Antiferromagnets with broken time-reversal symmetry, which are also termed altermagnets, have been attracting marked attention in condensed matter physics in recent years [1], because they potentially show emergent phenomena, such as large anomalous Hall effect or spin-current generation, which would be applicable to novel spintronics devices in the future. We have recently discovered a new altermagnet FeS, which exhibits antiferromagnetic (AFM) order and spontaneous Hall effect at room temperature [2]. This system is known to exhibit a spin reorientation transition at around 190 K. We found that the spontaneous Hall effect disappears at this transition. To investigate the magnetic structures above and below the transition temperature, we performed unpolarized and polarized neutron scattering measurements with a single crystal of FeS.

Unpolarized neutron diffraction measurements were carried out at High-Resolution Spectrometer HRC(BL12) in the Materials and Life-science experimental Facility (MLF) in Japan Accelerator Research Complex (J-PARC). A single crystal of FeS was mounted in a closed-cycle ⁴He refrigerator with the $(H,H,L)$ horizontal scattering plane. The sample was exposed to a pulsed polychromatic incident neutron beam. Bragg reflections from the sample were detected by the ³He position sensitive detectors. Figures 1(a) and 1(b) show the comparison between the temperature variations of magnetic susceptibilities and the integrated intensity of the 002 Bragg reflection, at which nuclear scattering is expected to be very weak. Since the neutron scattering cross section of a magnetic peak is proportional to a square of Fourier-transformed magnetization projected onto the plane perpendicular to the scattering vector, the intensity of the 002 reflection at high temperatures is attributed to the easy-plane type $q=0$ AFM structure as illustrated in the inset on the right side of Fig. 1b. At around 190 K, the intensity of the 002 reflection was abruptly reduced, which is exactly coincides with the sharp drop of the magnetic susceptibility along the $c$ axis. These changes can be interpreted that the system undergoes the spin-reorientation transition from the easy-plane type AFM to the easy-axis type AFM shown in the inset on the left side of Fig. 1(b).

To further corroborate the easy-axis type AFM order at low temperatures, we also performed polarized neutron scattering measurement at the POlarized Neutron Triple-Axis spectrometer PONTA installed at Japan Research Reactor 3. Similarly to the experiment at HRC, the $(H,H,L)$ horizontal scattering plane was selected. We measured polarized neutron scattering profile of the 112 reflection at 100 K. The polarization direction of the incident neutron beam was set to be parallel to the vertical direction. As shown in Fig. 2, we observed a strong spin-flip (SF) scattering, which arises from the Fourier-transformed magnetization perpendicular to both the scattering vector and the vertical direction, in addition to a non-spin-flip (NSF) scattering, which includes the nuclear scattering. By combining the results from HRC and PONTA, we confirmed that the system exhibits the $q=0$ AFM structure with magnetic moments parallel to the $c$ axis at low temperature.

References

• [1] L. Šmejkal et al., Phys. Rev. X 12, 031042 (2022).
• [2] R. Takagi et al., Nat. Mater. 24, 63-68 (2024).

Authors

• S. Seki^a, R. Takagi, A. Kitaori^a, H. Saito and T. Nakajima
• ^aThe University of Tokyo