Anisotropic Thermal Expansion and Cooperative Invar/Anti-Invar Effects in MnNi Alloy

T. Yokoyama 2 and K. Eguchi (IMS, The Grad. Univ. for Adv. Stud.) Thermal expansion of a tetragonal Mn88Ni12 alloy, which exhibits significant anisotropy, was investigated by EXAFS and path-integral quantum mechanical simulations. The EXAFS reveals that the Mn local environment is actually tetragonally distorted, while the Ni one retains its inherent cubic-like symmetry. The large thermal expansion along the a axis originates from the anti-Invar effect, while the negligibly small thermal expansion along the c axis originates from the cooperative Invar effect. Namely, the tetragonally distorted more stable antiferromagnetic Mn state gives a significantly smaller (slightly longer) atomic radius along a (c) than that of the spherical paramagnetic state.