Size-dependence of the heat capacity and thermodynamic properties of hematite (α-Fe2O3)

Abstract: The heat capacity of a 13nm hematite (α-Fe2O3) sample was measured from T =(1.5 to 350)K using a combination of semi-adiabatic and adiabatic calorimetry. The heat capacity was higher than that of the bulk which can be attributed to the presence of water on the surface of the nanoparticles. No anomaly was observed in the heat capacity due to the Morin transition and theoretical fits of the heat capacity below T =15K show a small T 3 dependence (due to lattice contributions) with no T 3/2 dependence. This suggests that there are no magnetic spin-wave contributions to the heat capacity of 13nm hematite. The use of a large linear term to fit the heat capacity below T =15K is most likely due to superparamagnetic contributions. A small anomaly within the temperature range (4 to 8)K was attributed to the presence of uncompensated surface spins. [Copyright &y& Elsevier]