STUDY OF THE CRITICAL DYNAMICS OF ISOTROPIC HEISENBERG FERROMAGNETS BY NEUTRON SCATTERING. S. Schorr F. Mezei,Hahn-Meitner-Institut Berlin, Glienicker Str.100, D-14109 Berlin, Germany.
The critical dynamic behavior in the isotropic Heisenberg ferromagnet EuO can be tested by inelastic neutron scattering. Below the ferromagnetic Curie temperature Tc we observed magnetic excitations (spin waves) and above this temperature the critical magnetic scattering, wich is reduced to the large spin fluctuations near the ferromagnetic phase transition, was measured. We can understand the critical dynamic behavior by taking into account the strong dipolar effects at wavenumbers q below ~0.1 Å-1. The dipolar interaction, i.e. the action of an internal magnetic field produced by critical fluctuations on the fluctuations themselves, plays a pronounced role, because it covers the dynamic scaling behavior in a certain well understood q-T-domain. We compared our results to recent mode coupling computional results [1], [2] taking into account both Heisenberg exchange and dipolar forces. Comparing various ferromagnetic materials such as EuO, EuS, Fe and Ni ([3], [4], [5]) there are big apparent differences between in the critical dynamics. We found that this differences can be explained if the dipolar perturbations are properly taken into account. Thus the principle of universality is maintained.
[1] E. Frey, H. Schwabl,Z. Phys. B 71 (1988) 355
[2] H. Schinz, H. Schwabl, to be published in J. Magn. Magn. Mat.
[3] P. Böni, to be published
[4] F. Mezei, J. Magn. Magn. Mat. 45 (1984) 67
[5] P. Böni et al., Phys. Rev. B 41 (1991) 575