1.
Introduction
With the development of the Internet of things and the application of wearable products, flexible electronic devices have received wide attention[1–3]. The realization of flexible electronics requires all building blocks from rigid electronics, such as active elements, optoelectronics, magneto-electronics, energy storage elements, can be reshaped on demand after fabrication[4]. Some commercially shapeable devices like electronic display[5–7], energy storage elements[8], and integrated circuitry[9], etc, is now available. From the beginning most of the research is devoted to the fabrication of shapeable high-speed electronics and optoelectronics. Only very recently, magnetic functionalities were added to the family of shapeable electronics. As we know, magnetic materials are important in the fabrication of electronic devices. For example, ferromagnetic materials play an important role in magnetic sensor and magnetic storage devices[10, 11]. So if we want to realize those devices as flexible, it is essential to realize the flexibility of the traditional functional magnetic materials and understand the evolution of the functional properties of the magnetic materials under the stress/strain environment. Furthermore, controlling the properties of materials and devices under the stress-related multiple fields is also a very important issue. Now some research on fabricating flexible magnetic materials has been done: flexible magnetic films prepared on flexible materials such as macromolecular polymers[12], ultra-thin metals[13], ultra-thin glasses[14], papers[15]. Therefore, the study of the physical properties of flexible magnetic thin films is a very important part of flexible magneto-electronics and an important foundation for the development of flexible magnetic devices. Magnetic anisotropy is one of the important internal parameters of magnetic materials, which not only determines the magnetic moment orientation and coercivity of magnetic materials, but also affects the working frequency and power consumption of magnetic devices. Therefore, the study of how to regulate the magnetic anisotropy and the understanding of the control mechanism have been the core problems in magnetic materials and magnetic physics.
In this review, we introduce recent progress in the fabrication of flexible magnetic films and devices, including the regulation of magnetic properties of magnetic thin films by stress-related multi-fields, and the design and fabrication of flexible magnetic devices.
2.
Flexible magnetic thin films
2.1
Stress dependence of magnetic anisotropy in flexible magnetic thin films
Magnetic thin films have been widely applied in magnetic sensors, magnetic tunnel junctions, microwave devices, and so on. Therefore, when those devices become shapeable, the magnetic films in devices will be affected by stress. In order to keep the performance of devices when they are subjected to stress, it is important to study the stress-dependent magnetic properties of magnetic films[11, 16–37]. Tang et al. systematically studied the strain-dependent magnetic anisotropy of amorphous Co40Fe40B20 films grown on flexible polyethylene terephthalate (PET) substrates[22]. Co40Fe40B20 thin films were grown on PET (~200 μm) by DC magnetron sputtering at room temperature, and fixed on several molds with different radius so that various tensile or compressive strains could be applied to the films, as shown in Figs. 1(a) and 1(b). When tensile (compressive) strain is applied, the normalized remanent magnetization Mr/Ms) in the perpendicular direction decreases (increases), which indicates that the anisotropy field of Co40Fe40B20 can be changed by strain [Figs. 1(c)–1(f)]. The strain dependence of the Mr/Ms and coercivity (Hc) for Co40Fe40B20 on PET substrate is shown in Fig. 2. The Mr/Ms of hard axis changes from 0.82 to 0.16 when strain is applied along the hard axis. Meanwhile Mr/Ms of easy axis changes from 0.88 to 0.52. These results indicate that the anisotropy field of Co40Fe40B20 film can be modified by strain through a magneto-mechanical coupling effect. In addition, Zhang et al. investigated the magnetic anisotropy of Fe81Ga19 crystalline films under application of external stress[38]. When the external magnetic field (H) is applied along the easy axis, Mr/Ms decreases drastically with increasing tensile strain from 0 to 0.3%. When H is applied along the hard axis, similar results are observed (Fig. 3).
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Figure1.
(Color online) The molds designed for applying strains[22]: (a) tensile strain, (b) compressive strain. Hysteresis loops of Co40Fe40B20 (70 nm)/PET under various external strains: (c) tensile strain, (d) compressive strain applied along the hard axis, (e) tensile strain, (f) compressive strain applied along the easy axis.
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Figure2.
(Color online) Strain dependence of normalized Mr/Ms and Hc for strains applied along the easy and hard axes, respectively[22].
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Figure3.
(Color online) Magnetic hysteresis loops of Fe81Ga19(50 nm)/Ta(10 nm)/PET with the direction of magnetic field along the (a) easy and (b) hard axes under compressive and tensile strains[38].
