Department of Physics Papers

Document Type

Journal Article

Date of this Version

9-1-1973

Publication Source

Physical Review B

Volume

8

Issue

5

Start Page

2166

Last Page

2184

DOI

10.1103/PhysRevB.8.2166

Abstract

The variational treatment of spin waves in the randomly diluted Heisenberg ferromagnet at zero temperature is studied. It is argued that a convenient variational principle for the excitation spectrum of an alloy is difficult to formulate rigorously. However, at extremely long wavelengths a simple variational calculatation probably has approximate validity providing localized excitations in isolated clusters of magnetic sites are excluded. Within the approximation which ignores this exclusion we obtain a second-order variational bound for the spin-wave energy which, unlike that found by Murray, is positive at all concentrations. Similar results for concentrations above the critical percolation concentration xp are obtained when the localized excitations are excluded in which case the restricted configuration averages can only be evaluated approximately. We point out that the critical concentration xc for the occurance of long-range order depends only on the properties of the infinite cluster. Thus the thermal stability of spin waves depends on the dimensionality of the infinite cluster. An argument is given to show that the infinite cluster is not one dimensional. The range of concentrations for which the infinite cluster is two dimensional is either nonexistent or small. We conclude, then, that xc is close, if not exactly equal, to xp. The condition for a discontinuity in TC(x) for x→xc is discussed in terms of a simple periodic model for dilution.

Comments

At the time of publication, author A. Brooks Harris was affiliated with Oxford University. Currently, he is a faculty member in the Department of Physics at the University of Pennsylvania.

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Date Posted: 12 August 2015

This document has been peer reviewed.