Diode-Like Behavior Arising from Antiferromagnetism
In a traditional diode, current flows in one direction only, thanks to an internal charge imbalance. Researchers have now shown a diode-like effect in an antiferromagnet with a zigzag magnetic structure [1]. The underlying mechanism is different from that in traditional diodes, as the zigzag pattern creates a combined magnetic and electric field that favors current flow in one direction. The strength of the diode effect in the antiferromagnet is relatively small, but rather than exploiting the effect to make a diode for conventional circuits, the team foresees possible applications in spintronics, devices that make use of electron spins.
A typical diode is a junction between two semiconductors having different charge carriers. The charge imbalance across this junction restricts current to flow in only one direction. Diode-like behavior can, in principle, occur in a single material, but it requires that the material’s internal structure is asymmetric in a particular way. This asymmetry should produce two effects: an internal electric field and an internal magnetic field. When those two fields are perpendicular to each other, they can exert a one-way force—called a toroidal moment—on electrons moving through the material, explains Kenta Sudo from Tohoku University in Japan.
There are materials—called multiferroics—that have both internal electric and internal magnetic fields, but they typically aren’t conductors, so they can’t work as diodes. However, recent experiments have produced diode effects in structurally asymmetric conductors in the presence of external magnetic fields. In these experiments, the external field combines with the internal asymmetry to produce the desired toroidal moment. Now Sudo and his colleagues have discovered a diode effect that doesn’t require an external field. The effect occurs in an antiferromagnet, which is a material whose magnetic elements, or spins, are arranged so that they alternate between spin-up and spin-down.
The material is NdRu2Al10, which is a conducting antiferromagnet with a zigzag arrangement of its magnetic spins. This spin arrangement, combined with the material’s crystal asymmetry, generates magnetic and electric fields that vary between layers in the crystal structure. Sudo and colleagues predicted that the combination of these fields forms a toroidal moment that drives diode-like behavior, also called nonreciprocal transport, in the antiferromagnet.
To explore this possibility, the researchers grew single crystals of NdRu2Al10 that they subsequently shaped into long, thin samples. They applied an alternating current to each sample and measured the resistance at various cryogenic temperatures. Above the transition temperature of 2.4 K, the material is nonmagnetic, and the resistance was equal in both directions. But below the transition, the resistance was higher in one direction than the other. The preferred direction depended on how the spins were arranged with respect to the crystalline structure.
The nonreciprocal conductivity σ(2) captures how strongly the electrical response differs between forward and backward current directions. The value of σ(2) that the team measured for NdRu2Al10 is far below that of traditional diodes, but it is more than a thousand times that of magnetically induced diode effects.
“The significance of our result is not in creating a practical diode,” Sudo says. “Instead, it shows that a single bulk material can exhibit a large nonreciprocal response purely from its intrinsic electronic and magnetic structure.” He says that this experiment reveals “a new intrinsic mechanism for nonreciprocal charge transport” and that the work will be helpful for studies of such effects in other materials.
In addition, Sudo says that the effect that he and his colleagues discovered could be useful in spintronics. The different spin arrangements in an antiferromagnet have the potential to store information, but measuring this internal magnetic structure can be challenging with traditional magnetism-detection techniques. In principle, measuring the nonreciprocal resistance of an antiferromagnet could offer a way to read out the stored information, Sudo says.
The team plans to search for other materials in which other symmetry-based mechanisms for nonreciprocal charge transport are present. They hope to build a theoretical framework that connects magnetic symmetry, electronic structure, and nonreciprocal transport.
“I consider this to be an outstanding research result,” says Youichi Yanase from Kyoto University in Japan, who has worked on nonreciprocal responses in magnetic materials and superconductors. He says that the researchers successfully observed the magnetic toroidal moment through its indirect effect on conductivity. “This achievement contributes to the development of antiferromagnetic spintronics utilizing the magnetic toroidal moment,” he says.
–Michael Schirber
Michael Schirber is a Corresponding Editor for Physics Magazine based in Lyon, France.
References
- K. Sudo et al., “Large spontaneous nonreciprocal charge transport in a zero-magnetization antiferromagnet,” Phys. Rev. Lett. 136, 016503 (2026).