Green's-Function Theory of a Disordered Heisenberg Ferromagnet

Abstract

Magnetic properties are calculated for a spin one-half Heisenberg ferromagnet in which the exchange interactions deviate randomly in strength from the mean interaction,, Disorder produces a low-energy peak and a high-energy tail in the density of spin-wave states for a simple-cubic geometry. The ferromagnetic Curie temperature is shown to decrease linearly with disorder; the temperature-dependent magnetization is not otherwise markedly altered.

Chemical and structural disorder effects on the Curie temperature

“…Data referring to pure mechanical treatments (for example, milling), tend to favor the decrease of T c with increasing disorder [6]. This agrees, qualitatively, with the earliest theoretical results [7]. In random alloys, most of the measured data are concerned with the nonmonotonic behavior of T c , as a function of the average concentration of various magnetic or non-magnetic components [8].…”

“…The earliest theories predict no disorder effects on the T c of Ising-like systems [11], and a depression of T c in Heisenberg-like systems [7]. More recently, contrasting theoretical results have been reported for ferromagnetic Anderson-Hubbard models, some pointing to an increase of T c [12,13], others pointing to a decrease [14].…”

“…In this case the overall effect on T c turns out to be model-dependent. This is due to the existence of a negative contribution [7], that is argued to reflect the interplay between structural disorder and spin waves.…”

Magnetostrictive behaviour of Fe73.5Si13.5B9Nb3−xMoxCu1 alloys

“…Montgomery et al [19] demonstrated that T C strongly depends on the exchange interaction (J 0 ) and at the same time, this one depends on the distance between the Fe atoms based on the Bethe-Slater curve. As previously observed [20], amorphous T C increased when the material was relaxed as a result of the structural order induced by the thermal treatment in the amorphous material.…”

Effect of the addition of Ge to the FINEMET alloy

“…Using Green's function theory for a Heisemberg ferromagnet, Montgomery et al [18] also demonstrated that if the deviation of the average value of the exchange interaction is taken into consideration for the calculation of the T C , this value diminishes in a factor proportional to j 2 =J 2 0 , where J 0 is the exchange interaction for equally spaced particles and j is its dispersion:…”