Uniaxial Negative Thermal Expansion in a Weak-Itinerant-Ferromagnetic Phase of CoZr₂H_3.49

We report an observation of uniaxial negative thermal expansion (NTE) that emerges exclusively in the weak-itinerant-ferromagnetic phase of hydrogenated CoZr₂H₃.₄₉. Powder synchrotron X-ray diffraction revealed that the c-axis contracts upon heating below the Curie temperature (T_C = 139 K), whereas the a-axis shows a positive thermal expansion. Magnetization measurements combined with Arrott plot analyses yielded a Rhodes-Wohlfarth ratio of 3.49, establishing the itinerant nature of the ferromagnetism. The observed uniaxial NTE in CoZr₂H₃.₄₉ clearly contrasts with the phonon-driven type anomalous thermal expansion in CoZr₂ superconductors, which demonstrates that hydrogen insertion switches both the electronic structure and the mechanisms of the uniaxial NTE. The uniaxial NTE is likely associated with an enhanced sharpening of the antibonding Co3dz² density of states, which favors the expansion of the one-dimensional Co-Co chain along the c-axis as the temperature decreases in the ferromagnetic state. These results establish the CoZr₂H_x system as a platform for investigating the interplay among superconductivity, itinerant ferromagnetism, and NTE.