Muon spin rotation and relaxation study on Nb${}_{1-y}$Fe${}_{2+y}$

Abstract

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 ($\hat{I}¼$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 NbFe${}_2$. 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 $\hat{I}¼$SR response for quantum critical behavior in Nb${}_{1.0117}$Fe${}_{1.9883}$ 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.