Speaker: Dr. Sergey A. Nikolaev, Institute of Innovative Research, Tokyo Institute of Technology, Japan

Host: Prof. Youwen Long, State Key Lab for Magnetism & Magnetic Materials

Time:  15:00, December 22, 2021

Abstract:

Multiferroic materials with coexisting magnetism and ferroelectricity provide a versatile platform for studying and developing novel technologies. The microscopic origin of multiferroicity is a fundamental physical problem and its understanding is an important step towards mutual control of electric polarization and magnetism in next generation electronic devices. The spin current theory of magnetically induced electric polarization has been widely used for the analysis of multiferroic activity in spiral magnets. Nevertheless, being largely phenomenological, this theory fails to account for various possible multiferroic scenarios in real materials, causing in some cases misinterpretations of their microscopic origin. In our recent studies we revisit the problem related to the emergence of electric polarization in magnetic systems and show in a generic form how it can be induced by a noncollinear alignment of spins. Starting from general considerations, we formulate a transparent toy theory for electric polarization in magnetic insulators and derive an invariant which couples to the spin current and remains finite even for centrosymmetric bonds, thus permitting finite polarization for noncollinear spins. Importantly, we claim that the phenomenological spin-current theory is a particular case of a more general spin-current theory proposed in our study and is correct only for particular symmetries of the Kramers states. We explicitly show that other symmetries can generally lead to different forms of magnetoelectric coupling, thus providing compelling alternatives for the analysis of multiferroics materials.

[1] Igor Solovyev, Ryota Ono, and Sergey Nikolaev. Phys. Rev. Lett. 127, 187601 (2021).

[2] S. A. Nikolaev and I. V. Solovyev. Phys. Rev. B 99, 100401(R) (2019)

Brief CV of Dr. Sergey A. Nikolaev:

Sergey A. Nikolaev received his PhD in condensed matter physics from the Ural Federal University in Russia and now works as a Specially Appointed Assistant Professor at the Tokyo Institute of Technology. His research interests are focused on theoretical studies of a broad spectrum of electronic and magnetic phenomena in strongly correlated systems by combining quantum models, field-theoretical methods, and first-principles calculations.