Es wird untersucht, warm sieh in der Quantenstatistik die freie Energie otme Kenntnis der stationiiren Zust~nde des Systems bereetmen ]~lSt. 2r den hierbei entwickelten Methoden wird die diamagnetische Suszeptibilit~t "con freien Elektronen, ihre Beeinflussung dureh die ZusammenstSBe und alas magnetisehe Verhalten gebundener Elektronen untersueht. Es wird festges~ellt, bis zu welchen Feldst~rken die Suszeptibiliti~t feldunabhiingig ist.

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Ising discussed the following model of a ferromagnetic body: Assume N elementary magnets of moment μ to be arranged in a regular lattice; each of them is supposed to have only two possible orientations, which we call positive and negative. Assume further that there is an interaction energy U for each pair of neighbouring magnets of opposite direction. Further, there is an external magnetic field of magnitude H such as to produce an additional energy of − μH (+ μH) for each magnet with positive (negative) direction.

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