Consecutive topological phase transitions and colossal magnetoresistance in a magnetic topological semimetal

Abstract The combination of magnetic symmetries and electronic band topology provides a promising route for realizing topologically nontrivial quasiparticles, and the manipulation of magnetic structures may enable the switching between topological phases, with the potential for achieving functional physical properties. Here, we report measurements of the electrical resistivity of EuCd₂As₂ under pressure, which show an intriguing insulating dome at pressures between p_c1 ~ 1.0 GPa and p_c2 ~ 2.0 GPa, situated between two regimes with metallic transport. The insulating state can be fully suppressed by a small magnetic field, leading to a colossal negative magnetoresistance on the order of 10⁵%, accessible via a modest field of ~ 0.2 T. First-principles calculations reveal that the dramatic evolution of the resistivity under pressure can be attributed to consecutive transitions of EuCd₂As₂ from a magnetic topological insulator to a trivial insulator, and then to a Weyl semimetal, with the latter resulting from a pressure-induced change in the magnetic ground state. Similarly, the colossal magnetoresistance results from a field-induced polarization of the magnetic moments, transforming EuCd₂As₂ from a trivial insulator to a Weyl semimetal. These findings underscore weak exchange couplings and weak magnetic anisotropy as ingredients for discovering tunable magnetic topological materials with desirable functionalities.