Topological van der Waals magnets

Recent remarkable discovery of graphene and topological insulators added another principle of quantum matter based on topological distinctions. Unlike ordinary quantum phases of matter, topological phases are not determined by a local order associated with broken symmetry but rather by a topological ingredient reflecting “twist” of electronic wave functions. In the course of exploring new low-dimensional materials with intriguing topological properties, we focused on magnetic materials, in which the interplay of magnetism and band topology can generate giant topological transport phenomena, potentially important for spin-related electronic applications. Our series of discoveries on new topological van der Waals semimetals would provide a material platform to investigate the rich topological structures of low dimensional magnets and their functionalities for next-generation topological electronics or spintronics.

Tunable spin injection and detection of Fe3GeTe2

Van der Waals heterostructures with two-dimensional magnets offer a magnetic junction with an atomically sharp and clean interface. Using a van der Waals assembly of ferromagnetic Fe3GeTe2 with non-magnetic hexagonal boron nitride and WSe2 layers, we demonstrate electrically tunable, highly transparent spin injection and detection across the van der Waals interfaces, which shows unprecetended multiple sign reveral of the tunnelling magnetoresistance.

"Tunable spin injection and detection across a van der Waals interface", K.-H. Min+, D. H. Lee+, S.-J. Choi, I.-H. Lee, J. Seo, D. W. Kim, K.-T. Ko, K. Watanabe, T. Taniguchi, D. H. Ha, C. Kim, J. H. Shim, J. Eom*, J. S. Kim*, and S. Jung*

Nature Materials, 21, 1144 (2022).

Colossal angular magnetoresistance in Mn3Si2Te6

We have proposed the concept of magnetic nodal-line semiconductor showing metal-insulator transition due to spin-orbit coupling effect on the nodal-line band degeneracy by spin rotation. The proposal is realized in a ferrimagnetic semiconductor Mn3Si2Te6 leading to a colossal angular magnetoresistance that has never been observed in other magnetic materials.

J. Seo+, C. De+, H. Ha+, J. E. Lee+, S. Park, J. Park, Y. Skourski, E. S. Choi, B. Kim, G. Y. Cho, H. W. Yeom, S.-W. Cheong, J. H. Kim*, B. -J. Yang*, K. Kim*, and J. S. Kim*

Nature 599, 576–581 (2021).

Tunable high-TN vdW antiferromagnet (Fe,Co)4GeTe2

We have succeeded to tune the interlayer magnetic interaction with chemical doping and thickness control in Fe₄GeTe₂, and induced a high-temperature antiferromagnetic phase in (Fe,Co)₄GeTe₂

J. Seo+, E. S. An+, T. Park+, S.-Y. Hwang, G.-Y. Kim, K. Song, G. S. Choi, E. Oh, M. Choi, K. Watanabe, T. Taniguchi, Y. J. Jo, H. W. Yeom, S.-Y. Choi*, J. H. Shim*, and J. S. Kim*

Nat. Commun. 12, 2844 (2021).

Fe4GeTe2, a new van der Waals ferromagnet with higher Tc

We discover another vdW ferromagnet Fe₄GeTe₂, using computation material search and high quality single crystal growth, which exhibits nearly room temperature ferromagnetism and flat topological nodal-line structure.

J. Seo+, D. Y. Kim+, E. S. An+, K. Kim, G.-Y. Kim, S.-Y. Hwang, D. W. Kim, B. G. Jang, H. Kim, G. Eom, S. Y. Seo, R. Stania, M. Muntwiler, J. Lee, K. Watanabe, T. Taniguchi, Y. J. Jo, J. Lee, B. I. Min, M. H. Jo, H. W. Yeom, S.-Y. Choi*, J. H. Shim*, J. S. Kim*

Sci. Adv. 6. 8912 (2020)

First topological van der Waals ferromagnet Fe3GeTe2

We firstly identified the topological nodal-line structure in a van der Waals (vdW) ferromagnet Fe₃GeTe₂ and presence of strong Berry curvature. As a result, a strong anomalous Hall effect is observed, which is much larger in strength than conventional ferromangets and robust against disorders.

K. Kim+, J. Seo+, E. Lee, K. -T. Ko, B. S. Kim, B. G. Jang, J. M. Ok, J. Lee, Y. J. Jo, W. Kang, J. H. Shim, C. Kim, H. W. Yeom, B. I. Min, B.-J. Yang*, and J. S. Kim*,

Nat. Mater. 17, 794 (2018).