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Broadband Terahertz Probes of Anisotropic Magnetoresistance Disentangle Extrinsic and Intrinsic Contributions

Lukáš Nádvorník, Martin Borchert, Liane Brandt, Richard Schlitz, Koen A. de Mare, Karel Výborný, Ingrid Mertig, Gerhard Jakob, Matthias Kläui, Sebastian T. B. Goennenwein, Martin Wolf, Georg Woltersdorf, and Tobias Kampfrath
Phys. Rev. X 11, 021030 – Published 7 May 2021
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Abstract

Anisotropic magnetoresistance (AMR) is a ubiquitous and versatile probe of magnetic order in contemporary spintronics research. Its origins are usually ascribed to extrinsic effects (i.e., spin-dependent electron scattering), whereas intrinsic (i.e., scattering-independent) contributions are neglected. Here, we measure AMR of polycrystalline thin films of the standard ferromagnets Co, Ni, Ni81Fe19, and Ni50Fe50 over the frequency range from dc to 28 THz. The large bandwidth covers the regimes of both diffusive and ballistic intraband electron transport and, thus, allows us to separate extrinsic and intrinsic AMR components. Analysis of the THz response based on Boltzmann transport theory reveals that the AMR of the Ni, Ni81Fe19, and Ni50Fe50 samples is of predominantly extrinsic nature. However, the Co thin film exhibits a sizable intrinsic AMR contribution, which is constant up to 28 THz and amounts to more than 2/3 of the dc AMR contrast of 1%. These features are attributed to the hexagonal structure of the Co crystallites. They are interesting for applications in terahertz spintronics and terahertz photonics. Our results show that broadband terahertz electromagnetic pulses provide new and contact-free insights into magnetotransport phenomena of standard magnetic thin films on ultrafast timescales.

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  • Received 12 October 2020
  • Revised 28 February 2021
  • Accepted 31 March 2021

DOI:https://doi.org/10.1103/PhysRevX.11.021030

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. Open access publication funded by the Max Planck Society.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Lukáš Nádvorník1,2,3,*, Martin Borchert1,2, Liane Brandt4, Richard Schlitz5, Koen A. de Mare6,7, Karel Výborný6, Ingrid Mertig4, Gerhard Jakob8, Matthias Kläui8, Sebastian T. B. Goennenwein5, Martin Wolf1, Georg Woltersdorf4, and Tobias Kampfrath1,2

  • 1Department of Physics, Freie Universität Berlin, 14195 Berlin, Germany
  • 2Department of Physical Chemistry, Fritz Haber Institute of the Max Planck Society, 14195 Berlin, Germany
  • 3Faculty of Mathematics and Physics, Charles University, 121 16 Prague, Czech Republic
  • 4Institut für Physik, Martin-Luther-Universität, Halle, Germany
  • 5Institut für Festkörper- und Materialphysik, Technische Universität Dresden, 01062 Dresden, Germany
  • 6Institute of Physics, Academy of Sciences of the Czech Republic, v.v.i., 162 00 Prague, Czech Republic
  • 7Department of Applied Physics, Eindhoven University of Technology, Eindhoven 5612 AZ, Netherlands
  • 8Institut für Physik, Johannes Gutenberg-Universität Mainz, 55128 Mainz, Germany

  • *nadvornik@karlov.mff.cuni.cz

Popular Summary

Electrical resistance arises from collisions of the conduction electrons with obstacles in the surrounding material and from their mass. However, in a magnetic metal, resistance is larger when the current flows parallel to the magnetization, and it is smaller when the electrons flow perpendicular to it. This “anisotropic magnetoresistance” (AMR) is usually explained by the assumption that electrons moving parallel to the magnetization collide with obstacles more frequently than electrons flowing perpendicularly. Here, we challenge that assumption and show experimentally that AMR can be strong even without electron collisions.

We use ultrashort electromagnetic pulses in the terahertz frequency range to measure AMR at rates both slower and faster than the average time between two collision events of conduction electrons. In this way, we separate contributions that are unrelated to electron collisions from those that are related. Based on experiment and modeling, we find that AMR unrelated to collisions emerges because the electrons moving along the magnetization direction are effectively heavier than electrons moving perpendicular to it.

AMR is an important probe of the magnetic state of prototypical ferromagnetic and antiferromagnetic spintronic devices, which convey and store information using electron spins rather than charge. Our findings show that collision-unrelated AMR has a large information bandwidth and thus is highly interesting for future ultrafast spintronic applications.

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Vol. 11, Iss. 2 — April - June 2021

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