# Laser-induced real-space topology control of spin wave resonances

Tim Titze  I. Physikalisches Institut, Universität Göttingen, 37077 Göttingen,
Germany  Sabri Koraltan  Physics of Functional Materials, Faculty of Physics,
University of Vienna, Vienna, Austria  Vienna Doctoral School in Physics,
University of Vienna, Vienna, Austria  Timo Schmidt  Institute of Physics,
University of Augsburg, 86135 Augsburg, Germany  Marcel Möller  Max Planck
Institute for Multidisciplinary Sciences, Göttingen, Germany  IV.
Physikalisches Institut, Universität Göttingen, 37077 Göttingen, Germany
Florian Bruckner  Physics of Functional Materials, Faculty of Physics,
University of Vienna, Vienna, Austria  Claas Abert  Physics of Functional
Materials, Faculty of Physics, University of Vienna, Vienna, Austria  Research
Platform MMM Mathematics-Magnetism-Materials, University of Vienna, Vienna,
Austria  Dieter Suess  Physics of Functional Materials, Faculty of Physics,
University of Vienna, Vienna, Austria  Research Platform MMM Mathematics-
Magnetism-Materials, University of Vienna, Vienna, Austria  Claus Ropers  Max
Planck Institute for Multidisciplinary Sciences, Göttingen, Germany  IV.
Physikalisches Institut, Universität Göttingen, 37077 Göttingen, Germany
International Center for Advanced Studies of Energy Conversion (ICASEC),
Universität Göttingen, 37077 Göttingen, Germany  Daniel Steil  I.
Physikalisches Institut, Universität Göttingen, 37077 Göttingen, Germany
Manfred Albrecht  Institute of Physics, University of Augsburg, 86135
Augsburg, Germany  Stefan Mathias  I. Physikalisches Institut, Universität
Göttingen, 37077 Göttingen, Germany  International Center for Advanced Studies
of Energy Conversion (ICASEC), Universität Göttingen, 37077 Göttingen, Germany

###### Abstract

Femtosecond laser excitation of materials that exhibit magnetic spin textures
promises advanced magnetic control via the generation of ultrafast and non-
equilibrium spin dynamics. We explore such possibilities in ferrimagnetic
[Fe(0.35 nm)/Gd(0.40 nm)]160 multilayers, which host a rich diversity of
magnetic textures from stripe domains at low magnetic fields, a dense
bubble/skyrmion lattice at intermediate fields, and a single domain state for
high magnetic fields. Using femtosecond magneto-optics, we observe distinct
coherent spin wave dynamics in response to a weak laser excitation allowing us
to unambiguously identify the different magnetic spin textures. Moreover,
employing strong laser excitation we show that we achieve versatile control of
the coherent spin dynamics via non-equilibrium and ultrafast transformation of
magnetic spin textures by both creating and annihilating bubbles/skyrmions. We
corroborate our findings by micromagnetic simulations and by Lorentz
transmission electron microscopy before and after laser exposure.

##  I Introduction

Magnetic spin textures are expected to be an important building block for
future spintronic- and magnonic-based memory and logic devices, and even for
unconventional computing techniques such as neuromorphic computing [1, 2, 3,
4, 5, 6]. Especially, localized magnetic solitons, known as skyrmions, have
attracted significant attention to be utilized in spin texture based devices
[7, 8, 9]. Potential applications of magnetic textures include the engineering
of spin wave dispersions in magnonic crystals, using magnetic textures as spin
wave phase shifters in logic devices, and employing them as nanoscale spin
wave emitters to name a few [10, 11, 12]. Obviously, the ability to actively
control such devices by non-destructive and fast means is highly desirable
from an application point of view. Ultrafast optical manipulation of spin
textures is a potential avenue to achieve these goals. However, optical
spectroscopy studies in spin-textured materials systems have so far either
focused on precessional spin dynamics present in low-temperature skyrmionic
phases [13, 14, 15, 16], or on statically observed laser-induced
transformations of spin textures [17, 18, 19, 20]. Here we show that we can
achieve ultrafast optical control of spin textures at room temperature and
thus laser-induced tuning of spin wave resonances to specific frequency
ranges. Our work opens new avenues for future actively controlled magnetic
spin texture-based devices.

