The rapid development of flexible smart wearable devices has put forward the requirements for flexibility of magnetoelectric functional devices. Due to the inverse magnetostrictive nature of the magnetic material, the stress/strain generated by the bent or stretched state changes the magnetic anisotropy of the magnetic film, thereby affecting the performance of the magnetic device. How to avoid the adverse effects of stress magnetic anisotropy on the performance of flexible magnetic devices is one of the important challenges in the development of flexible magnetic films and devices. Based on this research, the Chinese Academy of Sciences has prepared a flexible giant magnetoresistance spin valve sensor with high magnetic field sensitivity.

Magnetic film with periodic pleat structure prepared by different growth processes

In recent years, the research team of magnetoelectronic materials and devices of the Key Laboratory of Magnetic Materials and Devices of the Chinese Academy of Sciences has studied the regulation of stress/strain on the magnetic anisotropy of flexible magnetic films and flexible exchange-biased heterojunctions [Appl. Phys. Lett] 100, 122407 (2012), Appl. Phys. Lett. 102, 022412 (2013), Appl. Phys. Lett. 105, 103504 (2014)]. Using the inverse piezoelectric effect and anisotropic thermal expansion characteristics of flexible polyvinylidene fluoride (PVDF) piezoelectric film, the magnetic anisotropy of temperature field and electric field is realized in flexible FeGa/PVDF and CoFeB/PVDF composite film materials. Effective regulation, its magnetic anisotropy increases with increasing temperature, exhibits positive temperature coefficient characteristics, can solve the magnetic anisotropy of conventional magnetic materials with the increase of temperature, resulting in high-frequency magnetic devices at high temperatures The problem of performance degradation [Sci. Rep. 4, 6615 (2014), Sci. Rep. 4, 6925 (2014)]. Furthermore, the stress magnetic anisotropy of the magnetic film is improved by the binding action of the flexible substrate, and a high-frequency magnetic film having a ferromagnetic resonance frequency of 5.3 GHz and a reflection loss of 28 dB is obtained [Appl. Phys. Lett. 106, 162405 ( 2015)].

Parallel microstrip flexible giant magnetoresistance spin valve device with periodic pleat structure

For the spin valve device, the uniaxial magnetic anisotropy of the magnetic free layer is small, so that the direction of the magnetic moment is easily changed by the external magnetic field, exhibiting a high magnetic field sensitivity. However, for flexible spin valve devices, the stresses from the substrate during the fabrication process, as well as the stresses caused by deformations such as bending or stretching, will greatly reduce the magnetic field sensitivity of the flexible spin valve device. Recently, the research team compared two methods for preparing magnetic thin films having a surface periodic structure on a flexible polydimethylsiloxane (PDMS) substrate. The magnetic film directly grown on the stretched PDMS exhibits a regular surface wrinkle structure and a weak magnetic anisotropy; a surface periodic structure is preliminarily produced using a non-magnetic metal, and the deposited magnetic film exhibits a strong magnetic orientation. Heterosexual [Appl. Phys. Lett. 108, 102409 (2016)]. Based on this research, a flexible giant magnetoresistance spin valve sensor with high magnetic field sensitivity was prepared by direct growth on tensile PDMS. The longitudinal tensile strain can be released through the surface periodic structure, and the surface parallel microstrips can be designed. The transverse strain introduced by the Poisson effect can be released, thereby significantly reducing the influence of tensile strain on the magnetic anisotropy of the magnetic layer, and avoiding the fracture behavior of the metal film under tensile strain. The prepared spin valve magnetic sensor is 50. Within the range of % tensile strain, the magnetoresistance, magnetic field sensitivity, and sample resistance can be kept constant. [ACSNano10, 4403 (2016)]. The stretchable magnetic sensor with stable reliability can be used as a current sensor, a position sensor, an angle sensor, a gear sensor, etc., and is integrated in a flexible smart wearable device, and has an important application prospect.

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