Stress-Mediated Ferromagnetic Spinodal Decomposition for Giant Low-Field Magnetostriction

Nonequilibrium stress can trigger unconventional domain dynamics in magnetoelastic systems. Using phase‑field simulations, we reveal a stress‑driven spinodal decomposition in rhombohedral (R) Tb_0.3Dy_0.7Fe₂ (Tb-Dy-Fe) alloys with <111> easy axes. Gradual unloading of a [001] compressive stress induces a spontaneous, fully reversible transition from a uniform microdomain (MD) to an alternating nanodomain‑in‑microdomain (NIMD) pattern. Unlike classical nucleation‑and‑growth processes, this transformation proceeds via global, continuous small‑angle spin reorientation. Mechanistically, it originates from the negative curvature of the effective magnetic anisotropy (EMA) energy landscape, a direct analogue of thermodynamic spinodal instabilities. Magnetic exchange interactions oppose the decomposition, akin to chemical gradient energies, whereas magnetocrystalline anisotropy serves as the primary driving force. Remarkably, the resulting NIMD architecture delivers an ultrahigh magnetostriction of 721.5 ppm under a weak magnetic field of only 19.5 mT. This exceptional low‑field performance stems from the unique ability of R‑type nanodomains to undergo domain‑wall displacement, amplifying lattice deformation through energy minimization. Our findings establish a stress‑mediated magnetization control paradigm, opening new avenues for adaptive magnetoelastic materials in low‑field sensors and stress‑responsive nanotechnologies.