🔬 materials science

Confirmation of complex non-collinear Fe(II) spin ordering in the iron oxychalcogenide La $_2$ O $_2$ Fe $_2$ OSe $_2$ by broad range $^{139}$ La-NQR

This study confirms the complex non-collinear spin ordering in La $_2$ O $_2$ Fe $_2$ OSe $_2$ below 90 K through broad-range zero-field $^{139}$ La-NQR measurements, which align with previous NMR and Mössbauer results while revealing a significant modification of the electric field gradient at the La site below the Néel temperature.

Original authors: Tillmann Weinhold, Jerin Abraham George, Cornelius Herrmann, Domenic Nowak, Rajib Sarkar, Felix Seewald, Rowena Wachtel, Sabine Wurmehl, Bernd Büchner, Hans-Henning Klauss

Published 2026-10-22

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CC BY 4.0

Original authors: Tillmann Weinhold, Jerin Abraham George, Cornelius Herrmann, Domenic Nowak, Rajib Sarkar, Felix Seewald, Rowena Wachtel, Sabine Wurmehl, Bernd Büchner, Hans-Henning Klauss

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

Deep within the world of solid materials, there exists a hidden landscape where electrons do not simply flow like water, but align themselves in intricate, frozen patterns. This is the realm of magnetism, a force usually associated with the simple pull of a refrigerator magnet. However, in certain complex crystals, the magnetic behavior is far more subtle and sophisticated. Scientists are particularly interested in a specific class of materials called altermagnets. These are special because they possess a magnetic order that cancels itself out, leaving no overall magnetic pull, yet they still split their internal energy levels in a way that could revolutionize how we process information at incredibly high speeds. To understand these materials, researchers must look at how the tiny magnetic spins of atoms arrange themselves, a task that often requires peering into the atomic structure with extreme precision.

One such material, a crystal known as La2O2Fe2OSe2, has been a subject of intense study because it contains iron atoms arranged in a specific grid that promises these unique magnetic properties. For years, scientists have debated exactly how the iron atoms in this crystal line up when the material is cooled down. Early observations suggested the iron spins might point in a simple, straight line, but later, more detailed studies hinted at a more complex, twisted arrangement. The key to solving this mystery lies in the lanthanum atoms, which sit nearby in the crystal structure. These atoms act as silent witnesses; they do not carry a magnetic moment themselves, but they feel the magnetic influence of their iron neighbors. By listening to the subtle signals emitted by these lanthanum atoms, researchers can reconstruct the invisible magnetic map of the entire crystal.

In a recent investigation, a team of physicists set out to confirm the true nature of this magnetic arrangement using a technique called nuclear quadrupole resonance. Imagine listening to a radio station; usually, you tune into a single, clear frequency. However, in this experiment, the researchers were tuning into the natural "hum" of the lanthanum atoms inside the crystal, without applying any external magnetic field to disturb them. They scanned a wide range of frequencies, from 2.8 to 20 million cycles per second, while cooling the material from a warm 100 degrees down to a frigid 4 degrees above absolute zero. Their goal was to see how the signals changed as the iron atoms began to order themselves, a transition that happens around 90 degrees.

The results provided a clear and definitive picture. As the material cooled below the critical temperature, the single signal from the lanthanum atoms did not simply shift or split in a chaotic way. Instead, the researchers observed a very specific pattern: one signal remained steady, while three new signals appeared, spaced perfectly evenly from one another. This precise spacing is the fingerprint of a magnetic field acting on the atoms. By comparing these observations with detailed computer simulations, the team found that the data matched perfectly with a model where the iron spins are arranged in a non-collinear fashion. This means the iron atoms do not point in a single straight line; rather, they orient themselves along the bonds connecting them, creating a complex, four-sided pattern on the atomic grid.

Crucially, this study ruled out a competing theory that had been proposed by earlier researchers. That previous idea suggested that the crystal lattice itself might be physically distorting or splitting into two different types of atomic sites due to the magnetism, which would have caused the signals to behave differently. The new measurements showed no evidence of such a structural distortion. Instead, the data confirmed that the change in the signals comes entirely from the magnetic influence of the iron spins, which creates two distinct environments for the lanthanum atoms. One group of lanthanum atoms feels a strong magnetic push from the iron neighbors, while another group feels a much weaker push. This difference is exactly what creates the two sets of signals observed in the experiment.

The findings serve as a strong confirmation of the complex magnetic structure previously proposed by other groups using different methods. By combining the results of this new study with earlier work, the scientific community can now be confident that the iron spins in this material form a specific, non-straight pattern. This arrangement is not just a curiosity; it is the very feature that gives the material its potential as an altermagnet. The researchers demonstrated that by carefully listening to the quiet signals of the lanthanum atoms, they could map out the invisible magnetic architecture of the crystal without needing to guess or assume. The work stands as a solid piece of evidence, showing that the material's magnetic heart beats in a complex, non-collinear rhythm, opening the door for future studies into how these unique materials might be used in next-generation technology.

