Spontaneous Formation of Ordered Magnetic Domains by Patterning Stress

Smart Wrinkled Interfaces: Patterning, Morphing, and Coding of Polymer Surfaces by Dynamic Anisotropic Wrinkling

In contrast to traditional static surfaces, smart patterned surfaces with periodical and reversible morphologies offer limitless opportunities for encoding surface functions and properties on demand, facilitating their widespread application as functional building blocks in various devices. Advances in intelligently controlling the macroscopic properties of these smart surfaces have been accomplished through various techniques (such as three-dimensional printing, imprint lithography and femtosecond laser) and responsive materials. In contrast to the sophisticated techniques above, dynamic anisotropic wrinkling, taking advantage of dynamic programmable manipulation of surface wrinkling and its orientation, offers a powerful alternative for fabricating dynamic periodical patterns due to its spontaneous formation, versatility, convenient scale-up fabrication, and sensitivity to various stimuli. This review comprehensively summarizes recent advances in smart patterned surfaces with dynamic oriented wrinkles, covering design principles, fabrication techniques, representative types of physical and chemical stimuli, as well as fine-tuning of wrinkle dimensions and orientation. Finally, advanced applications of these smart patterned surfaces are presented, along with a discussion of current challenges and future prospects in this rapidly evolving field. This review would offer some insights and guidelines for designing and engineering novel stimuli-responsive smart wrinkled surfaces, thereby facilitating their sustainable development and progressing toward commercialization.

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.

Mechanical manipulation for ordered topological defects

Randomly distributed topological defects created during the spontaneous symmetry breaking are the fingerprints to trace the evolution of symmetry, range of interaction, and order parameters in condensed matter systems. However, the effective mean to manipulate topological defects into ordered form is elusive due to the topological protection. Here, we establish a strategy to effectively align the topological domain networks in hexagonal manganites through a mechanical approach. It is found that the nanoindentation strain gives rise to a threefold Magnus-type force distribution, leading to a sixfold symmetric domain pattern by driving the vortex and antivortex in opposite directions. On the basis of this rationale, sizeable mono-chirality topological stripe is readily achieved by expanding the nanoindentation to scratch, directly transferring the randomly distributed topological defects into an ordered form. This discovery provides a mechanical strategy to manipulate topological protected domains not only on ferroelectrics but also on ferromagnets/antiferromagnets and ferroelastics.

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.

Direct observation of tensile-strain-induced nanoscale magnetic hardening

Magnetoelasticity is the bond between magnetism and mechanics, but the intricate mechanisms via which magnetic states change due to mechanical strain remain poorly understood. Here, we provide direct nanoscale observations of how tensile strain modifies magnetic domains in a ferromagnetic Ni thin plate using in situ Fresnel defocus imaging, off-axis electron holography and a bimetallic deformation device. We present quantitative measurements of magnetic domain wall structure and its transformations as a function of strain. We observe the formation and dissociation of strain-induced periodic 180° magnetic domain walls perpendicular to the strain axis. The magnetization transformation exhibits stress-determined directional sensitivity and is reversible and tunable through the size of the nanostructure. In this work, we provide direct evidence for expressive and deterministic magnetic hardening in ferromagnetic nanostructures, while our experimental approach allows quantifiable local measurements of strain-induced changes in the magnetic states of nanomaterials.

Tuning Thermoelectric Conversion Performance of BiSbTe/Epoxy Flexible Films with Dot Magnetic Arrays

Embedding of magnetic functional units into the thermoelectric (TE) materials has been demonstrated to be an effective way to enhance the TE conversion performance. However, the magnetic functional units in TE materials are all randomly distributed. In this paper, to explore the effect of the ordering of the magnetic functional units on TE conversion performance, a series of BiSbTe/epoxy flexible thermoelectromagnetic (TEM) films with dot magnetic arrays were successfully prepared by a two-step screen printing combined with a hot pressing process. TEM films with dot magnetic arrays can achieve high carrier mobility, while the carrier concentration increases due to large coercivity. Therefore, its electrical conductivities are significantly improved on the condition that it maintains a high Seebeck coefficient. The TEM film with hexagonal-dot magnetic arrays exhibits the best electrical transport properties, for which the room-temperature power factor reaches 1.51 mW·m^-1·K^-2, increased by 33.6 and 36.1% as compared with those of the pristine TE film and the TEM film with a continuous magnetic layer, respectively. This work provides a new way to enhance the TE conversion performance of flexible TEM films through the ordered magnetic arrays.

Macroscopic Gradient Ordered α-Fe/Pr2Fe14B Nanocomposites with Ultrahigh Energy Density

Nanoparticle self-assembly enables the generation of complex ordered nanostructures with enhanced properties or new functionalities. However, the ordering is often limited to the micrometer scale with chemical strategies due to the relative weak supramolecular interactions that govern the self-assembly process. Here a physical strategy via temperature-gradient-assisted self-assembly is reported to create three-dimensional (3D) macroscopic ordered nanocomposites with different gradient variations in grain size, constituent content, and crystal orientation. The resulting α-Fe/Pr₂Fe₁₄B ordered nanostructure with reverse gradients in both the grain size and α-Fe content exhibits a record-high energy density of about 25 MGOe for isotropic α-Fe/Pr₂Fe₁₄B systems, approximately 130% higher than that of its disordered counterpart. Both experiments and micromagnetic simulations demonstrate that creating ordered nanostructures is an alternative approach to develop high-performance permanent-magnet materials. Our findings make a significant step toward creating 3D macroscopic ordered nanostructures and will stimulate the development of ordered nanomaterials.

Strain-Modulated Magnetism in MoS2

Since the experiments found that two-dimensional (2D) materials such as single-layer MoS2 can withstand up to 20% strain, strain-modulated magnetism has gradually become an emerging research field. However, applying strain alone is difficult to modulate the magnetism of single-layer pristine MoS2, but applying strain combined with other tuning techniques such as introducing defects makes it easier to produce and alter the magnetism in MoS2. Here, we summarize the recent progress of strain-dependent magnetism in MoS2. First, we review the progress in theoretical study. Then, we compare the experimental methods of applying strain and their effects on magnetism. Specifically, we emphasize the roles played by web buckles, which induce biaxial tensile strain conveniently. Despite some progress, the study of strain-dependent MoS2 magnetism is still in its infancy, and a few potential directions for future research are discussed at the end. Overall, a broad and in-depth understanding of strain-tunable magnetism is very necessary, which will further drive the development of spintronics, straintronics, and flexible electronics.