Abstract
A simple derivation of an effective Heisenberg Hamiltonian for itinerant ferromagnets is given and it is pointed out that the effective exchange integral Jeff(R) for a strong ferromagnet such as nickel decays exponentially with distance R, as in magnetic insulators. The standard Ruderman-Kittel-Kasuya-Yosida form of Jeff(R), typical of metals, is obtained only for weak ferromagnets such as iron.
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