Magnetic properties and magnetocaloric effect of R2Fe14B compounds with R = Y, Pr, Nd, Sm, Gd, Tb, Dy, Ho and High-Magnetic field phase transitions in the compounds with R = Gd, Dy

Abstract

The temperature and field dependences of the: magnetization, magnetic specific heat and magnetic entropy of the R2Fe14B compounds, are studied using the mean-field two-sublattice model, where R = Y, Pr, Nd, Sm, Gd, Tb, Dy, Ho, and B is boron. The isothermal entropy change ΔSm and the adiabatic temperature change ΔTad are calculated, for magnetic fields ≤ 5 T, by a Maxwell’s relation and a well-known integration involving the total heat capacity. The temperature range for these calculations is up to 700 K. Direct magnetocaloric effect (MCE), in all compounds, is present with maximum ΔSm and ΔTad in the range of 0.9–6.8 J/mol. K and 1.5–7.7 K, respectively, for a magnetic field change in the range of 1–5 T. The relative cooling power RCP(S) is in the range of 196–526 J/ kg. Ferrimagnetic compounds show both direct and small inverse MCE. It is found that Pr2Fe14B has the highest ΔSm of 6.8 J/mol. K and the largest ΔTad of 7.7 K at a magnetic field change of 5 T. The magnetization curves are calculated for Dy2Fe14B for temperatures in the range of 30–200 K and for Gd2Fe14B from 46 to 400 K for magnetic fields up to 300 T. It is found that these two compounds exhibit ferrimagnetic to ferromagnetic first-order phase transitions. Belov -Arrott plots, for these two compounds, show S-shaped feature which supports the occurrence of a first-order phase transition.