Muon spin rotation and relaxation study on Nb1 −yFe2 +y

We present a detailed study of the magnetic properties of weakly ferromagnetic/quantum critical Nb_{1 −y}Fe_{2 +y} using muon spin rotation and relaxation (μ SR ). By means of an angular dependent study of the muon spin rotation signal in applied magnetic fields on a single crystal in the paramagnetic state we establish the muon stopping site in the crystallographic lattice of NbFe₂. With this knowledge we develop models to describe the muon spin rotation and relaxation signals in the weakly ferromagnetic, spin density wave, and quantum critical phases of Nb_{1 −y}Fe_{2 +y} and fit the corresponding experimental data. In particular, we quantify the μ SR response for quantum critical behavior in Nb_1.0117Fe_1.9883 and extract the influence of residual weak structural disorder. From our analysis, Nb_{1 −y}Fe_{2 +y} emerges to be uniquely suited to study quantum criticality close to weak itinerant ferromagnetic order.