Spin electronics refers to the study of the intrinsic spin of electrons and their associated electronic charge and magnetic moments. Spintronics has provided a new approach for data storage and transmission, which requires the effective design of magnetic materials with unique electronic configuration. Specifically, manganese atoms have been used to design two-dimensional ferromagnetic monolayers with desirable ferromagnetic Currie temperatures, which can be further tuned through various methods, i.e., surface modification and strain. Recent reports about graphene-like hexagonal MnN monolayers being ferromagnetic with acceptable Currie temperature have inspired further research into the origin of ferrimagnetism, especially in transition metal-based two-dimensional materials.
For Mn-based two-dimensional ferromagnetic materials, the sensitivity of the magnetic coupling between the Mn atoms is mainly attributed to two factors: the distance between the atoms and the coordination. For instance, a reduction in the distance between the Mn atoms may result in a change from the ferrimagnetic phase into the antiferromagnetic phase in some monolayers like orthorhombic. Interestingly, the magnetic coupling sensitivity is largely responsible for the flexible, controllable, and diverse magnetic properties exhibited by Mn-based materials. Equipped with this knowledge, Kexian Zhao (graduate student) and Professor Qian Wang from Peking University, College of Engineering, reported a two-dimensional pentagon-based monolayer, penta-MnN2, and systematically investigated its magnetic properties. Their work is currently published in the journal, Applied Surface Science.
In brief, the robust intrinsic ferromagnetism in the penta-MnN2 sheet is attributed to the unique pentagonal geometric structure with N2 dimers. The magnetic properties of the proposed monolayer were studied using the first principle calculations coupled with Monte Carlo simulations. Also, the authors examined the change in the Currie temperature when changing from the hexagonal structural units to pentagonal units, that is, from Hex-MnN sheet to penta-MnN sheet.
Reports showed that apart from dynamic, thermal, and mechanical stability, the reported that penta-MnN exhibited half-metallicity property with a band-gap of 1.12 eV in spin-down channel. Moreover, penta-MnN2 exhibits a high Currie temperature of 913K, which is significantly higher than 368K exhibited by the Hex-MnN sheet. The Currie temperature could be further enhanced to 956K by applying a biaxial tensile strain of 3%. Furthermore, the authors noted that unlike the hexagonal MnN monolayer, the exchange interaction between the Mn atoms is significantly enhanced in the penta-MnN2 monolayer. This is attributed to the unique geometric configuration of the penta-MnN2 monolayer comprising of pentagons containing N2 dimers.
In summary, the study conducted by Kexian Zhao and Professor Qian Wang presented robust intrinsic ferromagnetism in the penta-MnN2 sheet comprising of pentagonal structural units. Due to its unique geometric configurations, the system is highly intriguing, exhibiting robust ferromagnetism and half-metallicity properties with a Curie temperature of 913K, which is significantly higher than that of Hex-MnN comprising of hexagonal structural units. Based on the diversity of the magnetism in Mn-based structures, Professor Qian Wang said in a statement to Advances in Engineering team that the presented approach is highly flexible and would aid in the design of new Mn-based magnetic materials with desirable properties for potential high-performance spintronic applications.
Zhao, K., & Wang, Q. (2020). High Curie temperature ferromagnetism in penta-MnN2 monolayer. Applied Surface Science, 505, 144620.