Stripe helical magnetism and two regimes of anomalous Hall effect in NdAlGe

We report the magnetic and electronic transport properties of the inversion and time-reversal symmetry breaking Weyl semimetal NdAlGe. This material is analogous to NdAlSi, whose helical magnetism presents a rare example of a Weyl-mediated collective phenomenon, but with a larger spin-orbit coupling. Our neutron diffraction experiments revealed that NdAlGe, similar to NdAlSi, supports an incommensurate spin density wave (T-inc = 6.8 K) whose spins are predominantly pointing along the out-of-plane direction and have a small helical spin canting of 3 degrees. The spin density wave has a long wavelength of approximate to 35 nm and transitions to a commensurate ferrimagnetic state below T-com = 5.1 K. Using small-angle neutron scattering, we showed that the zero-field cooled ferrimagnetic domains form stripes in real space with characteristic length scales of 18 and 72 nm parallel and perpendicular to the [110] direction, respectively. Interestingly, for the transport properties, NdAlSi does not exhibit an anomalous Hall effect (AHE) that is commonly observed in magnetic Weyl semimetals. In contrast to NdAlSi, we identify two different AHE regimes in NdAlGe that are, respectively, governed by intrinsic Berry curvature and extrinsic disorders/spin fluctuations. Our paper suggests that Weyl-mediated magnetism prevails in this group of noncentrosymmetric magnetic Weyl semimetals NdAlX, but transport properties including AHE are affected by material-specific extrinsic effects such as disorders, despite the presence of prominent Berry curvature.

Automated summary

We investigate the magnetic and electronic transport properties of NdAlGe, a Weyl semimetal which breaks inversion and time-reversal symmetries. The material displays an incommensurate spin density wave and transitions to a commensurate ferrimagnetic state at low temperatures. Small-angle neutron scattering reveals the formation of ferrimagnetic domains with different characteristic length scales in real space. Interestingly, unlike NdAlSi, NdAlGe exhibits two different regimes of anomalous Hall effect, one governed by intrinsic Berry curvature and the other by extrinsic disorders/spin fluctuations.