  While ferromagnets are at the heart of daily life applications, their large
magnetization and resulting energy cost for switching bring into question their
suitability for reliable low-power spintronic devices. Non-collinear
antiferromagnetic systems do not suffer from this problem and often possess
remarkable extra functionalities: non-collinear spin order may break
space-inversion symmetry and thus allow electric-field control of magnetism, or
produce emergent spin-orbit effects, which enable efficient spin-charge
interconversion. To harness these unique traits for next-generation
spintronics, the nanoscale control and imaging capabilities that are now
routine for ferromagnets must be developed for antiferromagnetic systems. Here,
using a non-invasive scanning nanomagnetometer based on a single
nitrogen-vacancy (NV) defect in diamond, we demonstrate the first real-space
visualization of non-collinear antiferromagnetic order in a magnetic thin film,
at room temperature. We image the spin cycloid of a multiferroic BiFeO$_3$ thin
film and extract a period of $\sim70$ nm, consistent with values determined by
macroscopic diffraction. In addition, we take advantage of the magnetoelectric
coupling present in BiFeO$_3$ to manipulate the cycloid propagation direction
by an electric field. Besides highlighting the unique potential of NV
magnetometry for imaging complex antiferromagnetic orders at the nanoscale,
these results demonstrate how BiFeO$_3$ can be used as a versatile platform for
the design of reconfigurable nanoscale spin textures.
