Tuning intrinsic anomalous Hall effect from large to zero in two ferromagnetic states of SmMn2Ge2

The intrinsic anomalous Hall conductivity (AHC) in a ferromagnetic metal is completely determined by its band structure. Since the spin orientation direction is an important band–structure tuning parameter, it is highly desirable to study the anomalous Hall effect in a system with multiple spin reorientation transitions. We study a layered tetragonal room temperature ferromagnet $\mathrm{Sm}{\mathrm{Mn}}_2{\mathrm{Ge}}_2$, which gives us the opportunity to measure magnetotransport properties where the long $c$-axis and the short $a$-axis can both be magnetically easy axes depending on the temperature range we choose. We show a moderately large fully intrinsic AHC up to room temperature when the crystal is magnetized along the $c$-axis. Interestingly, the AHC can be tuned to completely extrinsic with extremely large values when the crystal is magnetized along the $a$-axis, regardless of whether the $a$-axis is magnetically easy or hard axis. First-principles calculations show that nodal line states originate from Mn-$d$ orbitals just below the Fermi energy ($E_{\mathrm{F}}$) in the electronic band structure when the spins are oriented along the $c$-axis. Intrinsic AHC originates from the Berry curvature effect of the gapped nodal lines in the presence of spin-orbit coupling. AHC almost disappears when the spins are aligned along the $a$-axis because the nodal line states shift above $E_{\mathrm{F}}$ and become unoccupied. Since the AHC can be tuned from fully extrinsic to intrinsic even at 300 K, $\mathrm{Sm}{\mathrm{Mn}}_2{\mathrm{Ge}}_2$ becomes a potential candidate for room-temperature spintronics applications.