Enhancement of the magnetocaloric effect in Nd0.6-xGdxSr0.4MnO3 (0.02 ≤ x ≤ 0.1) perovskite manganites: The role of Gd3+ ionic substitution

Abstract

The influence of Gd³⁺ doping on the magnetocaloric properties of Nd_0.6-xGd_xSr_0.4MnO₃ (0.02 ≤ x ≤ 0.1) compounds, prepared using auto-combustion sol-gel technique, has been studied. Rietveld refinement of the X-ray diffraction (XRD) data has shown all compounds to be nanocrystalline with single-phased orthorhombic structures that index to the Pnma space group. The unit cell volumes reduce as the Gd³⁺ ions gradually substitute the Nd³⁺ones. The tuning Mn⁴⁺/Mn³⁺ ratio for all compounds, which are slightly lower than that of the Gd-free compound, demonstrates almost equal amounts of both ions. A ferromagnetic-to-paramagnetic transition is observed with rising temperature wherein the Curie temperature (T_C) gradually drops with increasing Gd³⁺ concentration (x). All materials exhibit ferromagnetism at 2 K, with saturation magnetization values that increase slightly with x. Both the calculated maximum magnetic entropies ($\left|\Delta S_M^{\mathrm{Max}}\right|$) and relative cooling powers (RCP) increase to relatively large values with increasing temperature and x values. The values obtained for $\left|\Delta S_M^{\mathrm{Max}}\right|$ and RCP for Nd_0.6-xGd_xSr_0.4MnO₃ (0.02 ≤ x ≤ 0.1) are comparable with those of standard pure Gd revealing their potential as magnetic refrigeration systems.