Mathematical model modulates the anomalous Hall angle in a magnetic topological semimetal

When an electric current passes through some materials, it generates a voltage perpendicular to the direction in which the current is flowing and of an applied magnetic field. This physical phenomenon, known as the anomalous Hall effect, has been linked to the intrinsic properties of some materials.

The efficiency with which a longitudinal current drives a transverse spin-polarized current in these materials is referred to as the anomalous Hall angle (θ^A). In many conventional magnetic materials, this angle is typically very small, which in turn limits the sensitivity of sensors and other devices developed using these materials.

Researchers at the Chinese Academy of Sciences have introduced a new mathematical model that allows them to modulate the θ^A in the magnetic topological semimetal Co₃Sn₂S₂.

Their approach, outlined in a paper published in Nature Electronics, could contribute to the future development of more sensitive sensors that leverage the anomalous Hall effect.

“In 1881, physicist E. H. Hall found a unique electromagnetic phenomenon in magnetic materials, the anomalous Hall effect, which to some extent behaves like the normal Hall effect but closely relates to the magnetism of conductors,” Enke Liu, co-author of the paper, told Phys.org.

“However, this effect always remains in a low level, with one of its key parameters—anomalous Hall angle—being rather small. This small angle results in a low driving efficiency of applied longitudinal current density to the transverse Hall current density, which obstructs the applications of anomalous Hall effect in Hall sensors and spintronic devices.”

Seven years ago, Liu and his colleagues uncovered the existence of magnetic Weyl fermions, exotic, chiral and mass-less quasiparticles, in a magnetic topological semimetal. These intriguing quasiparticles were found to generate a topologically enhanced Berry curvature, a pseudomagnetic field in the momentum space of condensed matter, which plays a crucial role in the generation of the anomalous Hall effect.

“Since the discovery of this fermion, we have been expecting to attain the giant anomalous Hall effect and the Hall angle by exploiting the magnetic Weyl physics,” said Liu. “We hope to see the large anomalous Hall effect can be utilized in the advanced spin devices.”

In their paper, Liu and his colleagues introduce a two-variable mathematical model for the anomalous Hall angle. The unique characteristic of this model is that it expresses the anomalous Hall angle as a function of the product of longitudinal resistivity and anomalous Hall conductivity for the first time.

“According to the rules deduced from the model, we implemented the designed experimental schemes on magnetic Weyl semimetal Co₃Sn₂S₂ to realize the modulation on the angle, from the views of intrinsic and extrinsic mechanisms including topological state, slight doping, temperature, and dimensionality,” explained Liu.

Using their approach, the team obtained a zero-field giant anomalous Hall angle of 25° (46%), which is an order higher than those in conventional materials over the past 70 years.

Subsequently, they developed a novel anomalous Hall sensor, achieving a low-frequency magnetic field detection capability of 23 nT/Hz^0.5@1Hz and a Hall sensitivity of 7028 μΩcm/T, which are 3 times and 10 times higher, respectively, than those of currently known anomalous Hall sensors.

The recent work by this team of researchers introduces a new universal strategy for tuning the anomalous Hall angle in magnetic topological materials. In the future, it could pave the way for the development of increasingly advanced and sensitive sensors based on magnetic materials with larger anomalous Hall effects.

“We provide a proof-of-principle demonstration of topology-enhanced high-performance magnetic sensing and is expected to advance the application of magnetic topological physics in next-generation magneto-electronics,” added Liu.

“We will now continue to find new magnetic topological materials and novel physical phenomena and promote our explorations of magnetic topological physics on advanced topological quantum devices.”

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