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Exchange modes in ferromagnetic superlattices.

The spin-wave spectrum of infinite, semi-infinite, and finite ferromagnetic superlattices with arbitrary elementary units is analyzed theoretically in the exchange-dominated region. The general dispersion equations for spin-wave modes are derived in the framework of the transfer-matrix formalism. Infinite structures with layered defects are also analyzed. Some numerical results are presented for structures with N=1, 2, and 6 atomic planes in the elementary unit. The considerations are restricted to simple cubic structures with the exchange coupling between nearest neighbors.