Increasing Magnetic Anisotropy at The Nanoscale for Micromagnetic Actuators

When scaled down to micrometer dimensions, magnetic actuators along with other micromagnetic devices strongly depend upon their nanoscale behavior. For example, magnetorheological elastomers (MREs), which are composites of a polymer matrix and a magnetic filler, will stretch, bend, and generally behave in a manner that is intrinsically linked to the properites of the magnetic filler. However, the extent by which a magnetic material can bend and deform is proportional to its elasticity and inversely proportional to the cube of its thickness. Thus, high-resolution actuation and bending of MREs at the microscale requires an MRE of thickness similarly at the microscale and a magnetic filler that is much smaller – on the order of nanometers.<br/>In this talk, we will present a direct comparison of actuation by microparticle-filled and nanoparticle-filled elastomer thin films, illustrating that higher actuation response at small applied magnetic fields (&lt;100 mT, typical of integrated devices) can be achieved by the nanoparticle films [1]. Therefore, MRE microactuators using magnetic nanoparticles are desirable. However, magnetic particles are known to lose their anisotropy and ferromagnetic behavior as the dimensions are reduced to the nanoscale. Thus, nanoparticle MRE actuation can be further increased with improved nanostructures having increased remanent magnetic moment.<br/>We will show that synthesis of high saturation magnetization Fe₆₅Co₃₅ (230 emu/g) magnetic nanoparticles addresses this obstacle head on, bringing both anisotropy and strong ferromagnetism down to this size regime and achieving <i>6x higher remanent magnetization</i> than conventional ferrous magnetic nanoparticles (maghemite, steel, and pure Fe). Furthermore, synthesis of self-assembled Fe-Co nanochains (theoretical 1-dimensional nanostructures) doubles the nanoscale anisotropy by adding shape anisotropy to the existing magnetocrystalline anisotropy [2, 3]. Of particular importance is that these nanochain structures with aspect ratios exceeding 10:1 can be synthesized in high concentration solutions without the need for an external field, thereby simultaneously allowing yield to increase while simplifying the synthesis process.<br/>Finally, these anisotropic 1-dimensional nanostructures, i.e. nanochains, are hypothesized to produce the maximimum actuation response from the MRE microactuators when the filler concentration is optimized. We will present our experiments that show that an optimal volume concentration of nanochains in the elastomer matrix exists, at 6 vol.%, such that it minimizes the antiferromagnetic chain-to-chain interactions while maximizing remanence (0.43*M_s). Capitalizing on these results, an optimized soft actuator was made with the nanochains and shown to provide large actuation at fields &lt;100 mT. Multiple modes of actuation will be illustrated including, lateral bending, twisting, and vertical bending.<br/><br/>[1] L. Cestarollo, S. Smolenski, A. El-Ghazaly, <i>ACS Applied Materials & Interfaces</i>, 2022.<br/>[2] Y. Chen, A. El-Ghazaly, <i>Small</i>, 2023.<br/>[3] Y. Chen, … A. El-Ghazaly, <i>Advanced Functional Materials</i>, 2023.