Magnetic properties of Ge, Re and Cr substituted Fe$_5$SiB$_2$

Abstract: One of the possible approaches to decrease the demand for critical elements such as rare earths is to develop new sustainable magnets. Iron-based materials are suitable for gap magnets applications since iron is the most abundant ferromagnetic element on Earth. Fe$5$SiB$_2$ is a candidate as gap magnet thanks to its high Curie temperature (T${\text{C}} \sim$ 800 K) and saturation magnetization (M${\text{S}}\sim$ 140 Am$^2$kg$^{-1}$). However its anisotropy field is too low for applications (H${\text{A}} \sim$ 0.8 T). In order to increase the anisotropy value, we synthesized a series of Ge, Re and Cr substituted Fe$5$SiB$_2$ samples and studied their magnetic properties. They all crystallize in the Cr$_5$B$_3$-type tetragonal structure with the $I4/mcm$ space group. Curie temperature (T${\text{C}}$ = 803 K) and saturation magnetization (M${\text{S}}$ = 138 Am$^2$kg$^{-1}$) are slightly decreased by elemental substitution with Re having the largest effect. Despite being reduced, T${\text{C}}$ and M${\text{S}}$ still maintain significant values (T${\text{C}}>$ 750 K and M${\text{S}}$ = 118 Am$^2$kg$^{-1}$). The room temperature anisotropy field has been measured by Singular Point Detection (SPD) and increases by about 15% upon Re substitution, reaching 0.92 T for Fe${4.75}$Re$_{0.25}$SiB$_2$. We have also used Nuclear Magnetic Resonance and SPD measurements to study the spin reorientation transition which takes place at 172 K and we have found that it is partially suppressed by substitution of Ge from 172 K to 140 K and completely suppressed upon Cr and Re substitution.