🔬 materials science

Tilted $p$ -wave magnet candidate CeNiAsO

By utilizing $^{75}$ As nuclear quadrupole/magnetic resonance to overcome the challenge of small ordered moments, this study identifies CeNiAsO as a rare "tilted $p$ -wave magnet" characterized by a commensurate antiferromagnetic order with a small out-of-plane moment that rotates the spin polarization axis and enhances non-relativistic spin splitting.

Original authors: Zhuo Wang, Zheng Liu, Shuo Zou, Hua-Xun Li, Jin-Xin Hu, Zhuolun Qiu, Ze Wang, Jiamin Gong, Lucheng Wei, Kangjian Luo, Hai Zeng, Meng Zhang, Chao Dong, Chuanyin Xi, Junfeng Wang, Jiakun Fang, Xiaotao H

Published 2026-08-21

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Zhuo Wang, Zheng Liu, Shuo Zou, Hua-Xun Li, Jin-Xin Hu, Zhuolun Qiu, Ze Wang, Jiamin Gong, Lucheng Wei, Kangjian Luo, Hai Zeng, Meng Zhang, Chao Dong, Chuanyin Xi, Junfeng Wang, Jiakun Fang, Xiaotao Han, Guang-Han Cao, Liang Li, Yongkang Luo

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Magnetism is a force we encounter daily, from the simple compass needle to the hard drive in a computer. For centuries, scientists have sorted magnetic materials into two main families: ferromagnets, like the iron in a fridge door that pulls everything together with a strong, unified pull, and antiferromagnets, where the tiny magnetic arrows inside the material point in opposite directions, canceling each other out so that the material feels magnetically invisible to the outside world. In recent years, a third, more exotic category has emerged in the scientific imagination: altermagnets. These are materials that, like antiferromagnets, show no net pull to the outside world, yet internally they possess a hidden, organized structure that splits their electrons into two distinct groups based on their spin. This hidden order makes them incredibly promising for the next generation of ultra-fast, low-energy electronics. However, finding and understanding these materials is difficult because their internal magnetic arrangements are often subtle and easily confused.

A team of researchers has now turned their attention to a specific crystal called CeNiAsO, a compound that had been suspected of being one of these rare altermagnets, but whose internal magnetic structure remained a puzzle. Previous studies had suggested that the magnetic moments inside this crystal were arranged in a flat, coplanar pattern, but the measurements were too faint to be certain. The researchers faced a significant challenge: the magnetic signal from the atoms in this crystal was surprisingly weak, making it nearly impossible to determine exactly how the tiny magnetic arrows were tilted. To solve this, they employed a technique that acts like a highly sensitive local probe, listening to the radio-frequency signals emitted by the arsenic atoms within the crystal. By analyzing how these signals changed under different conditions, they were able to map the magnetic landscape with a precision that previous methods could not achieve.

What they discovered was that the magnetic structure was not perfectly flat as previously thought. Instead, the magnetic arrows were slightly tilted out of their plane, creating a small but crucial vertical component. This tilt, though tiny—amounting to a moment of only about 0.05 micro-magnets per atom—was enough to break a specific symmetry that had been thought to be preserved. This small tilt reorients the axis along which the electrons are split, effectively turning the material into what the authors call a "tilted p-wave magnet." This is a rare and specific type of magnetic order where the internal symmetry allows for unique electronic behaviors that are distinct from standard magnets. The researchers confirmed this finding by combining their experimental data with theoretical calculations, showing that this tilted arrangement is stable and consistent with the material's observed properties.

The significance of this discovery lies in how it changes our understanding of how these materials work. In the standard view of such magnets, the direction of the electron spin splitting is locked to the crystal's physical shape. However, this study shows that a slight tilt in the magnetic order can rotate this direction, opening up new possibilities for controlling the material's electronic properties. The team also observed that when they applied a strong external magnetic field, the material began to behave differently, showing a specific electrical response known as the anomalous Hall effect, which only appears when the internal symmetry is broken. This behavior matched their theoretical predictions, confirming that the tilted structure is the key to the material's unique characteristics.

By resolving the exact magnetic structure of CeNiAsO, the researchers have provided a clear benchmark for the field of altermagnetism. They have shown that even a minute tilt in the magnetic order can have profound effects on the material's symmetry and its electronic behavior. This work suggests that the search for new magnetic materials for future technologies should look closely at these subtle tilts, as they may hold the key to unlocking more efficient ways to manipulate electron spins. The study does not claim to have solved all the mysteries of this material, particularly regarding why the magnetic moments are so small to begin with, but it has successfully mapped the terrain where those mysteries lie. The findings offer a solid foundation for future experiments and calculations, guiding scientists toward a deeper understanding of how magnetic order can be engineered to create new functional materials.

