Magnetocaloric Effect and Mossbauer Spectroscopy Study for Defect Analysis in Gadolinium Garnets*

This study discusses the effects of dopants, such as Praseodymium (Pr³⁺) and Calcium (Ca⁺²), on Gd₃Fe₅O₁₂ garnet. Samples are prepared using the auto-combustion method; all samples maintain cubic crystal structure. Praseodymium doping reduces the coordination of Fe³⁺ ions, forming penta-coordinated Fe³⁺ ions in the lattice. Similarly, maintaining charge balance, Ca²⁺ doping changes Fe³⁺ to Fe⁴⁺. The formation of Fe⁴⁺ at the down spin state (tetrahedral site) enhances the net magnetic moment of the compound. These different types of doping share similar mechanisms of lattice disorder resulting from the direct interaction of iron and oxygen tetrahedra. Thus, these defects alter the structural and magnetic order of the compound. The study explores the structural and magnetocaloric properties of the doped garnet. The magnetocaloric properties of doped garnet were assessed via isothermal magnetization curves in the field up to 5T. Gd_2.2Pr_0.8Fe₅O₁₂ compound exhibits a maximum entropy change of 2.33 J kg^-1K^-1 and a Relative Cooling power (RCP) value of 263 J kg^-1.  Meanwhile, the Gd_2.5Ca_0.5Fe₅O12 compound demonstrates even better magnetocaloric properties compared to the pure Gd₃Fe₅O₁₂ compound, with a maximum entropy change of 3.11 J kg^-1K^-1 and a notably high RCP value of 349 J kg^-1. Mossbauer studies are in progress to identify the effect of defects on iron coordination and hyperfine parameters. These findings suggest it is possible to fine-tune the magnetocaloric properties of Gd₃Fe₅O₁₂ via defects, thus offering a novel mechanism for fine-tuning the magnetocaloric properties of the compound.