Preparation and Physical Properties of Stretchable FeRh Films with Periodic Wrinkle Structure

Stretchable magnetic materials are highly desirable for developing flexible high‐performance magnetoelectronic devices. However, it is still challenge to fabricate stretchable magnetic films with high growth temperature. In this work, metamagnetic FeRh films are grown on mica substrates at high temperature and then transferred onto the prestrained polydimethylsiloxane (PDMS). Due to the large modulus mismatch between FeRh films and PDMS, FeRh films with periodic wrinkle pattern is formed after releasing the prestrain, enabling a controllable stretchability up to a few tens of percent. Moreover, the meta‐magnetic phase transition temperature can be effectively controlled by the prestrain, with an increase of ≈4 K for every 10% increase in the prestrain. In case of the applied tensile strain less than the prestrain, the obtained flexible FeRh films can maintain stable performance after thousands of stretches, making it promising for applications in flexible magnetoelectronic devices.


DOI: 10.1002/admi.202202487
flexible electronics which can be integrated with 3D freedom surface is vitally important. [5] As one of the most important electronic materials, metamagnetic material FeRh alloy with CsCl-type structure have drawn people's attention because of its first-order phase transition near room temperature. [6][7][8][9] This material is antiferromagnetic (AF) at room temperature and undergoes a phase transition to the ferromagnetic (FM) phase upon heating above the transition temperature. [10,11] Due to this transition, FeRh show interesting phenomena such as large magnetoresistance effect, magnetocaloric effect, [12][13][14][15] barocaloric effect, [16] heat assisted, or electrically assisted magnetic recording, [17][18][19][20][21] etc. These phenomena endow it broad potential applications in future flexible spintronic devices. Previous studies revealed that the magnetic phase transition of FeRh is very sensitive to strain. [22][23][24][25][26] In the process of flexibility, FeRh film will inevitably be affected by strain, which will change its properties. Therefore, it is vitally important to develop a method for preparing flexible FeRh films that are insensitive to strain.
In order to obtain a magnetic film with stable performance under strain, it is necessary to release the stress of the flexible magnetic film through structural design. The most common method is to prepare magnetic thin films with "wrinkle structure" on elastic substrates such as polydimethylsiloxane (PDMS). [27][28][29][30][31][32] For example, Oliver et al. produced a magnetic sensor with a constant giant magnetoresistance (GMR) at 29% tensile strain by sputtering the spin valve on prestrained PDMS. [33] Subsequently, Li et al. prepared a spin valve device with a periodic wrinkle structure that could maintain sensitivity, magnetoresistance, and other properties under 30% strain by sputtering GMR sensors on prestrained PDMS. [34] However, this method encounters severe challenges when preparing flexible FeRh thin films. The most important one is that the preparation of high quality FeRh films often requires high growth temperature, and most organic flexible substrates cannot withstand it. Therefore, it is necessary to explore new methods to prepare flexible FeRh which is insensitive to strain. In recent years, people have explored several methods to prepare flexible magnetic thin films at high growth temperature. One method is to directly grow it on flexible inorganic substrates that can withstand high temperature. For example, Fina et al. obtained flexible FeRh films by sputtering MgO on Stretchable magnetic materials are highly desirable for developing flexible high-performance magnetoelectronic devices. However, it is still challenge to fabricate stretchable magnetic films with high growth temperature. In this work, metamagnetic FeRh films are grown on mica substrates at high temperature and then transferred onto the prestrained polydimethylsiloxane (PDMS). Due to the large modulus mismatch between FeRh films and PDMS, FeRh films with periodic wrinkle pattern is formed after releasing the prestrain, enabling a controllable stretchability up to a few tens of percent. Moreover, the meta-magnetic phase transition temperature can be effectively controlled by the prestrain, with an increase of ≈4 K for every 10% increase in the prestrain. In case of the applied tensile strain less than the prestrain, the obtained flexible FeRh films can maintain stable performance after thousands of stretches, making it promising for applications in flexible magnetoelectronic devices.