Technical Summary: Tilted p-wave magnet candidate CeNiAsO

Problem Statement
The magnetic structure of the heavy-fermion compound CeNiAsO, a proposed candidate for a correlated $p$ -wave magnet (altermagnet), has remained elusive due to its unexpectedly small ordered magnetic moments. Previous neutron scattering studies suggested a commensurate antiferromagnetic (CAFM) order with a coplanar, non-collinear arrangement of Ce moments ( $q = (0.5, 0, 0)$ ) and a spin polarization axis aligned with the crystallographic $c$ -axis ( $S_z \parallel c$ ). However, the ordered moment size ( $\sim 0.37 \, \mu_B/\text{Ce}$ ) is significantly smaller than theoretical expectations for the $|\pm 1/2\rangle$ Kramers doublet and the structural analog CeFeAsO. Furthermore, the magnetic order exhibits extreme resistance to external magnetic fields, reaching only $0.17 \, \mu_B/\text{Ce}$ at 58 T. These factors, combined with the inability of conventional neutron techniques to definitively resolve the out-of-plane component ( $m_z$ ) of the moments, left open critical questions regarding the preservation of $C_s^{2z}T$ symmetry and the nature of the spin-splitting mechanism. Specifically, it was unclear whether the moments were strictly coplanar or if a small canting existed that would break specific symmetries and alter the spin polarization axis.

Methodology
To resolve the magnetic structure with high precision, the authors employed $^{75}\text{As}$ nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) spectroscopy. These techniques were chosen for their high sensitivity to local internal magnetic fields ( $B_{\text{int}}$ ) at the Arsenic sites, which are governed by transferred hyperfine interactions with neighboring Ce-4f spins.

Experimental Setup: Measurements were conducted on high-quality single crystals and aligned powders. NQR was performed in zero field, while NMR was conducted under an external field of $\mu_0 H = 10$ T. High-field Hall effect and magnetization measurements were also performed up to 58 T (steady and pulsed fields) to probe the field-induced evolution of the magnetic state.
Theoretical Framework: The experimental spectra were analyzed using a total Hamiltonian ( $H_{\text{tot}} = H_Z + H_{\text{int}} + H_Q$ ) incorporating Zeeman terms and nuclear quadrupole interactions. Numerical simulations were used to fit the resonance frequencies and transition probabilities to determine the magnitude and orientation of $B_{\text{int}}$ .
First-Principles Calculations: Density Functional Theory with Hubbard $U$ correction (DFT+U) was utilized to model the electronic band structure and anomalous Hall conductivity (AHE) for various magnetic configurations, including the proposed tilted CAFM state and a field-polarized ferromagnetic state.

Key Results

Determination of Magnetic Structure: The analysis of $^{75}\text{As}$ NQR and NMR spectra revealed that the ground state is not a strictly coplanar order with $S_z \parallel c$ , but rather a tilted coplanar CAFM order. The internal field analysis yielded a polar angle $\theta_{\text{int}} \approx 44^\circ$ and an azimuthal angle $\phi_{\text{int}} \approx 26.5^\circ$ . By combining these with the known ordered moment magnitude ( $m \approx 0.37 \, \mu_B$ ), the authors determined a small out-of-plane canting component of $m_z \approx 0.05 \, \mu_B$ .
Symmetry Breaking and Reconstruction: This small $m_z$ component breaks the $C_s^{2z}T$ symmetry (which requires $m_z=0$ ) while preserving the combined time-reversal and translation symmetry ( $T\tau$ ). Consequently, the spin polarization axis $\hat{n}$ is reoriented away from the crystallographic $c$ -axis to a new direction $\hat{n} \approx (0, 0.231, 0.973)$ .
Enhanced Spin Splitting: DFT+U calculations demonstrated that this magnetic tilting leads to a non-monotonic dependence of the spin band splitting ( $\Delta S$ ) on the canting angle. Notably, the splitting increases with small canting angles, reaching a maximum enhancement of 23% near $\theta_m \approx 32^\circ$ compared to the non-canted case. This suggests that magnetic tilting can actively promote non-relativistic spin-splitting effects.
Anomalous Hall Effect (AHE): Consistent with the preserved $T\tau$ symmetry, zero-field AHE is forbidden. However, high-field Hall measurements revealed the emergence of AHE loops at temperatures below $T_{N2}$ when the magnetic field exceeds $\sim 13$ T. This onset coincides with a rapid drop in the NMR central peak splitting ( $\Delta f$ ), indicating a field-induced reorientation of the Ce moments (likely breaking $T\tau$ symmetry by generating a net $m_z$ ), which allows for the appearance of AHE.

Significance and Claims
The paper claims to establish CeNiAsO as a rare example of a "tilted $p$ -wave magnet." By resolving the small out-of-plane moment, the authors demonstrate that the magnetic symmetry reconstruction in this system leads to a reorientation of the spin polarization axis, distinct from the crystallographic axes. This finding challenges the conventional view that spin textures in altermagnets are strictly constrained by crystallographic symmetry.

The work provides an essential benchmark for understanding how magnetic canting impacts the stability of symmetry-enforced spin-splitting in altermagnetic materials. The authors posit that this tilted configuration enhances non-relativistic spin-splitting phenomena, such as the Edelstein effect, offering a new route to tune spintronic functionalities in heavy-fermion systems. Additionally, the study validates the utility of NQR/NMR techniques for determining magnetic structures in systems with small ordered moments where conventional neutron scattering may be insufficient, particularly under extreme conditions like high magnetic fields.

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