##  II Results

###  II.1 Static magnetic properties of
[Fe($0.35\text{\,}\mathrm{nm}$)/Gd($0.40\text{\,}\mathrm{nm}$)]160 thin films

Figure 1: Static magnetic sample properties: a) Out-of-plane $M$-$H$
hysteresis loop of the
[Fe($0.35\text{\,}\mathrm{nm}$)/Gd($0.40\text{\,}\mathrm{nm}$)]160 thin film
at room temperature. Solid lines correspond to $M$-$H$ data for increasing
(red) and decreasing (blue) field, black squares mark assignments of LTEM
images (I)–(V) in b) to magnetic field regions in the $M$-$H$ loop. The
coloration of magnetic field regimes represent the underlying spin textures.
b) LTEM images (grayscale) and micromagnetic simulations (color) of the
magnetization for an upsweep (bottom, red arrow) and downsweep (top, blue
arrow) of the magnetic field reveal the spin texture corresponding to
different magnetic fields (I)-(V) in the hysteresis loop. The direction of $M$
in the micromagnetic simulation data is given by the color code. c) Legend of
different spin objects indicating the field-induced transformations possible
for a magnetic field upsweep with non-chiral and chiral domains walls, type-2
bubbles and clockwise (CW) and counter-clockwise (CCW) skyrmions.

The studied sample system is a multilayer stack of Fe and Gd with composition
[Fe(0.35 nm)/Gd(0.40 nm)]160 deposited on a thermally-oxidized Si(100)
substrate. Static magnetic characterization of this sample was performed using
Superconducting Quantum Interference Device - Vibrating Sample Magnetometry
(SQUID-VSM). Samples deposited on thin commercial Si3N4 membranes were used
for Lorentz Transmission Electron Microscopy (LTEM) studies to image the
static magnetic texture in applied out-of-plane (oop) magnetic fields.

Figure 1a shows the $M$-$H$ hysteresis loop from SQUID-VSM for positive
magnetic fields and Fig. 1b displays the corresponding LTEM images (gray-
scaled middle panels) for representative field values together with
micromagnetic simulations of the field-dependent magnetic texture (color-
scaled top and bottom panels). Without applied field (Figs. 1a and 1b, left),
the sample is in a demagnetized state, and the magnetic texture consists
solely of oop stripe domains with Bloch domain walls (region I). We find two
different types of domain walls, similar to previous studies [21, 22]: Narrow
stripes with domain walls that do not exhibit any chirality, and broader
stripes with twice the periodicity featuring the same chirality. Applying an
increasing oop magnetic field (Fig. 1a, red part of $M$-$H$ loop), the oop
magnetization of the sample initially increases linearly with the applied
magnetic field. In LTEM, this process is visible as a decrease of the density
of domain walls (Fig. 1b, II, bottom images) and the nucleation of cylindrical
spin objects from collapsed narrow stripe domains (so called type-2 magnetic
bubbles). For higher magnetic fields (region III), dipolar Bloch-type
skyrmions start to nucleate from the broad stripes. In region IV, in a field
range of $\mu_{0}H\approx$190\text{\,}$-$250\text{\,}\mathrm{mT}$$, a bubble
and skyrmion (B/SK) lattice with characteristic spacing and groups of spin
objects of the same type is formed. Close to saturation, the bubbles vanish
and only skyrmions remain (see SI Fig. S4). For even higher applied fields
(region V) magnetic saturation is achieved. Decreasing the magnetic field from
saturation does not lead to the appearance of a B/SK lattice in region IV, as
reflected in the LTEM images (Fig. 1b, IV, top) and $M$-$H$ loop (Fig. 1a,
blue line) and corroborated by the micromagnetic simulation. The measured oop
magnetization in this region is larger than for the upsweep of the magnetic
field, as no cylindrical spin objects reducing the oop magnetization exist.
The small hysteresis in this field region in the $M$-$H$ loop is thus
indicative of the disordered B/SK lattice. For further decreasing field
strengths (regions III to I), the evolution of the magnetic texture is again
comparable to the upsweep, except that, at least in region III, the initial
domain wall density is lower in the decreasing field case. Please note that
small differences observed in the magnetic sample properties between SQUID-VSM
and LTEM measurements, i.e., higher magnetic field values for the same spin
texture in LTEM, arise from the fact that two different substrates are used
for the sample preparation resulting in different growth conditions of the
films. Micromagnetic simulations match the magnetic field values of the
corresponding SQUID-VSM data and are discussed further below.