Introduction
Flexible electronic devices have attracted extensive attention in the fields of intelligent robots, human-computer interaction health care and intelligent display because of their unique advantages such as deformability, portability, and conformal attachment with complex surfaces. [1][2][3][4] Fabrication of www.advmatinterfaces.de HASTELLOY. [20] However, flexible FeRh films obtained by this method can only withstand limited strain. Another method to obtain flexible magnetic films is to use sacrificial layers. [35,36] For instance, Li et al. grew FeRh film on the NaCl sacrificial layer and spin coated PDMS on the FeRh film. They obtained a flexible FeRh/PDMS film after dissolved the sacrificial layer substrate. [37] However, the flexible FeRh films also cannot withstand large strain and cracks form as the radii of curvature is lower than 1.5 mm. Recently, Liu et al. fabricated a high quality BaTiO 3 film on the SrAlO 3 sacrificial layer and then transferred it to the prestrained PDMS. After dissolving the sacrificial layer, a flexible BaTiO 3 film with a periodic wrinkle structure was obtained, [36] which provided a new perspective for the preparation of flexible FeRh films that are insensitive to strain.
In this work, flexible FeRh films with periodic wrinkle structure were obtained by transferring FeRh films to flexible PDMS substrate, which utilize the weak van der Waals bonding force between artificial fluoro-crystalline mica substrate and FeRh film. This method for preparing flexible film can overcome the problem of high growth temperature needed in many magnetic films (The characteristic of different preparation method for flexible films are listed in Table 1). The wavelength and amplitude of FeRh wrinkle structure can be controlled by the prestrain of substrate and the thickness of FeRh film. The obtained flexible FeRh film exhibits a significant AF to FM phase transition near 370 K. In case of the applied tensile strain less than the prestrain, no cracks appear in the stretched film, rendering stable electrical and magnetic properties upon deformation, which shall be benefiting for the construction of stretchable high-performance magnetoelectronic devices.

Figure 1a
schematically shows the processes of fabricating flexible FeRh films with a wrinkling morphology. FeRh thin films with different thickness was primarily grown on Mica substrate (5 × 5 mm 2 ) by magnetron sputtering system, and the thin film was then adhered to the prestrained PDMS (8 × 20 mm 2 ). Polyimide (PI) tape was used to separate Mica from FeRh. To ensure a firm adhesion between PI tape and Mica, the PI/Mica/FeRh/PDMS laminate was firmly pressed with forceps, the PI tape was subsequently removed from one end. Consequently, the Mica substrate was removed, leaving the FeRh film on the PDMS. This separation between FeRh with mica implies that the adhesion between FeRh and mica is weaker than that between mica and PMDS. The interaction between the FeRh layer and PDMS comes from the Van der Waals' force between the FeRh films and the surface of PDMS.
After releasing the prestrain, the FeRh film forms a wrinkling morphology because of the large mismatch of Young's moduli between the compliant PDMS substrates and the rigid metal layers. The obtained FeRh films can keep performance stable over a period of several months. Figure 1b,c shows that flexible FeRh films can withstand bending and stretching strain, respectively.
To confirm that the Mica on the back of the FeRh film was stripped cleanly, i.e., the cleavage occurred at the interface between FeRh and Mica. XRD-2θ scanning was performed on FeRh/Mica and FeRh/PDMS surfaces at room temperature, as shown in Figure 2. The typical X-ray θ-2θ scan for a FeRh/ Mica shows clear (001), (002), and (110) peaks, indicating the B2 structure of FeRh, as shown in Figure 2a,b shows the X-ray θ-2θ scan for the FeRh/PDMS. There is no diffraction peak of Mica in the flexible FeRh films prepared at different prestrain ratios, indicating that Mica is completely stripped when FeRh films are transferred from Mica to PDMS. The diffraction peak of the FeRh/PDMS is same as that of the FeRh film on mica substrate. Prestrain of PDMS basically do not affect the diffraction peak position of the film.
The thickness of the rigid film, and the prestrain of the substrate play a crucial role in the formation of wrinkle structure. The flexible FeRh films were prepared by pasting FeRh films (30 nm) on 20%, 30%, 40%, and 50% prestrained PDMS. The morphology of the wrinkle structures was characterized by atomic force microscopy (AFM) in 60 × 60 µm 2 . Figure 3a(iv) shows the cross-sectional morphology of the wrinkle structure by scanning electron microscope (SEM). The amplitude (A) was the half distance from peak to valley, and the wavelength was the distance between two nearby valleys, as shown in Figure 3a(iv). Figure 3a(I-IV) shows the morphology of 30 nm thick FeRh thin film prepared with different prestrained PDMS. The wavelength of flexible FeRh films decreases with the increase of prestrain. Wrinkles are arranged parallel to each other, showing good periodicity. Figure 3a(i-iii) shows the morphology of 20% prestrained FeRh thin film prepared with different thickness. The wavelength of flexible FeRh films increase with the increase of prestrain. Furthermore, the dependence of the wrinkle structure wavelength and amplitude on the applied pre-strain and film thickness are obtained, as shown in Figure 3b. As the pre-strain ratio increases from 20% to 50%, the amplitude of the wrinkle structure increases from 4.6 to 7.4 µm, while the wavelength decreases from 22.9 to 21.0 µm. For the film thickness increase from 30 to 50 nm, the wrinkle wavelength increases from 22.9 to 38.5 µm, and the wrinkle amplitude increases from 4.6 to 7.6 µm. The above experimental results can be theoretically predicted by an elastic model, which describes the wavelength and amplitude of the wrinkle by minimizing the total strain energy composed of the where t f is the thickness of metal films, ε pre is the prestrain ratio of flexible substrate, which have the same variation trend, and there is a multiple relationship between them, that is λ Experiment = µ 1 ·λ Calculated , A Experiment = µ 2 ·A Calculated , where µ 1 and µ 2 is related to the specific experimental cases. For instance, the film transferred to the prestrained flexible substrate may not be strain-free, and the flexible substrate is also not strain-free after the prestrain release. In addition, the poorly defined interfaces between film and flexible substrate, and the unknown spatial uniformity differences in mechanical properties may also contributed.

