Stretching-Tunable High-Frequency Magnetic Properties of Wrinkled CoFeB Films Grown on PDMS

Advances in Flexible Magnetosensitive Materials and Devices for Wearable Electronics

Emerging fields, such as wearable electronics, digital healthcare, the Internet of Things, and humanoid robots, highlight the need for flexible devices capable of recording signals on curved surfaces and soft objects. In particular, flexible magnetosensitive devices garner significant attention owing to their ability to combine the advantages of flexible electronics and magnetoelectronic devices, such as reshaping capability, conformability, contactless sensing, and navigation capability. Several key challenges must be addressed to develop well-functional flexible magnetic devices. These include determining how to make magnetic materials flexible and even elastic, understanding how the physical properties of magnetic films change under external strain and stress, and designing and constructing flexible magnetosensitive devices. In recent years, significant progress is made in addressing these challenges. This study aims to provide a timely and comprehensive overview of the most recent developments in flexible magnetosensitive devices. This includes discussions on the fabrications and mechanical regulations of flexible magnetic materials, the principles and performances of flexible magnetic sensors, and their applications for wearable electronics. In addition, future development trends and challenges in this field are discussed.

Thickness-Dependent Gilbert Damping and Soft Magnetism in Metal/Co-Fe-B/Metal Sandwich Structure

The achievement of the low Gilbert damping parameter in spin dynamic modulation is attractive for spintronic devices with low energy consumption and high speed. Metallic ferromagnetic alloy Co-Fe-B is a possible candidate due to its high compatibility with spintronic technologies. Here, we report thickness-dependent damping and soft magnetism in Co-Fe-B films sandwiched between two non-magnetic layers with Co-Fe-B films up to 50 nm thick. A non-monotonic variation of Co-Fe-B film damping with thickness is observed, which is in contrast to previously reported monotonic trends. The minimum damping and the corresponding Co-Fe-B thickness vary significantly among the different non-magnetic layer series, indicating that the structure selection significantly alters the relative contributions of various damping mechanisms. Thus, we developed a quantitative method to distinguish intrinsic from extrinsic damping via ferromagnetic resonance measurements of thickness-dependent damping rather than the traditional numerical calculation method. By separating extrinsic and intrinsic damping, each mechanism affecting the total damping of Co-Fe-B films in sandwich structures is analyzed in detail. Our findings have revealed that the thickness-dependent damping measurement is an effective tool for quantitatively investigating different damping mechanisms. This investigation provides an understanding of underlying mechanisms and opens up avenues for achieving low damping in Co-Fe-B alloy film, which is beneficial for the applications in spintronic devices design and optimization.

Deformation-Induced Photoprogrammable Pattern of Polyurethane Elastomers Based on Poisson Effect

Elastomers with high aspect ratio surface patterns are a promising class of materials for designing soft machines in the future. Here, a facile method for fabricating surface patterns on polyurethane elastomer by subtly utilizing the Poisson effect and gradient photocrosslinking is demonstrated. By applying uniaxial tensile strains, the aspect ratio of the surface patterns can be optionally manipulated. At prestretched state, the pattern on the polyurethane elastomer can be readily constructed through compressive stress, resulting from the gradient photocrosslinking via selective photodimerization of an anthracene-functionalized polyurethane elastomer (referred to as ANPU). The macromolecular aggregation structures during stretching deformation significantly contribute to the fabrication of high aspect ratio surface patterns. The insightful finite element analysis well demonstrates that the magnitude and distribution of internal stress in the ANPU elastomer can be regulated by selectively gradient crosslinking, leading to polymer chains migrate from the exposed region to the unexposed region, thereby generating a diverse array of surface patterns. Additionally, the periodic surface patterns exhibit tunable structural color according to the different stretching states and are fully reversible over multiple cycles, opening up avenues for diverse applications such as smart displays, stretchable strain sensors, and anticounterfeiting devices.

Enhanced Stress Stability in Flexible Co/Pt Multilayers with Strong Perpendicular Magnetic Anisotropy

Due to the magnetoelastic coupling, the magnetic properties of many flexible magnetic films (such as Fe, Co, and Ni) are sensitive to mechanical stress, which deteriorates the performance of flexible magnetoelectronic devices. We show that by stacking Co and Pt alternatively to form multilayers with strong perpendicular magnetic anisotropy (PMA), both magnetic hysteresis and magnetic domain measurements reveal robust PMA against external stress. As the PMA weakens at increased Co thickness, the magnetic anisotropy is vulnerable to external stress. These results were understood based on a micromagnetic model, which suggests that the strength of magnetoelastic anisotropy with respect to initial effective magnetic anisotropy affects the stress-stability of the film. Although the stress coefficient of magnetoelastic anisotropy is enhanced at reduced Co thickness, the concomitant increase of initial effective magnetic anisotropy guarantees a robust PMA against external stress. Our results provide a route to constructing flexible magnetoelectronic devices with enhanced stress stability.

