Quantum-well-induced engineering of magnetocrystalline anisotropy in ferromagnetic films

• Published in: NPG Asia materials 2017-08, Vol.9 (8), p.e424-e424
• Main Authors: Chang, Ching-Hao, Dou, Kun-Peng, Guo, Guang-Yu, Kaun, Chao-Cheng
• Format: Article
• Language: English
• Subjects: 119/118; 639/301/1034/1038; 639/766/119/1001; 639/766/25; 639/925/357/997; Anisotropy; Biomaterials; Charge injection; Chemistry and Materials Science; Data storage; Density of states; Electron spin; Electronic devices; Energy Systems; Fermi level; Ferromagnetic films; Ferromagnetic materials; Magnetic anisotropy; Materials Science; Memory devices; Optical and Electronic Materials; original-article; Physical properties; Power consumption; Quantum wells; Silver; Structural Materials; Surface and Interface Science; Thin Films
• ISSN: 1884-4049; 1884-4057
• DOI: 10.1038/am.2017.148

Tuning quantum well states (QWSs) to govern physical properties in nanoscale leads to the development of advanced electronic devices. Here, we propose that QWSs can be engineered to control magnetocrystalline anisotropy energy (MCAE) which dominates the magnetization orientation (that is, the easy axis) of a ferromagnetic thin film. We investigate from first-principles the MCAE of the bcc Fe film on an Ag substrate. The calculated MCAE oscillates largely as Fe thickness increases agreeing well with experiments, and reaches oscillation extremes as the Fe d-orbital QWSs approach the Fermi level ( E F ). Crucially, we find that this phenomenon stems from the combined effect of intrinsic spin-orbit interaction (SOI) and Rashba SOI field on the Fe QWSs, which modulates the density of states at E F as the Fe thickness varies. Moreover, this effect offers a way to tune not only the strength of magnetic anisotropy but also the easy axis of a Fe film by shifting E F within ten meV via moderately charge injection, which could realize advanced memory devices with ultra-low power consumption. Magnetic materials: A slimming effect The thickness of a ferromagnetic film may be used to control its magnetic properties for spintronic applications, a theoretical study finds. Ferromagnetic thin films could be incorporated into conventional electronic devices to impart a magnetic degree of freedom. But a way of controlling the properties of such films is needed to optimize device performance and functionality. In a theoretical analysis, Ching-Hao Chang and Chao Cheng Kaun from the Academia Sinica in Taiwan and colleagues have now shown that the magnetocrystalline anisotropy energy of iron films on a silver substrate can be engineered by varying the film's thickness. This effect stems from the so-called quantum well states that arise due to electron motion being effectively confined to a two-dimensional plane. It could be used to realize advanced memory devices with ultralow power consumption. Electronic and geometric controlling the magnetization orientation of a material in nanoscale is key in developing spintronics, which correlates with tuning its magnetocrystalline anisotropy energy (MCAE). Although the MCAE of a Fe thin film is measured to oscillate with the film thickness and to vary with the amount of injected charges, switching the magnetization orientation electrically is desired. In this work, we provide a microscopic picture based on Fe quantum-well states and spin