Physical analysis of the through-ligand long-distance magnetic coupling: spin-polarization versus Anderson mechanism

Abstract

Physical analysis of the through-ligand long-distance magnetic coupling: spin-polarization versus Anderson mechanismAuthors: T. Terencio (a) , R. Bastardis (b) , N. Suaud (a) , D. Maynau (a) , J. Bonvoisin (c) , J.P.Malrieu (a) , C. J. Calzado (d) and N. Guihéry (a) Affiliations:(a AbstractThe physical factors governing the magnetic coupling between two magnetic sites are analyzed and quantified as functions of the length of the bridging conjugated ligand. Using wave-function-theory based ab initio calculations, it has been possible to separate and calculate the various contributions to the magnetic coupling, i.e. the direct exchange, the spin polarization and the kinetic exchange. It is shown on model systems that while the Anderson mechanism brings the leading contribution for short-length ligands, the spin polarization dominates the through-long-ligand couplings. Since the spin polarization decreases more slowly than the kinetic exchange, highly spin polarizable bridging ligand would generate a good magneto-communication between interacting magnetic units. 2 I IntroductionThe electronic communication between two magnetic sites connected through bridges has been a topic of large interest in molecular electronics [1][2][3][4] and spintronics [5,6]. Much effort from experimentalists has been invested in the elucidation of the role of extended bridges between two interacting units, both concerning the electron transfer in mixed valence compounds (electro-communication) and the magnetic coupling of unpaired electrons in pure magnetic compounds (magneto-communication).From a theoretical point of view, several works reviewed in references [7,8] Furthermore, the separation of both mechanisms enables us to quantify all effective interactions involved in the magnetic coupling, i.e. direct exchange, spin polarization, hopping integral t and on-site electron repulsion U.In section II, the various physical factors governing the magnetic interaction are briefly summarized and the methodology required to accurately reproduce them is recalled. Section III is devoted to an ab initio study of the different contributions to the magnetic coupling as

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“…However, in the anti and syn series presented here, this contribution takes negative values. A more complete theoretical treatment, according to the double‐spin‐polarization mechanism issued from the alternative valence bond model of the EE interaction, replaces the original ferromagnetic spin delocalization contribution given by the direct exchange integral ( K >0) by an effective one ( K eff ) that includes an additional antiferromagnetic spin polarization contribution that would be predominant in these two series 23. Interestingly, the absolute value of the effective exchange integral is smaller for the syn series than for the anti one, which suggests that the spin polarization effects are less important for a syn disposition of the two facing anthraquinone spacers in which the intramolecular π–π stacking (“through‐space”) interactions are maximized.…”

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“…An important example of electronic communication are mixed‐valence compounds, where adding more subunits to the bridge usually leads to an exponential decay of the electronic coupling, , where β is a decay constant and d the distance between the redox moieties A and B . A similar exponential decay has also been observed or predicted theoretically for electron transfer in donor–bridge–acceptor complexes, for electron transport through molecular wires connected to metal electrodes, and for Heisenberg exchange coupling in dinuclear transition metal complexes and in organic diradicals . Although it is a frequently observed behavior, it should be noted that exponential decay with increasing bridge length is not universal; first, the decay parameter is not only a bridge parameter as it depends on the type of property studied, and, for a given property, on the electronic structure of the subunits linked by the bridge .…”

“…In the conjugated bridges studied here, the ferromagnetic contribution to exchange coupling resulting from the magnetic orbitals' exchange integral is usually negligible. Again, this simple model may not be applicable in certain cases, as, for example, spin polarization is neglected, which becomes relevant if doubly occupied or virtual orbitals come close in energy to the singly occupied ones, and which for long bridges may be as important as the orbital splitting and exchange contributions …”