Electrical Manipulation of a Topological Antiferromagnetic State

Abstract: Electrical manipulation of emergent phenomena due to nontrivial band topology is a key to realize next-generation technology using topological protection. A Weyl semimetal is a three-dimensional gapless system that hosts Weyl fermions as low-energy quasiparticles. It exhibits various exotic phenomena such as large anomalous Hall effect (AHE) and chiral anomaly, which have robust properties due to the topologically protected Weyl nodes. To manipulate such phenomena, the magnetic version of Weyl semimetals would be useful as a magnetic texture may provide a handle for controlling the locations of Weyl nodes in the Brillouin zone. Moreover, given the prospects of antiferromagnetic (AF) spintronics for realizing high-density devices with ultrafast operation, it would be ideal if one could electrically manipulate an AF Weyl metal. However, no report has appeared on the electrical manipulation of a Weyl metal. Here we demonstrate the electrical switching of a topological AF state and its detection by AHE at room temperature. In particular, we employ a polycrystalline thin film of the AF Weyl metal Mn$3$Sn, which exhibits zero-field AHE. Using the bilayer device of Mn$_3$Sn and nonmagnetic metals (NMs), we find that an electrical current density of $\sim 10^{10}$-$10^{11}$ A/m$^2$ in NMs induces the magnetic switching with a large change in Hall voltage, and besides, the current polarity along a bias field and the sign of the spin Hall angle $\theta{\rm SH}$ of NMs [Pt ($\theta_{\rm SH} > 0$), Cu($\theta_{\rm SH} \sim 0$), W ($\theta_{\rm SH} < 0$)] determines the sign of the Hall voltage. Notably, the electrical switching in the antiferromagnet is made using the same protocol as the one used for ferromagnetic metals. Our observation may well lead to another leap in science and technology for topological magnetism and AF spintronics.

• The paper demonstrates electrical switching of topological antiferromagnetic states in Mn3Sn thin films using bilayer devices with nonmagnetic metals.
• The experiments use devices with differing spin Hall angles to reveal sharp Hall voltage changes and confirm the existence of Weyl nodes.
• The findings lay a foundation for energy-efficient spintronic devices and enhance our understanding of real-space spin dynamics in topological materials.

Electrical Manipulation of a Topological Antiferromagnetic State

The paper presents significant findings on the electrical manipulation of a topological antiferromagnetic (AF) state in Mn$_3$Sn thin films. The study dives deep into the domain of condensed matter physics, emphasizing the interplay of topology and magnetism in electronic systems. The notion of controlling topological states in magnetic systems holds vast implications for the future of spintronics, particularly in terms of speed and density of information storage devices.

Key Findings

The research demonstrates, for the first time, the electrical switching of a topological AF state using a thin film of Mn$_3$Sn, a Weyl semimetal, and detects the anomalous Hall effect (AHE) in these systems. Weyl semimetals are characterized by the presence of Weyl nodes—points in the momentum space where conduction and valence bands meet—and show unusual transport phenomena such as large AHE. The ability to electrically control these topological states opens new possibilities for electronic devices that can harness the robust properties of Weyl nodes.

The experiments reveal that AHE can be effectively manipulated in AF Weyl metals through electrical means, marked by sharp changes in Hall voltage. This manipulation occurs using bilayer devices combining Mn$_3$Sn with nonmagnetic metals like Platinum (Pt), Copper (Cu), and Tungsten (W). The electrical switching in these devices depends on the polarity of the current and the sign of the spin Hall angle ($\theta_{SH}$) of the nonmagnetic metal layer. The authors note that the same protocol employed for conventional spintronics can be extended to antiferromagnetic systems, presenting an intriguing avenue for future research and technological advancement.

Experimental Approach

Key experimental efforts included employing bilayer devices that consist of Mn$_3$Sn and a nonmagnetic metal to experimentally confirm electrical switching. The use of nonmagnetic layers with distinct spin Hall angles allowed for the observation of different switching polarities and critical current densities ($J_c$), indicative of the effect of spin-orbit torque (SOT) on the AF domain's perpendicular switching.

In confirming the presence of a magnetic Weyl semimetal state in Mn$_3$Sn films, the researchers used chiral anomaly signatures and anomalous Nernst effect (ANE) measurements. The chiral anomaly, evidenced by the angular dependence of magnetoconductivity and planar Hall conductivity, provides robust affirmation of Weyl nodes' presence in the polycrystalline films. Furthermore, the ANE measurements supported the existence of substantial transverse thermoelectric responses, reinforcing the link between the large Berry curvature and the intrinsic topological nature of the system.

Implications and Future Directions

The study's implications are multifaceted. On a practical level, demonstrating room-temperature electrical switching using AF materials indicates the potential for Mn$_3$Sn and similar materials in future spintronic devices that require less power and offer increased data processing speed compared to current technology utilizing ferromagnetic elements. Theoretical perspectives also gain from the study’s insights into manipulating Weyl fermions in momentum space via real-space spin dynamics, paving the way for novel ways to harness topological properties in other material systems.

There exists a promising trajectory for future research to explore analogous effects in other AF materials with similar symmetry properties, extending the utility of these findings across various magnetic systems with potential technological applications. Understanding the dynamics of these systems could advance neuromorphic computing technologies, as suggested by the memristive behavior observed in the Hall voltage changes, a step towards devices capable of mimicking synaptic processes.

Conclusion

The ability to electrically manipulate topological AF states in polycrystalline Mn$_3$Sn films represents a significant stride forward in condensed matter physics and spintronics. By leveraging the unique properties of Weyl semimetals, this research proposes new pathways for developing advanced electronic devices that could revolutionize data storage and processing technologies. The insights into the deterministic switching mechanisms and topological responses lay a rich groundwork for future exploration in the growing field of topological antiferromagnetic spintronics.