  Chromia (Cr2O3) is a magnetoelectric oxide which permits voltage-control of
the antiferromagnetic (AFM) order, but it suffers technological constraints due
to its low Neel Temperature (TN ~307 K) and the need of a symmetry breaking
applied magnetic field to achieve reversal of the Neel vector. Recently, boron
(B) doping of Cr2O3 films led to an increase TN > 400 K and allowed the
realization of voltage magnetic-field free controlled N\'eel vector rotation.
Here, we directly image the impact of B doping on the formation of AFM domains
in Cr2O3 thin films and elucidate the mechanism of voltage-controlled
manipulation of the spin structure using nitrogen vacancy (NV) scanning probe
magnetometry. We find a stark reduction and thickness dependence of domain size
in B-doped Cr2O3 (B:Cr2O3) films, explained by the increased germ density,
likely associated with the B doping. By reconstructing the surface
magnetization from the NV stray-field maps, we find a qualitative distinction
between the undoped and B-doped Cr2O3 films, manifested by the histogram
distribution of the AFM ordering, i.e., 180 degree domains for pure films, and
90 degree domains for B:Cr2O3 films. Additionally, NV imaging of
voltage-controlled B-doped Cr2O3 devices corroborate the 90 degeree rotation of
the AFM domains observed in magnetotransport measurement.
