Tokyo Tech materials scientists have produced a significant, unusual anomalous Hall resistance in a new magnetic semiconductor in the absence of large-scale magnetic ordering, confirming a recent theoretical prediction. The researchers’ results shed new light on the anomalous Hall effect, a quantum phenomenon previously linked to long-range magnetic order.
When charged particles, such as electrons, move under the influence of electric and magnetic fields, they can interact. When a magnetic field is applied perpendicular to the plane of a current-carrying wire, for example, the electrons begin to deviate sideways due to magnetic force, and a voltage difference arises across the conductor soon after. The “Hall effect” is a well-known example of this phenomena.
The Hall effect, on the other hand, does not necessitate the use of magnets. It can even be detected for free in magnetic materials with long-range magnetic order, such as ferromagnets!
This phenomenon, dubbed “anomalous Hall effect” (AHE), appears to be a close relative of the Hall effect. Its mechanism, on the other hand, is far more complicated.
The most widely accepted theory now holds that the AHE is caused by a feature of the electronic energy bands known as “Berry curvature,” which is caused by an interaction between the electron’s spin and its motion inside the material, also known as “spin-orbit interaction.”
It has been theoretically proposed that a large AHE is possible even above the temperature at which the magnetic order vanishes, especially in magnetic semiconductors with low charge carrier density, strong exchange interaction between electrons, and finite spin chirality, which relates to the spin direction with respect to the direction of motion.
Masaki Uchida
Is magnetic ordering necessary for AHE? A recent theory suggests otherwise.
“It has been theoretically proposed that a large AHE is possible even above the temperature at which the magnetic order vanishes, especially in magnetic semiconductors with low charge carrier density, strong exchange interaction between electrons, and finite spin chirality, which relates to the spin direction with respect to the direction of motion,” explains Associate Professor Masaki Uchida from Tokyo Institute of Technology (Tokyo Tech), whose research focus lies in condensed matter physics.
Dr. Uchida and his Japanese associates were intrigued and determined to put this idea to the test. They investigated the magnetic properties of a new magnetic semiconductor EuAs, which is only known to have a peculiar distorted triangular lattice structure, and found antiferromagnetic (AFM) behavior (neighboring electron spins aligned in opposite directions) below 23 K, according to a new study published in Science Advances.
Furthermore, they discovered that in the presence of an external magnetic field, the material’s electrical resistance decreased substantially with temperature, a phenomenon known as “colossal magnetoresistance” (CMR). More intriguingly, the CMR was found even at temperatures beyond 23 K, where the AFM order had evaporated.
“It is naturally understood that the CMR observed in EuAs is caused by a coupling between the diluted carriers and localized Eu2+ spins that persist over a wide range of temperatures,” comments Dr. Uchida.
The rise in Hall resistivity with temperature, which peaked at 70 K, considerably above the AFM ordering temperature, was the star of the show, indicating that significant AHE was indeed feasible without magnetic order.
The scientists utilized model calculations to figure out what generated this unusually large AHE, and discovered that it was caused by skew scattering of electrons by a spin cluster on the triangular lattice in a “hopping regime,” in which electrons did not flow but rather “hopped” from atom to atom.
These findings help us get a better grasp of how electrons behave inside magnetic materials.
“Our findings have helped shed light on triangular-lattice magnetic semiconductors and could potentially lead to a new field of research targeting diluted carriers coupled to unconventional spin orderings and fluctuations,” comments an optimistic Dr. Uchida.
