  In this work, we develop a nanoscale magnetic field sensor based on a
spin-Hall nano-oscillator (SHNO) device. We fabricate constriction-based SHNO
devices using Ni$_{81}$Fe$_{19}$/Au$_{0.25}$Pt$_{0.75}$ bilayer and use them to
demonstrate quasi-DC sensing up to kilohertz-scale frequencies under a bias
field of 400 Oe in the sample plane. The magnetic field sensing is based on the
linear dependence of SHNO oscillation frequency on external magnetic field. The
detectivity of the sensor is as low as 0.21 $\mu$T/$\sqrt{\mbox{Hz}}$ at 100
Hz, with an effective sensing area of 0.32 $\mu$m$^2$, making SHNO devices
suitable for scanning probe nanoscale magnetometry.
