Spin Waves in an Amorphous Heisenberg Ferromagnet
Theoretical investigations of the ferromagnetic properties of noncrystalline solids have taken two directions - mean-field theories and Green-function calculations. The Green-function calculations1–3 have been applied only to disordered alloys or, at least, to systems in which a Bravais lattice exists; on the other hand, only mean-field theories have been applied to truly amorphous systems. As an approach to investigate the properties of an amorphous ferromagnet, the mean-field calculations4–7 have been deficient in several respects. They have not taken into proper consideration the structure of the amorphous material. Furthermore, although experiments have indicated that structural order reduces the spontaneous magnetization σ and Curie temperature Tc of the amorphous material below that of the corresponding crystalline problem, some mean-field calculations predict an increase in σ and Tc. We now present a Green-function calculation of the spectrum of an amorphous Heisenberg Ferromagnet which takes into consideration the characteristic structure of an amorphous material and which also predicts a reduction of σ and Tc.
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