Magnetic Response function of the Itinerant Ferromagnet CeFe2.

Neutron inelastic scattering experiments on single crystals of the itinerant ferromagnet CeFe2 show that there is a strong competition between the ferromagnetic ground state and an antiferromagnetic (AF) ground state with the wavevector q = [111/222]. The ferromagnetic spin-wave has a small temperature independent gap of 0.25 meV and a reduced (compared to other rare-earth Fe2 Laves phases) stiffness constant of D = 155(5) me V-A2. The strong fluctuations around the AF wavevector give rise to a spin wave dispersion relationship that can be followed across the reduced AF Brillouin zone. The gap in the AF excitation spectrum is ~1 me V at 15K, and rises to ~5meV above 100 K. At low temperature with a window of +- 20 GHz observe an apparent static component of ~ 0.05 muB superimposed on the ferromagnetic component of 1.2 muB per Fe. The spatial correlations of these AF fluctuations extends over many unit cells at low temperature. Our measurements have not detected any response directly from the Ce moments, so we assume that their response is spread over a wide energy range. Mossbauer spectra show an anomalous behavior of the Lamb-Mossbauer factor as a function of temperature, and also show that the magnetic system is not saturated even at a large (5T) fields, suggesting that short-range AF order may persist to higher temperatures than the medium-range order observed in the neutron experiments.JRC.E-Institute for Transuranium Elements (Karlsruhe