Ultrafast Amplification and Nonlinear Magnetoelastic Coupling of Coherent Magnon Modes in an Antiferromagnet

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論文 / 著書情報
Article / Book Information
Title

Ultrafast Amplification and Nonlinear Magnetoelastic Coupling of
Coherent Magnon Modes in an Antiferromagnet

Authors

D. Bossini, M. Pancaldi, L. Soumah, M. Basini, F. Mertens, M. Cinchetti,
T. Satoh, O. Gomonay, S. Bonetti

Citation

Physical Review Letters, Vol. 127, Issue 7, pp. 077202-1-6

Pub. date

2021, 8

Pub. version

https://doi.org/10.1103/PhysRevLett.127.077202

Copyright

(C) 2021 American Physical Society

Powered by T2R2 (Tokyo Institute Research Repository)

PHYSICAL REVIEW LETTERS 127, 077202 (2021)

Ultrafast Amplification and Nonlinear Magnetoelastic Coupling of Coherent Magnon
Modes in an Antiferromagnet

1

D. Bossini ,1,* M. Pancaldi,2 L. Soumah,2 M. Basini,2 F. Mertens ,3 M. Cinchetti ,3
T. Satoh ,4 O. Gomonay ,5 and S. Bonetti 2,6

Department of Physics and Center for Applied Photonics, University of Konstanz, D-78457 Konstanz, Germany
2
Department of Physics, Stockholm University, 106 91 Stockholm, Sweden
3
Experimentelle Physik VI, Technische Universität Dortmund, Otto-Hahn Straße 4, 44227 Dortmund, Germany
4
Department of Physics, Tokyo Institute of Technology, Tokyo 152-8551, Japan
5
Institut für Physik, Johannes Gutenberg Universität Mainz, D-55099 Mainz, Germany
6
Department of Molecular Sciences and Nanosystems, Ca’ Foscari University of Venice, 30172 Venezia-Mestre, Italy
(Received 5 November 2020; revised 22 May 2021; accepted 24 June 2021; published 9 August 2021)
We investigate the role of domain walls in the ultrafast magnon dynamics of an antiferromagnetic NiO
single crystal in a pump-probe experiment with variable pump photon energy. Analyzing the amplitude of
the energy-dependent photoinduced ultrafast spin dynamics, we detect a yet unreported coupling between
the material’s characteristic terahertz- and gigahertz-magnon modes. We explain this unexpected coupling
between two orthogonal eigenstates of the corresponding Hamiltonian by modeling the magnetoelastic
interaction between spins in different domains. We find that such interaction, in the nonlinear regime,
couples the two different magnon modes via the domain walls and it can be optically exploited via the
exciton-magnon resonance.
DOI: 10.1103/PhysRevLett.127.077202

Antiferromagnets (AFMs) have recently surged as candidates for a novel paradigm of spintronics devices able to
outperform ferromagnetic and ferrimagnetic materials in
terms of operational frequency, storage density, and resilience to external fields [1–4]. Intrinsically the long-range
antiferromagnetic order presents domains, which can
hardly be manipulated. This magnetic texture and the
magnetoelastic coupling—which is intimately interconnected to the domain structure—have been very recently
shown to play a major role in the mechanism allowing
electric manipulations of the N´eel vector [5–7]. The quest
for an ever faster and more energy efficient control of
AFMs motivates the use of ultrashort light pulses as
stimulus to drive (sub)picosecond spin dynamics [8–17].
However, the role of domain walls in magnetoelastic AFMs
on the ultrafast N´eel vector dynamics has been hitherto not
addressed, although being a crucial issue, since the overwhelming majority of AFMs in nature displays a multidomain magnetoelastic ground state.
Here, we demonstrate that the domain walls can activate
a novel functionality in an antiferromagnetic crystal,
namely a nonlinear magnetoelastic domain-walls-mediated
coupling between coherent spin wave modes belonging to
different branches of the magnon dispersion, affecting the
ultrafast dynamics of the N´eel vector. We realize experimentally the tailored amplification of coherent terahertz
oscillations of the N´eel vector by pumping a magnon mode
in an antiferromagnetic NiO crystal. This process is
triggered by driving a combined electronic and magnetic
0031-9007=21=127(7)=077202(6)

transition and results even in the amplification of a different
gigahertz magnon mode via the aforementioned coupling.
Finally, we formulate a macroscopic phenomenological
model able to explain the observations by taking into
account the role of the domain walls in the ultrafast
dynamics of the N´eel vector.
Our specimen is a 100 μm-thick freestanding single
crystal of NiO, cut along the h111i direction and has a
multidomain structure. A specimen in a multidomain state
can be described invoking as many antiferromagnetic
vectors (defined as n ≡ M ⇑ − M ⇓ , where M ⇑;⇓ represent
the magnetization of the two sublattices), each belonging to
a T domain [Fig. 1(a)].
The domain structure of NiO, comprising spin (S) and
twin (T) domains, is tightly connected with the magnetoelastic coupling, since when the crystal enters the magnetic
phase strained magnetic domains are formed [18,19]. The
magnetoelastic energy is also the major contribution to the
anisotropy gap in the magnon dispersion [20].
As a matter of fact, the magnon dispersion of NiO can be
described in the first approximation in terms of two
branches, so that at the center of the Brillouin zone two
modes are active [Figs. 1(b) and 1(c)]. A 1.07 THz mode,
which we are going to refer to as the high-frequency (hf)
mode, has already been excited by means of two different
approaches: resonant terahertz excitation [10,11] and
nonresonant impulsive stimulated Raman scattering
[9,13,15,16,21] (ISRS) mainly by means of optical laser
pulses. The ISRS mechanism succeeded also in inducing a

