Effect of Gd doping on magnetic and MCE properties of M-type barium hexaferrite.

• Gadolinium doping in BHF results in modification of magnetic properties. • "Law of Approach to Saturation" gives the magnetocrystalline anisotropy constant. • Para-ferrimagnetic transition is observed around 700 K. • Maximum entropy change and RCP are found to be 0.72 J/kgK and 27.5 J/kg, respectively, near Curie temperature. • Possibility to use the Gd doped BHF at high temperature as a cooling agent. Gd doped barium hexaferrite (BaFe 12−x Gd x O 19 , x = 0.0–0.7) has been synthesized by the sol-gel method to explore its magnetic and MCE (magnetocaloric effect) properties. The materials crystallize to hexagonal magnetoplumbite phase. Average particle size decreases with the increase in Gd concentration in barium hexaferrite (BHF). The coercive field increases from 3.2 to 4.8 kOe, and saturation magnetization decreases from 68.21 to 54.23 emu/g with the increase in Gd concentration from x = 0.0 to x = 0.7. These large changes in magnetic parameters reveal the effect of Gd concentration in BHF. The saturation magnetization monotonously reduces with an increase in Gd concentration in BHF due to a decrease in average particle sizes. The saturation magnetization is found to be higher at a lower temperature (60 K) compared to that of room temperature (300 K). It is due to a reduction in thermal energy at low temperature which is smaller compared to the magnetic Gibbs free energy at low temperature. Hence, the magnetic spins are freezing along the applied magnetic field direction at the low temperature. Also, the magnetocrystalline anisotropy constant (obtained by the "Law of Approach to Saturation method") is found to be more at low temperature compared to that of room temperature due to an increase in the strength of spin-orbit coupling with the decrease in temperature (i.e. thermal energy). The M-T curves and M-H hysteresis loops reveal paramagnetic to ferromagnetic transition at the Curie temperature. The maximum entropy change was found to be in the range of 0.12–0.72 J/kgK in a window of the applied magnetic field of 0.5–3 T, and the corresponding RCP max was found to be 2.5–27.5 J/kg. The present study opens a window to explore the MCE on BHF based material. [ABSTRACT FROM AUTHOR]