Researchers at the Institute of Science Tokyo, led by Hiroaki Ishizuka, have developed a quantum-mechanical theory explaining a puzzling electrical behavior in chiral magnets, tracing it to a scattering mechanism related to the decades-old Kondo effect.
Image credit: Institute of Science Tokyo, from Phys.org
Chiral magnets are materials whose atomic-scale magnetic moments twist into helices, vortices and other complex spin textures. One consequence of this chirality is nonreciprocal current: electric current flows more easily in one direction through the material than the other, an effect of growing interest for magnetic sensing and spintronic devices. Most prior theoretical treatments modeled the magnetic moments in these systems as classical, localized spins, even though experiments in materials such as MnSi suggest quantum fluctuations play a significant role - a gap the new theory sets out to close.
Using the Kondo lattice model, the team analyzed how conduction electrons scatter off localized, quantum-mechanical magnetic moments in a chiral magnetic environment, cross-checking the result with two independent methods: a Green's function calculation and a scattering-theory approach combined with semiclassical Boltzmann transport theory. Both approaches converged on the same result for the electrical magnetochiral effect, a nonreciprocal response in which current contains a component proportional to the square of the applied electric field: its conductivity develops a distinctive logarithmic dependence on temperature at low temperatures, a signature with no analog in classical treatments of chiral magnetism.
Tracing the microscopic origin of this behavior, the researchers found it arises from quantum interference between an electron scattering once off a magnetic moment and scattering twice off that same moment - a mechanism closely related to the Kondo effect, first described in 1964 for the anomalous scattering of electrons off magnetic impurities at low temperatures. "Our results indicate that a nontrivial effect unique to quantum fluctuations exists in the transport phenomena related to spin chirality," said Ishizuka. The team points to MnSi and other known chiral magnets as candidates for experimentally testing the prediction. "By demonstrating that Kondo-type quantum effects can significantly enhance nonreciprocal responses, our study opens new avenues for designing quantum-driven spintronic functionalities and improving the performance of magnetic devices based on nonlinear transport properties," Ishizuka added.