2.2
Control of magnetic anisotropy of magnetic thin films by stress-related multi-fields
Magnetic anisotropy of magnetic materials usually decreases with increasing temperature, showing negative temperature coefficients[39, 40], which is harmful to the performance of magnetic devices[41, 42]. From the applications point of view, it is necessary to fabricate materials whose magnetic anisotropy shows zero or positive temperature coefficients. Recently, Liu et al. realized positive temperature coefficients of magnetic anisotropy of 60 nm Co40Fe40B20 films grown on polyvinylidene fluoride (PVDF) substrates[43]. The magnetic hysteresis loops along the easy axis (x direction) and hard axis (y direction) at different temperatures were measured. The results show that the easy axis become easier and the hard axis become harder with increasing temperature [Figs. 4(a) and 4(b)]. Fig. 4(c) shows the Mr/Ms versus temperature for H parallel to the easy (x) and hard (y) directions, respectively. The crossover occurs near 298 K, which means that the sample is magnetically isotropic around 298 K. When the temperature decreases from 298 K, the easy axis is switched from the x to the y direction. When the temperature is higher than 298 K, Mr/Ms difference between the easy and hard axes increases with increasing temperature. Namely, the easy axis becomes easier while the hard axis becomes harder, indicating an abnormal enhanced magnetic anisotropy with increasing temperature.
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Figure4.
(Color online) Temperature dependence of magnetic properties of Co40Fe40B20/PVDF[43]. (a) In-plane magnetic hysteresis loops measured along x direction at different temperatures. (b) In-plane magnetic hysteresis loops measured along y direction at different temperatures. (c) Temperature dependence of the remanent magnetization with H along x and y directions, respectively. The inset: angular dependence of Mr/Ms at T = 290 and 310 K. (d) Temperature dependence of the remanent magnetization with H along x and y directions for the samples prepared at different temperatures.
The control of magnetization direction by stress-related multi-fields was investigated in flexible Fe81Ga19/PVDF structures[23]. The samples with different initial magnetic anisotropies were realized by applying compressive strains along the x direction of the PVDF during deposition processes. The hysteresis loops of Fe81Ga19/PVDF structures under different strains are measured at various field directions. The angular dependence of Mr/Ms is shown in Fig. 5, where the uniaxial magnetic anisotropy increases with increasing compressive strain. Fig. 6 shows the angular dependence of Mr/Ms ratios for the Fe81Ga19/PVDF film with ε = 0.06% and ε = 0.00% measured at different electric fields applied along the thickness direction of PVDF. Under zero electric field, there is a uniaxial magnetic anisotropy along the y direction [Fig. 6(a)]. When the electric field is increased to 267 kV/cm, magnetic isotropy appears. When the electric field is varied from 0 to ?267 kV/cm, the uniaxial magnetic anisotropy along the d32 direction of film were enhanced. When an electric field of 267 kV/cm is applied, the uniaxial magnetic anisotropy with an easy axis along the x direction is induced [Fig. 6(b)]. In contrast, when an electric field of ?267 kV/cm is applied, the uniaxial magnetic anisotropy with an easy axis along the y direction is induced. These experimental results suggest that the reorientation of uniaxial magnetic anisotropy by electric fields can be realized in flexible multiferroic heterostructures when the strain generated by the electric field is strong enough to cancel out the initial strain. The thermal cycling associated with an alternatively positive and negative magnetic field can switch the magnetization of FeGa/PVDF film between the positive and negative directions [Fig. 7].
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Figure5.
(Color online) Angular dependence of normalized remanent magnetization for Fe81Ga19/PVDF films with different compressive strains along the x direction[23].
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Figure6.
(Color online) Angular dependence of normalized remanent magnetization for the Fe81Ga19/PVDF film with a compressive strain of (a) 0.06% and (b) 0.00% along the x direction under different electric fields[23].
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Figure7.
(Color online) (a) Magnetization of FeGa/PVDF film reversed by thermal cycling under an alternatively positive and negative magnetic field. Top: thermal cycles between 280 and 320 K. Middle: an alternatively positive and negative magnetic field used for the measurements. Bottom: measurements of magnetization under the sequence of thermal cycles and magnetic field[23].
