Heat capacity studies of the magnetic phase transition in sodium-rich NaxCoO2 (0.73⩽x⩽0.87)

Abstract

Specific heat measurements in the temperature region from 2 to 50 K in magnetic field up to 10 T, oriented parallel and perpendicularly to the CoO 2 layers were carried out on a series of high-quality single-crystals of Na x CoO 2 (x = 0. 73, 0.76, 0.77, 0.78 and 0.87). Surprisingly, sharp lambda type anomaly was observed only for the concentration x = 0.76 at temperature (21.80 ± 0.02) K, for all the remaining doping levels round anomaly in experimental data was visible at temperature~20 K, indicating a smeared magnetic phase transition. While the magnetic field oriented perpendicularly to the CoO 2 layers shifts the temperature of this anomaly to lower values, parallel magnetic field has no influence on it, what indirectly supports the idea of A-type antiferromagnetic ordering in studied systems.

Thermodynamic and Magnetotransport Properties of High Quality Na_{0.77}CoO_{2} Single Crystals

“…The broad bump around 20 K observed in heat capacity, indicates a smeared magnetic phase transition. Magnetic elds up to 9 T, oriented perpendicularly to ab plane, shift the transition temperature to lower values, while parallel magnetic elds have no inuence on it [4]. This fact is in accordance with the assumption of A-type antiferromagnetic ordering, suggested for Na x CoO 2 at x > 0.75 in ref.…”

Defining a critical temperature of a crossover from BEC to the normal phase in anisotropic quantum magnets

“…This point may be interpreted as a critical temperature of the crossover. It would be interesting to study the number (or density) fluctuations for other smeared phase transitions such as BCS-BEC [28,29] and magnetic transitions in sodium rich materials, e.g., Na x CoO 2 [30].…”