Ground state magnetic structure of Mn3Sn

TL;DR

This work determines the zero-field ground-state magnetic structure of Mn$_3$Sn using spherical neutron polarimetry, identifying an inverse triangular Type III order with spins along $<100>$, distinct from Mn$_3$Ge's Type IV. Density functional theory shows negligible energy difference between Type III and IV, implying sixth-order anisotropy governs the selection rather than a large exchange energy discrepancy. In the high-temperature phase, a moderate magnetic field partially selects three of six magnetic domains, revealing controllable domain population; in the low-temperature incommensurate phase the domain structure is reset and becomes field-decoupled, preventing domain control by known methods. These results clarify the anisotropy landscape and have implications for AHE control and antiferromagnetic spintronics in Mn$_3$Sn.

Abstract

We use spherical neutron polarimetry to determine the ground state magnetic structure of Mn3Sn. We find that Mn3Sn adopts an inverse triangular structure with spins parallel to <100> (Type III) rather than spins parallel to <110> (Type IV). Density functional theory calculations reveal no energy difference between these two structures, suggesting that the selection is caused by subtle effects such as sixth-order anisotropy. Partial control of the magnetic domain population through a moderate magnetic field is key to distinguish between the two models. We find that three of the six domains are approximately equally populated, while the others have negligible population. Upon entering the low temperature incommensurate phase, the domain structure is lost. The domains decouple from the magnetic field, and can therefore not be controlled by any known method.

Figures (7)

• Figure 1: The crystal structure of Mn$_3$Sn, with Mn atoms marked in blue and Sn atoms marked in red. Faded atoms are at a lower layer than non-faded atoms. (a) The unit cell of Mn$_3$Sn. (b) Several unit cells viewed along the $c$ axis, showing how each layer of Mn atoms forms a kagome lattice.
• Figure 2: Four symmetry-allowed magnetic structures of Mn$_3$Sn. The Mn atoms are shown with colored circles, while Sn atoms are shown with gray. Faded atoms are at a lower layer than non-faded atoms. We find that type III gives the best agreement with data.
• Figure 3: Comparison between the observed and calculated polarization matrix elements $P_{ij}$ for the Bragg peaks measured in the (a) $(hk0)$ and (b) $(h0l)$ scattering planes. For each reflection, the symbol (fit) and vertical bar (data) represent (from left to right) $P_{xx}$, $P_{yx}$, $P_{zx}$, $P_{xy}$, $P_{yy}$, $P_{zy}$, $P_{xz}$, $P_{yz}$, and $P_{zz}$. Reflections marked with an asterisk (*) are measurements that were repeated with the incident polarization reversed. The measured data are plotted as columns with errorbars, and the red and blue points represent the best fits for type III and IV structures, respectively. The difference between data and fit is shown at the bottom of the figures. (c) The domain distribution from our fit to (b). Each section in the pie chart shows the relative population of the domain, with the corresponding structure given outside the pie chart. The black arrows show the direction of the small magnetic moment in the domain. The magnetic field was applied along ${\bf b}$.
• Figure 4: (a): The position and (b) the intensity of the peaks in the incommensurate phase as a function of temperature. Blue, green and orange markers indicate data at zero field, 10 T and -1 T, respectively. Neither the peak position nor the integrated intensity is affected by the magnetic field. Lines are guides to the eye. In a region around 230 K, the peaks are too close to be fitted separately, and a single peak was fitted instead. In (b), we show half the peak intensity in this region with faded markers. Outside that region, the faded markers indicate the sum of the two peaks.
• Figure 5: (a) Magnetic susceptibility of Mn$_3$Sn with the magnetic field of strength $\mu_0H=0.1$ T parallel to the $a$ (blue points) and $c$ (red points) axes. (b) The Anomalous Hall resistance of Mn$_3$Sn at 300 K (red points) and at 200 K (blue points), showing the absence of AHE in the incommensurate phase.