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

TL;DR

This study investigates the magnetic and thermal properties of Mn$_{1.4}$Fe$_{3.6}$Si$_{3}$, revealing large negative thermal expansion, significant magnetocaloric effect, and itinerant ferromagnetism with quasi-2D Ising critical behavior.

Contribution

It provides new insights into the anisotropic magnetoelasticity, large negative thermal expansion, and the quasi-2D long-range interaction criticality in Mn$_{1.4}$Fe$_{3.6}$Si$_{3}$, highlighting its itinerant electron nature.

Findings

Large negative volume thermal expansion across transition

Significant magnetocaloric effect near transition

Negative magnetoresistance attributed to spin disorder suppression

Abstract

We report the thermal expansion, critical behavior, magnetocaloric effect (MCE), and magnetoresistance ($MR$) on the polycrystalline Mn$_{1.4}$Fe$_{3.6}$Si$_{3}$ compound around the ferromagnetic transition. A large negative volume thermal expansion ($\alpha_{\rm V}\sim -20 \times 10^{-6}$ 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 ($\Delta S_{\rm m} \simeq -6$ J/Kg-K) across the magnetic transition in this compound. The large and negative $MR$ ($\sim -3\%$ 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 $MR$ 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 $J(r)\sim r^{-3.41}$. 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.