  The generation and control of nanoscale magnetic fields are of fundamental
interest in material science and a wide range of applications. Nanoscale
magnetic resonance imaging quantum spintronics for example require single spin
control with high precision and nanoscale spatial resolution using fast
switchable magnetic fields with large gradients. Yet, characterizing those
fields on nanometer length scales at high band width with arbitrary orientation
has not been possible so far. Here we demonstrate single electron and nuclear
spin coherent control using the magnetic field of a hard disc drive write head.
We use single electron spins for measuring fields with high spatial resolution
and single nuclear spins for large band width measurements. We are able to
derive field profiles from coherent spin Rabi oscillations close to GHz in
fields with gradients of up to 10 mT/nm and measure all components of a static
and dynamic magnetic field independent of its orientation. Our method paves the
way for precision measurement of the magnetic fields of nanoscale write heads
important for future miniaturization of the devices.
