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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 φ(T )2β with β = 0.103 ± 0.018; a value near the β = 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.

Topological defects, potential information carriers, were written into and erased from a solid with femtosecond light pulses.

Heat-capacity, X-ray diffraction, and resistivity measurements on a high-quality BaFe2As2 sample show an evolution of the magneto-structural transition with successive annealing periods. After a 30-day anneal the resistivity in the (ab) plane decreases by more than an order of magnitude, to 12 µΩcm, with a residual resistance ratio ∼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 ∼0.15 K, and is unchanged in a magnetic field µ0H = 14 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.