Axial Zero Thermal Expansion with Good Mechanical Strength in the Ni-Doped Kagome Ho₂Fe₁₇ Magnets

Zero thermal expansion (ZTE) metals have drawn considerable scientific and practical interest due to their excellent dimensional stability. However, the narrow temperature window and inherent brittleness of the ZTE compounds severely limit their processing and application. Herein, an axial ZTE alloy (Ho₂Fe_13.8Ni_3.2, α_l = -0.3 × 10^-6 K^-1 and 110-545 K) is realized in the Ni-doped Ho₂Fe₁₇ magnets, which effectively broadens the ZTE temperature window. Combining the variable-temperature neutron diffraction and magnetization measurements, we reveal that the content of Ni tailors the ferromagnetic ordering of the Fe sublattice and regulates lattice negative thermal expansion. Good compressive strength is subsequently achieved by introducing excess Fe in the axial ZTE composition, namely, the dual-phase alloy of Ho₂Fe_13.8Ni_3.2-Fe₅ (α_l = +0.3 × 10^-6 K^-1, 110-535 K, and 1.19 ± 0.2 GPa). Neutron diffraction and scanning electron microscopy reveal that the α phase (Fe-Ni) precipitates in the hexagonal phase matrix play a critical role in maintaining ZTE characteristics and enhancing compressive strength. The present chemical design approach may be applicable for obtaining high-performance axial ZTE alloys.