Research Progress in Flexible Magnetic Sensing Thin Film Materials and Devices

The rapid development of flexible smart wearable devices puts forward the requirements for the flexibility of magnetoelectric functional devices. Due to the inverse magnetostrictive properties of magnetic materials, the stress/strain generated by the bending or stretching state can change the magnetic anisotropy of the magnetic thin film, thereby affecting the performance of magnetic devices. 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.

In recent years, our research team has systematically studied the regulation of stress/strain on the magnetic anisotropy of flexible magnetic thin 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 properties of flexible polyvinylidene fluoride (PVDF) piezoelectric films, the combined effect of temperature and electric fields on magnetic anisotropy was realized in flexible FeGa/PVDF and CoFeB/PVDF composite films. Effective regulation, its magnetic anisotropy increases with the increase of temperature, showing a positive temperature coefficient characteristic, which can solve the problem that the magnetic anisotropy of conventional magnetic materials decreases with the increase of temperature, which leads to high-frequency magnetic devices under high temperature. The problem of performance degradation [Sci. Rep. 4, 6615 (2014), Sci. Rep. 4, 6925 (2014)]. Furthermore, the stress magnetic anisotropy of the magnetic film was improved by the confinement of the flexible substrate, and a high-frequency magnetic film with a ferromagnetic resonance frequency of 5.3 GHz and a reflection loss of 28 dB was obtained [Appl. Phys. Lett. 106, 162405 (2015)].

Magnetic thin films with periodic wrinkle structures prepared by different growth processes

Figure 1: Magnetic thin films with periodic wrinkle structures prepared by different growth processes

Parallel microstrip flexible giant magnetoresistive spin valve device with periodic wrinkle structure

Figure 2: Parallel microstrip flexible giant magnetoresistive spin valve device with periodic wrinkle structure

For the spin valve device, the uniaxial magnetic anisotropy of the magnetic free layer is very small, so that the direction of the magnetic moment is easily changed by the external magnetic field, showing a high magnetic field sensitivity. However, for the flexible spin valve device, the stress from the substrate during the fabrication process and the stress generated by deformation such as bending or stretching during use will greatly reduce the magnetic field sensitivity of the flexible spin valve device. Recently, the research team compared two methods for fabricating magnetic films with periodic surface structures on flexible polydimethylsiloxane (PDMS) substrates (Fig. 1). The magnetic film directly grown on the stretched PDMS exhibits a regular surface wrinkle structure and weak magnetic anisotropy; a surface periodic structure is pre-generated by using a non-magnetic metal, and then the deposited magnetic film exhibits a strong magnetic anisotropy Heterosexual [Appl. Phys. Lett. 108, 102409 (2016)]. On the basis of this research, a flexible giant magnetoresistive spin valve sensor with high magnetic field sensitivity was fabricated by directly growing on stretched PDMS. The surface periodic structure can release longitudinal tensile strain and design surface-parallel microstripes. The transverse strain introduced by the Poisson effect can be released, thereby significantly reducing the effect of tensile strain on the magnetic anisotropy of the magnetic layer and avoiding the fracture behavior of metal films under tensile strain. Within the tensile strain range of %, the magnetoresistance, magnetic field sensitivity, and sample resistance can remain stable. [ACS Nano 10, 4403 (2016)] (Figure 2). Stretchable magnetic sensors with stable and reliable performance can be used as current sensors, position sensors, angle sensors, gear sensors, etc., integrated in flexible smart wearable devices, and have important application prospects.

This project is supported by the National Natural Science Foundation of China, the Chinese Academy of Sciences and the Ningbo Science and Technology Bureau.

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