Recently, the quantum functional materials and advanced photonics research team led by Lu Yalin, a professor at the University of Science and Technology of China, has made important progress in quantum functional materials research. The team associate researcher Xiaofang Xiao, associate professor Fu Zhengping, and the American Lorenz Berkeley National Experiment Dr. Jinghua Guo, Ph.D., Professor, China University of Science and Technology, Zhao Wei, and Professor Ma Chao, Hunan University, collaborated in the research of new high-temperature, high-symmetry ferromagnetic insulators, the preparation of high-quality oxide films and synchrotron radiation, advanced photoelectric detection, first-rate The combination of principle calculations, etc., successfully discovered a highly symmetric ferromagnetic insulator higher than the liquid nitrogen temperature (77K), and explained the new mechanism of high-temperature ferromagnetic transition. Related research results were published in the “Proceedings of the National Academy of Sciences”. on.
In general, magnetic materials can be classified into ferromagnetic and antiferromagnetic, and in real materials, ferromagnetic materials are usually conductive, and antiferromagnetic materials are usually insulated. With the development of quantum technology, the performance of quantum functional materials There are more and more requirements, such as the need for insulating ferromagnetic materials (ferromagnetic insulators) in quantum topological devices, and the need for ferromagnetic insulators to have high lattice symmetry to facilitate epitaxial growth with other materials into future quantum The device needs to have as high a ferromagnetic transition temperature as possible in order to facilitate a practical working environment that is closer to the device.
Most of the ferromagnetic insulators found in previous studies are different from each other by occupying different positions of two magnetic atoms to promote their orbital dominance. This type of ferromagnetic insulator is best known as Y3Fe5O12 (YIG). However, this type of ferromagnetic insulator has complexities. With a low-symmetry lattice structure, the same atoms can easily occupy different lattice sites, making the preparation of high-quality ferromagnetic insulators very difficult, and seriously affecting the performance of their ferromagnetic insulators. More seriously, When these complex structures of ferromagnetic insulators are applied to magnetic quantum devices or tunneling devices, they are difficult to epitaxially grow with other highly symmetric materials, causing difficulties in the preparation and integration of future devices. At the same time, it is known that Ferromagnetic transition temperatures of highly symmetric non-doped ferromagnetic insulators are very low, most of which lie below 16K, far below the minimum required liquid nitrogen temperature. The low temperature ferromagnetic insulation exhibited thus may be due to 4f The track is too narrow, and the superexchange effect between oxygen is too weak. Usually, the rarity of quantum functional materials is subject to the basic objective physical laws. To achieve a breakthrough, we must start with a deep physical mechanism and design and develop new quantum materials that can produce new types of performance. This places high demands on physical mechanism research and material preparation.
In order to obtain ferromagnetic insulators with high symmetry and easy epitaxial growth ability, which can work at high temperatures, the team conducted a thorough material screening, and it was thought that LaCoO3 thin films could be a research object of a highly symmetrical ferromagnetic insulator. However, the source of ferromagnetism of LaCoO3 thin films is full of controversy. Due to the high requirements for preparation, a large number of defects often appear in thin films. Therefore, many people thought that these defects caused ferromagnetism, leading to unstable performance and Uncontrollable. In this study, the team developed a high-quality, nearly defect-free LaCoO3 thin film based on the advantages of high-quality single crystal thin film preparation and studied the source of ferromagnetism. It was found that LaCoO3 thin film is indeed a rare high-temperature ferromagnetic material. Insulators, whose ferromagnetic transition temperature can be as high as 85K, are five times higher than previously studied materials and are higher than liquid nitrogen temperature. By preparing LaCoO3 films with different oxygen contents, different stresses, and different thicknesses, it has been found that the concentration of oxygen deficiency increases. Causes the weakening of ferromagnetism, and when the content of Co2+ caused by oxygen deficiency reaches about 10%, the ferromagnetism will disappear completely; The first-principles calculations found the conclusion that is basically consistent with the experiment. When the oxygen defect is introduced into the LaCoO3 film under tensile stress, the resulting high spin state of Co2+ (t2g3eg2) and the adjacent Co3+ high-spin state or Co2+ high-spin state formation The antiferromagnetic interaction of the domain weakens the ferromagnetism. And when the concentration of Co2+ reaches 12.5%, the antiferromagnetic interaction replaces the ferromagnetic interaction and becomes a new long process, and the ferromagnetism thus disappears completely. The LaCoO3 thin film ferroelectric insulation mechanism was explained and proved, providing a much-needed new material for the future development of high-quality magnetic quantum devices.
China University of Science and Technology Hefei University of Hefei micro-scale National Center for Physicians Meng Dechao, Guo Hongli as the co-first author, Yan Xiaofang, Lu Yalin as the author of the correspondence. The research was funded by the Ministry of Science and Technology, the National Natural Science Foundation of China, the Chinese Academy of Sciences and the Ministry of Education.