077202-1

© 2021 American Physical Society

PHYSICAL REVIEW LETTERS 127, 077202 (2021)
(a)

(a)

T2
Domain
D
main

1600

Absorption (1/cm)

Domain Wall

T1
Domain

0

(c)

(b)

hf mode

lf mode

Exciton
peak

Magnon
sideband

1200

T=5K
T = 40 K
800

T = 60 K
T = 77 K
400

n

T = 90 K

n

T = 110 K
0
0.96

0.97

0.975

0.98

Energy (eV)

FIG. 1. (a) Two of the four possible T domains and the
corresponding orientation of the N´eel vector (green arrows). The
magenta area represents the wall; the order parameter (dark arrows)
rotates in this region.At the center of thewall (ξ ≈ 0) the orientations
of n in the T1 and T2 domain are parallel with each other. (b) In- and
(c) out-of-plane dynamics of the order parameter induced by the
low- and high-frequency magnon mode.

(b)
1.4

5.4

E (meV)

1.3
5.0
1.2
4.6

1.1

Frequency (THz)

lower frequency magnon mode, which will be labeled as
low frequency (lf), with a frequency on the order of
130 GHz [9,13]. The previous time-resolved investigations
reporting the photoactivation of both modes, mostly performed focusing the pump and probe beams into a single T
domain, do not show any form of coupling or interaction
between the two magnetic eigenmodes.
An unexplored pathway to the femtosecond optical
generation of the hf mode relies on the exciton-magnon
(X-M) transition [22,23]. This process consists in the
simultaneous excitation of a spin-forbidden (i.e.,
ΔS ¼ 1) electronic transition and of a magnon (i.e.,
ΔS ¼ −1), restoring the overall conservation of spin as
required for electric dipole transitions [22,23]. We thus
measured the absorption spectrum of our sample as a
function of temperature (for details, see [24]). The spectra
obtained for T < 100 K display a peak centered at approximately 0.97 eV and a sideband at higher energy [Fig. 2(a)].
The position of the sideband is temperature dependent, and
the energy shift between the two spectral features
(≈4 meV) is consistent with the energy of the hf mode
[Fig. 2(b)] observed by Raman spectroscopy [36]. Our
observations are in excellent agreement with the literature
[25,36], so resonantly pumping our sample in the 0.97 eV
spectral range is expected to result in the generation of the
hf mode. We aim at answering two open scientific questions. First, whether the X-M transition can actually
resonantly induce coherent magnons on the femtosecond
timescale. Second, whether the domain walls play a role in
the ultrafast spin dynamics of a multidomain AFM and, in
case they do, what this role is.
We tackle these questions in a magneto-optical pumpprobe experiment, in which the pump photon energy can be

0.965

1.0

4.2
20

40

60

80

100

T (K)

FIG. 2. (a) Absorption coefficient of NiO around the excitonmagnon resonance detected at different values of temperature.
Traces are displaced vertically for the sake of clarity. (b) Blue
symbols: temperature-dependent energy shift between the electronic peak and the sideband [24]. Green symbols: reported
values of the temperature dependence of the 1 THz magnon
mode, detected with Raman spectroscopy [36].

tuned in the 0.92–1.07 eV spectral range [Fig. 3(a)],
allowing us to compare the spin dynamics triggered by a
resonant pumping of the X-M with the signal detected by
exciting NiO nonresonantly (setup described in [24]). The
rotation of the polarization detected in every trace shown in
Fig. 3(b) reveals oscillations at the frequency of approximately 110 GHz. Considering the value of the frequency,
we ascribe this harmonic component of the signal to the lf
mode [9,13]. The slight deviation of the frequency from the
reported value is due to magnetostriction induced by both
internal and external strains of the sample, which can
significantly affect the magnon frequency in antiferromagnets [37]. Additionally, some of the traces in Fig. 3(b)
display also a faster oscillatory component, whose frequency matches the reported 1.07 THz value of the hf
mode. In the inset of Fig. 3(b) the power spectra (i.e.,
square modulus of the Fourier transform) of the 0.92 and
0.97 eV time traces are shown, displaying the presence and
absence of the 1 THz magnon. The discussion of the

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PHYSICAL REVIEW LETTERS 127, 077202 (2021)
1

NiO

Max
absorption

Normalized amplitude

0.8

111

(b)

E = 1.07 eV
E = 1.01 eV

-0.02

-0.04

-0.06

-0.08

Bandwidth
pulses
PS2

0.4

PS1

Absorption

0.2

E = 0.92 eV

X-M

Amplitude (arb. ...

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