Influence of side groups on 90° superexchange: A modification of the Goodenough-Kanamori-Anderson rules
Wiebe Geertsma and D. Khomskii*
Solid State Physics Laboratory, Groningen University, Nijenborgh 4, 9747 AG Groningen, The Netherlands
~Received 29 November 1995!
Abstract:
A mechanism is suggested which can modify in certain cases the Goodenough-Kanamori-Anderson rules determining the character of superexchange in magnetic insulators; namely, side groups coupled to ligands, which are often present but are usually ignored, may contribute significantly, and may in certain cases even lead to a change in sign of the superexchange interaction. Thus this factor can make the 90° superexchange of half-filled shells antiferromagnetic, in contrast to the usual case. Qualitative arguments and numerical estimates show that this mechanism may be important in the inorganic spin-Peierls compound CuGeO3 .
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http://journals.aps.org.sci-hub.cc/prb/abstract/10.1103/PhysRevB.54.3011
As is well known, the exchange interaction in magnetic insulators is predominantly caused by the so-called superexchange — which is due to the overlap of the localized orbitals of the magnetic electrons with those of intermediate ligands. There exist different processes contributing to the superexchange interaction which appear under various names in the various calculational schemes: delocalization superexchange, potential exchange, indirect exchange, kinetic exchange, correlation exchange, ring exchange, etc. ~see, e.g.,1–4 and below!. Nevertheless, usually most of these partial processes give results which follow the so-called Goodenough-Kanamori-Anderson ~GKA! rules. According to these rules for instance, a 180° superexchange ~the magnetic ion–ligand–magnetic ion angle is 180°! of two magnetic ions with partially filled d shells is strongly antiferromagnetic, whereas a 90° superexchange is ferromagnetic and much weaker.
Recently, several materials were studied in which these rules seem to be violated, or at least their validity is questionable. One of such systems which now attracts considerable attention is CuGeO3—the first inorganic compound showing a spin-Peierls transition.5
As is well known, the exchange interaction in magnetic insulators is predominantly caused by the so-called superexchange — which is due to the overlap of the localized orbitals of the magnetic electrons with those of intermediate ligands. There exist different processes contributing to the superexchange interaction which appear under various names in the various calculational schemes: delocalization superexchange, potential exchange, indirect exchange, kinetic exchange, correlation exchange, ring exchange, etc. ~see, e.g.,1–4 and below!. Nevertheless, usually most of these partial processes give results which follow the so-called Goodenough-Kanamori-Anderson ~GKA! rules. According to these rules for instance, a 180° superexchange ~the magnetic ion–ligand–magnetic ion angle is 180°! of two magnetic ions with partially filled d shells is strongly antiferromagnetic, whereas a 90° superexchange is ferromagnetic and much weaker.
Recently, several materials were studied in which these rules seem to be violated, or at least their validity is questionable. One of such systems which now attracts considerable attention is CuGeO3—the first inorganic compound showing a spin-Peierls transition.5
Hello, my name is Dr Abderrahmane Reggad. I'm a 51 year old and I am a researcher in materials' sciences.
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