Recently, quantum functional materials and advanced photonic technology research teams led by Lu Yalin, a professor at the University of Science and Technology of China, have made important progress in quantum functional materials research. The team associate researcher Miao Xiaofang, associate professor Fu Zhengping and others, collaborated with Dr. Jinghua Guo of Dr. Lorenz Berkeley National Laboratory, Professor Zhao Yan of China University of Science and Technology and Professor Ma Chao of Hunan University to study the process of new high temperature and high symmetry ferromagnetic insulators. In addition, the combination of high-quality oxide film preparation with synchrotron radiation, advanced optoelectronic detection, and first-principles calculations, successfully discovered a highly symmetrical ferromagnetic insulator with a temperature higher than liquid nitrogen (77K), and explained the high temperature The new mechanism of ferromagnetic transition. The relevant research results were published in the "Proceedings of the National Academy of Sciences". Magnetic materials are generally classified into ferromagnetic and antiferromagnetic, and in real materials, ferromagnetic materials are generally electrically conductive, and antiferromagnetic materials are generally insulative. With the development of quantum technology, there is an increasing demand for the performance of quantum functional materials, such as ferromagnetic materials (ferromagnetic insulators) that need to be insulated in quantum topology devices, and at the same time, the ferromagnetic insulators are required to have a high crystal lattice. The symmetry is conducive to epitaxial growth with other materials into future quantum devices; it is necessary 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 the difference in occupation sites of two magnetic atoms to promote their orbital dominance. The most famous of these ferromagnetic insulators is Y3Fe5O12 (YIG). However, this type of ferromagnetic insulator has a complex, low-symmetry lattice structure, and the same atom can easily occupy different lattice sites, making the preparation of a high-quality ferromagnetic insulator very difficult and seriously affecting its ferromagnetic insulator. Performance. What is more serious is that 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, resulting in difficulties in device preparation and integration in the future. At the same time, the currently known ferromagnetic transition temperatures with high symmetry non-doped ferromagnetic insulators are very low, mostly below 16 K, far below the minimum required liquid nitrogen temperature. The low-temperature ferromagnetic insulation exhibited in this way may be due to the narrowness of the 4f orbit and the too weak superexchange between oxygen. In general, the rarity of quantum functional materials is subject to basic objective physical laws. Therefore, breakthroughs must be made from deep physical mechanisms to design and develop new quantum materials that can produce new types of properties. These are proposed for physical mechanism research and material preparation. Extremely high demands. In order to obtain ferromagnetic insulators with high symmetry and easy epitaxial growth capability that can operate at high temperatures, the team carried out a thorough material screening and considered that LaCoO3 thin films could be a research object for a highly symmetrical ferromagnetic insulator. . However, the source of the ferromagnetic properties of LaCoO3 films is full of controversy. Due to the high requirements for preparation, many defects often appear in the films. Many people thought that these defects caused ferromagnetism, leading to unstable performance and incompatibility. control. 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 further studied its source of ferromagnetism. It was found that LaCoO3 thin film is indeed a rare high-temperature ferromagnetic insulator. The ferromagnetic transition temperature can be as high as 85K, which is 5 times that of the materials studied in the past and is higher than the liquid nitrogen temperature. By preparing LaCoO3 films with different oxygen contents, different stresses and different thicknesses, it was found that an increase in the oxygen defect concentration causes ferromagnetism, and the ferromagnetism disappears completely when the oxygen content causes the Co2+ content to reach about 10%; The first-principles calculation 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) is adjacent to the high spin state of Co3+ or the high spin state of Co2+. 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 completely disappears. This study fully explained and proved the LaCoO3 thin film ferromagnetic insulation mechanism, providing a much-needed new material for the future development of high-quality magnetic quantum devices and other applications. He Dachao and Guo Hongli, Ph.D. students at the China National University of Science and Technology Hefei National Center for Physical Sciences at the Microscale, were the co-first authors, and Miao Xiaofang and Lu Yalin were the authors of the correspondence. The study 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. 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