Emergent Magnetic Phase in Pressure-Tuned Antiferromagnetic Topological Insulator

Zintl material EuCd2As2 attracts much recent research interest as it can manifest various ideal topological quantum states under certain conditions just within the same host. While the ground state of this material is expected to be an antiferromagnetic (AFM) topological insulator or axion insulator as outlined by the Zintl concept and theoretical band calculations, yet all samples from previous works experimentally show an AFM semimetal state. Based on successful synthesis of single crystals of EuCd2As2 having an AFM insulating state and a negative colossal magnetoresistance (CMR, ∼10,000%) effect, here we present a comprehensive high-pressure study of this unique material. We found that the AFM transition temperature is raised linearly from ∼9.5 K at ambient pressure to ∼55.0 K at 24.0 GPa; meanwhile, the CMR persists whereas its maximum magnitude first increases, reaching ∼40,000% at 12.6 GPa, and then decreases significantly. Beginning at ∼24.0 GPa, a ferromagnetic metallic phase with an ordinary negative MR shows up, which is accompanied by a trigonal P3m1 to monoclinic C2/m structural transition. The emergent phase dominates over the pristine AFM insulating one upon completion of the structural transition beyond ∼40 GPa. Our results demonstrate the tunability of quantum states by pressure and may offer crucial insights into understanding of the interplay of magnetism and transports in EuCd2As2 and its derivatives.