Spontaneous two-ion magnetostriction of the Hubbard itinerant ferromagnet Y₂Fe₁₄B

We show that the large spontaneous, i.e., when the spontaneous magnetization lies along the easy c axis, magnetostrictive strains ε^α,1 (volume distortion) and ε^α,2 (c-axis pure shape distortion), below the Curie temperature T_C≅571 K, for the Invar intermetallic tetragonal ferromagnet Y₂Fe₁₄B can be explained using a degenerate t-J Hubbard model. The strictions can be explained as arising alone from a double two-ion interaction source: the strain dependence of the one-electron hopping t matrix between the Fe pair e sites and the strain dependence of the exchange interaction J of the e Fe pair atoms with its four nearest neighbor k₁-site Fe atoms. Temperature effects are introduced via the spontaneous magnetization induced Stoner gap open between spin polarized rigid 3d bands. Good theoretical agreement is found for the thermal variations of both strains, and information about the spatial dependencies of t and J is extracted.