Dynamics of itinerant ferromagnets above T C (invited)

A review is given of our current studies of the magnetic scattering, above TC, from Ni, Fe, and Fe (4%-Si). Reliable scattering functions S(Q,ω) are obtained up to 50 meV of energy transfer by combining polarized and unpolarized neutron techniques. Although constant E scans show magnon like peaks in agreement with the previous Oak Ridge data, no peaks are observed in our constant Q scans up to 0.5 Å−1, in contrast to the well defined peak reported by Lynn for Fe(12%-Si). Unexpectedly, for a wide temperature range, the scattering function S(Q,ω) takes a simple Lorentzian form S(Q,ω)∝(1/κ21+q2) ⋅ (Γ/Γ2+ ω2) , with Γ=Λq2, where Λ is the diffusion constant, and κ1 the inverse correlation range. At TC, where Γ=Aq2.5, this can be reduced to a universal scattering function q0.5/[q5+(ω/A)2]. This scaling gives an entirely new and unified interpretation of magnetic scattering from Ni, Fe, and all other isotropic cubic ferromagnets.