Exchange local moment

Superexchange describes interaction between localized moments of ions in insulators that are too far apart to interact by direct exchange. It operates through the intermediary of a nonmagnetic ion. Superexchange arises from the fact that localized-electron states as described by the formal valences are stabilized by an admixture of excited states involving electron transfer between the cation and the anion. A typical example is the 180° cation-anion-cation interaction in oxides of rocksalt structure, where antiparallel orientation of spins on neighbouring cations is favoured by covalent... 

Fig. 3.8 Left-hand panel The on-site atomic energy levels for up and down spin electrons due to the exchange splitting Im where / and m are the Stoner exchange integral and local moment respectively. Right-hand panel The local magnetic moment m, as a function of //2 / where / and h are the exchange and bond integrals respectively. Compare with the self-consistent LSDA solution in the upper panel of Fig. 3.6.

A simple and asymptotically correct [26-28] model is the Ruderman-Kittel-Kasuya-Yosida or RKKY exchange between two localized moments in a Pauli-paramagnetic matrix. For a free-electron gas of wave-vector kF,... 

It has been pointed out that any relationship between the exchange integral and the Weiss field is only valid at 0 K, since the former considers magnetic coupling in a pair-wise manner and the latter results from a mean-field theory (Goodenough, 1966). Finally, it is also essential to understand that Eq. 8.43 is strictly valid only for localized moments (in the context of the Heitler-London model). One might wonder then whether the Weiss model is applicable to the ferromagnetic metals, in which the electrons are in delocalized Bloch states, for example, Fe, Co, and Ni. This will be taken up later. 

It was shown in section 3 that the transport properties of magnetic R-non-transition metal compounds can be understood in terms of a direct exchange interaction between conduction electrons and R localized moments. This interaction leads to an indirect RKKY interaction between the localized moments which describes the magnetic properties of these compounds. 

A more positive result was obtained with one another quasi 1-D conductor, a copper phthalocyanine iodide [82], where it has been shown that the interactions between Cu " local moments arise from indirect exchange, involving the n charge carriers below 2K a spin glass state is apparently present. 

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