NdAlGe中的条带螺旋磁性和两种反常霍尔效应区
Stripe Helical Magnetism and Two Regimes of Anomalous Hall Effect in NdAlGe
摘要：
我们报道了反转和时间反演对称性破缺Weyl半金属NdAlGe的磁性和电子输运性质。这种材料类似于NdAlSi，其螺旋磁性是Weyl介导的集体现象的罕见例子，但具有更大的自旋轨道耦合。我们的中子衍射实验表明，NdAlGe，类似于NdAlSi，支持一个无公度伊辛自旋密度波（$T_{\text{inc}}=6.8$ K）与一个小的螺旋自旋倾斜的3 $^\circ$和长波长的$\sim$ 35 nm，转换到一个相称的亚铁磁态低于$T_{\text{com}}=5.1$ K。利用小角中子散射，我们发现零场冷却的亚铁磁畴在实空间中形成条纹，其特征长度尺度分别为18 nm和72 nm，平行于和垂直于[110]方向。有趣的是，对于输运性质，NdAlSi不表现出异常霍尔效应（AHE），通常观察到的磁性外尔半金属。在对比NdAlSi，我们确定了两个不同的AHE制度NdAlGe分别由内在的Berry曲率和外在的障碍/自旋波动。我们的研究表明，外尔介导的磁性普遍存在于这组非中心对称的磁性外尔半金属NdAl $X$，但运输性能，包括AHE的影响，材料特定的外在效应，如障碍，尽管存在突出的Berry曲率。
英文摘要：
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.
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