Stripe Helical Magnetism and Two Regimes of Anomalous Hall Effect in NdAlGe

TL;DR

This study investigates the magnetic structures and electronic transport in NdAlGe, revealing two regimes of anomalous Hall effect influenced by intrinsic Berry curvature and extrinsic disorders, highlighting complex Weyl-mediated magnetism.

Contribution

It uncovers two distinct anomalous Hall effect regimes in NdAlGe, driven by Berry curvature and disorder, and details its unique magnetic phases and domain structures.

Findings

NdAlGe exhibits incommensurate helical and ferrimagnetic phases.

Two AHE regimes are identified, governed by Berry curvature and extrinsic effects.

Magnetic domains form stripe patterns with characteristic length scales.

Abstract

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 Ising spin density wave ($T_{\text{inc}}=6.8$ K) with a small helical spin canting of 3$^\circ$ and a long-wavelength of $\sim$ 35 nm, which transitions to a commensurate ferrimagnetic state below $T_{\text{com}}=5.1$ 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 nm 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 study suggests that Weyl-mediated magnetism prevails in this group of noncentrosymmetric magnetic Weyl semimetals NdAl$X$, but transport properties including AHE are affected by material-specific extrinsic effects such as disorders, despite the presence of prominent Berry curvature.