Structure-Property Correlations in Sb, Ge, and Ga Doped AlFe$_2$B$_2$ for Magnetocaloric Applications

This study investigates the effects of Sb, Ge, and Ga doping in AlFe$_2$B$_2$ on magnetic and magneto-caloric properties. Samples of AlFe$_2$B$_2$ and AlFe$_{1.9}$M$_{0.1}$B$_2$ (M= Ge, Ga and Sb) with 20\% excess Al were synthesized by arc melting, and the powder processed were investigated for their phase constituents, microstructure, magnetic and magneto-caloric effect. The parent compounds prepared showed the AlFe$_2$B$_2$ phase with a FeB secondary phase. However, in Sb and Ga-doped samples, an additional impurity phase, Al$_{13}$Fe$_4$, was observed apart from FeB, while in Ge-doped, only the AlB$_2$ impurity phase was present. The Curie temperature of AlFe$_2$B$_2$ is 277 K, increasing with Sb, Ge, and Ga doping to 287\,K, 297\,K, and 296\,K, respectively. The magnetization ($M$) is also higher with Ge and Ga addition in the 100-300\,K range; however, with Sb doping, the $M$ decreases significantly compared to parent AlFe$_2$B$_2$. The magnetic entropy change under 2\,T reached 2.93 JKg$^{-1}$K$^{-1}$ near 274\,K in AlFe$_2$B$_2$, which decreases to 2.53 JKg$^{-1}$K$^{-1}$ and 1.92 JKg$^{-1}$K$^{-1}$ with Ge and Ga, respectively. With Sb doping, the MC change was affected dramatically to 0.32 JKg$^{-1}$K$^{-1}$. However, the relative cooling power of Ge doped is the same as that of parent AlFe$_2$B$_2$. This research advances the understanding of the relationship between doping elements and magnetic properties in AlFe$_2$B$_2$ and opens pathways for designing magneto-caloric materials with tailored magnetic characteristics.