  Dynamics and textures of magnetic domain walls (DWs) may largely alter the
electronic behaviors in a Weyl semimetal system via emergent gauge fields.
However, very little is known about even the basic properties of these domain
walls in Weyl materials. In this work, we imaged the spontaneous magnetization
and magnetic susceptibility of a ferromagnetic (FM) Weyl semimetal CeAlSi using
scanning SQUID microscopy. We observed the ferromagnetic DWs lined-up with the
[100] direction (or other degenerate directions). We also discovered the
coexistence of stable and metastable domain phases, which arise likely due to
magnetoelastic and magnetostriction effects and are expected to be highly
tunable with small strains. We applied an in-plane external field as the CeAlSi
sample was cooled down to below the magnetic phase transition of 8.3K, showing
that the pattern of FM domains is strongly correlated with both the amplitude
and the orientation of the external field even for weak fields of a few Gauss.
The area of stable domains increases with field and reaches maximum when the
field is parallel to the main crystallographic axes of the CeAlSi crystal. Our
results suggest that the manipulation of these heterogeneous phases can provide
a practical way to study the interplay between magnetism and electronic
properties in Weyl systems, and that these systems can even serve as a new
platform for magnetic sensors.
