Room-Temperature Skyrmions at Zero Field in Exchange-Biased Ultrathin Films

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Room-Temperature Skyrmions at Zero Field in Exchange-Biased Ultrathin Films

K. Gaurav Rana, A. Finco, F. Fabre, S. Chouaieb, A. Haykal, L. D. Buda-Prejbeanu, O. Fruchart, S. Le Denmat, P. David, M. Belmeguenai, T. Denneulin, R. E. Dunin-Borkowski, G. Gaudin, V. Jacques, and O. Boulle

Phys. Rev. Applied 13, 044079 – Published 30 April 2020

Abstract

We demonstrate that magnetic skyrmions with a mean diameter around 60 nm can be stabilized at room temperature and zero external magnetic field in an exchange-biased $Pt$ / $Co$ / ${Ni}_{80} {Fe}_{20}$ / ${Ir}_{20} {Mn}_{80}$ multilayer stack. This is achieved through an advanced optimization of the multilayer-stack composition in order to balance the different magnetic energies controlling the skyrmion size and stability. Magnetic imaging is performed both with magnetic force microscopy and scanning nitrogen-vacancy magnetometry, the latter providing unambiguous measurements at zero external magnetic field. In such samples, we show that exchange bias provides an immunity of the skyrmion spin texture to moderate external-magnetic-field perturbations, in the tens-of-millitesla range, which is an important feature for applications such as memory devices. These results establish exchange-biased multilayer stacks as a promising platform toward the effective realization of memory and logic devices based on magnetic skyrmions.

Received 24 January 2020
Accepted 30 March 2020

DOI:https://doi.org/10.1103/PhysRevApplied.13.044079

Physics Subject Headings (PhySH)

Exchange bias Skyrmions Spintronics Surface & interfacial phenomena

Antiferromagnets Devices Ferromagnets Magnetic multilayers

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

K. Gaurav Rana¹, A. Finco ², F. Fabre ², S. Chouaieb ², A. Haykal², L. D. Buda-Prejbeanu¹, O. Fruchart ¹, S. Le Denmat³, P. David³, M. Belmeguenai⁴, T. Denneulin ⁵, R. E. Dunin-Borkowski ⁵, G. Gaudin¹, V. Jacques^2,*, and O. Boulle^1,†

¹Université Grenoble Alpes, CNRS, CEA, Grenoble INP, Spintec, Grenoble, France
²Laboratoire Charles Coulomb, Université de Montpellier and CNRS, 34095 Montpellier, France
³Université Grenoble Alpes, CNRS, Institut Néel, 38000 Grenoble, France
⁴Université Sorbonne Paris Nord, LSPM, CNRS, UPR 3407, 93430 Villetaneuse, France
⁵Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons and Peter Grünberg Institute, Forschungszentrum Jülich, 52425 Jülich, Germany

^*vincent.jacques@umontpellier.fr
^†olivier.boulle@cea.fr

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Vol. 13, Iss. 4 — April 2020

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Images

Figure 1
(a) Central panel: the schematic structure of the exchange-biased multilayer stack. Left panel: a scanning-transmission-electron-microscopy image of a cross section of the sample obtained in a high-angle annular dark-field imaging (STEM-HAADF) mode. Right panel: energy-dispersive x-ray (EDX) elemental map. The white arrow indicates the direction of the internal exchange-bias field $B_{int}$ across the ${Ni}_{80} {Fe}_{20}$ / ${Ir}_{20} {Mn}_{80}$ interface. (b)–(d) Magneto-optical Kerr rotation recorded as a function of the magnetic field applied perpendicularly to the film plane for various ${Ir}_{20} {Mn}_{80}$ thicknesses: (b) $t_{{Ir}_{20} {Mn}_{80}} = 4$ nm, (c) $t_{{Ir}_{20} {Mn}_{80}} = 4.1$ nm, and (d) $t_{{Ir}_{20} {Mn}_{80}} = 4.2$ nm. For these experiments, the ${Ni}_{80} {Fe}_{20}$ thickness is $t_{{Ni}_{80} {Fe}_{20}} = 2.1$ nm. (e)–(g) Corresponding magnetic-force-microscopy (MFM) images recorded at zero external magnetic field. We note that no additional magnetic field is applied to the sample after the annealing procedure. (h) Line profile across the skyrmion marked with the white dashed circle in (g). The red solid line is a Gaussian fit, leading to a full width at half maximum (FWHM) $w_{MFM} = 34 \pm 3$ nm.
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Figure 2
(a) A $N$ - $V$ magnetometry image recorded in photoluminescence- (PL) quenching mode above the optimized exchange-biased multilayer stack at room temperature and zero external magnetic field. In these experiments, the distance between the $N$ - $V$ sensor and the ferromagnetic layer is approximately 65 nm. (b) Line profile across the skyrmion marked with the dashed circle in (a). The profile is taken along the fast-scan direction (horizontal) and the red solid line is a Gaussian fit, leading to a FWHM $w = 58 \pm 8$ nm and a PL quenching contrast $C = 27 \pm 2 %$ . (c),(d) Statistical distributions of (c) the width $w$ and (d) the quenching contrast $C$ , inferred from measurements over a set of 51 isolated skyrmions. The dashed lines are data fitting with normal distributions. The values written on top of the histograms correspond to the mean value $\pm$ standard deviation of the distributions. (e) Simulations of the PL-quenching image resulting from the stray field produced by skyrmions with diameters $d_{sk} = 40$ , 60, and 80 nm (from left to right). Line profiles taken across the skyrmions (black dashed lines) are displayed below the PL maps, indicating the width and the quenching contrast.
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Figure 3
(a)–(c) MFM images recorded at zero external magnetic field (a) before and (b),(c) after the application of an out-of-plane field pulse of a few seconds’ duration of (b) $- 12$ mT and (c) $+ 12$ mT. (d)–(f) The same experiment before (d) and after the application of a field pulse of (e) $- 200$ mT and (f) $+ 200$ mT. The skyrmion size distributions extracted for these MFM images are shown in Appendixes pp1 and pp1. (g) The percentage change in skyrmion density as a function of the magnetic-field-pulse amplitude. The top-left inset is an enlargement of the weak magnetic field amplitudes. The bottom-right inset is the hysteresis loop of the exchange-biased multilayer stack measured by recording the magneto-optical Kerr rotation as a function of the out-of-plane magnetic field. The letters (d)–(f) refer to the MFM images.
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Figure 4
(a) The frequency of the Stokes $f_{s}$ (black) and anti-Stokes $f_{as}$ (red) BLS lines as a function of $k_{SW}$ . (b) $Δ f = f_{s} - f_{as}$ as a function of $k_{SW}$ .
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Figure 5
Skyrmion size distributions extracted from the MFM images shown in Fig. 3.
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