Heat capacity study of ${\text{BaFe}}_2{\text{As}}_2$: Effects of annealing

Heat capacity, x-ray diffraction, and resistivity measurements on a high-quality ${\text{BaFe}}_2{\text{As}}_2$ sample show an evolution of the magnetostructural transition with successive annealing periods. After a 30-day anneal the resistivity in the $\left(ab\right)$ plane decreases by more than an order of magnitude, to $12\mu \Omega \text{cm}$, with a residual resistance ratio $\sim 36$; the heat capacity anomaly at the transition sharpens, to an overall width of less than K, and shifts from 135.4 to 140.2 K. The heat capacity anomaly in both the as-grown sample and after the 30-day anneal shows a hysteresis of $\sim 0.15\text{K}$, and is unchanged in a magnetic field ${\mu }_{0}H=14\text{T}$. The x-ray and heat capacity data combined suggest that there is a first-order jump in the structural order parameter. The entropy of the transition is reported.

Figure 1

Characterization of the as grown ${\text{BaFe}}_2{\text{As}}_2$ single crystal: magnetization in magnetic field of 5 $5\text{T}\parallel \left(ab\right)$ (upper window) and zero-field normalized resistivity in $\left(ab\right)$ plane (lower window).

Figure 2

(Color online) The zero-field $C/T$ of the as-grown ${\text{BaFe}}_2{\text{As}}_2$ crystal. The inset shows the same data near the transition on an expanded scale.

Figure 3

(Color online) The low-temperature specific heat of as-grown ${\text{BaFe}}_2{\text{As}}_2$ crystal. The data, in magnetic fields $H\parallel c$, are displaced by progressive increments of $5\text{mJ}{\text{K}}^{-2}{\text{mol}}^{-1}$ for clarity. For each field the curve represents a simultaneous fit to all the data.

Figure 4

(Color online) Reciprocal-lattice scans across the ${\left(228\right)}_T$, ${\left(408\right)}_O$, and ${\left(048\right)}_O$ reflections above and below the structural phase transition. The vertical arrows in panels (c) and (d) show the ${\left(408\right)}_O$ and ${\left(048\right)}_O$ reflections. Panels (b) and (c) show a continuous splitting of the central ${\left(228\right)}_T$ reflection which precedes and coexists with the ${\left(408\right)}_O$ and ${\left(048\right)}_O$ reflections. All reflections are fitted with a Lorentzian-squared peak profile.

Figure 5

(Color online) The structural order parameter, $\delta =\left(a-b\right)/\left(a+b\right)$, as a function of temperature. Filled symbols are for cooling data and open symbols are for the warming data.

Figure 6

(Color online) (a) Normalized resistivity of ${\text{BaFe}}_2{\text{As}}_2$ single crystal, as grown and after 1, 4, 14, and 30 days of annealing. (b) The same data as in (a) near the transition. The data were shifted in $\rho \left(T\right)/\rho \left(300\text{K}\right)$ to facilitate comparison of the transition temperature after annealing. (c) Residual resistivity ratio of in-plane resistivity, $\text{RRR}\equiv \rho \left(300\text{K}\right)/\rho \left(2\text{K}\right)$, versus annealed time periods.

Figure 7

(Color online) $C/T$ near the transitions of the ${\text{BaFe}}_2{\text{As}}_2$ sample, as grown and after annealings.

Figure 8

(Color online) $C/T$ near the transitions of the ${\text{BaFe}}_2{\text{As}}_2$ sample after the 30-day anneal in zero field, heating (◼) and cooling (◆); in 14 T, heating (●). The inset shows the relaxation curves of the sample temperature versus time of a measurement spanning the transition. The arrows point to regions of the curve that have a reduced $dT/dt$, showing the presence of the latent heat, a signature of the first-order transition.

Figure 9

(Color online) Zero-field, low-temperature $C/T$ versus $T^2$ of the as-grown and 30-day-annealed ${\text{BaFe}}_2{\text{As}}_2$ samples. The solid curve represents the collective fit to the data for the as-grown sample in all fields.