Magnon polarons in the van der Waals antiferromagnet $\mathrm{Fe}{\mathrm{PS}}_{3}$

Magnon polarons in the van der Waals antiferromagnet $Fe {PS}_{3}$

D. Vaclavkova, M. Palit, J. Wyzula, S. Ghosh, A. Delhomme, S. Maity, P. Kapuscinski, A. Ghosh, M. Veis, M. Grzeszczyk, C. Faugeras, M. Orlita, S. Datta, and M. Potemski

Phys. Rev. B 104, 134437 – Published 29 October 2021

Abstract

The hybridization of magnons (spin waves) with phonons, if sufficiently strong and comprising of long wavelength excitations, may offer a new playground when manipulating the magnetically ordered systems with light. Applying a magnetic field to a quasi-two-dimensional antiferromagnet, $Fe {PS}_{3}$ , we tune the magnon-gap excitation to coincide with the initially lower-in-energy phonon modes. Hybrid magnon-phonon modes, the magnon polarons are unveiled with the demonstration of a pronounced avoided crossing between the otherwise bare magnon and phonon excitations. The magnon polarons in $Fe {PS}_{3}$ are traced with Raman scattering experiments. However, as we show, they also couple directly to terahertz photons, evoking their further explorations in the domain of antiferromagnetic optospintronics. The magnon-phonon coupling is also discussed as a possible reason of the magnon mode splitting observed in the absence of a magnetic field.

Received 24 August 2021
Accepted 18 October 2021

DOI:https://doi.org/10.1103/PhysRevB.104.134437

Physics Subject Headings (PhySH)

Antiferromagnetism Magnetism Magnons

Antiferromagnets Van der Waals systems

Infrared spectroscopy Raman spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

D. Vaclavkova ^1,*, M. Palit^2,*, J. Wyzula¹, S. Ghosh², A. Delhomme¹, S. Maity², P. Kapuscinski ^1,3, A. Ghosh², M. Veis^1,4, M. Grzeszczyk^1,5, C. Faugeras¹, M. Orlita^1,4, S. Datta ^2,†, and M. Potemski^1,5,‡

¹Laboratoire National des Champs Magnétiques Intenses, LNCMI-EMFL, CNRS UPR3228, Université Grenoble Alpes, Université Toulouse, Université Toulouse 3, INSA-T, Grenoble and Toulouse, France
²School of Physical Sciences, Indian Association for the Cultivation of Science, 2A & B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India
³Department of Experimental Physics, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland
⁴Institute of Physics, Charles University, Ke Karlovu 5, Prague, 121 16 Czech Republic
⁵Institute of Experimental Physics, Faculty of Physics, University of Warsaw, ul. Pasteura 5, 02-093 Warszawa, Poland

^*These authors contributed equally to this work.
^†subhanano@gmail.com
^‡marek.potemski@lncmi.cnrs.fr

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Vol. 104, Iss. 13 — 1 October 2021

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Figure 1
(a) False-color map of the Raman scattering response of $Fe {PS}_{3}$ antiferromagnet, together with a few selected characteristic spectra, measured as a function of temperature in the range from 4.2 K up to 300 K. $P_{i}$ ( $i = 1, ..., 4$ ) and $F_{i}$ ( $i = 1, ..., 4$ ) resonances are due to phonon modes and the $M$ -feature corresponds to the magnon-gap excitation. (b) False-color map of the evolution of the low-temperature ( $T = 4.2$ K) magneto-Raman scattering response of $Fe {PS}_{3}$ with an applied magnetic field oriented perpendicular to the plane of the layers, together with a few selected spectra, in the spectral region from 60 to 180 ${cm}^{- 1}$ . The coupling between the lower magnon branch $M_{-}$ and three $P_{i}$ ( $i = 1, ..., 3$ ) phonon modes is clearly observed. An additional $R_{1}$ phonon mode is activated at high magnetic fields. The energy of the upper $M_{+}$ magnon branch smoothly develops with the magnetic field and does not reach the energy of the $P_{4}$ phonon in the range of magnetic fields investigated.
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Figure 2
Low-temperature ( $T = 4.2$ K) far-infrared transmission spectra for selected values of the magnetic field applied perpendicular to the layer planes, in the spectral region 60–180 ${cm}^{- 1}$ . Transmission minima denoted as $P_{i}$ ( $i = 1, ..., 3$ ) and $M_{+}$ / $M_{-}$ have their counterparts in resonances observed in Raman scattering spectra. $I_{i}$ ( $i = 1, ..., 3$ ) transitions are only visible in FIR transmission spectra. The pronounced minimum at $\sim$ 80 ${cm}^{- 1}$ is due to absorption resonance in the polyethylene foil used in our experimental setup to filter the high-frequency radiation.
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Figure 3
Magnetic field dependence of the energy positions of hybrid magnon-phonon modes. Full circles represent the experimental data extracted from magneto-Raman scattering measurements. These data are reproduced with solid lines, following the theoretical modeling described in the text. Crosses account for the energy positions of transitions observed in FIR transmission spectra. The resonances observed in Raman scattering and FIR transmission spectra overlap within the experimental error. Dashed lines show the field dependence of energy positions of magnon modes without coupling to phonons.
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Figure 4
(a) High-resolution magneto-Raman scattering and FIR transmission spectra of magnon resonance measured at low magnetic fields from 0 to 1.2 T. The transmission minimum, slightly above $M_{+}^{*}$ resonance, is only visible in FIR spectra measured under high resolution and is unaffected by the magnon excitation. (b) Energies of the two components of the magnon mode as a function of the magnetic field, extracted from the spectra shown in (b) together with the results of data simulation (solid lines) applied to reproduce the evolution of hybrid magnon-phonon modes in the full range of magnetic field investigated (see Fig. 3).
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Physical Review B

covering condensed matter and materials physics

Magnon polarons in the van der Waals antiferromagnet $Fe {PS}_{3}$

D. Vaclavkova, M. Palit, J. Wyzula, S. Ghosh, A. Delhomme, S. Maity, P. Kapuscinski, A. Ghosh, M. Veis, M. Grzeszczyk, C. Faugeras, M. Orlita, S. Datta, and M. Potemski

Phys. Rev. B 104, 134437 – Published 29 October 2021

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Physical Review B

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Magnon polarons in the van der Waals antiferromagnet FePS3

D. Vaclavkova, M. Palit, J. Wyzula, S. Ghosh, A. Delhomme, S. Maity, P. Kapuscinski, A. Ghosh, M. Veis, M. Grzeszczyk, C. Faugeras, M. Orlita, S. Datta, and M. Potemski

Phys. Rev. B 104, 134437 – Published 29 October 2021

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Magnon polarons in the van der Waals antiferromagnet $Fe {PS}_{3}$