Nanoscale magnetic field sensing with spin-Hall nano-oscillator devices

Spin-Hall nano-oscillators (SHNOs) are magnetic bilayer devices that convert dc charge current to microwave frequency magnetic oscillations under external magnetic field. The operational principle is based on spin Hall effect: a charge current in a nonmagnetic layer with spin-orbit interaction generates a transverse pure spin current that flows into an adjacent magnetic layer and compensates its magnetic damping. This leads to auto-oscillations in the gigahertz frequency regime. The oscillation frequency of SHNOs is tuneable with dc current and external magnetic field, enabling their applications as agile microwave signal generators. Here we investigate their application as nanoscale magnetic field sensors. We fabricated SHNO devices based on Ni₈₁Fe₁₉/Au_0.25Pt_0.75 with a single 150 nm constriction and four 150 nm constrictions in an array separated by 350 nm. These devices are designed to operate in a bias field of ~400 Oe in the sample plane. The 1-constriction device has a detectivity below 1 μT/√Hz for ac magnetic field frequency > 100 Hz (Figure 3) with an effective sensing area of 0.071 μm². The 4-constriction device has a detectivity below 1 μT/√Hz for ac magnetic field frequency > 20 Hz, but with a slightly larger effective sensing area of 0.32 μm². The devices are able to sense small magnetic field variation around the bias field in both time and frequency domain. The nanoscale sensing area of these devices is interesting for local sensors such as in scanning probe magnetometry.