The study of topological materials, particularly those incorporating magnetic symmetries, is a burgeoning field. The interplay between magnetic structures and electronic band topology offers a pathway to create novel functionalities. The manipulation of magnetic structures allows for the switching between topological phases, leading to potentially useful physical properties. This research focuses on EuCd₂As₂, an f-electron magnetic topological material with an orthorhombic structure. In its paramagnetic state, EuCd₂As₂ exhibits a single Dirac cone at the Fermi level. Below its antiferromagnetic (AFM) transition temperature (Tₙ ≈ 9 K), Eu²⁺ moments align in the ab-plane, forming A-type AFM order. This magnetic structure opens a gap at the Dirac point, potentially making it a small-gap magnetic topological insulator (MTI). Applying a magnetic field can induce a fully polarized state along the c-axis, leading to a Weyl semimetal (WSM) state. Previous studies suggested a Weyl state in the paramagnetic phase and the possibility of other phases like axion insulators, depending on the magnetic ground state. The tunable magnetism of EuCd₂As₂, demonstrated by changes in synthesis or application of hydrostatic pressure, makes it an ideal system to investigate the relationship between magnetism and topology.

Existing literature highlights the potential of magnetic topological insulators and the role of magnetism in influencing topological properties. Studies on EuCd₂As₂ have shown the presence of a Dirac cone in the paramagnetic phase and the opening of a gap in the AFM phase. The possibility of achieving a Weyl semimetal state through magnetic field application has also been explored. Previous research suggests a strong coupling between magnetism and electronic transport in this material. The proximity of competing magnetic ground states in EuCd₂As₂ highlights its suitability for studying the manipulation of its electronic topology.

Single crystals of EuCd₂As₂ were grown using the Sn-flux method. Electrical resistivity measurements under pressure were performed using a piston-cylinder device and a diamond anvil cell, with a pressure-transmitting medium ensuring hydrostaticity. Measurements were conducted down to 0.3 K in a ⁴He refrigerator with a 1 T magnetic field. Magnetization measurements were carried out using a vibrating sample magnetometer. First-principles electronic structure calculations were performed using density functional theory (DFT) with the VASP code, employing the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation and including an on-site Coulomb interaction (U = 6 eV) for Eu-f orbitals. The electronic structures were calculated for different magnetic ground states and pressures, and band structures were fitted to tight-binding Hamiltonians using the Wannier function method. The resulting Hamiltonians were used to calculate surface states and analyze their electronic topology.

Resistivity measurements under pressure revealed a clear insulating dome between approximately 1.0 GPa (ρc₁) and 2.0 GPa (ρc₂). Below these critical pressures, metallic behavior was observed. The insulating state was easily suppressed by a magnetic field of ~0.2 T (for H||c), leading to a colossal negative magnetoresistance (~10⁻³ at 0.3 K). DFT calculations showed that the pressure-induced changes in resistivity could be explained by consecutive topological phase transitions. The first transition was from a magnetic topological insulator (MTI) to a trivial insulator (TrI) at ρc₁, while the second was from a TrI to a Weyl semimetal (WSM) at ρc₂, driven by a pressure-induced AFM-FM transition. The colossal magnetoresistance resulted from the field-induced polarization of the Eu²⁺ moments, transforming the system from a TrI to a WSM. The calculations showed that the insulating state is closely related to the AFM order and that its suppression under pressure or magnetic field is coupled to a transition to a WSM phase. The transition from MTI to TrI was attributed to enhanced electronic hopping suppressing band inversion. The transition from TrI to WSM involved a change in the magnetic ground state from AFM to FM. The colossal magnetoresistance showed a strong dependence on both pressure and temperature, with a significant increase observed at lower temperatures.

The findings demonstrate a strong interplay between topology and magnetism in EuCd₂As₂. The consecutive topological phase transitions observed highlight the tunability of the material's electronic properties through pressure and magnetic field. The colossal magnetoresistance, achieved with remarkably low magnetic fields, indicates a highly sensitive response to magnetic perturbations. The near degeneracy of AFM and FM states, facilitated by weak magnetic exchange couplings and weak magnetic anisotropy, is crucial for the observed tunability. The robustness of the colossal magnetoresistance regardless of field orientation suggests potential for technological applications. The possibility of replacing hydrostatic pressure with chemical pressure to achieve similar results at ambient pressure is suggested.

This study demonstrates consecutive topological phase transitions and colossal magnetoresistance in EuCd₂As₂. The findings underscore the importance of weak exchange couplings and weak magnetic anisotropy in creating tunable magnetic topological materials. The colossal magnetoresistance observed, particularly its response to low magnetic fields, suggests potential applications in low-temperature technologies. Future research could explore chemical pressure methods to achieve similar results without hydrostatic pressure and investigate the behavior of this material at even lower temperatures.

The study primarily focuses on a limited number of samples. While consistent behavior across the samples was observed, further investigations with larger sample sizes are recommended. The theoretical calculations rely on approximations within DFT, and the exact nature of the interactions might require more advanced computational techniques. The study was mainly conducted down to 0.3 K; extending the experiments to lower temperatures could potentially reveal further insights.