Successive Magnetic Transitions Enable an Ultrawide Temperature Window of Zero Thermal Expansion in (Hf,Ti)Fe _2+x

Zero thermal expansion (ZTE) alloys hold significant potential for a variety of precise applications, yet the working temperature range remains limited, generally lower than 500 K. Herein, enabled by high-temperature annealing, we realize heterogeneous elemental distribution and thus multiple magnetic orders in nonstoichiometric kagome metal (Hf,Ti)Fe_2+<i>x</i>. Successive magnetic transitions yield ZTE in Hf_0.6Ti_0.4Fe_2.54 over an ultrawide temperature window, with an average linear coefficient of thermal expansion <i>α̅</i>_<i>l</i> = 0.76 × 10^-6 K^-1 (112-525 K). Magnetization measurements, Lorentz TEM, neutron powder diffraction, and Mössbauer spectra reveal that antisite Fe introduces extra magnetic exchange interactions and stabilizes the magnetic ordering. A spin reorientation emerges near 370 K when the direction of magnetic moments in the 2<i>a</i> and 6<i>h</i> sites transform from in-plane to out-of-plane. Then, the moments show an abrupt decrease at 470 K, and turn completely paramagnetic around 525 K. The compensation between magnetic order and the phonon effect enables such an ultrawide ZTE across room temperature. The present study provides insight into unconventional ZTE behaviors for metallic magnets with promising applications.