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Journal of Physics C Solid State Physics

Dynamics of a random ferromagnet with long-range interactions

M. A. Contínentino, N. Rivier

Published Sep 28, 1977• 10 citations
Publisher

Abstract

The dynamics of a random ferromagnet with infinite-range exchange interactions and Gaussian distribution is studied. The Green function for the propagation of magnetic excitations is obtained in an exponential form which permits the use of techniques associated with problems involving random matrices. The magnetic properties of this system are obtained and compared with the ordered case.

DOI
10.1088/0022-3719/10/18/025

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The dynamics of a random ferromagnet with infinite-range exchange interactions and Gaussian distribution is studied. The Green function for the propagation of magnetic excitations is obtained in an exponential form which permits the use of techniques associated with problems involving random matrices. The magnetic properties of this system are obtained and compared with the ordered case.
In a previous paper [E. Milotti, Phys. Rev. E 63, 026116 (2001)]. I have shown how to derive both thermodynamical and dynamical properties of an infinite-range Ising spin system with binary ferromagnetic interactions from the master equation for magnetization obtained from a simple spin dynamics. The same method can be adapted to different spin interactions: here I discuss the case of antiferromagnetic interactions. This model permits a study of the static properties of the antiferromagnetic lattice, and it displays very clearly the differences between the antiferromagnetic and the ferromagnetic case with long-range interactions. The dynamical behavior of the antiferromagnetic system is simpler, and the magnetization always relaxes exponentially.
The one-dimensional Ising model in an external magnetic field with uniform long-range interactions and random short-range interactions satisfying bimodal annealed distributions is studied. This generalizes the random model discussed by Paladin et al. (J. Phys. I France 4, 1994, p. 1597). Exact results are obtained for the thermodynamic functions at arbitrary temperatures, and special attention is given to the induced and spontaneous magnetization. At low temperatures the system can exist in a “ferrimagnetic” phase with magnetization 0 < σ < 1, in addition to the usual paramagnetic, ferromagnetic and antiferromagnetic phases. For a fixed distribution of the random variables the system presents up to three tricritical points for different intensities of the long-range interactions. Field-temperature diagrams can present up to four critical points. 1
Within a random phase approximation,the quantum Heisenberg ferromagnetic chain with long-range interaction proportional to r-p was studied by Green-function method.The results indicated that for exchange interaction decaying as r-p for 1p2,there existed a phase transition at finite temperature,and that no transition to a ferromagnetic phase at any finite temperature occurs for p≥2.The results are reasonable agreement with those of the Monte Carlo calculations and the other theoretical approaches.
The thermodynamic properties of a classical spin-SSS Heisenberg ferromagnetic chain with long-range interactions in the presence of an external magnetic field are investigated by means of the spectral density method within the framework of the two-time Green's functions in classical statistical mechanics. For exchange interactions decaying as ${r}^{\ensuremath{-}\ensuremath{\alpha}},$ it is shown that long-range order exists at finite temperature for $1<\ensuremath{\alpha}<2$ and that no transition to a ferromagnetic phase at any finite temperature occurs for $\ensuremath{\alpha}>~2,$ consistently with a recent extension of the Mermin-Wagner theorem to spin models with long-range interactions. Besides, by means of a suitable modification of the lowest-order moment equations for the spectral density, the critical temperature and the main critical exponents for $1<\ensuremath{\alpha}<2$ and the low-temperature behavior of the paramagnetic susceptibility for $\ensuremath{\alpha}>~2$ are also obtained. Our results appear in a reasonable agreement with the available exact ones and Monte Carlo calculations.
A Green-function method is employed within a random phase approximation to study the one-dimensional quantum Heisenberg model with long-range ferromagnetic interaction proportional to ${r}^{\ensuremath{-}p}.$ We have shown that for 1<p<21<p<21<p<2 there exists a phase transition at finite temperatures and estimate its critical temperature.