A New Kind of Magnet



 There is a new class of magnetic materials known as altermagnets. This new material displays a unique type of magnetism. Altermagnets are different from regular ferromagnetic and antiferromagnetic reactions. The electron spin of altermagnets varies depending on their momentum. Altermagnets are unique and hold promise for new electronic and spintronic devices.

Scientists at Stony Brook University are studying the nonlinear response of planar altermagnets. They have published their work in Physical Review Letters. The researchers report viewing a non-linear response in these materials, using their quantum geometry.

Ali Akbar Ghorashi is co-author of this paper. He reports, "Recently two experiments have confirmed the predictable role of quantum geometry in the second-order response of the conventional PT-symmetric antiferromagnets. In these materials, due to the combination of parity (P) and time-reveal (T) symmetries, the Berry curvature (the imaginary component of quantum geometric tensor) vanishes, and it is shown that the second-order response is governed by the quantum metric (the real component of quantum geometric tensor)."

Altermagnets do not have the combined PT symmetry. Because of this, the quantum geometry involved with the nonlinear response in this material was elusive to scientists. 

Ghorashi stated, "The goal of our work was to derive the nonlinear response of altermagnets and distinguish the contributions from the Berry curvature and from the quantum metric. Our findings ended up being more dramatic than anticipated." 

At first, Ghorashi and his team were studying the nonlinear responses of altermagnets and the influences driving this response. The team used the semiclassical Boltzman theory. They computed all of the possible contributions to the nonlinear response of altermagnets up to the third order in an electric field.

Ghorashi said, "We uncovered the quantum geometric origin of each term by oder in scattering time. Next, for each planar alter magnet, we used symmetry to determine which contributions survive in the longitudinal and Hall components of the third-order conductivity."

The scientists involved in the study found surprising results. The team found non-linear responses in planar altarmagnets that are caused by the materials' quantum geometry.

Ghorashi explains, "Remarkably, due to inversion symmetry, altermagnets have vanishing second-order response, Therefore, to the best of our knowledge, they are the first class of materials where the third-order response is their leading nonlinear response. Furthermore, we showed this response is giant due to the large spin-splitting in these materials. Moreover, the weak spin-orbit coupling (compared to magnetic exchange term) of altermagnets also shows up in their nonlinear response, providing a novel transport characterization for this new class of materials, which was previously limited to searching for linear anomalous Hall conductivity."

He summarizes, "One immediate future research direction for us will be to go beyond the relaxation time approximation and investigate the effect of disorder which has already been shown to enrich the physics of PT-symmetric antiferromagnets."

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