• Open Access

Weakly-Correlated Nature of Ferromagnetism in Nonsymmorphic CrO2 Revealed by Bulk-Sensitive Soft-X-Ray ARPES

F. Bisti, V. A. Rogalev, M. Karolak, S. Paul, A. Gupta, T. Schmitt, G. Güntherodt, V. Eyert, G. Sangiovanni, G. Profeta, and V. N. Strocov
Phys. Rev. X 7, 041067 – Published 19 December 2017

Abstract

Chromium dioxide CrO2 belongs to a class of materials called ferromagnetic half-metals, whose peculiar aspect is that they act as a metal in one spin orientation and as a semiconductor or insulator in the opposite one. Despite numerous experimental and theoretical studies motivated by technologically important applications of this material in spintronics, its fundamental properties such as momentum-resolved electron dispersions and the Fermi surface have so far remained experimentally inaccessible because of metastability of its surface, which instantly reduces to amorphous Cr2O3. In this work, we demonstrate that direct access to the native electronic structure of CrO2 can be achieved with soft-x-ray angle-resolved photoemission spectroscopy whose large probing depth penetrates through the Cr2O3 layer. For the first time, the electronic dispersions and Fermi surface of CrO2 are measured, which are fundamental prerequisites to solve the long debate on the nature of electronic correlations in this material. Since density functional theory augmented by a relatively weak local Coulomb repulsion gives an exhaustive description of our spectroscopic data, we rule out strong-coupling theories of CrO2. Crucial for the correct interpretation of our experimental data in terms of the valence-band dispersions is the understanding of a nontrivial spectral response of CrO2 caused by interference effects in the photoemission process originating from the nonsymmorphic space group of the rutile crystal structure of CrO2.

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  • Received 27 February 2017

DOI:https://doi.org/10.1103/PhysRevX.7.041067

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

F. Bisti1,2,*, V. A. Rogalev1, M. Karolak3, S. Paul4, A. Gupta4, T. Schmitt1, G. Güntherodt5, V. Eyert6, G. Sangiovanni3, G. Profeta7,8, and V. N. Strocov1

  • 1Swiss Light Source, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland
  • 2ALBA Synchrotron Light Facility, 08290 Cerdanyola del Vallès, Spain
  • 3Institut für Theoretische Physik und Astrophysik, Universität Würzburg, Am Hubland Campus Süd, 97074 Würzburg, Germany
  • 4MINT Center, University of Alabama, Tuscaloosa, Alabama 35487, USA
  • 5II. Physikalisches Institut, RWTH Aachen University, 52074 Aachen, Germany
  • 6Materials Design SARL, 42, Avenue Verdier, 92120 Montrouge, France
  • 7Dipartimento di Scienze Fisiche e Chimiche, Università dell’Aquila, Via Vetoio 10, 67100 L’Aquila, Italy
  • 8CNR-SPIN L’Aquila, Via Vetoio 10, 67100 L’Aquila, Italy

  • *Corresponding author. fbisti@cells.es

Popular Summary

Chromium dioxide (CrO2) belongs to class of materials with a unique property—electrons with one spin direction are electrically conductive, while those with the opposite spin direction are insulating or semiconducting. This particular “half-metallic” behavior for CrO2 was theoretically predicted more than 30 years ago. But the role of correlations among the electrons and how that relates to possible changes in this spin behavior with temperature remains under debate because of a lack of exhaustive experimental evidence. Fully understanding this behavior could lead to advances in the field of spintronics, which envisions novel electronic devices that rely on manipulations of electron spin rather than charge. We provide clear answers to these issues by revealing the momentum-resolved electronic structure of CrO2 with soft-x-ray angle-resolved photoemission measurements.

This technique allows us to penetrate through a native amorphous layer on the surface and probe the CrO2 electronic structure for the first time. Using first-principles calculations, we demonstrate that only a small amount of on-site Coulomb interaction is required for catching the salient experimental features. Methodologically, correct interpretation of the experimental data requires a nontrivial theoretical treatment that is able to reproduce the interference effect due to the nonsymmorphic crystal structure of CrO2. This method can be extended to any nonsymmorphic crystals, which are presently a hot topic because of their topological protection.

Our results put the conventional picture of the spin depolarization process into perspective and give new insight into the half-metallicity of this material. Furthermore, we provide a solid basis for the realization of theoretically predicted new electronic phases in CrO2 thin films.

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Vol. 7, Iss. 4 — October - December 2017

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