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.