Neutron diffraction study of the magnetic and structural phase transitions in BaFe2As2

We present the results of an investigation of both the magnetic and structural phase transitions in a high quality single crystalline sample of the undoped iron pnictide compound BaFe2As2. Both phase transitions are characterized via neutron diffraction measurements which reveal simultaneous, continuous magnetic and structural orderings with no evidence of hysteresis, consistent with a single second-order phase transition. The onset of long-range antiferromagnetic order can be described by a simple power-law dependence phi(T)(2)proportional to(1-T/T-N)(2 beta) with beta=0.103 +/- 0.018; a value near the beta=0.125 expected for a two-dimensional Ising system. Biquadratic coupling between the structural and magnetic order parameters is also inferred along with evidence of three-dimensional critical scattering in this system.