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Pure Spin Current and Magnon Chemical Potential in a Nonequilibrium Magnetic Insulator

Kevin S. Olsson, Kyongmo An, Gregory A. Fiete, Jianshi Zhou, Li Shi, and Xiaoqin Li
Phys. Rev. X 10, 021029 – Published 6 May 2020
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Abstract

Nonequilibrium phenomena are ubiquitous in nature and in a wide range of systems, including cold atomic gases and solid-state materials. While these phenomena are challenging to describe both theoretically and experimentally, they are essential for the fundamental understanding of many-body systems and practical devices. In the context of spintronics, when a magnetic insulator (MI) is subjected to a thermal gradient, a pure spin current is generated in the form of magnons without the presence and dissipation of a charge current—attractive for reducing energy consumption and central to the emerging field of spin caloritronics. However, the experimental methods for directly quantifying a spin current in insulators and for probing local phonon-magnon nonequilibrium and the associated magnon chemical potential are largely missing. Here, we apply a heating laser to generate a thermal gradient in the MI yttrium iron garnet (YIG), Y3Fe5O12, and evaluate two components of the spin current, driven by temperature and chemical potential gradients, respectively. The experimental method and theory approach for evaluating quasiparticle chemical potential can be applied for analogous phenomena in other many-body systems.

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  • Received 2 June 2019
  • Revised 18 February 2020
  • Accepted 27 February 2020

DOI:https://doi.org/10.1103/PhysRevX.10.021029

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Kevin S. Olsson1, Kyongmo An2, Gregory A. Fiete1,3,4, Jianshi Zhou5,6, Li Shi5,6, and Xiaoqin Li1,5,*

  • 1Department of Physics, Center of Complex Quantum Systems, The University of Texas at Austin, Austin, Texas 78712, USA
  • 2Laboratory of Nanoscale Magnetic Materials and Magnonics, Institute of Materials, School of Engineering, École Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland
  • 3Department of Physics, Northeastern University, Boston, Massachusetts 02115, USA
  • 4Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
  • 5Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, USA
  • 6Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas 78712, USA

  • *Corresponding author. elaineli@physics.utexas.edu

Popular Summary

Spintronics aims to develop devices using spin, instead of charge, to store and process information. In magnetic insulators, a spin current is carried by localized spins that are coupled to each other and form magnons—collective excitations of electron spin. Heating a magnetic insulator provides an effective way to generate spin current. Understanding spin transport in such a nonequilibrium system is thus a fascinating problem from the point of view of both fundamental science and information technology. Here, we characterize the nonequilibrium between two important types of quasiparticles—magnons and phonons—that exists in such a system.

We use a heating laser to generate a thermal gradient in a magnetic insulator. Our combined theoretical and experimental studies demonstrate that an accurate system description requires—in addition to phonon and magnon temperatures—the concept of magnon chemical potential, which we measure for the first time in this type of nonequilibrium system. Based on the measured temperature and chemical potential gradients, we also quantitatively evaluate two distinct contributions to the total spin current.

The measured magnon chemical potential provides previously unavailable insight that will better inform spintronic device development. More broadly, our theory and experimental measurements of quasiparticle transport are also applicable to other types of nonequilibrium systems.

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Vol. 10, Iss. 2 — April - June 2020

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