Giant-Exchange-Driven Vectorial Control of a Minimal Topological Magnet in Eu(3)In(2)As(4).

The interplay between magnetism and band topology provides a route to controlling quantum states of matter, yet its realization in materials is often constrained by weak exchange coupling and complex electronic structures. Here, a giant exchange coupling is identified in the newly predicted topological magnet EuInAs, giving rise to magnetization-dependent band shifts of up to 300 meV. Together with its intrinsically soft magnetic response, this strong coupling enables systematic tuning of topological phases by both the magnitude and orientation of applied magnetic fields. The magneto-topological phase diagram is mapped out in which an antiferromagnetic topological insulator ground state evolves, under modest fields, into a proposed intermediate 2/3-ferrimagnetic phase, and further into fully polarized ferromagnetic states predicted to host either Weyl or nodal-ring semimetals. Notably, the Weyl phase corresponds to a minimal model hosting a single pair of Weyl nodes. Quantum oscillations, anomalous Hall transport and magneto-infrared spectroscopy consistently reveal exchange-driven band reconstruction across these transitions. Rotation of the magnetization theoretically provides an efficient means to tune the momentum-space positions and separations of the Weyl nodes. These results establish EuInAs as a model system for exploring how strong exchange coupling can be used to control topological band structures with minimal complexity.