Magnetic correlations influence the energy of conduction electrons through the exchange coupling. A magnetization leads to a splitting of the band where, however, the states of the higher band are unstable against spin-flip with spin-wave production. The rather unknown ion-position dependence of the spin Hamiltonian couples the spins to the phonons. For weak spin-spin couplings an adiabatic principle is assumed. The effect of spin order on the phonon frequency, on the infrared and Raman spectra, and the isotope effect of magnetic couplings is discussed.
For the coupling of two ferromagnets separated by a nonmagnetic layer, the Ruderman–Kittel [C. Kittel, Quantum Theory of Solids (Wiley, New York, 1963)] theory gives an expression which oscillates as a function of the thickness of the laser and asymptotically decreases as the reciprocal square of the thickness. The prefactor is independent of the electron density.
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