Edge localization of spin waves in antidot multilayers with perpendicular magnetic anisotropy

S. Pan, S. Mondal, M. Zelent, R. Szwierz, S. Pal, O. Hellwig, M. Krawczyk, and A. Barman

Phys. Rev. B 101, 014403 – Published 3 January 2020

Abstract

We study the spin-wave dynamics in nanoscale antidot lattices based on Co/Pd multilayers with perpendicular magnetic anisotropy. Using time-resolved magneto-optical Kerr effect measurements we demonstrate that the variation of the antidot shape introduces significant change in the spin-wave spectra, especially in the lower frequency range. By employing micromagnetic simulations we show that additional peaks observed in the measured spectra are related to narrow shell regions around the antidots, where in-plane domain structures are formed. This is because the magnetic anisotropy in these regions is reduced due to the $G a^{+}$ ion irradiation during the focused ion beam milling process of the antidot fabrication. The results point at possibilities for exploitation of localized spin waves in out-of-plane magnetized thin films, which are easily tunable and suitable for magnonic applications.

Received 25 June 2019
Revised 5 October 2019

DOI:https://doi.org/10.1103/PhysRevB.101.014403

Physics Subject Headings (PhySH)

Magnetic anisotropy Magnetic domains Spin dynamics

Magnetic multilayers Nanostructures

Ferromagnetic resonance Landau-Lifschitz-Gilbert equation Micromagnetic modeling Optically detected magnetic resonance

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

S. Pan ^1,*, S. Mondal^1,*, M. Zelent ^2,†, R. Szwierz², S. Pal¹, O. Hellwig ^3,4, M. Krawczyk ^2,‡, and A. Barman¹

¹Department of Condensed Matter Physics and Material Sciences, S. N. Bose National Centre for Basic Sciences, Block JD, Sector III, Salt Lake, Kolkata 700 106, India
²Faculty of Physics, Adam Mickiewicz University, Uniwersytetu Poznanskiego 2, 61–614 Poznań, Poland
³Institute of Physics, Chemnitz University of Technology, Reichenhainer Straße 70, D-09107 Chemnitz, Germany
⁴Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, Germany

^*These authors contributed equally to the paper.
^†Corresponding author: mateusz.zelent@amu.edu.pl
^‡Corresponding author: krawczyk@amu.edu.pl

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Vol. 101, Iss. 1 — 1 January 2020

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Images

Figure 1
A schematic of the multilayered ADL structure composed of Co and Pd layers under investigation and the definition of the coordinate system used throughout the paper. Here the antidot has a triangular shape with side length $D$ and a shell with width $d$ around it with reduced PMA is marked by the grey color. The parameters $a_{1}$ and $a_{2}$ are the $x$ and $y$ lattice constants of the antidot array, respectively.
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Figure 2
(a) The SEM images of the four types of ADLs investigated in this article. (b) TR-MOKE signals after subtracting a biexponential background and (c) its FFT spectra at $H = 2.23 kOe$ for ADLs with different shapes of the antidots: circular: C, square: S, triangular: T, and diamond: Di. The vertical red-dashed line points at the SW frequency of the unpatterned ML film.
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Figure 3
(a) The simulated spectrum of SWs in the C antidot with the zero PMA value in the shell of 10- nm width around the antidot, with $H = 2.23 kOe$ along the $z$ axis. (b) The static configuration of the magnetization stabilized in the structure. (c), (e) The amplitude of the in-plane component of the dynamical part of the magnetization, and (d), (f) its phase of high intensity SWs at 9.01 and 10.44 GHz, indicated as i and ii in (a), respectively.
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Figure 4
Simulated frequency spectra as a function of the shell width $d$ around the antidots in the sample C. The color map indicates the intensity of the peaks. The red-dahsed line indicates the frequency of the SW measured from the unpatterned ML film.
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Figure 5
Simulated SW spectra for the S- (a), T- (b) and Di-ADL (c) structure with $d = 10 nm$ with the respective static magnetization configurations and the profiles of the most intense lines in the spectra. The out-of-plane magnetic field $H = 2.23 kOe$ is used in simulations, and the SW spectra are calculated from the in-plane component of the magnetization.
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Figure 6
The spectra for the Di-ADL structure with a shell of $d = 20 nm$ . (a) The magnetization in the shell forms the closure domain structure, and (b) the head-to-head and tail-to-tail domain walls are present in the bottom and top of the shell. (c) The bias magnetic field has been tilted by 10° from the normal direction towards the $y$ axis. The magnetization at the shell forms a closure domain structure and the magnetic field used to excite SW is along the $x$ axis. (d) The same as in (c), but with the excitation field exactly along the out-of-plane direction and the spectra calculated from the $m_{z}$ component of the magnetization. In simulations a bias magnetic field of 2.23 Oe is assumed.
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