Critical fluctuations in the weak itinerant ferromagnet Ni3Al: A comparison between self-consistent renormalization and mode-mode coupling theory

The magnetic excitations in the weak itinerant ferromagnet Ni3Al have been investigated in the ordered phase as well as in the paramagnetic phase by m eans of inelastic neutron scattering, in order to search for critical fluct uations. The magnetic excitations have been interpreted within the framewor k of both itinerant and mode-mode coupling theory (MMT) for an isotropic fe rromagnet. The parametrization of the linewidth for the critical scattering shows that the dynamical critical exponent z similar or equal to 2.47 +/- 0.16 is close to the value expected for an isotropic Heisenberg model, prov ing that critical fluctuations are also important in weak-itinerant ferroma gnets. Moreover, the temperature dependence of the spin waves compares well with the predictions of MMT. The correlation length in weak itinerant magn ets is very large leading to pronounced short-range order and to critical f luctuations far away from Te. The results: show that it is necessary to inc lude critical fluctuations in the theory for itinerant magnetism.