Thermal Expansion Measurements of the Antiferromagnetic Transition in Na$_x$CoO$_2$ Single Crystals (x=0.75 and 0.80)

Thermal expansion, heat capacity and magnetic susceptibility measurements of Na$_x$CoO$_2$ (x=0.75 and 0.80) single crystals are reported. The results show second-order antiferromagnetic transition near $T_N$ = 22 K. The high-resolution thermal expansion measurements, performed with a fused quartz dilatometer cell in both basal plane and c-axis directions, reveal strong anisotropy that is attributed to the hexagonal structure. Above and below $T_N$, the thermal expansion coefficients and specific heat measurements exhibit $\lambda$- like peaks which are similar to those reported for other antiferromagnetic compounds. Analysis of the critical behavior for both samples will be discussed.