Observation of the Anomalous Hall Effect in a Collinear Antiferromagnet

Abstract: Time-reversal symmetry breaking is the basic physics concept underpinning many magnetic topological phenomena such as the anomalous Hall effect (AHE) and its quantized variant. The AHE has been primarily accompanied by a ferromagnetic dipole moment, which hinders the topological quantum states and limits data density in memory devices, or by a delicate noncollinear magnetic order with strong spin decoherence, both limiting their applicability. A potential breakthrough is the recent theoretical prediction of the AHE arising from collinear antiferromagnetism in an anisotropic crystal environment. This new mechanism does not require magnetic dipolar or noncollinear fields. However, it has not been experimentally observed to date. Here we demonstrate this unconventional mechanism by measuring the AHE in an epilayer of a rutile collinear antiferromagnet RuO$_2$. The observed anomalous Hall conductivity is large, exceeding 300 S/cm, and is in agreement with the Berry phase topological transport contribution. Our results open a new unexplored chapter of time-reversal symmetry breaking phenomena in the abundant class of collinear antiferromagnetic materials.

• The paper establishes that collinear antiferromagnetic ordering in RuO2 yields a significant anomalous Hall effect without requiring a ferromagnetic dipole moment.
• The investigation uses epitaxial thin films and advanced structural and magnetic measurements to confirm (110) orientation and robust antiferromagnetic order.
• The research reports an anomalous Hall conductivity up to 330 S·cm⁻¹ and a Néel temperature above 300 K, highlighting RuO2’s potential for room-temperature spintronic applications.

Observation of the Anomalous Hall Effect in a Collinear Antiferromagnet

The paper under review successfully demonstrates the experimental observation of the anomalous Hall effect (AHE) in a collinear antiferromagnet, specifically in rutile RuO$_2$. This study presents a paradigm shift in the understanding of AHE, challenging the conventional notion that this effect requires either a ferromagnetic dipole moment or a complex noncollinear magnetic ordering. This new mechanism emerges from collinear antiferromagnetism within an anisotropic crystal framework, devoid of significant magnetic dipolar fields.

Historically, AHE is associated with ferromagnetic or noncollinear antiferromagnetic systems where time-reversal symmetry breaking (TRSB) occurs in the presence of a magnetic dipole or a structured noncollinear geometry. However, these configurations either disturb topological states or limit quantum coherence, affecting their utility in nanoscale devices. The present research identifies collinear antiferromagnetic alignment in RuO$_2$ as a viable environment for AHE, overturning the prior perception of collinear antiferromagnets as excluding AHE.

Notably, the experimental investigation of RuO$_2$ includes the growth of epitaxial thin films on MgO substrates, achieving a room-temperature resistivity of approximately 64 μΩ·cm under optimized growth conditions. High-quality X-ray diffraction and transmission electron microscopy confirm the (110) orientation and reveal the structural integrity of the films. Through magnetization and exchange bias measurements, the study further corroborates the presence of collinear antiferromagnetic order with a Néel temperature exceeding 300 K, highlighting the material's suitability for room-temperature applications.

The experimentally observed AHE in RuO$_2$ is characterized by an anomalous Hall conductivity reaching up to 330 S·cm$^{-1}$ at low temperatures. This conductivity exceeds the values reported for noncollinear antiferromagnets like Mn$_3$Sn and even approaches that of ferromagnetic Fe films. The anomalous signal manifests in (110)-oriented films, whereas (100)-oriented RuO$_2$/SrTiO$_3$ films demonstrate negligible AHE, reinforcing the orientation dependence of the effect. Importantly, DFT calculations align with these experimental observations, predicting substantial Berry curvature and intrinsic anomalous conductivity as the result of spin-orbit coupling and TRSB, without reliance on a ferromagnetic dipole.

The research findings are significant, demonstrating that TRSB and related topological phenomena can indeed manifest in collinear antiferromagnets without the conventional drawbacks of ferromagnetic or noncollinear systems. These results imply broader possibilities for collinear antiferromagnets in spintronic applications and topological electronics, particularly given their high ordering temperatures and compositional simplicity.

Future directions could explore similar AHE phenomena in other collinear antiferromagnets, potentially uncovering new material classes suitable for next-generation devices. Moreover, understanding the interplay between structural crystallography and magnetic properties remains crucial in optimizing these antiferromagnetic systems for practical implementation. The study thus posits RuO$_2$ and its ilk as promising candidates in the exploration of topological quantum states and advanced magnetic materials.