Antiferromagnetic And Nematic Phase Transitions In Bafe2(As1=Xpx)(2) Studied By Ac Microcalorimetry And Squid Magnetometry

We study the antiferromagnetic (AFM) and structural phase transitions in single-crystal BaFe2(As-1-(x) P-x)(2) (x = 0, 0.3) at temperatures TN and TS, respectively, by high-resolution ac microcalorimetry and SQUID magnetometry. The specific heat measurements of both as-grown and annealed BaFe2As2 display a sharp peak at the AFMs-tructural transitions. A kink in the entropy of annealed BaFe2As2 gives evidence for splitting of the two transitions by approximately 0.5 K. No additional features could be identified in the specific heat of both BaFe2As2 and BaFe2(As0.7P0.3)(2) in the temperature regions around T* > TS where torque measurements [S. Kasahara et al., Nature 486, 382 (2012)] had revealed the "true" nematic phase transition, even though the Ginzburg-Landau model used to fit the magnetic torque data indicates that the expected thermal anomaly should be easily observable with our experimental resolution.