Large Topological Hall Effect in a Short-Period Helimagnet MnGe

We have observed an unconventional, likely topological, Hall effect over a wide temperature region in the magnetization process of a chiral-lattice helimagnet MnGe. The magnitude of the topological Hall resistivity is nearly temperature-independent below 70 K, which reflects the real-space fictitious magnetic field proportional to a geometric quantity (scalar spin chirality) of the underlying spin texture. From the neutron diffraction study, it is anticipated that a relatively short-period (3–6 nm) noncoplanar spin structure is stabilized from the proper screw state in a magnetic field to produce the largest topological Hall response among the B20-type (FeSi-type) chiral magnets.

Figure 1

(a) Temperature dependence of resistivity and magnetic susceptibility ($\chi =M/H$ at $H=100\mathrm{Oe}$) in MnGe. (b) Temperature dependence of the integrated intensity of magnetic Bragg peaks indexed with the propagation vector ${\mathit{Q}}_m=\{\delta 00\}$. The integrated intensity is normalized by that at 20 K. Inset shows neutron powder diffraction patterns of 20, 100, 160, and 200 K as subtracted by that at $270\mathrm{K}>T_N$. (c) Temperature dependence of the helical period $\lambda$ and the magnitude of ${\mathit{Q}}_m$. The dashed and solid lines in (b) and (c) are the guide to the eyes.

Figure 2

Magnetic-field dependence of (a) magnetization $M$, (b) resistivity ${\rho }_{xx}$ as normalized by the zero-field value ${\rho }_{xx}(0)$, and (c),(d) Hall resistivity ${\rho }_{yx}$ at various temperatures. Solid lines in panels (c) and (d) are the fitted curves of ${\rho }_{yx}$ using the relation that ${\rho }_{yx}=R_{0}B+S_A{\rho }_{xx}^{2}M$ with the fitting parameters $R_0$ and $S_A$ (see text).

Figure 3

(a) and (b) Comparison between $T$-dependence of (a) anomalous and (b) topological components of Hall resistivity (solid circles) and conductivity (open circles). ${\rho }_{yx}^A$ is here defined by the value of ${\rho }_{yx}(14\mathrm{T})$, while $|{\rho }_{yx}^T{|}^{\max }$ here by the negative maximal values of ${\rho }_{yx}^T\mathrm{\text{−}}H$ curves in panel (c). (c) Magnetic-field dependence of topological Hall resistivity ${\rho }_{yx}^T$. (d) A contour map of ${\rho }_{yx}^T$ in the plane of temperature and magnetic field. The white curve represents the temperature variation of the critical field $H_C$, at which the ferromagnetic spin-collinear state is realized.

Figure 4

Comparison between the hysteresis behavior of ${\rho }_{yx}^T$, $M$, and $I_{\mathrm{mag}}(1-\delta 10)/I_{\mathrm{nu}}$ at 30 K. $I_{\mathrm{mag}}(1-\delta 10)$ and $I_{\mathrm{nu}}$ represent the neutron magnetic reflection of the screw structure and the nuclear one, respectively. Inset in panel (b) shows neutron powder diffraction patterns at 30 K at various magnetic fields. The small triangles, the solid line and the dashed line in the inset are the marks for new magnetic reflections, reflections of the helical structure, and the impurity Bragg peaks, respectively. Solid squares are the data points of the integrated intensity of the new magnetic reflection ($q=16.1{\mathrm{nm}}^{-1}$) from the field-induced magnetic texture. Thin arrows denote directions of field scans. The dashed lines are the guide to the eyes.