We present a detailed study of the magnetic properties of weakly ferromagnetic/quantum critical ${\mathrm{Nb}}_{1-y}{\mathrm{Fe}}_{2+y}$ using muon spin rotation and relaxation ($\mu \mathrm{SR}$). 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 ${\mathrm{NbFe}}_2$. 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 ${\mathrm{Nb}}_{1-y}{\mathrm{Fe}}_{2+y}$ and fit the corresponding experimental data. In particular, we quantify the $\mu \mathrm{SR}$ response for quantum critical behavior in ${\mathrm{Nb}}_{1.0117}{\mathrm{Fe}}_{1.9883}$ and extract the influence of residual weak structural disorder. From our analysis, ${\mathrm{Nb}}_{1-y}{\mathrm{Fe}}_{2+y}$ emerges to be uniquely suited to study quantum criticality close to weak itinerant ferromagnetic order.

• Received 6 April 2022
• Revised 21 July 2022
• Accepted 26 September 2022

DOI:https://doi.org/10.1103/PhysRevB.106.134408