Exchange coupling in MnBi/Fe-Co thin film bilayers

ORAL

Abstract

To achieve enhanced energy products of MnBi for rare-earth free permanent magnet applications, we studied the exchange coupled soft/hard bilayers based on MnBi films. By using DC magnetron sputtering, we fabricated pure MnBi films with magnetization of 500 emu/cc and coercivity of 1.6 T. A (BH)$_{\mathrm{max}}$ of 6.2 MGOe is obtained for pure MnBi films. A large enhancement in (BH)$_{\mathrm{max}}$ due to exchange coupling in MnBi/Fe-Co bilayers is observed with Fe-Co thicknesses between 2 and 5 nm. The highest (BH)$_{\mathrm{max}}$ obtained is 14.0 MGOe at room temperature with a single phase magnetization curve for a MnBi (20 nm)/Co (2 nm) bilayer. TEM and XPS studies indicate there is no oxidation between soft/hard interface. The XMCD results show that the soft moments of Fe/Co at a thickness of 2 nm are perpendicular to the MnBi plane, indicating nearly perfect hard-soft coupling. Moreover, a micromagnetic calculation on perpendicularly-coupled MnBi/Fe-Co bilayers suggests a critical coupling thickness of 4 nm of the soft layer. We will also discuss results from polarized neutron reflectometry measurements performed on the bilayers.

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Authors

  • Lei Fang

    University of Maryland

  • Tieren Gao

    University of Maryland

  • Sean Fackler

    University of Maryland

  • Shingo Maruyama

    University of Maryland

  • Ichiro Takeuchi

    Luigi Colombo, Texas Instruments Incorporated, University of Maryland

  • Jun Cui

    Pacific Northwest National Laboratory

  • M.J. Krammer

    Ames laboratory

  • Duane D. Johnson

    Ames Laboratory, Ames laboratory, Department of Materials Science and Engineering, Iowa State University

  • Elke Arenholz

    Lawrence Berkeley National Laboratory

  • Julie Borchers

    NIST Center for Neutron Research, National Institute of Standard and Technology, Gaithersburg, MD, NIST

  • Brian Kirby

    National Institute of Standards and Technology, National Institute for Standard Technology, NIST, National Institute of Standards and Technology, Center for Neutron Research

  • William Ratcliff

    NIST Center for Neutron Research, NIST, NIST

  • Ralph Skomski

    Department of Physics and Astronomy \& NCMN, University of Nebraska, University of Nebraska, Univ of Nebraska - Lincoln

  • Samuel Lofland

    Rowan University, Rowan University, Department of Physics and Astronomy