Understanding Unconventional Hall Resistance In Magnetic Semiconductor

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The findings provide insights into anomalous Hall effect, a quantum phenomenon previously associated with long-range magnetic order

Ferromagnets exhibit the property of the Hall effect wherein deviation of electrons occurs when a magnetic field is applied perpendicular to the plane of a current-carrying conductor, thus creating a voltage difference.

Another phenomenon called the “anomalous Hall effect” (AHE) is also known to occur, caused due to property of electronic energy bands called “Berry curvature” creating interaction between the electron’s spin and its motion inside the material, more commonly known as “spin-orbit interaction.”

Theoretically, a large AHE is possible above temperatures at which the magnetic order vanishes, especially in magnetic semiconductors with low charge carrier density.

To put this theory to test, researchers from the Tokyo Institute of Technology (Tokyo Tech) investigated the magnetic properties of a new magnetic semiconductor EuAs that is known to exhibit a peculiar distorted triangular lattice structure and an antiferromagnetic (AFM) behaviour below 23 K.

Furthermore, the material’s electrical resistance dropped dramatically with temperature in the presence of an external magnetic field, a behaviour known as “colossal magnetoresistance” (CMR). However, more interestingly, the CMR was observed even above 23 K, where the AFM order vanished.

“It is naturally understood that the CMR observed in EuAs is caused by a coupling between the diluted carriers and localised Eu2+ spins that persist over a wide range of temperatures,” said Dr Masaki Uchida, Associate Professor at Tokyo Institute of Technology (Tokyo Tech).

The rise in Hall resistivity with temperature up to 70 K (far above the AFM ordering temperature) demonstrated that large AHE was indeed possible without magnetic order.

To understand this unconventional occurrence, large AHE, model calculations were performed that showed the effect could be attributed to a skew scattering of electrons by a spin cluster on the triangular lattice where the electrons “hopped” rather than flow from atom to atom.

These final results will help better understand the electron behaviour inside magnetic solids.

“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,” commented Dr Uchida.

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