2.3
Stress dependence of exchange bias effect in flexible magnetic bilayers
Exchange bias effect is induced by interfacial exchange coupling between ferromagnet layer and antiferromagnet layer, which are considered as one of the most crucial factors in determining the performance of flexible spintronic sensors[44]. So it is necessary to study how external stress affects the magnetic characteristics in exchange biased bilayer[45–51]. Zhang et al. investigated the effects of mechanical strains on magnetic properties of Fe81Ga19/Ir20Mn80 bilayers grown on flexible polyethylene terephthalate substrates[20]. The main results are shown in Fig. 8. The strength and orientation of the uniaxial anisotropy can be modified by mechanical strains, and the exchange bias field decrease drastically when a compressive strain parallel to the pinning direction (PD), but only a slight decrease under a tensile strain.
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Figure8.
(Color online) Strain dependence of (a) loop squareness, (b) Hc, (c) Heb for H parallel and ε perpendicular to the PD, and (d) Hc for H perpendicular and ε parallel to the PD in FeGa(10 nm)/IrMn(tIrMn) bilayers with different tIrMn[20].
Zhang et al. investigated the influences of thermal deformation on the magnetic properties of flexible exchange bias (EB) grown on PVDF substrates[52]. They fabricated Fe81Ga19/Ir20Mn80 bilayers with pinned direction along the x and y directions of PVDF, respectively. The magnetic hysteresis loops of FeGa/IrMn EB heterostructures were measured at various temperatures while applied H along the x and y directions of PVDF. The results are shown in Fig. 9. The values of Hc, Heb, and Mr/Ms change with the variation in temperature. Therefore, temperature controls the magnetic properties of Fe81Ga19/Ir20Mn80 EB heterostructures.
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Figure9.
(Color online) Hysteresis loops measured at different temperatures for the FeGa/IrMn bilayers deposited on flexible PVDF membranes with the PD set (c, d) along the x direction and (e, f) along the y direction. The magnetic field is applied at (c, e) θ = 0° and (d, f) θ = 90° with respect to the PD. The configurations of measurement are schematically indicated in the insets of (c) and (e), respectively[52].
2.4
Fabrication of flexible magnetic films with controllable magnetic anisotropy
Qiao et al. propose a method to grow flexible magnetic films with controllable magnetic anisotropy[53]. Amorphous Co40Fe40B20 (CoFeB) thin films grown on flexible polyimide (PI) substrates, which were fixed on convex molds with different curvatures during the magnetron sputtering deposition [Fig. 10(a)]. After deposition the samples are released from the convex molds and flattened, a compressive stress is thus induced in the film as shown in Fig. 10(b). Due to the positive magneto-striction of CoFeB, the compressive strain induces a magnetic anisotropy in the film. Fig. 10(c) shows the angular dependence of normalized remanent magnetization (Mr/Ms), which oscillates with 180? periodicity showing a uniaxial magnetic anisotropy in the CoFeB film. They summarize the factors that influence the magnetic anisotropy. The compressive strain induced in the film increases when the curvature of the mold increases, which gives rise to a larger uniaxial magnetic anisotropy as shown in Fig. 10(d). Investigations on films with difference thicknesses show that the anisotropy field is enhanced with increasing CoFeB film thickness at a given mold curvature [Fig. 10(e)]. Furthermore, the magnetic anisotropy also increases when the thickness of substrate is increased as shown in Fig. 10(f). Furthermore, for FeGa/polycrystalline films an obvious in-plane uniaxial magnetic anisotropy can be enhanced by increasing the applied pre-strains on the substrates during film growth[54].
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Figure10.
(Color online) (a) A schematic illustration of the experimental setup for sample fabrication (b) diagram of the flattened state of magnetic film (c) angular dependence of normalized Mr/Ms of 70 nm CoFeB film (d) the curvature radii dependence of the anisotropy field (Hk) for CoFeB films with different thicknesses (e) the thickness of film dependence of the anisotropy field (Hk) with different curvature radii, (f) the thickness of substrate dependence of the anisotropy field (Hk) for CoFeB films with different curvature radii[53].