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The magnetic phase transition of the stretchable FeRh films was investigated. Figure 4a shows the temperature-dependent magnetization of flexible FeRh films measured with an in-plane magnetic field of 4 T. During the heating process, the magnetization increases sharply from 70 to 1020 emu cm −3 around 370 K, indicating an AF to FM phase transition. As the   Figure 4b shows the dependence of Tt on the prestrain. It is found that the phase transition temperature increases about 4 K at 10% increase of the prestrain ratio. It can be seen from the results in Figure 2 that the FeRh film will suffer small compressive strain during the release of PDMS prestrain. The compressive stress could stabilize the AF phase in FeRh film and thus suppress the process of AF-FM phase transition. [23] Consequently, the phase transition temperature shifts toward higher temperature with the increase of prestrain.
In addition, uniaxial strain was applied to characterize the electrical and magnetic stability of the flexible FeRh films. To do that, the flexible FeRh film was etched by argon ion to obtain a thin film strip with a size of about 200 × 3000 µm 2 . Liquid metal was used to draw electrodes from the endpoints of the obtained FeRh film strip to the edge of PDMS. Figure 5a shows the dependence of the tensile strain on the resistances change for the 30 nm thick FeRh film. When the strain applied to the flexible FeRh film is within the PDMS prestrain range, the resistance of the sample is stable, and the resistance variation is within 1%, while for strain exceeds the prestrain, cracks will form within the flexible FeRh strips and the resistance will sharply increase to an undetectable value. In addition, the influence of strain on the phase transition temperature of the flexible FeRh film is explored, and the results are shown in Figure 5b. The phase transition temperature of the flexible FeRh film remains unchanged after thousands of cycles. In the process of stretching, the wrinkle morphology of the flexible film can be gradually flattened to the original state, and new wrinkle morphology can be adaptively formed in the process of strain release as shown in the inset of Figure 5b. The conductive path does not change, so that the resistance stability of the flexible FeRh in the process of stretching is guaranteed.

Conclusion
In summary, we prepared flexible FeRh films on PDMS. The wrinkle structures on the film surface changed regularly with wavelength varied from 22.9 to 21.0 µm and amplitude changed from 4.6 to 7.5 µm by changing the prestrain of PDMS from 20% to 50%, the wavelength and amplitude also changed regularly by changing the thickness of FeRh films, showing agreement with the theoretical calculation. The phase transition temperature increased by about 4 K at 10% increase in the prestrain. As the applied tensile strain is less than the prestrain, the resistance, and phase transition temperature of flexible FeRh films remain stable and show excellent fatigue performance. This result provides an important basis for the construction of stretchable high-performance magnetoelectronic devices.

Experimental Section
FeRh thin films were prepared by ultrahigh vacuum magnetron sputtering on commercial mica substrates (Harbin Tebo Technology) with a base pressure below1 × 10 −8 Torr. The films were grown at 600 °C and then in situ annealed at 700 °C for 1 h. The layer thicknesses were controlled by the deposition time, which were calibrated by X-ray reflectivity (XRR).
The PDMS substrate used for the flexible FeRh film is Dow Corning 184. The basic components and curing agent are mixed together at a ratio of 10:1, coated on silicon wafers, and cured in the oven at 100 °C for 1 h before use.
The wrinkling morphology of flexible thin films was characterized by atomic force microscopy (AFM). The scan size is 60 × 60 µm 2 , the resolution is 256 × 256 pixels, and the scan frequency is 1 Hz. Raw data processing (background subtraction, flattening, and filtering) and subsequent atomic force micrograph analysis were performed using Nano-Scope analysis software.
The crystal structures of FeRh films on mica and PDMS were characterized by a high-resolution X-ray diffraction (HRXRD). During the test, A copper target was used as the incident X-ray source, the incident wavelength was 1.5406 A, the voltage was 40 kV, the current was 40 mA, the measurement Angle range was 20-80°, and the scanning step was 0.01°.
The temperature-dependent magnetic behaviors of FeRh films on mica and PDMS were measured by Superconducting Quantum Interference Device (SQUID-VSM) at a fixed magnetic field of 4 T. The test temperature range is 150-400 K, and the test step is 2 K.