Spontaneous Anomalous Hall Effect at Room Temperature in Antiferromagnetic Material NbMnAs

Technical Summary: Spontaneous Anomalous Hall Effect at Room Temperature in Antiferromagnetic Material NbMnAs

Problem and Context
The anomalous Hall effect (AHE), a spontaneous Hall effect occurring without an external magnetic field, is traditionally associated with ferromagnetic (FM) materials due to broken time-reversal symmetry. However, recent theoretical and experimental work has established that certain antiferromagnetic (AFM) materials with specific symmetry breakings—symmetrically equivalent to FM states—can also generate large AHEs. While materials like Mn$_3$Sn and Mn$_3$Ge have demonstrated large anomalous Hall conductivities (AHCs) in bulk crystal forms, the search for new AFM materials exhibiting significant AHEs at room temperature remains a critical challenge. Previous work by the authors identified a large AHC in the AFM metal NbMnP ($T_N = 244$ K), which crystallizes in an orthorhombic TiNiSi-type structure (space group $Pnma$) with a magnetic point group $m'm2'$. This symmetry allows for both AFM ordering and a small spontaneous magnetization, enabling the AHE. The study aims to explore whether substituting Phosphorus (P) with Arsenic (As) in this system preserves the AFM state while potentially increasing the Néel temperature ($T_N$) to reach room temperature, despite the fact that the fully substituted compound NbMnAs had not been previously reported.

Methodology
The researchers synthesized polycrystalline samples of NbMnP$_{1-x}$As$_x$ ($x = 0.2, 0.4, 0.6, 0.8, 1$) and stoichiometric NbMnAs via solid-state reaction. To optimize purity, excess Nb and Mn were used for the NbMnAs synthesis. Single crystals of NbMnAs were grown using the Ga–In flux method. Structural characterization was performed using powder and single-crystal X-ray diffraction (XRD) to confirm crystal symmetry, lattice parameters, and site occupancies. Elemental composition was analyzed via X-ray fluorescence (XRF). Magnetic properties were measured using a SQUID magnetometer, and electrical transport measurements (resistivity and Hall effect) were conducted on both polycrystalline pellets and single crystals. Hall resistivity was antisymmetrized with respect to the magnetic field to isolate the transverse component.

Key Results

1. Crystal Structure and Composition:
   • Polycrystalline NbMnAs was confirmed to crystallize in the orthorhombic $Pnma$ structure, identical to NbMnP. The unit-cell volume increased monotonically with As content, confirming successful substitution.
   • Single crystals of NbMnAs exhibited a significant deficiency at the As site (occupancy $\approx$ 85.2% by single-crystal XRD and $\approx$ 72% by XRF), whereas the polycrystalline sample was close to stoichiometric (Nb$_{0.92}$Mn$_{1.07}$As$_{0.96}$).
2. Magnetic Properties:
   • The Néel temperature ($T_N$) increased monotonically with As content, reaching 354 K for polycrystalline NbMnAs.
   • Polycrystalline NbMnAs exhibited a small spontaneous magnetization of approximately $6 \times 10^{-3} \mu_B$/Mn at low temperatures, consistent with an AFM state having a weak FM component (likely along the $a$-axis via Dzyaloshinskii–Moriya interaction).
   • Single crystals showed a much larger spontaneous magnetization ($8 \times 10^{-2} \mu_B$/Mn) and a reduced $T_N$ range (290–340 K), which the authors attribute to the As deficiency and sample quality rather than a change in the fundamental magnetic point group.
3. Transport Properties:
   • Polycrystalline NbMnAs exhibited a clear anomalous Hall effect (AHE) below $T_N = 354$ K, even at zero magnetic field. The Hall resistivity showed hysteresis similar to the magnetization curve.
   • Single crystals also displayed a clear hysteresis in Hall resistivity below their respective $T_N$ (294 K).
   • The anomalous Hall conductivity ($\sigma_{AHE}$) for the single crystal reached approximately 60 $\Omega^{-1}$cm$^{-1}$ at low temperatures. While this is lower than values reported for NbMnP (230–450 $\Omega^{-1}$cm$^{-1}$) and Mn$_3$Ge (380 $\Omega^{-1}$cm$^{-1}$), it is significantly larger than those of other room-temperature AFM AHE materials like MnTe and FeS (< 1 $\Omega^{-1}$cm$^{-1}$).

Significance and Claims
The paper claims the discovery of NbMnAs as a novel antiferromagnetic material capable of generating a large anomalous Hall effect at room temperature. Key contributions include:

• Stability of the AFM State: Demonstrating that substituting P with As in the NbMnP system preserves the $Pnma$ crystal structure and the specific magnetic symmetry ($m'm2'$) required for the AHE.
• Room Temperature Operation: Achieving a Néel temperature of 354 K, which allows for the observation of the AHE at room temperature in a bulk AFM material.
• Symmetry-Driven Response: Confirming that the large AHE arises from the AFM ordering symmetry rather than net ferromagnetism, evidenced by the small spontaneous magnetization in the stoichiometric polycrystalline sample.
• Potential for Applications: The authors posit that NbMnAs is a promising candidate for future spintronic and thermoelectric technologies, as it is expected to exhibit other ferromagnetic-like responses (such as the anomalous Nernst effect and magneto-optical Kerr effect) at room temperature.

The study acknowledges that while the single crystals currently suffer from As deficiency and lower $T_N$, the polycrystalline results confirm the intrinsic potential of the stoichiometric compound. The work underscores the importance of bulk material exploration in identifying new topological antiferromagnets with high transition temperatures.