Low-field reversible magnetostriction sign switching in single-phase TbFe_2-xCo_x alloys

REFe₂ (RE=rare-earth) alloys exhibit large positive or negative magnetostrictive strains and are widely used in functional devices; however, achieving sign-changing magnetostriction within a single phase remains challenging. In this work, single-phase TbFe_2-xCo_x(1.4 ≤x ≤ 1.9) alloys were synthesized by arc melting and their structural, magnetic, and magnetostrictive properties were systematically investigated. Rietveld refinement of X-ray diffraction (XRD) data and high-resolution transmission electron microscopy (HRTEM) analyses confirm the formation of single-phase MgCu₂-type cubic Laves phase structure, accompanied by spontaneous magnetostriction-induced rhombohedral distortion. Inverse Fast Fourier Transform (IFFT) analysis reveals the coexistence of nanoscale multi-variant rhombohedral domains, which strongly influence the magnetostrictive response. The magnetostriction coefficient λ₁₁₁ decreases from ∼2040 ppm for TbFe_0.6Co_1.4 to ∼990 ppm for TbFe_0.1Co_1.9 with increasing Co concentration. The TbFe_0.6Co_1.4, TbFe_0.4Co_1.6, and TbFe_0.2Co_1.8 alloys exhibit conventional positive V-shaped magnetostriction with low hysteresis (∼2.1–2.3%). In contrast, the TbFe_0.1Co_1.9 alloy exhibits unusual reversible sign-changing magnetostriction, showing a transition from negative strain (∼−132 ppm at 0.24 T) to positive strain (∼166 ppm at 1 T) within a single rhombohedral phase. The sign reversal is attributed to the competition between non-180° variant switching at low magnetic fields and magnetization rotation along the 〈111〉 easy axis at higher fields. These results demonstrate an intrinsic mechanism for reversible strain control in single-phase magnetostrictive materials and provide a promising pathway for advanced strain-mediated functional devices.