Magnetic-field-controllable elasticity of helical spring magnets composed of magnetic-particle-polymer composites

We present an experimental study demonstrating the ability to control the spring constant of helical mechanical springs using a magnetic field, achieved by embedding ferrimagnetic Fe3O4 nanoparticles within in a silicone polymer matrix. The composite material, in its gel form, was injected into a 3D-printed mold featuring a helical-spring-shaped cavity. An external magnetic field applied perpendicular to the coil axis of the spring allows the aligmment of the magnetic nanoparticle assemblies (chain axis) in the field direction. This alignment process determines the preferred magnetization orientation of the particle assembly chain, thereby balancing the magnetic force between the magnetic anisotropy field and the Zeeman field under a given external field. When the spring is subjected to compression or stretching loads under an externally applied magnetic field, these two magnetic fields modify the effective spring constant of the helical spring magnets (HSMs) by similar to 31%, incresing it from 8.7 N m-1 (under no field) to 11.5 N m-1 at 300 mT. Analytical modeling using a simplified rod geometry aptly explains the experimental results, demonstrating that the spring constant linearly increases with the field strength up to 300 mT. Such composite HSMs could be utilized as active vibration absorbers or isolators due to their field-controllable elasticity.