Abstract: Mean-field description of spin glasses: Comparison of theory and experiment

Abstract

The results of mean-field models 1–3 of the spin-glass state are reviewed with emphasis on the temperature and magnetic field dependence of the susceptibility and the nonlinear magnetization near the spin-glass temperature Tg. These models fail below the temperature defined by De Almeida and Thouless4 where the projection hypothesis3 is made. In order to test this overall description, a good knowledge of the equilibrium magnetizaton of the spin glass is needed. We present our results for the field cooled magnetization of a Ag-Mn alloy (10.6 at. % Mn) from which a tentative H-T phase diagram is drawn for the existence of the spin-glass phase.5 Measurements of the nonlinear component of the magnetization near Tg allow us to define the critical exponents γ and δ for the divergence of the field-induced order parameter above Tg. We find5 γ = 1.50±0.5 and δ∼2, to be compared with the values of 1 and 2, respectively, from the mean-field model.2 These results are compared to other determinations6–8 on similar systems from which a scaling function for the nonlinear magnetization is determined. There is no agreement at present between these experimental results. 1D. Sherrington and S. Kirkpatrick, Phys. Rev. Lett. 32, 1792 (1975). 2M. Suzuki, Prog. Theor. Phys. 58, 1151 (1977). 3G. Parisi and G. Toulouse, J. Phys. Lett. 41, 361 (1980). 4J. R. L. De Almedia and D. J. Thouless, J. Phys. A 11, 983 (1978). 5H. Bouchiat, thèse de 3e cycles, Orsay, 1981 (unpublished); P. Munod and H. Bouchait, Disordered Systems and Localized Conference Proceedings, Rome, 1981 (in press) and J. Phys. Lett. (in press). 6B. Barbara, A. P. Malozemoff, and Y. Imry, Phys. Rev. Lett. (unpublished). 7A. Berton, J. Chaussy, J. Odin, R. Rummal, and R. Tournier (unpublished). 8Y. Miyako, S. Chikazawa, T. Saito, and Y. G. Yochunas, J. Appl. Phys. 52, 1779 (1981).