Anomalous and topological Hall effects in epitaxial thin films of the noncollinear antiferromagnet ${\mathrm{Mn}}_3\mathrm{Sn}$

Noncollinear antiferromagnets with a $D{0}_{19}$ ($\mathrm{space}\mathrm{group}=194$, $P{6}_3/mmc$) hexagonal structure have garnered much attention for their potential applications in topological spintronics. Here, we report the deposition of continuous epitaxial thin films of such a material, ${\mathrm{Mn}}_3\mathrm{Sn}$, and characterize their crystal structure using a combination of x-ray diffraction and transmission electron microscopy. Growth of ${\mathrm{Mn}}_3\mathrm{Sn}$ films with both (0001) $c$-axis orientation and ($40\overline{4}3$) texture is achieved. In the latter case, the thin films exhibit a small uncompensated Mn moment in the basal plane, quantified via magnetometry and x-ray magnetic circular dichroism experiments. This cannot account for the large anomalous Hall effect simultaneously observed in these films, even at room temperature, with magnitude ${\sigma }_{xy}\left({\mu }_{0}H=0\mathrm{T}\right)=21{\mathrm{\Omega }}^{-1}\mathrm{c}{\mathrm{m}}^{-1}$ and coercive field ${\mu }_0{H}_c=1.3\mathrm{T}$. We attribute the origin of this anomalous Hall effect to momentum-space Berry curvature arising from the symmetry-breaking inverse triangular spin structure of ${\mathrm{Mn}}_3\mathrm{Sn}$. Upon cooling through the transition to a glassy ferromagnetic state at around 50 K, a peak in the Hall resistivity close to the coercive field emerges. This indicates the onset of a topological Hall effect contribution, arising from a nonzero scalar spin chirality that generates a real-space Berry phase. We demonstrate that the polarity of this topological Hall effect, and hence the chiral nature of the noncoplanar magnetic structure driving it, can be controlled using different field-cooling conditions.

Figure 1

(a) $2\theta \text{−}\theta$ x-ray diffraction scans measured for a 70-nm ${\mathrm{Mn}}_3\mathrm{Sn}$ (0001) film grown on a ${\mathrm{SrTiO}}_3$ (111) substrate, and for a 60-nm ${\mathrm{Mn}}_3\mathrm{Sn}$ ($40\overline{4}3$) film grown on a MgO (001) substrate. Inset shows azimuthal $\varphi$ scans of the ${\mathrm{SrTiO}}_3$ {202}, Ru {$10\overline{1}1$}, and ${\mathrm{Mn}}_3\mathrm{Sn}$ {$20\overline{2}1$} reflections for a 70-nm ${\mathrm{Mn}}_3\mathrm{Sn}$ (0001) sample. The corresponding positions of in-plane crystallographic directions are indicated. (b) Cross-section scanning-TEM micrograph of a 70-nm ${\mathrm{Mn}}_3\mathrm{Sn}$ (0001) film, viewed along the $\left[\overline{1}10\right]$ zone axis of the ${\mathrm{SrTiO}}_3$ (111) substrate. Inset shows a wide-view image over an extended region of the same lamella. (c) Representative crystal structure of ${\mathrm{Mn}}_3\mathrm{Sn}$ (0001) films grown on ${\mathrm{SrTiO}}_3$ (111) substrates using a Ru buffer layer. (d) $\chi \text{−}\varphi$ x-ray pole figures measured for a 60-nm ${\mathrm{Mn}}_3\mathrm{Sn}$ ($40\overline{4}3$) sample. The MgO [100] crystalline axis was aligned with $\varphi =0^{\circ }$. Inset shows an atomic force microscopy topographic map for a similar 50-nm ${\mathrm{Mn}}_3\mathrm{Sn}$ ($40\overline{4}3$) film, capped with 2-nm Ru. (e) Cross-section high-resolution TEM micrograph of a 60-nm ${\mathrm{Mn}}_3\mathrm{Sn}$ ($40\overline{4}3$) film, viewed along the [010] zone axis of the MgO (001) substrate. Inset shows an overview TEM image measured for the same sample. (f) Representative crystal structure of ${\mathrm{Mn}}_3\mathrm{Sn}$ ($40\overline{4}3$) films grown on MgO (001) substrates using a Ru buffer layer.

Figure 2

(a) Magnetization measured as a function of magnetic field for a 60-nm ${\mathrm{Mn}}_3\mathrm{Sn}$ ($40\overline{4}3$) film at 300 K. Magnetic field was applied either out of plane or along one of two orthogonal in-plane directions (closed and open symbols represent down and up field sweeps, respectively). Inset shows magnetization as a function of out-of-plane magnetic field, measured at different temperatures. (b) X-ray absorption spectroscopy and XMCD spectra for a 70-nm ${\mathrm{Mn}}_3\mathrm{Sn}$ ($40\overline{4}3$) film, recorded using ${\sigma }_{-}$ polarized x-rays in ${\mu }_0{H}_{\pm }=\pm 8\mathrm{T}$ magnetic fields applied out of plane, at 100 K. Inset shows XMCD, calculated as the difference between spectra recorded with ${\sigma }_{\pm }$ polarized x-rays, measured in different out-of-plane magnetic fields at 100 K, as well as that measured in −8 T at 300 and 5 K.

Figure 3

Magnetotransport for a 70-nm ${\mathrm{Mn}}_3\mathrm{Sn}$ (0001) film patterned into $75\times 25\text{−}\mu {\mathrm{m}}^2$ Hall bars, with $500\text{−}\mu \mathrm{A}$ current parallel to the $\left[01\overline{1}0\right]$ in-plane crystallographic direction. (a) Longitudinal resistivity measured as a function of temperature during either zero-field cooling, or cooling in a 7-T magnetic field applied out of plane (closed symbols), and subsequent zero-field warming (open symbols). Diagram illustrates measurement geometry in relation to different crystallographic directions in the ${\mathrm{Mn}}_3\mathrm{Sn}$ (0001) film. (b) Hall resistivity measured as a function of magnetic field applied out of plane at different temperatures. Transverse resistivity was measured along the $\left[\overline{2}110\right]$ crystalline axis (closed and open symbols represent down and up field sweeps, respectively).

Figure 4

Magnetotransport for a 60-nm ${\mathrm{Mn}}_3\mathrm{Sn}$ ($40\overline{4}3$) film patterned into $75\times 25\text{−}\mu {\mathrm{m}}^2$ Hall bars. Transverse resistivity was measured normal to the (0001) crystal plane, with $500\text{−}\mu \mathrm{A}$ current parallel to the $\left[01\overline{1}0\right]$ in-plane crystallographic direction (closed and open symbols represent down and up field sweeps, respectively). Isothermal Hall resistivity measured as a function of magnetic field applied out of plane in the (a) higher-temperature regime and (b) lower-temperature regime. Diagram illustrates measurement geometry in relation to different crystallographic directions in the ${\mathrm{Mn}}_3\mathrm{Sn}$ ($40\overline{4}3$) film. Inset shows the Hall effect measured for a 30-nm ${\mathrm{Mn}}_3\mathrm{Sn}$ ($40\overline{4}3$) film at 5 K, after cooling under different field conditions.