Technical Summary: Confirmation of Complex Non-Collinear Fe(II) Spin Ordering in La₂O₂Fe₂OSe₂ via Broad Range ¹³⁹La-NQR

Problem and Context
The transition metal oxychalcogenide La₂O₂Fe₂OSe₂ is a candidate material for altermagnetism, a class of compensated antiferromagnets exhibiting large spin-splitting in their electronic band structures. While the material is known to order antiferromagnetically below a Néel temperature ( $T_N \approx 90$ K), the precise nature of the magnetic structure has been a subject of debate. Earlier neutron diffraction studies suggested a collinear antiferromagnetic order. However, a 2014 combined local probe study (utilizing ⁵⁷Fe Mössbauer spectroscopy, ¹³⁹La-NMR, and µSR) by Günther et al. proposed a complex non-collinear spin-ice-like ordering where Fe²⁺ spins align along O-Fe-O bonds with 90-degree in-plane non-collinearity on the Lieb lattice.

A subsequent 2021 zero-field ¹³⁹La-NQR study by Hussain et al. reported a discontinuous splitting of resonance signals below $T_N$ . They interpreted this as evidence of a magnetically induced local lattice anomaly creating two distinct La sites with different electric field gradients (EFG). This interpretation challenged the purely magnetic origin of the spectral changes and suggested a structural distortion accompanying the magnetic ordering.

Methodology
This work presents a comprehensive investigation combining experimental ¹³⁹La-Nuclear Quadrupole Resonance (NQR) measurements with theoretical simulations to resolve the nature of the magnetic ordering and the origin of the spectral features observed below $T_N$ .

Sample Synthesis: Polycrystalline La₂O₂Fe₂OSe₂ was synthesized via solid-state reaction using La₂O₃, Fe, and Se powders. The process involved drying precursors, pressing pellets in an Ar-filled glovebox, and sintering in a multi-step temperature sequence (up to 1100 °C) with a regrinding step to ensure homogeneity. Phase purity was verified via X-ray diffraction and electron microscopy.
Experimental Measurements: Zero-field ¹³⁹La-NQR measurements were conducted on polycrystalline samples across a broad frequency range (2.8–20 MHz) and temperature range (4.2–100 K). The study utilized pulsed NQR techniques with Gaussian apodization to suppress noise, followed by Fourier transformation to obtain frequency-dependent intensities.
Simulations: The authors simulated ¹³⁹La NMR/NQR spectra for the ¹³⁹La nucleus ( $I=7/2$ $I = 7/2$ ) using a Hamiltonian that includes both the Zeeman interaction and the quadrupole interaction. Simulations were performed for two specific geometric cases corresponding to the proposed non-collinear model:
1. La 2: Magnetic field ( $\vec{B}$ ) parallel to the principal component of the EFG ( $V_{zz}$ , along the c-axis).
2. La 1: Magnetic field ( $\vec{B}$ ) perpendicular to $V_{zz}$ (in the a-b plane).
  The simulations accounted for random grain orientations to model powder spectra and calculated relative peak intensities based on 2000 random orientations of the RF excitation pulse.

Key Results

Spectral Observations: Above $T_N$ , a single resonance line at 9.4 MHz (identified as the $3\nu_Q$ transition) was observed. Below $T_N$ , three additional resonance peaks appeared. These new peaks were equidistantly separated by the quadrupole frequency $\nu_Q$ and exhibited a temperature-dependent shift characteristic of an order parameter. The original $3\nu_Q$ peak remained at a constant frequency (9.4 MHz) but with reduced intensity.
Field Determination: The absence of resonance peaks above 16.3 MHz allowed the authors to determine the magnetic saturation field at the high-field site to be approximately 1.127 T at $T \approx 0$ K.
Model Validation: The experimental data, including the temperature dependence of the peak positions and the relative intensities, were fully consistent with the non-collinear magnetic model proposed by Günther et al. and McCabe et al.
- The "La 2" site corresponds to the large magnetic field parallel to $V_{zz}$ , explaining the shifted, temperature-dependent peaks.
- The "La 1" site corresponds to a small magnetic field (estimated $<50$ mT) in the a-b plane, explaining the temperature-independent peak at 9.4 MHz.
Refutation of Lattice Anomaly: The study demonstrates that the observed spectral features (specifically the splitting and shifts) can be explained entirely by the transferred magnetic hyperfine fields arising from the non-collinear Fe-spin ordering. A substantial modification of the EFG (a lattice anomaly) or a discontinuous splitting of the resonance signals into two distinct subspectra with different EFGs, as suggested by Hussain et al., is inconsistent with the current broad-range measurements and simulations.

Significance and Claims
The paper claims to provide strong confirmation of the complex non-collinear spin ordering in La₂O₂Fe₂OSe₂. By combining broad-range ¹³⁹La-NQR with ⁵⁷Fe Mössbauer spectroscopy and theoretical simulations, the authors resolve previous ambiguities regarding the magnetic structure.

The primary significance lies in demonstrating that the spectral changes observed below $T_N$ are driven by magnetic hyperfine interactions rather than a magnetically induced structural phase transition (lattice anomaly). The results support the model where Fe spins form a non-collinear, spin-ice-like arrangement on the Lieb lattice, generating two distinct La sites with different transferred magnetic fields. The authors conclude that the combination of nuclear probe methods (NMR and Mössbauer) is a powerful tool for distinguishing between collinear and non-collinear magnetic structures in polycrystalline specimens, particularly when electric quadrupole and magnetic Zeeman interactions are of comparable magnitude.

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