Low-energy excitations near a field induced magnetic quantum-critical line in Nb1-yFe2+y

Our previous neutron scattering on ferromagnetic (FM) Nb1-yFe2+y revealed that chemical-composition (y) tuning at zero field leads to a FM quantum-critical point (QCP) masked by a spin-density-wave (SDW). The excitation spectrum at zero field is characterised by enhanced soft modes with an exotic dispersion that reflects the presence of emerging SDW order. In contrast, transverse-field tuning of a FM Nb1-yFe2+y sample has revealed an unmasked FM QCP with a more conventional excitation spectrum expected by spin-fluctuation theory. A plausible assumption is that the unmasked FM QCP belongs to a transverse-field induced quantum critical line, which a critical concentration (y_c) terminates in a quantum multi-critical point. We propose to explore the evolution of the low-energy excitation spectrum along this quantum critical line and ask for 6 days of beamtime.