Spectroscopic study of metallic magnetism in single-crystalline Nb1-y Fe2+y

We have investigated single crystals and polycrystals from the series Nb1-yFe2+y, -0.004≤y≤0.018, by electron spin resonance, muon spin relaxation, and Mössbauer spectroscopy. Our data establish that at lowest temperatures all samples exhibit bulk magnetic order. Slight Fe excess induces low-moment ferromagnetism, consistent with bulk magnetometry, while Nb-rich and stoichiometric NbFe2 display spin density wave order with small magnetic moment amplitudes of order ∼0.001-0.01μB/Fe. This provides microscopic evidence for a modulated magnetic state on the border of ferromagnetism in NbFe2.