First-order magnetic and structural phase transitions in Fe$_{1+y}$Se$_x$Te$_{1-x}$

TL;DR

This study investigates the structural and magnetic phase transitions in Fe$_{1+y}$Se$_x$Te$_{1-x}$, revealing first-order transitions with unique magnetic and structural properties distinct from FeAs-based pnictides, challenging existing theoretical models.

Contribution

It provides detailed experimental evidence of first-order magnetic and structural transitions in FeSe$_{1-x}$Te$_x$, highlighting differences from FeAs-based materials and questioning current density functional theory assumptions.

Findings

Fe$_{1.068}$Te shows a first-order transition near 67 K.

The magnetic order and lattice distortion differ from FeAs-based pnictides.

Results challenge existing theoretical predictions about Fermi surfaces and magnetic order.

Abstract

We use bulk magnetic susceptibility, electronic specific heat, and neutron scattering to study structural and magnetic phase transitions in Fe$_{1+y}$Se% $_x$Te$_{1-x}$. Fe$_{1.068}$Te exhibits a first order phase transition near 67 K with a tetragonal to monoclinic structural transition and simultaneously develops a collinear antiferromagnetic (AF) order responsible for the entropy change across the transition. Systematic studies of FeSe$%_{1-x}$Te$_x$ system reveal that the AF structure and lattice distortion in these materials are different from those of FeAs-based pnictides. These results call into question the conclusions of present density functional calculations, where FeSe$_{1-x}$Te$_x$ and FeAs-based pnictides are expected to have similar Fermi surfaces and therefore the same spin-density-wave AF order.