Ultrafine magnetic particles push soft actuators to 350 degrees Celsius

Soft robots that crawl, grip and swim under the pull of magnetic fields have long promised a future of untethered machines that can slip through blood vessels, sort delicate objects and explore disaster zones no wheeled robot could reach. The heart of these devices is a simple recipe: hard-magnetic particles embedded in a rubbery elastomer, magnetized in a programmed pattern so that an external field can bend the composite into precise, repeatable shapes. Yet the recipe has a stubborn Achilles heel. The neodymium-iron-boron magnets that give most magnetoactive composites their strength begin to lose their magnetism as soon as things heat up, and a new study now shows how to remove that bottleneck altogether.

Researchers led by Kangmo Koo and Jimin Lee, working with Young-Tae Kwon of the Korea Institute of Materials Science and Yong-Ho Choa of Hanyang University, report in Advanced Composites and Hybrid Materials a soft actuator built from ultrafine particles of samarium-iron-nitride, Sm2Fe17N3, that keeps working reliably at temperatures up to 350 degrees Celsius. For comparison, actuators made from conventional Nd2Fe14B particles suffered severe degradation beyond 200 degrees Celsius. The gap of 150 degrees Celsius is not a marginal improvement; it opens an entirely new operating envelope for magnetic soft machines in environments such as engine bays, industrial furnaces, aerospace structures and minimally invasive medical devices that must survive sterilization.

The reason the field has been stuck with thermal fragility lies in the physics of ferromagnetism. Every ferromagnetic material has a Curie temperature, the point at which thermal energy overwhelms the quantum-mechanical exchange interactions that align atomic magnetic moments, and the material loses its permanent magnetization. Nd2Fe14B, despite holding the highest maximum energy product of any commercial permanent magnet, has a relatively modest Curie temperature, and even well below that point its coercivity, the field needed to demagnetize it, drops rapidly with heat. In a soft actuator, the embedded particles must retain their remanent magnetization to experience torque in an applied field; once heat erodes that magnetization, the programmed deformation collapses and the device simply stops working.

Samarium-based magnets have always been the obvious alternative. Sm2Fe17N3 combines a high Curie temperature with large magnetocrystalline anisotropy, the intrinsic property that anchors magnetization against reversal and gives the material high coercivity. The compound also offers strong remanent magnetization and a competitive energy product, making it a candidate to match the performance of neodymium magnets while shrugging off heat that would cripple them. The problem has been making it in the right form. High-performance magnetic composites need fine, uniform particles so they can be dispersed evenly in elastomers and magnetized into intricate patterns; coarse or irregularly shaped particles produce weak spots, rough surfaces and unpredictable actuation.

Synthesizing ultrafine Sm2Fe17N3 has proved notoriously difficult. The conventional route runs through a reduction-diffusion process, in which samarium oxide is reduced in the presence of iron and calcium, allowing samarium atoms to diffuse into the iron lattice to form the Sm-Fe intermediate phase, which is subsequently nitrogenated. At the fine particle sizes the field needs, the samarium species tend to migrate and coarsen aggressively, producing overgrown Sm-Fe intermediate phases that ruin the uniformity of the product. The result has historically been powders that are either too coarse for high-quality composites or too chemically heterogeneous to deliver the intrinsic magnetic performance the compound promises.

The team’s solution is an elegant materials-chemistry trick: a calcium oxide core-shell strategy. By engineering a CaO shell around the reacting particles, the researchers created a physical barrier that suppresses the overgrowth of the Sm-Fe intermediate phase during reduction-diffusion. The shell confines the diffusion pathway, so each particle transforms uniformly rather than cannibalizing its neighbors. After nitrogenation, the process yields ultrafine Sm2Fe17N3 particles with systematically enhanced magnetic properties. The measured intrinsic coercivity reached 12.59 kilo-oersted, and both the remanent magnetization and the maximum energy product improved markedly relative to control samples made without the CaO shell. That combination matters because an actuator needs high coercivity to survive heat and demagnetizing fields, but it also needs strong remanence and energy product to generate large forces and large bending deformations.

With the ultrafine particles in hand, the researchers embedded them in elastomer matrices and programmed the magnetization profiles needed for shape-morphing soft actuators. When they drove the devices under elevated temperature, the Sm2Fe17N3 composites maintained robust actuation all the way to 350 degrees Celsius, while otherwise comparable Nd2Fe14B-based actuators showed severe degradation beyond 200 degrees Celsius. The demonstration effectively redefines the thermal ceiling of magnetoactive soft robotics, replacing the neodymium workhorse with a samarium alternative that no longer has to be babied away from heat.

The implications stretch well beyond the laboratory bench. Soft actuators built on this material could operate inside industrial machinery, near combustion engines or within thermal processing equipment, where untethered magnetic control was previously impractical. In aerospace, magnetic soft structures that survive thermal cycling could enable adaptive surfaces and deployable mechanisms without the servo motors and wiring that add weight and failure points. In medicine, instruments that must endure autoclave sterilization at elevated temperatures could retain full magnetic functionality afterward. The core-shell synthesis strategy itself may also generalize: if a sacrificial oxide shell can tame diffusion during reduction-diffusion in samarium magnets, similar approaches could refine other hard-magnetic or reactive compounds where fine particle synthesis has been blocked by coarsening.

There are still engineering questions ahead. Scaling the CaO core-shell process from laboratory batches to tonnage production, integrating the ultrafine powders into existing composite fabrication workflows, and optimizing elastomer matrices so they survive the same 350-degree environments as the particles will all demand further work. But the central result stands on its own: the thermal limit that has constrained hard-magnetic soft actuators since their inception is not a law of nature but a limitation of one particular magnet. By combining a carefully engineered synthesis route with a magnetically superior compound, the team has shown that soft machines can be programmed with magnetic fields and still take the heat. For a field whose ambitions run from microsurgery to space exploration, that is a genuine step change.

Subject of Research: Development of thermally stable ultrafine Sm2Fe17N3 magnetic particles for high-temperature soft actuators

Article Title: Ultrafine Sm2Fe17N3 soft actuators enabling robust operation at elevated temperature

Article References: Ultrafine Sm2Fe17N3 soft actuators enabling robust operation at elevated temperature. (n.d.). https://doi.org/10.1007/s42114-026-02082-1

Image Credits: AI Generated

DOI: 10.1007/s42114-026-02082-1

Keywords: Sm2Fe17N3, soft actuators, hard-magnetic soft composites, Nd2Fe14B, thermal stability, coercivity, CaO core-shell strategy, reduction-diffusion process, soft robotics, magnetic materials, Curie temperature, elevated temperature actuation