Results of Mode Coupling Theory for the Paramagnetic and Critical Spin Fluctuations in Heisenberg Magnets
The progress in the understanding of the general features of static and dynamic critical phenomena, allowed by the use of renormalization group methods [1–2], raised in recent years the interest in experiments on the dynamics of spin fluctuations in simple paramagnets. The interpretation of the experimental data of magnetic neutron scattering on simple ferromagnets, however, revealed to be a tough challenge to available theoretical methods. In fact the performed experiments gave us not only the proof of the validity of the predictions of renormalization group concerning dynamic scaling and associated critical exponents , but provided us also with new more detailed informations on the shape of the magnetic response function which cannot be interpreted by renormalization group theory. In fact the latter gives at most an asymptotic form for the scaling function, but does not provide a means of calculation. The magnetic correlation functions can be obtained only within other approximation schemes, which can be divided basically in two classes: mode-coupling theories  and approximations developed starting from the consistency with low order frequency sum-rules . The latter approaches give parametrized functional expressions for the response function which revealed very useful to fit experimental data when convoluted with instrumental resolution function, but they are not consistent with dynamic scaling.
KeywordsSpin Fluctuation Mode Coupling Theory Renormalization Group Theory Heisenberg Ferromagnet Europium Oxide
Unable to display preview. Download preview PDF.
- S.W. Lovesey, Condensed Matter Physic: Dynamic Correlations, Frontiers in Physics vol. 61, Benjamin Cummings (1986)Google Scholar
- S.W. Lovesey and R.A. Meserve, Jour. Phys. C 6, 79 (1972)Google Scholar
- F. Mezei, Physica 136B, 417 (1986)Google Scholar
- H.G. Bohn, A. Kollmar and W. Zinn, Phys. Rev. B 11, 6504 (1984)Google Scholar