Ningbo Magnetic Materials Co., Ltd. obtained a series of advances in magnetic control of flexible magnetic films
Flexible electronic devices have the advantages of flexibility, portability, and potential low-cost manufacturing, and have important application prospects in the medical, information, energy, defense, and other fields, and have attracted widespread attention. The most desirable flexible electronic devices, such as flexible wearable devices, require all of their components to be flexible, including flexible power supplies, flexible circuits, flexible displays, flexible sensing, flexible storage, and the like.
It can be seen that how to realize the flexibility of traditional functional materials, to understand the evolutionary laws of the material's functional characteristics in the stress/strain environment, and to master the methods of controlling the material and device functions under multiple physical fields becomes a very important issue. On the other hand, magnetic materials are an important part of electronic devices.
Preparing a magnetic thin film on a flexible substrate and studying its magnetoelectric characteristics is an important basis for the development of flexible magnetic electron/spintronic devices. The Magnetic Materials and Devices Key Laboratory of the Chinese Academy of Sciences (Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences) The magnetic electron materials and devices team has been working on flexible magnetically functional materials and devices in recent years, including the development of flexible magnetic and electrical functional materials. Technology to study the evolution and regulation of physical properties of magnetically functional materials and devices under the influence of multiple physical fields (flexible state). In the past two years, a series of advances have been made in the control of magnetic anisotropy and magnetic moment orientation.
Magnetic anisotropy is one of the important internal parameters of magnetic materials. It not only determines the magnetic moment orientation and coercive force of magnetic materials, but also affects the operating frequency and power consumption of magnetic devices. Studying how to control magnetic anisotropy and its regulation mechanism has always been one of the core issues in magnetic materials and magnetic physics.
The team prepared a magnetostrictive film on a flexible substrate and studied the stress/strain regulation of magnetic anisotropy and exchange-bias effects in magnetostrictive films (CoFeB, FeGa, etc.). It was found that the magnetic anisotropy of the magnetostrictive film can be controlled by applying stress, and the difficult axis and the easy axis can be interchanged under certain stress conditions. For a film having a positive magnetostriction coefficient, the easy axis of magnetization tends to be in the tensile stress direction, and the hard axis of magnetization tends to be in the compressive stress direction. (FIG. 1) [Appl. Phys. Lett. 100, 122407 (2012); Appl. Phys. Lett. 105, 103504 (2014)]. In a flexible FaGa/IrMn magnetically exchange biased bilayer film, it was found that the exchange bias field of the heterojunction decreased with the increase of the applied stress/strain, while the coercivity increased with the applied stress/strain. And increase [Appl. Phys. Lett. 102, 022412 (2013)]. This result indicates that the response of the ferromagnetic layer and the antiferromagnetic layer to the applied stress/strain is inconsistent in the direction of the magnetic moment, resulting in the weakening of the pinning effect of the antiferromagnetic layer on the ferromagnetic layer and reducing the size of the exchange bias field.
The above research results not only help people to understand the magnetic anisotropy of flexible magnetic films, but also lay a foundation for the development of flexible spin valves and magnetic tunnel junction devices, and more importantly, provide new ideas for regulating the magnetic anisotropy of materials.
Magnetic anisotropy generally decreases with increasing temperature, resulting in insufficient thermal stability of some magnetic devices, and some applications closely related to magnetic anisotropy are greatly limited. In response to this problem, based on the above work, the team skillfully utilized the thermal properties of PVDF's anisotropic thermal expansion and the inverse magnetostrictive effect to successfully design a class of magnetic composites with positive temperature coefficient magnetic anisotropy. Thin film material. By systematically studying the temperature-varying hysteresis loops and ferromagnetic resonance spectra of different composite films of CoFeB/PVDF, FeGa/PVDF and Ni/PVDF, it was confirmed that the magnetic anisotropy of PVDF-based magnetic composite films has a positive temperature coefficient, that is, As the temperature rises, the effect of magnetic anisotropy increases (Figure 2).
The results of this study have important implications for improving the thermal stability of magnetic materials [Sci. Rep. 4, 6615 (2014)]. At the same time, the team also used the ferroelectric and anisotropic thermal expansion characteristics of PVDF to achieve effective control of magnetic anisotropy and magnetization direction by multiphysics. For example, in the FeGa/PVDF heterostructure, the easy axis transformation of magnetic anisotropy of FeGa is realized by applying an electric field at 295K (Fig. 3), and with the assistance of a small magnetic field (less than coercivity), The temperature controls the 180° flip of the magnetic moment. It can be seen that the flexible magnetic composite film material based on PVDF has many degrees of freedom such as temperature, electric field and magnetic field, and has potential application value in the field of flexible spintronic devices [Sci. Rep. 4, 6925 (2014)].
Related work has been published successively in Appl. Phys. Lett. 100, 122407 (2012), Appl. Phys. Lett. 102, 022412 (2013), J. Appl. Phys. 113, 17A901 (2013), J. Appl. Phys .113, 17C705 (2013), J. Appl. Phys. 114, 173913 (2013), Chin. Phys. B. 22, 127502 (2013) (Invitation Review), Appl. Phys. Lett. 105, 103504 (2014) , Sci. Rep. 4, 6615 (2014), Sci. Rep. 4, 6925 (2014). The research work was supported by the National Natural Science Foundation of China, the 973 subproject, projects in Zhejiang Province, and Ningbo City.
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