Calorimetric study of Nd2Fe14B: Heat capacity, standard Gibbs energy of formation and magnetic entropy

Experimental Cp, m°T−1 vs T for Nd2Fe14B(cr) and a hypothetical substance La2Fenm14B. [Display omitted] •Nd2Fe14B is a main constituent of rare-earth magnets.•We investigated its thermodynamic properties from absolute zero to high temperature.•Predicted values of magnetic moment from heat capacity agree with measurement.•Evidence suggests Nd 4f component of projected density of states is localized in the core.•These insights will help find microstructures with optimal intrinsic magnetic properties. The thermodynamic properties for Nd2Fe14B, the main constituent of rare-earth magnets, were investigated from absolute zero to high temperature. Standard entropies Δ0TSm○ were determined by measuring the molar heat capacity Cp, m° from 2 K via the Debye-Einstein-spin wave functions. The temperature dependence of the standard Gibbs energy of formation ΔfGm° (T) from absolute zero to high temperature was determined by combining Δ0TSm○ with reference formation energies from first principles calculations. We found that the ΔfGm° (T) based on the open core approach, in which the Nd 4f components of the projected density of states are localized in the core, agrees well with that estimated from phase equilibria. Our measurements of the Sommerfeld parameter provided further support for the open-core concept. The magnetic moment, calculated as a function of Cp, m°, agrees with the result of direct magnetization measurement, suggesting that separating the spin and lattice vibration terms is necessary from very low to high temperature via the Curie temperature at 585.3 K. This work is expected to contribute significantly to magnetic materials design.