In addition, Qiao et al. designed another method to fabricate CoFeB films with enhanced stress-invariance[55]. They fixed PI substrates on the convex molds to apply stress and apply magnetic fields during deposition. Both the applied stress and magnetic fields induce magnetic anisotropy in the magnetic thin films. They fabricated the flexible magnetic film by DC magnetron sputtering as shown in Fig. 11(a). After deposition the magnetic thin films are released from the convex molds and flattened, a compressive stress is induced in the film as shown in Fig. 11(b). The angular dependence of hysteresis loops under different tensile and compressive strain generated via outward and inward bending of the sample were measured at room temperature, the results are shown in Fig. 11(c). It shows that normalized Mr/Ms of film prepared without pre-induced magnetic anisotropy sharply changes from 0.77 to 0.14 under tensile stress, while the Mr/Ms along the easy axis only changes from 0.92 to 0.43 for the film with pre-induced magnetic anisotropy. So they have fabricated magnetic thin film with enhanced stability of magnetization direction. Besides, they study the stress-coefficient of CoFeB film by controlling the magnetic anisotropy on PVDF substrate, the stress-coefficient of magneto-elastic anisotropy in amorphous CoFeB film is obtained to be 170.7 × 103 erg/cm3 GPa?1[56].
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Figure11.
(Color online) (a) A schematic illustration of the experimental setup for sample fabrication during depositing processes, both bowed growth and magnetic field are used to induce the anisotropy of magnetic materials. (b) Diagram of the flattened state of magnetic film after deposition, a compressive stress is produced in film when substrates are shaped from convex to flat. (c) The strain dependence of normalized Mr/Ms for bending CoFeB films with and without pre-induced magnetic anisotropy, respectively[55].
For the spin valve device the magnetic free layer has a small uniaxial magnetic anisotropy, so that the direction of the magnetic moment can be easily modified by external magnetic fields, exhibiting a high magnetic field sensitivity. However, for the flexible spin valve device the stress from the substrates and the strains induced by bending or stretching would greatly reduce the magnetic field sensitivity of the flexible spin valve device. Therefore, it is important to fabricate magnetic films that are insensitive to external stress. Zhang et al. compared two methods of preparing magnetic films with periodic structures on a flexible polydimethylsiloxane (PDMS) substrate[57]. For the method A, as shown in Fig. 12(a), the PDMS substrates were stretched 30% by using a homemade stretching apparatus before deposition. Subsequently, Ta layer and an FeGa layer were deposited on the pre-strained PDMS by using direct current magnetron sputtering. After the pre-strain was removed from the sample, an ordered wrinkled nanostructure appears on the surface. In the method B, as shown in Fig. 12(b), Ta deposited on 30% pre-strained PDMS, and then, FeGa films with various thicknesses were deposited on the relaxed Ta/PDMS patterned surface. The magnetic properties of the wrinkled FeGa films were measured and shown in Fig. 13. The magnetic films directly grown on the stretched PDMS exhibit regular surface fold structures and weak magnetic anisotropy, and the films by depositing FeGa on a wrinkled Ta/PDMS surface show a remarkable uniaxial magnetic anisotropy.
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Figure12.
(Color online) (a) Schematic illustration of the method A used to fabricate wrinkled FeGa films. (b) Schematic representation of the method B used to fabricate wrinkled FeGa films[57].
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Figure13.
(Color online) Hysteresis loops for FeGa films with different thicknesses obtained by the method A when the magnetic field is applied (a) parallel and (b) perpendicular to wrinkles, (c) coercive field Hc as a function of the magnetic field orientation with respect to the wrinkles. Method B when the magnetic field is applied (d) parallel and (e) perpendicular to wrinkles, (f) coercive field Hc as a function of the magnetic field orientation with respect to the wrinkles[57].
3.
Flexible magnetic devices
3.1
Stress dependence of GMR effect in magnetic spin valve devices
The emerging field dubbed “straintronics”[58–64] involves the integration of strain with spintronic devices including magnetic tunnel junctions and magnetic spin valves, therefore it is necessary to study stress dependence of GMR effect in magnetic spin valve devices[65–73]. The giant magnetoresistance (GMR) effect[74, 75], discovered in 1988, is widely applied in read heads of ultrahigh density magnetic recording systems and magnetic sensors; the deposition of GMR films onto flexible substrates was first demonstrated by Parkin et al. in 1992[76]. From the practical application viewpoint, two critical issues need to be addressed for flexible GMR devices: (i) the limited stretchability due to the fragile nature of the magnetic thin film, and (ii) the unstable magnetic field sensitivity due to the large residual strain. Some studies were performed using plastic polyimide substrates[80], which allowed for magnetoelectric measurements at tensile strains of up to 0.75% directly applied by stretching[71]. Higher levels of deformation were obtained by Chen et al., who deposited extended GMR films on buffer-coated polyester transparency[20], as shown in Fig. 14(a). No performance degradation was observed in the magneto-resistive elements after 1000 cycles of bending. Furthermore, tensile testing [Fig. 14(b)] of the flat GMR film up to 2% strain revealed a mechanism for a mechanical fine tuning of the magnetoelectric properties.
