Study of antiferromagnetic and nematic phase transitions in $BaFe_2(As_{1-x}P_x)_2$ by AC micro-calorimetry and SQUID magnetometry

TL;DR

This study investigates the antiferromagnetic and nematic phase transitions in BaFe2(As_{1-x}P_x)_2 using high-resolution calorimetry and magnetometry, clarifying the nature and temperature of these transitions.

Contribution

It provides detailed calorimetric and magnetic evidence for the splitting and nature of AFM and structural transitions in BaFe2(As_{1-x}P_x)_2, clarifying the phase transition behavior.

Findings

Sharp peaks at AFM/structural transitions in specific heat.

Splitting of AFM and structural transitions by ~0.5K in annealed samples.

No evidence of a second-order nematic transition at T* in calorimetric data.

Abstract

We study the antiferromagnetic (AFM) and structural phase transitions in single crystal $BaFe_2(As_{1-x}P_x)_2$ $(x=0, 0.3)$ at temperatures $T_N$ and $T_S$, respectively, by high resolution ac microcalorimetry and SQUID magnetometry. The specific heat measurements of both as grown and annealed $BaFe_2As_2$ displays a sharp peak at the AFM/Structural transitions. A kink in the entropy of annealed $BaFe_2As_2$ gives evidence for splitting of the two transitions by approximately 0.5K. No additional features could be identified in the specific heat of both $BaFe_2As_2$ and $BaFe_2(As_{0.7}P_{0.3})_2$ in the temperature regions around $T^*$ > $T_S$ where torque measurements [S. Kasahara et al., Nature 486, 382 (2012)] had revealed the "true" nematic phase transition, indicating that the behavior at $T^*$ does not represent a 2nd order phase transition, and that the phase transition of $BaFe_2(As_{1-x}P_x)_2$ into the orthorhombic phase does occur at $T_S$.