared of mathematical details addressing the review, that we referrer as tut