Unconventional magnetic materials, including non-collinear antiferromagnets, p-wave magnets and altermagnets, are an emerging frontier for spintronics and hybrid quantum devices. Critical to the application of these materials is control over the magnetic domain state, as their unique, symmetry-driven properties vanish in a multi-domain limit. Understanding the mechanisms governing domain formation in systems with compensated local moments remains a critical challenge for device realisation.

Here, we investigate the domain morphology of the non-collinear antiferromagnet Mn₃NiN across different length scales using complementary spatial imaging techniques. By employing a focused scanning laser to map the spatial distribution of the anomalous Nernst voltage in a two micrometer-size Hall cross, we identify a robust checkerboard pattern indicative of octupole macrodomains on the order of 1 μm². Then, using scanning nitrogen-vacancy centre magnetometry, we provide nanoscale mapping of the magnetic domain evolution on cooling, probing the ferrimagnetic to antiferromagnetic phase transition in unpatterned samples. We observe the formation of a disordered, dendritic domain structure whose roughness is quantified using its fractal dimension. We show the domain structure cannot be explained by the balance of demagnetisation energy and domain wall energy, and conclude elastic contributions and defects are a critical factor to explain the domain size.