Tunable Magnetic Domain Patterns in Thickness-Gradient Nickel Films on Flexible PDMS Substrates

Flexible magnetoelectronic devices (based on magnetic films) have great application prospects in the fields of information storages, bionic robotics, and electronic skins. The intrinsic stress and external loading are very important to modulate the structures and properties of flexible magnetic films due to the magnetoelastic coupling effect. Here, we report on tunable magnetic domain patterns in thickness-gradient nickel (Ni) films deposited on flexible polydimethylsiloxane substrates. It is found that stripe magnetic domains spontaneously form in the Ni films and their sizes increase with the film thickness. The internal stress evolves from tensile to compressive states with decreasing film thickness, leading to the formation of cracks in thicker regions and wrinkles in thinner regions. Meanwhile, the orientations of stripe magnetic domains change from the gradient direction to the orthogonal direction. The structural features, evolution behaviors, and physical mechanisms of the cracks, wrinkles, and magnetic domains are analyzed based on the stress theory and magnetoelastic coupling. Periodic gradient Ni films with large-scale regulations of stripe magnetic domains are also realized by masking of copper grids. This study helps to better understand the magnetoelastic coupling effect in gradient flexible magnetic films and provides a technique to modulate anisotropic magnetic properties by designing specific film systems.

Wrinkled Interfaces: Taking Advantage of Anisotropic Wrinkling to Periodically Pattern Polymer Surfaces

Periodically patterned surfaces can cause special surface properties and are employed as functional building blocks in many devices, yet remaining challenges in fabrication. Advancements in fabricating structured polymer surfaces for obtaining periodic patterns are accomplished by adopting "top-down" strategies based on self-assembly or physico-chemical growth of atoms, molecules, or particles or "bottom-up" strategies ranging from traditional micromolding (embossing) or micro/nanoimprinting to novel laser-induced periodic surface structure, soft lithography, or direct laser interference patterning among others. Thus, technological advances directly promote higher resolution capabilities. Contrasted with the above techniques requiring highly sophisticated tools, surface instabilities taking advantage of the intrinsic properties of polymers induce surface wrinkling in order to fabricate periodically oriented wrinkled patterns. Such abundant and elaborate patterns are obtained as a result of self-organizing processes that are rather difficult if not impossible to fabricate through conventional patterning techniques. Focusing on oriented wrinkles, this review thoroughly describes the formation mechanisms and fabrication approaches for oriented wrinkles, as well as their fine-tuning in the wavelength, amplitude, and orientation control. Finally, the major applications in which oriented wrinkled interfaces are already in use or may be prospective in the near future are overviewed.

Stabilizing High-Frequency Magnetic Properties of Stretchable CoFeB Films by Ribbon-Patterned Periodic Wrinkles

The intrinsic nonstretchable feature of magnetic films has significantly limited its applications on wearable high-frequency devices. Recent studies have proved that the wrinkling surface structure based on the growth on polydimethylsiloxane (PDMS) is an effective route to obtain stretchable magnetic films. However, it is still a great challenge to simultaneously achieve a desired stretchability and stretching-insensitive high-frequency properties of magnetic films. Herein, we reported a convenient method to stabilize the high-frequency properties of stretchable magnetic films by depositing magnetic ribbon-patterned films on prestrain PDMS membranes. The ribbon-patterned wrinkling CoFeB films have far fewer cracks than the continuous film, which indicates a nice strain-relief effect and thus confers the stability of high-frequency properties for the films under stretching. However, the wrinkle bifurcation and the uneven thickness at the ribbon edge could adversely affect the stability of its high-frequency properties. The 200 μm wide ribbon-patterned film shows the best stretching-insensitive behaviors and maintains a constant resonance frequency of 3.17 GHz at strain from 10% to 25%. Moreover, a good repeatability has been demonstrated by performing thousands of stretch-release cycles, which did not significantly deteriorate its performances. The ribbon-patterned wrinkling CoFeB films with excellent stretching-insensitive high-frequency properties are promising for application in flexible microwave devices.