Scholars successfully prepared wafer-level room-temperature two-dimensional ferromagnetic materials

The team led by Wu Shuxiang, associate professor at the School of Materials Science and Engineering of Sun Yat-sen University, in collaboration with the Institute of Physics of the Chinese Academy of Sciences, the Hong Kong University of Science and Technology, the National University of Singapore, and Hangzhou Normal University, has made important progress in the research of wafer-scale room-temperature two-dimensional ferromagnetic materials. The relevant results were recently published in Nature Communications.

Exploring the long-range magnetic order of two-dimensional ferromagnets at the thickness limit is of great significance for advancing basic physics research and developing new magnetic and spintronic devices that manipulate magnetization with low energy consumption. In three-dimensional systems, magnetism is mainly affected by exchange interactions, which leads to magnetic phase transitions at finite temperatures. In the two-dimensional Heisenberg model, due to the limitations of the Mermin–Wagner theorem, thermal fluctuations will inhibit the formation of two-dimensional ferromagnetic long-range magnetic order. However, strong magnetocrystalline anisotropy can open the spin wave excitation energy gap, effectively suppress thermal fluctuations, and promote the emergence of two-dimensional ferromagnetism at finite temperatures. Whether the room-temperature long-range ferromagnetic order observed in bulk crystals can be maintained in a few unit cells (uc) or 1 uc films remains an open question, because thermal fluctuations at high temperatures can easily destroy the two-dimensional ferromagnetic order. Maintaining the long-range ferromagnetic order of ferromagnetic materials at the thickness limit is related to magnetic exchange coupling and is also closely related to the strength of magnetic anisotropy.

The two-dimensional ferromagnets reported so far exhibit soft magnetic properties and low magnetic anisotropy. In addition, these two-dimensional ferromagnetic materials are all thin sheets peeled from bulk single crystals, which makes it difficult to control their thickness and size, hindering their in-depth research and practical application in spin electronics. Therefore, it is particularly urgent to explore wafer-level two-dimensional ferromagnetic materials with intrinsic ferromagnetism, Curie temperature above room temperature, strong magnetic anisotropy, and compatibility with traditional microelectronic devices.

To this end, Wu Shuxiang's team, funded by the Guangdong Natural Science Foundation and other projects, successfully prepared a wafer-level two-dimensional ferromagnetic Fe3GaTe2 film with controllable single-cell layer number through molecular beam epitaxy. The epitaxially grown Fe3GaTe2 film exhibits strong ferromagnetism with perpendicular magnetic anisotropy. The Curie temperature of the 9 uc epitaxial film is as high as 420 K, and the perpendicular magnetic anisotropy at 300 K is several times higher than that of the CoFeB film that is currently widely studied. When the thickness of the epitaxial film is reduced to the limit, the ferromagnetic order of the 1 uc film is still maintained, and its Curie temperature can reach 345 K, which is due to its strong perpendicular magnetic anisotropy. Compared with the mechanically peeled Fe3GaTe2 flakes, the Curie temperature of the epitaxial film at the same thickness exceeds 60 K, which may be attributed to the tensile strain effect caused by the substrate. First-principles calculations found that the enhancement of ferromagnetism in the epitaxial Fe3GaTe2 film is not only due to the stress-enhanced magnetic exchange coupling, but also related to the stress-enhanced magnetic anisotropy. The strong magnetic anisotropy can stabilize the ferromagnetism by suppressing thermal fluctuations and avoiding the destruction of long-range ferromagnetic order.

The successful preparation of wafer-level room-temperature two-dimensional ferromagnetic materials marks an important breakthrough in fields such as magnetic materials science, and lays a solid material foundation for the development of spin electronics devices based on two-dimensional magnets.