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Theory Seminar: Electromagnetic response of noncollinear antiferromagnets

Date : Wednesday, August 31st, 2016 11:00 AM - 12:00 PM Place : Seminar Room 5 (A615), 6th Floor, ISSP Lecturer : Dr. Hua Chen Affiliation : University of Texas at Austin Committee Chair : Masaki Oshikawa (63275)
e-mail: oshikawa@issp.u-tokyo.ac.jp

In spite of rich properties and significant academic interest, antiferromagnets have always been overshadowed by ferromagnets in real-life applications based on magnetism, including spintronics. This is primarily due to the fact that antiferromagnet order parameter couples weakly to external magnetic field and has therefore been difficult to manipulate. In this talk I will discuss a number of recent theoretical and experimental developments that counter this conventional wisdom in a class of antiferromagnets with stable noncollinear magnetic order. As an introduction I will talk about the surprising discovery of the anomalous Hall effect (AHE), the generation of a voltage perpendicular to current in the absence of a magnetic field, in noncollinear antiferromagnets. In these materials the AHE can be used as an efficient probe to determine the global orientation of the noncollinear antiferromagnetic order. I will then discuss the coupling between the noncollinear order parameter and external electric and magnetic fields. Electric fields or currents can change the magnetization direction and induce collective dynamics through current- or electric-field-induced spin-orbit torques. I will show through both toy model and first principles calculations that the spin-orbit torque is nonzero in a prototypical noncollinear antiferromagnet, Mn3Sn, in spite of its global inversion symmetry. As for external magnetic fields, although their direct coupling with noncollinear spin magnetization is rather weak, I will show that there is a large orbital moment in these noncollinear antiferromagnets. Coupling between the external magnetic field and the orbital moment can lead to a torque on the noncollinear order parameter through a response function that can be viewed as dual to the current-induced torque.


(Published on: Wednesday August 24th, 2016)