Critical fluctuations in the weak itinerant ferromagnetNi3Al:A comparison between self-consistent renormalization and mode-mode coupling theory

The magnetic excitations in the weak itinerant ferromagnet ${\mathrm{Ni}}_{3}\mathrm{Al}$ have been investigated in the ordered phase as well as in the paramagnetic phase by means of inelastic neutron scattering, in order to search for critical fluctuations. The magnetic excitations have been interpreted within the framework of both itinerant and mode-mode coupling theory (MMT) for an isotropic ferromagnet. The parametrization of the linewidth for the critical scattering shows that the dynamical critical exponent $z\ensuremath{\simeq}2.47\ifmmode\pm\else\textpm\fi{}0.16$ is close to the value expected for an isotropic Heisenberg model, proving that critical fluctuations are also important in weak-itinerant ferromagnets. Moreover, the temperature dependence of the spin waves compares well with the predictions of MMT. The correlation length in weak itinerant magnets is very large leading to pronounced short-range order and to critical fluctuations far away from ${T}_{C}.$ The results show that it is necessary to include critical fluctuations in the theory for itinerant magnetism.