Amorphization-induced Mott-like transition in van der Waals ferromagnets CrGeTe₃ and CrSiTe₃

Increasing the Curie temperature (T_c) of van der Waals ferromagnets has long been a highly regarded research topic for developing their applications in spintronic devices. In recent years, the T_c of 2D semiconductors CrGeTe₃ (CGT) and CrSiTe₃ (CST) has been enhanced through several methods such as external pressure [1] and carrier doping [2]. In addition, CGT and CST are charge-transfer type Mott insulators and an insulator-metal transition occurs simultaneously with the increase in T_c. In this talk, we report that the amorphous CGT and CST created by high-energy ion irradiation surprisingly exhibit similar high-T_c ferromagnetic metallic behavior. Magnetization and transport measurements revealed that the ferromagnetic semiconductor CST (CGT) transforms into a ferromagnetic metal after amorphization, with its T_c increasing from 32K (67 K)to 100K (200K [3]). Local structural analysis using XAFS measurements revealed that the Cr-Te-Cr bonding persists in the amorphous state, but the Cr-Te bond length is shorter than that of crystalline phase. The shortened bond length can modify the super-exchange interaction, resulting in enhanced conductivity and increased T_c, similar to the Mott transition observed under external pressure in single crystals [1].

[1] D. Bhoi, J. Gouchi, N. Hiraoka, Y. Zhang, N. Ogita, T. Hasegawa, K. Kitagawa, H. Takagi, K. H. Kim, Y. Uwatoko, Phys. Rev. Lett. 127, 217203 (2021)

[2] I. A. Verzhbitskiy, H. Kurebayashi, H. Cheng, J. Zhou, S. Khan, Y. P. Feng, G. Eda, Nat Electron 3, 460 (2020)

[3] S. Zhang, K. Harii, T. Yokouchi, S. Okayasu, and Y. Shiomi, Adv. Electron. Mater. 10, 2300609 (2024).