QUANTUM CRITICAL-BEHAVIOR OF CLEAN ITINERANT FERROMAGNETS

We consider the quantum ferromagnetic transition at zero temperature i n clean itinerant electron systems, We find that the Landau-Ginzburg-W ilson order parameter field theory breaks down since the electron-elec tron interaction leads to singular coupling constants in the Landau-Gi nzburg-Wilson functional, These couplings generate an effective long-r ange interaction between the spin or order parameter fluctuations of t he form 1/r(2d-1), With d the spa tial dimension. This leads to unusua l scaling behavior at the quantum critical point in 1 < d less than or equal to 3, which we determine exactly, We also discuss the quantum-t o-classical crossover at small but finite temperatures, which is chara cterized by the appearance of multiple temperature scales, A compariso n with recent results on disordered itinerant ferromagnets is given.