Thermodynamic nature of the antiferromagnetic transition in NaxCoO2

High-resolution thermal expansion measurements of single-crystalline Na0.80CoO2 reveal continuous behavior of the paramagnetic-antiferromagnetic phase transition at T-M=21.7(1) K with critical exponent alpha=0.18(2). The thermal expansion is found to be highly anisotropic. Thermodynamic analysis provides the hydrostatic pressure derivative dT(M)/dP=+4.6(2) K/GPa. Similar measurements and analysis are presented for Na0.75CoO2.