Magnetocaloric effects and critical behavior of La0.65Ca0.35−xGdxMnO3 (0≤x≤0.15)

In this study, we investigate the magnetocaloric effects (MCE) and critical behavior of Gd-doped La${}_{0.65}$Ca${}_{0.35-x}$Gd_xMnO₃ ($x=0,0.10,0.15$) polycrystalline materials. Our results reveal excellent MCE in the Gd-doped samples. Under a magnetic field of 7$$T, the full-width at half maximum ($\mathrm{\Delta }{T}_{\mathrm{\text{FWHM}}})$ increases from 41$$K ($x=0.00$) to 121$$K ($x=0.10$) and 112$$K ($x=0.15$). Additionally, the refrigerant capacity (RC) is enhanced by 149% and 145% at $x=0.10$ and $x=0.15$, respectively, compared to the parent phase. We propose that these improvements can be attributed to the introduction of Gd${}^{3+}$ ions, which possess smaller ionic radii. This reduction in the average A-site ionic radius weakens the double exchange (DE) interactions, resulting in a more continuous phase transition within the system. Supporting this view, we observe a decrease in magnetization strength after doping, a reduction in Curie temperature ($T_C)$ from 250$$K ($x=0.00$) to 134$$K ($x=0.10$) and 130$$K ($x=0.15$) and a transformation from a first-order to a second-order phase transition in the doped samples. To characterize the critical behavior of the phase transition in the doped samples, we employ the K-F method. The obtained critical exponents for $x=0.10$ and $x=0.15$ are $\beta =0.380\pm 0.006$, $\gamma =1.323\pm 0.003$, $\delta =4.482\pm 0.005$ and $\beta =0.477\pm 0.008$, $\gamma =1.083\pm 0.004$, $\delta =3.270\pm 0.002$, respectively. Furthermore, the calculation of the n values suggests a transition of the phase transition in the doped samples from short-range ordering to long-range ordering.