  Sensitive detection of weak magnetic moments is an essential capability in
many areas of nanoscale science and technology, including nanomagnetism,
quantum readout of spins, and nanoscale magnetic resonance imaging. Here, we
show that the write head of a commercial hard drive may enable significant
advances in nanoscale spin detection. By approaching a sharp diamond tip to
within 5 nm from the pole and measuring the induced diamagnetic moment with a
nanomechanical force transducer, we demonstrate a spin sensitivity of 0.032
Bohr magnetons per root Hz, equivalent to 21 proton magnetic moments. The high
sensitivity is enabled in part by the pole's strong magnetic gradient of up to
28 million Tesla per meter and in part by the absence of non-contact friction
due to the extremely flat writer surface. In addition, we demonstrate
quantitative imaging of the pole field with about 10 nm spatial resolution. We
foresee diverse applications for write heads in experimental condensed matter
physics, especially in spintronics, ultrafast spin manipulation, and mesoscopic
physics.
