Synergistic Weakening of Magnetoelastic Coupling and Enhancement of Exchange Interactions Enable High‐Temperature Zero Thermal Expansion in a Lightweight and Hard Monoboride Ferromagnet

Severe thermal expansion at high temperatures can induce internal stress accumulation and interfacial failure, leading to dimensional instability in structural and functional components. Yet, high‐temperature zero thermal expansion (ZTE) alloys, which are of technological importance, remain rare. In this work, we developed a lightweight ferromagnetic MnB‐based alloy by isostructural alloying, which exhibits a stable high‐temperature ZTE ( α l = 1.95 × 10 −6 K −1 , 595–705 K; α l = −1.99 × 10 −6 K −1 , 705–815 K) and high hardness of 16.5 GPa. The incorporation of Fe simultaneously weakens the magnetoelastic coupling strength and increases the Curie temperature ( T C ), leading to this rare Invar effect. First‐principles calculations further elucidate the underlying physical mechanism. The introduction of Fe modifies the electronic density of states and reduces the bonding strength associated with negative thermal expansion (NTE), thereby softening the strong NTE and enabling ZTE behavior. Meanwhile, ferromagnetic Fe brings about strong Mn‐Fe exchange interactions, resulting in an overall increase in exchange interaction strength and an elevated T C . Additionally, the strong covalent bonding between B atoms provides the fundamental origin of the outstanding mechanical hardness. Our work thus not only proposes a high‐temperature ZTE alloy system but also deepens the understanding of thermal expansion behavior in magnetic NTE/ZTE materials.