# Uniaxial pressure effect on the magnetic ordered moment and transition temperatures in BaFe$_{2-x}T_x$As$_2$ ($T=$Co, Ni)

## TL;DR

This study investigates how in-plane uniaxial pressure influences the magnetic ordered moment and transition temperatures in BaFe$_{2-x}T_x$As$_2$ compounds, revealing increases in magnetic order and temperature linked to Fermi surface nesting and electronic nematicity.

## Contribution

It provides experimental and theoretical insights into how uniaxial pressure enhances magnetic properties near optimal superconductivity in iron-based superconductors.

## Key findings

- Uniaxial pressure increases magnetic ordered moment by up to 15%.
- Transition temperature $T_N$ increases by about 0.25 K/MPa.
- Pressure effects are explained by Fermi surface nesting and quantum fluctuations.

## Abstract

We use neutron diffraction and muon spin relaxation to study the effect of in-plane uniaxial pressure on the antiferromagnetic (AF) orthorhombic phase in BaFe$_2$As$_2$ and its Co- and Ni-substituted members near optimal superconductivity. In the low temperature AF ordered state, uniaxial pressure necessary to detwin the orthorhombic crystals also increases the magnetic ordered moment, reaching an 11$\%$ increase under 40 MPa for BaFe$_{1.9}$Co$_{0.1}$As$_2$, and a 15$\%$ increase for BaFe$_{1.915}$Ni$_{0.085}$As$_2$. We also observe an increase of the AF ordering temperature ($T_N$) of about 0.25 K/MPa in all compounds, consistent with density functional theory calculations that reveal better Fermi surface nesting for itinerant electrons under uniaxial pressure. The doping dependence of the magnetic ordered moment is captured by combining dynamical mean field theory with density functional theory, suggesting that the pressure-induced moment increase near optimal superconductivity is closely related to quantum fluctuations and the nearby electronic nematic phase.