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Temperature dependence of the exchange splitting in ferromagnetic metals Lonzarich, Gilbert G.

Abstract

The temperature dependence of the exchange splitting of the energy bands in ferromagnetic iron has been studied at low temperatures by carefully examining the de Haas-van Alphen frequency associated with the minority-spin electron 'lens' sheets of the Fermi surface as a function of temperature between 1° and 4°K. A high resolution phase measurement technique revealed that the variation of the lens frequency over this temperature range was less than one part in 10⁵ and was virtually identical to that measured for the corresponding electron lens in molybdenum. By contrast, a variation of one part in 10⁴ would be expected for the iron lens on the basis of a literal interpretation of the Stoner model, in which the exchange splitting of the energy bands is proportional to the magnetization at all temperatures. The absence of any significant change of the frequency with temperature gives strong evidence that the magnetization in iron decreases almost entirely by spin-wave excitations and that spin waves have negligible effect on the exchange splitting. The latter conclusion is consistent with a recent theory by Edwards for the electron-magnon interaction in itinerant-electron ferromagnets. On the basis of Edwards' theory the experimental technique described in this dissertation can be used to systematically study the single-particle magnetization in metallic ferromagnets without any interference from spin-wave excitations. In particular our experimental results yield an upper bound for the single-particle magnetization in iron at low temperatures. A new set of low-frequency de Haas-van Alphen oscillations has been studied in iron and the experimental results obtained so far suggest the oscillations originate from very small ellipsoidal surfaces centered on the points N of the Brillouin zone. This result points to a particular ordering of the energy bands at N, a feature of the band structure that cannot be predicted reliably from first principles.

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