In the Weiss phenomenological theory of ferromagnetism, each magnetic atom experiences the action of an effective field Hef, while in the Heisenberg-Frenkel quantum theory, the interaction energy of atoms is expressed by the relation Uext = -2JSiSj (SiSj are the spins of interacting atoms). Taking into account the interaction of an atom only with n nearest neighbors and considering it identical with them, find the connection between the phenomenological Weiss constant and the exchange integral J. The volume V per one atom is considered given.

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