Nano weaving creates 'Chinese knot' magnetism for powerful microwave shielding
(Nanowerk Spotlight) While investigations into magnetism predate 1820, it was Hans Christian Ørsted's discovery in that year that truly inaugurated the field of electromagnetism by demonstrably linking electric currents and magnetic fields.
Following the development of our modern understanding of magnetism arising from electron quantum properties, harnessing tailored nanoscale magnetic materials like metal-organic frameworks for advanced applications has faced two key challenges.
First, reliably constructing these frameworks with specific and programmable architectural arrangements remains difficult. While recent advancements in chemical synthesis offer some promise, conventional methods often result in random agglomerates of nanoparticles, lacking the sub-micron resolution and control needed for desired functionalities.
Second, effectively characterizing these nanostructures presents another hurdle. Traditional electron microscopy techniques often lack the spatial resolution and magnetic field sensitivity to directly probe nanoscale magnetic behaviors within these engineered structures. Establishing clear correlations between the architectural details of the frameworks and their magnetic responses remains a critical gap in our knowledge.
Now, an international team led by scientists at Fudan University in China has reported a breakthrough magnetic nano-framework that enables the deliberate reconfiguration and visualization of nanoscale magnetic flux lines.
Schematic illustration of the synthesis process and simplified mode. a) Schematic diagram of the preparation procedure for Co@HMC composites; b) The structure simplification of prepared composite to a mode of staggered nanoporous layers loaded with Co nanoparticles. (Reprinted with permission by Wiley-VCH Verlag)
“We strategically disperse magnetic cobalt nanoparticles into a nanoporous carbon framework with top and bottom pores arranged in a staggered formation,” explained Dr. Renchao Che, senior author. “This intricate structure shapes the magnetic flux lines into a ‘Chinese knot’ pattern, significantly altering the material’s magnetic properties.”
Importantly, the team employed advanced electron holography techniques to directly visualize the reconstructed magnetic field lines within their nano-framework. “The innovation of magnetic holography was crucial for observing the reorganization of magnetic flux caused by our tailored 3D architecture,” said Dr. Che.
The researchers demonstrated that the staggered nanopores strongly enhanced the nanostructure’s absorption of microwave radiation, making it an extremely efficient electromagnetic shielding material. According to simulations, the novel framework exhibits unique dynamic behaviors, with magnetic vortices shifting in response to external fields.
Magnetism arises from the quantum mechanical spins of electrons and their interactions. Although magnetic behaviors had been investigated since ancient times, scientific understanding only developed with Ørsted's discovery in 1820.
In the 20th century, scientists achieved accurate descriptions of magnetism through quantum mechanics. However, reliably controlling magnetism remained out of reach. Natural magnetic materials contain complex domains with semi-random alignments of electron spins. Moreover, directly observing the intricate fluxes at nanoscales pushed against the limits of microscopy.
“The core challenges have been finding ways to deliberately organize magnetic nanoparticles so their spins interact in a controllable manner, and directly visualizing the resulting magnetic flux lines,” explained Dr. Che.
Previous approaches had attempted to arrange magnetic nanoparticles into two-dimensional sheets and three-dimensional disordered clusters. However, these structures showed limited programmability over the magnetic couplings between nanoparticles. The random arrangements yielded only statistical aggregate behaviors rather than predictable engineered responses.
Moreover, prior to recent breakthroughs, even observing the basic magnetic signatures from nanoscale volumes posed major difficulties. Conventional microscopy lacks both the spatial resolution to isolate small numbers of nanoparticles and the magnetic field sensitivity to detect subtle signals. Correlating the nanostructure arrangements to the unseen magnetic flux lines they produced was therefore impossible.
These persistent barriers in precision nano-assembly and nanoscale imaging stymied progress on nanomagnetic control for over 50 years. Both creating intricate magnetic nanostructures and revealing their detailed responses confounded legions of scientists across generations. The challenges seemed perhaps insurmountable.
Dr. Che and his colleagues realized they could combine deliberate nano-architecture with cutting-edge holographic microscopy to reveal a hidden realm of magnetic behaviors. They set out to create a meticulous nanoporous matrix filled with magnetic cobalt that would interact with external fields in distinctive ways. Moreover, direct visualization may elucidate the microscopic underpinnings.
“We induced the confined growth of a zeolitic imidazolate framework between layers of silica nanoparticles,” described Dr. Che. “Carefully controlling the self-assembly and etching processes resulted in a carbon structure with staggered top and bottom nanopores decorated with cobalt nanoparticles.”
Critically, this geometry forced the reconstructed magnetic field lines to take on a “Chinese knot” morphology along the nanopore edges. Theoretical simulations suggest the intricate knotting patterns significantly enhance magnetic coupling interactions between nanoparticles. This morphology strengthens the overall ferromagnetic behavior which is crucial for the material’s exceptional microwave absorption properties.
The team utilized aberration-corrected transmission electron microscopy (ACTEM) and advanced magnetic holography hardware to image magnetic behaviors within the nano-framework. ACTEM employs specialized lens configurations and detection schemes to minimize optical distortions and enhance resolution down to nanometer scales. Magnetic holography entails using an electron biprism to create splitting and interference of electron waves passing through vacuum and the sample. Researchers can digitally reconstruct precise magnetic flux lines and field strengths from the recorded holographic interference patterns.
This technique enabled the Fudan team to directly demonstrate the presence and realignment of magnetic field lines along the nanopores, as cobalt nanoparticles emitted intense flux taking on Chinese knot shapes.
To assess if the tailored magnetic geometry yielded useful properties, the team evaluated its absorption of microwave radiation - which many electromagnetic devices aim to suppress.
Strikingly, the staggered cobalt-infused nanoporous layers exhibited marked improvements in dissipating microwaves compared to an unstructured cobalt nanoparticle composite. Its absorption bandwidth spanning 12.5 - 17.3 GHz surpassed previous materials under two millimeters thickness.
The researchers suggest two synergistic mechanisms underlie the strong performance - intrinsic magnetism of the cobalt nanoparticles couples with the reconstructed flux lines enhancing electron spin interactions, while the conductive graphitic interlayers contribute dielectric loss. The visualized vortex dynamics also indicate robust magnetic resonance behaviors causing energy dissipation.
Upon further adjusting geometric and materials parameters, the optimal configuration displayed resonant absorption covering nearly the entire 12.4 - 17.7 GHz Ku microwave band at an ultrathin 1.7 mm thickness.
According to the researchers, these findings provide a promising avenue for developing lightweight, broadband microwave-absorbing materials for use shielding electromagnetic sensitive instrumentation and even wearable tech. The bio-compatible nanoporous structure could help mitigate growing public concerns over excessive environmental microwave radiation.
Moreover, the breakthroughs fundamentally advance capabilities in deliberately organizing magnetic nanoparticles and tailoring their interactions with external fields. Mastering magnetism at the nanoscale unlocks a wide spectrum of other possible applications like ultra-high density data storage, magnetic sensors for portable diagnostics, contrast agents for MRI cancer detection, and catalysts to improve chemical manufacturing.
“Looking ahead, these breakthroughs provide a valuable roadmap for engineering advanced magnetic nanomaterials not only for shielding devices but also transformative technologies such as high-capacity solid-state hard drives, point-of-care biosensors, wearable MRI devices, and efficient magnetic catalyst systems,” concluded Dr. Che.
However, designing new magnetic nanoarchitectures and predicting their emergent dynamics remains highly challenging. The researchers emphasize that combining precise self-assembly methods with advanced holographic imaging will likely continue unlocking new horizons.