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Figure14.
(Color online) (a) Schematic illustration of (Co/Cu)N MLs deposited on Si and flexible substrates, and a photographic image of circularly bent (Co/Cu)20 MLs deposited on polyester substrate. (b) Strain-dependent GMR of (Co-1 nm/Cu-t nm)30 MLs deposited on polyester substrates under 0.2 Tesla[16].
Melzer et al. obtained a highly stretchable spin valve sensor which shows a stable GMR magnitude during stretching, by means of the predetermined periodic fractures and random wrinkling on the super-elasticpoly (dimethylsiloxane) (PDMS) substrate[11]. The fabrication process is schematically shown in Fig. 15(a), the results of testing the GMR performance under uniaxial tensile deformations are shown in Figs. 15(b)–15(e). The results show that tensile strains of up to 29% could be applied without major changes of the GMR magnitude of around 7%. A sensitivity of 0.8% Oe-1 was achieved for the as-prepared spin valve, which decreases down to 0.2% Oe-1 upon stretching. In situ cyclic loading experiments were conducted, which revealed a very stable behavior over 500 cycles, accounting for the elasticity and reliability of the prepared elements.
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Figure15.
(Color online) (a) Fabrication process of stretchable magnetic sensors with random wrinkling and periodic fracture of the spin valve stack. In situ GMR measurements during tensile deformations of the prepared SV sensor elements. (b) Full loop and (c) minor loop GMR curves at different applied strains. (d) Magnitude of the GMR ratio and absolute sample resistance in dependence of tensile strain up to the point where the electrical contact is lost. (e) Coercivity and sensitivity of the free layer over applied tensile strain[11].
Li et al. employed a strain-relief architecture based on the strain-engineered wrinkles to fabricate flexible magneto-electronics[78], which can be realized simply by direct sputtering with metal mask on pre-strained PDMS substrates [Fig. 16(b)]. In that work, they fabricated GMR dual spin-valve sensor exhibiting nearly unchanged MR ratio of 9.9% and a magnetic field sensitivity up to 0.69%/Oe, and zero-field resistance in a wide range of stretching strain, making it promising for applications in conformal shapes or a movement parts. They employed a dual spin valve structure [Fig. 16(a)] to construct multilayer magnetic sensors, due to the spin-dependent scattering on both sides of the free layer. Dual spin valve structure has been demonstrated to show higher GMR ratio and higher magnetic field sensitivity than those of conventional simple spin valves.
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Figure16.
(Color online) (a) The layer structure of the dual spin valve (not to scale). (b) An illustrative drawing of the experimental setup for the sample fabrication. (c) A schematic diagram for the appearance of as-fabricated sample after releasing the tensile pre-strain[78].
The GMR performance of the wrinkled spin valve ribbons (SVRs) fabricated under various pre-strain was measured by the standard four wire method with an in-plane magnetic field applied perpendicular to the long axis of SVRs, i.e., parallel to the exchange bias [the inset of Fig. 17(c)]. The maximum GMR ratios (ΔR/R0) of 9.8%, 10.2%, and 7.9% are obtained in the spin valves of SVR-0%, SVR-30%, and SVR-50%, respectively [Figs. 17(a)–17(c)]. The almost constant GMR ratio of 7.9%, magnetic field sensitivity of 0.4%/Oe and zero field resistance of 92 Ω are achieved in the tensile strain range of 0?25% [Figs. 17(d)–17(f)]. To assess the stretching fatigue of the wrinkled SVRs, they conducted two sequential cyclic stretching tests on the sample of SVR-30%, the result reveals an excellent stability of the GMR performance during at least hundreds of times of stretching-releasing cycles.
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Figure17.
(Color online) GMR curves of (a) SVR-0% without applied strain, (b) SVR-30% with 0 (solid circles) and 25% (open circles) applied tensile strain, and (c) SVR-50% with 0 (solid circles) and 25% (open circles) applied tensile strain. Inset in (c) shows the GMR measurement geometry: both the current and the applied tensile strain are parallel to the ribbons, while the magnetic field is applied perpendicular to the ribbons. The applied tensile strain dependence of (d) the GMR ratio (ΔR/R0)max (squares), (e) the magnetic field sensitivity S (circles), and (f) the zero-field resistance R0 (triangles) for SVR-30% (solid symbols) and SVR-50% (open symbols). The uniaxial tensile strain from 0 to 35% is applied along the ribbons[78].
