Impact of interface structure on magnetic exchange coupling in $\mathrm{MnBi}/{\mathrm{Fe}}_{x}{\mathrm{Co}}_{1\ensuremath{-}x}$ bilayers

Impact of interface structure on magnetic exchange coupling in $MnBi / {Fe}_{x} {Co}_{1 - x}$ bilayers

S. Sabet, A. Moradabadi, S. Gorji, M. Yi, Q. Gong, M. H. Fawey, E. Hildebrandt, D. Wang, H. Zhang, B.-X. Xu, C. Kübel, and L. Alff

Phys. Rev. B 98, 174440 – Published 29 November 2018

Abstract

Magnetic exchange coupling behavior was investigated in MnBi/FeCo bilayer system at the hard/soft magnetic interface. We performed a combined study of cross-sectional high resolution transmission electron microscopy (HR-TEM), DFT calculations, and micromagnetic simulations to elucidate effect of interface structure on exchange coupling. Exchange spring MnBi/ ${Fe}_{x} {Co}_{1 - x} (x = 0.65$ and 0.35) bilayers with various thicknesses of the soft magnetic layer were deposited in a dc magnetron sputtering unit from alloy targets. According to magnetic measurements, using a Co-rich layer leads to a more coherent exchange coupling with optimum soft layer thickness of about 1 nm. Our DFT calculations predicted formation of a polycrystalline FeCo layer with coexisting crystalline and disordered (110) phases. The indexed FFTs from HR-TEM images confirmed a crystalline FeCo(110) layer, with slight misorientation in some areas, and a disordered region close to the interface which deteriorates interface exchange coupling. Moreover, our micromagnetic simulations showed how the thickness of the FeCo layer and the interface roughness both control the effectiveness of exchange coupling in MnBi/FeCo bilayer.

Received 10 May 2018
Revised 3 October 2018

DOI:https://doi.org/10.1103/PhysRevB.98.174440

Physics Subject Headings (PhySH)

Exchange interaction Magnetic coupling

Multilayer thin films

Density functional theory Micromagnetic modeling Sputtering Transmission electron microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

S. Sabet^1,*, A. Moradabadi^1,2, S. Gorji^3,4, M. Yi¹, Q. Gong¹, M. H. Fawey^3,4, E. Hildebrandt¹, D. Wang^3,5, H. Zhang¹, B.-X. Xu¹, C. Kübel^3,5, and L. Alff^1,†

¹Institute of Materials Science, Technische Universität Darmstadt, 64287 Darmstadt, Germany
²Institut für Chemie und Biochemie, Freie Universität Berlin, 14195 Berlin, Germany
³Institute of Nanotechnology (INT), Karlsruhe Institute of Technology (KIT), 76344 Eggenstein-Leopoldshafen, Germany
⁴Joint Research Laboratory Nanomaterials (KIT and TUD) at Technische Universität Darmstadt, Jovanka-Bontschits-Strasse 2, 64287 Darmstadt, Germany
⁵Karlsruhe Nano Micro Facility, Karlsruhe Institute of Technology (KIT), 76344 Eggenstein-Leopoldshafen, Germany

^*sabet@oxide.tu-darmstadt.de
^†alff@oxide.tu-darmstadt.de

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Vol. 98, Iss. 17 — 1 November 2018

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Figure 1
The XRD patterns from exchange spring bilayers of ${Mn}_{55} {Bi}_{45} / {Fe}_{35} {Co}_{65}$ (at. %) with various thicknesses of FeCo soft magnetic layer between 1 and 3 nm. The LTP-MnBi thin films were annealed at $T_{ann} = 365^{\circ} C$ (638 K) followed by deposition of FeCo layer at a substrate temperature of $T_{sub} = 100^{\circ} C$ (373 K). The spectra have vertical offset for clarity. The peaks originating from residual bismuth in the films or Al capping layer are labeled with (*) and (+), respectively.
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Figure 2
Out-of-plane magnetization data for MnBi/FeCo bilayers with different FeCo thicknesses from 0 to 3 nm measured at 300 K. (a) With a Fe-rich and (b) with a Co-rich soft magnetic FeCo layer. The dashed line in (b) shows the in-plane magnetization for a single MnBi layer.
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Figure 3
(a) Cross-sectional high resolution transmission electron microscopy (HR-TEM) image from a MnBi/FeCo bilayer sample $(c$ -axis textured MnBi hard magnetic layer with a thickness of $\sim 50$ nm and polycrystalline Co-rich FeCo soft magnetic layer with a thickness of $\sim 5$ nm). (b) STEM image from a cross section of the layers along with EDX elemental map from Mn, Bi, Fe, Co, Al, Pt, and O across the layers.
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Figure 4
Micromagnetic simulation results of a MnBi/ ${Fe}_{3} {Co}_{5}$ model system with disordered FeCo(110) [d-(110)] and crystalline FeCo(111) [c-(111)] interfaces. Magnetic reversal curves: (a) No interface roughness with the $A^{int}$ value listed in Table 1. (b) Interface roughness with the same $A^{int}$ as in (a). (c) Interface roughness with $A^{int}$ reduced to $10 %$ of that in (a). The external magnetic field $μ_{0} H_{ex}$ is applied along the $z$ direction. Inset of (a): Model geometry with in-plane periodic boundary condition. Inset of (b): Interfacial roughness of MnBi with a maximum dent height of 0.4 nm. (a-i) and (a-ii), (b-i) and (b-ii), and (c-i) and (c-ii) present the magnetic configurations $(y z$ surface at $x = 0)$ corresponding to the marked circles of reversal curves in (a), (b), and (c), respectively, which belongs to the disordered ${Fe}_{3} {Co}_{5}$ (110).
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Figure 5
Hysteresis plots obtained from micromagnetic simulations for MnBi(001)/FeCo(110) double layers (a) without and (b) with interface roughness. (c) Variation of the magnetization with respect to the applied field around zero field for the theoretical and experimental hysteresis plots as a function of FeCo thickness. For the case of interfaces without roughness two regions are evident in which at 1 nm FeCo thickness incoherent coupling between the hard and soft magnetic layers appears. The rough interfaces in both theoretical and experimental cases behave incoherently for all thicknesses. For comparison, the experimental data for the epitaxial case of MnGa(001)/FeCo(001) bilayer are also presented.
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Physical Review B

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Impact of interface structure on magnetic exchange coupling in $MnBi / {Fe}_{x} {Co}_{1 - x}$ bilayers

S. Sabet, A. Moradabadi, S. Gorji, M. Yi, Q. Gong, M. H. Fawey, E. Hildebrandt, D. Wang, H. Zhang, B.-X. Xu, C. Kübel, and L. Alff

Phys. Rev. B 98, 174440 – Published 29 November 2018

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Physical Review B

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Impact of interface structure on magnetic exchange coupling in MnBi/FexCo1−x bilayers

S. Sabet, A. Moradabadi, S. Gorji, M. Yi, Q. Gong, M. H. Fawey, E. Hildebrandt, D. Wang, H. Zhang, B.-X. Xu, C. Kübel, and L. Alff

Phys. Rev. B 98, 174440 – Published 29 November 2018

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Impact of interface structure on magnetic exchange coupling in $MnBi / {Fe}_{x} {Co}_{1 - x}$ bilayers