Negative thermal expansion and itinerant ferromagnetism in Mn$_{1.4}$Fe$_{3.6}$Si$_{3}$

Abstract

We report the thermal expansion, critical behavior, magnetocaloric effect (MCE), and magnetoresistance (M R) on the polycrystalline Mn1.4Fe3.6Si3 compound around the ferromagnetic transition. A large negative volume thermal expansion (αV ∼ −20 × 10 −6 K −1 ) is observed across the transition temperature with a strong anisotropic variation of lattice parameters in the ab-plane. The anisotropic magnetoelasticity arises from the competition between magnetic ordering and structural deformation which could be responsible for the large MCE (∆Sm ≃ −6 J/Kg-K) across the magnetic transition in this compound. The large and negative M R (∼ −3% in 80 kOe) is also observed at the transition temperature which can be attributed to the suppression of spin disorder. Further, the Rhodes-Wolfarth ratio (RWR > 1) and identical field dependence of M R and MCE isotherms indicate the itinerant character of the 3d electrons. The critical exponents determined from the analysis of magnetization and MCE are consistent with the quasi-two-dimensional (2D) Ising model with long range exchange interactions which decays as J(r) ∼ r −3.41 . This unconventional quasi-2D Ising character with long-range interactions can be ascribed to strong ab-plane anisotropy and the delocalized 3d electrons in the studied compound.