Structure‐Encoded Magnetic Anisotropy in Super‐Tough Magnetoelastic Aramid–Silk–Magnetite Fibers

ABSTRACT

Magnetic anisotropy is essential for controlling magnetic responses in soft matter, yet it is conventionally imposed using external magnetic fields. Here, we show that magnetic anisotropy can instead be intrinsically encoded and continuously tuned through hierarchical structural organization, establishing a route by which anisotropic dipolar interactions, rather than conventional shape effects, dominate the magnetic energy landscape. This mechanism is achieved by engineering composite nanofibrils in which magnetite nanoparticles are grown in situ within an aligned aramid–silk matrix, inspired by biomineralization in chiton teeth. The resulting aramid–silk–magnetite fibers combine high strength and modulus with exceptional toughness up to 820 MJ m ^{− 3} , while exhibiting tunable magnetic anisotropy spanning axial to radial easy‐axis configurations through structural relaxation. Quantitative analysis reveals that collective dipolar coupling dominates magnetic anisotropy by orders of magnitude over demagnetization‐field contributions, identifying nanoparticle interaction networks as the primary origin of anisotropy. This built‐in anisotropy enables strain‐mediated reconfiguration of magnetic properties, giving rise to magnetoelastic coupling that scales with fiber modulus and reflects the available structural mobility for magnetic reorientation. By amplifying structure‐defined anisotropy under mechanical loading, we further demonstrate strain‐controlled magnetic torque for actuation. These findings establish a structural design principle for programming magnetic anisotropy in mechanically robust magnetoelastic materials.