3.2
Imperceptible GMR sensor
Flexible wearable electronics and biomedical devices have become the subject of active research area in recent years. Sensor devices are the fundamental components for wearable electronics and biomedical devices. Therefore sensor devices should not only be flexible, but also elastic and ideally even withstand high biaxial deformations.
Melzer et al. constructed highly sensitive giant magnetoresistive (GMR) sensor elements on ultrathin 1.4 umpolyethylene terephthalate (PET) foils[79]. They demonstrated magneto-electronics that can reversibly attain tensile strains up to 270%, nearly a tenfold increase over previously reported concepts[11]. They prepared magnetosensitive elements that are compliant to uniaxial and biaxial deformation, creating a universal potential for applications in stretchable electronic systems. Moreover, their devices are remarkably stable, withstanding 1000 stretch cycles without fatigue. Our ultrathin magneto-electronic elements can be made stretchable in a one-step post-fabrication transfer process by laminating them onto a pre-stretched elastomer (3 M very high bond (VHB)), as illustrated in Fig. 18(a) for uniaxial strain. The magneto-sensitive capabilities of the presented elements are not affected by the post-processing [Fig. 18(b)], on the example of a GMR multilayer device measured before and after the lamination and wrinkling on the VHB tape. GMR characterization of devices is investigated by in situ stretching stage [Fig. 19(a)]. The recorded GMR curves at different tensile strain levels up to 270% along the direction of pre-strain are congruent with each other [Fig. 19(b)]. Imperceptible magneto-electronic elements were readily worn directly on the palm of the hand [Fig. 20]. Here, a set of five GMR sensors intimately conforms to the inner hand and simultaneously follows the motions and deformations of the skin when the hand is moved [Figs. 20(a) and 20(b)]. One sensor element is electrically contacted with thin copper wires and the resistance of the on-skin sensor is recorded while moving the fingers, opening and closing the hand, applying a magnetic field with a permanent magnet, and alternating the distance to the magnet [Fig. 20(c)]. The recorded signal is shown in Fig. 20(d).
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Figure18.
(Color online) (a) The post fabrication step to obtain ultra-stretchable GMR sensors from imperceptible elements by face-down lamination onto a highly pre-stretched stripe of VHB tape. Four contact pads are reaching beyond the tape (top). Relaxing the elastomer results in out-of-plane wrinkling of the sensor foil and enables re-stretching (bottom). (b) GMR curves of an imperceptible Co(1 nm)/[Co(1 nm)/Cu(2.2 nm)]30 multilayer element at a flat state before lamination onto the pre-stretched VHB support and at a highly wrinkled state after the release of pre-strain[79].
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Figure19.
(Color online) (a) A Co(1 nm)/[Co(1 nm)/Cu(2.2 nm)]30 sample mounted to the in situ stretching stage relaxed (left) and fully elongated (up). The arrow in the left image indicates the axis of the applied magnetic field. (b) GMR curves recorded for strains from 0% to 250% in increments of 50%, plus 270% and 275%, according to the legend[79].
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Figure20.
(Color online) Imperceptible GMR sensors: (a)–(c) Imperceptible GMR sensor array on a human palm with one element connected to a readout circuit during rest, moving the hand and in proximity to a permanent magnet, respectively. All scale bars: 20 mm. (d) recorded resistance of the sensor element for panels (a)–(c)[79].
4.
Summaries and outlook
The stress dependence of magnetic anisotropy in flexible magnetic thin films has been studied, showing that the magneto-elastic coupling effect would change magnetic anisotropy of flexible magnetic thin films. Based on stress engineering, flexible magnetic thin films with controlled magnetic anisotropy have also been designed and fabricated. Besides, the regulation of magnetic anisotropy in magnetic thin films by stress-related multi-physics fields has also been achieved. In the aspect of flexible magnetic devices, design and fabrication of strain-invariance device have also studied from different ways. In order to further study magnetic anisotropy of magnetic thin films, it is necessary to study the microscopic mechanism of magnetic anisotropy induced by stress. From the point of view of application, it is inevitable to fabricate stress-insensitive magnetic thin films. Besides ferromagnetic materials, antiferromagnetic materials also play an important role in magneto-electric devices, so it is essential to study the dependence of stress and antiferromagnetic magnetic moment.