###  II.2 Coherent spin dynamics in response to weak optical excitations

After the thorough characterization of the ground state magnetic spin
textures, we now focus on the magnetic response of the sample to an ultrashort
optical excitation. In particular, we seek correlations between the temporal
response and the different spatial textures in Fig. 1. For these measurements,
we use a comparably weak perturbation (fluence
$F=$300\text{\,}\mathrm{\SIUnitSymbolMicro J}\text{\,}{\mathrm{cm}}^{-2}$$,
pulse duration $\tau_{p}<40$ fs) leading to a total demagnetization of the
sample by only $3.5\text{\,}\mathrm{\char 37\relax}$. Magnetization dynamics
$M(t)$ were detected by changes in the light polarization upon reflection of
the sample using the magneto-optical Kerr effect (MOKE) in polar geometry with
the magnetic field applied in oop direction. For details see the methods
section.

Figure 2: Out-of-plane magnetization dynamics for a weak perturbation in
dependence of the applied oop magnetic field. Magnetization dynamics for a)
upsweep and b) downsweep of the magnetic field. The colors of the curves
represent the regions of different spin texture as given in Fig. 1a: Stripe
domains (blue), B/SK lattice (green), and magnetic saturation (red). Data is
normalized to the Kerr signal in magnetic saturation in oop direction.

Figure 2 depicts the magnetization dynamics in response to the optical
excitation and in dependence of the applied oop magnetic field for both an
upsweep (Fig. 2a) and a downsweep of the magnetic field (Fig. 2b). Four
processes can be identified: Immediately after photoexcitation, the
magnetization is quenched on a timescale of only $300\text{\,}\mathrm{fs}$.
This ultrafast demagnetization process is followed by a fast remagnetization
on a few picosecond timescale. During the remagnetization, about half of the
initial magnetization is recovered, and the magnetization stays constant
thereafter for up to $\approx$$40\text{\,}\mathrm{ps}$ depending on the
applied magnetic field. Finally, the magnetization recovers on a
$\text{\,}\mathrm{ns}$-timescale by thermal transport out of the film. Most
notably, in addition to this well-known and incoherent magnetic response [23,
24], a coherent oscillatory signal component is clearly visible in the stripe
domain phase (blue transients) and the B/SK lattice phase for the upsweep of
the magnetic field (green transient).

Figure 3: Coherent magnetization dynamics in the experiment and micromagnetic
simulation in dependence of the applied oop magnetic field. Coherent
magnetization dynamics for a) upsweep and c) downsweep of the magnetic field
with the corresponding Fourier spectra b) and d), respectively. In case of the
upsweep a), b) a unique spin wave mode is observed for
$\mu_{0}H=$190\text{\,}$-$240\text{\,}\mathrm{mT}$$.

The main question now is to what extent this coherent response is indicative
of the specific spin texture in the Fe/Gd sample. To answer this question, we
performed micromagnetic simulations of the spin dynamics after a magnetization
quench of less than $10\text{\,}\mathrm{\char 37\relax}$ (see Methods). Figure
3 depicts a full comparison of the experimentally found and simulated coherent
oscillatory signal components after subtraction of the incoherent background
for both the upsweep (Fig. 3a) and downsweep (Fig. 3c) of the magnetic field.
The corresponding Fourier transforms for experiment and micromagnetic
simulation are shown in Fig. 3b and Fig. 3d, respectively.

Evidently, experiment and simulation match well and show a clear
correspondence of the underlying spin textures with the observed coherent spin
dynamics. The Fourier spectra for magnetic fields of $\mu_{0}H=0-190$ mT,
i.e., the stripe domain state, show a strong oscillatory mode $m_{1}$ with
$f_{1}\approx 2.3$ GHz, which is identified as a breathing mode of stripe
domains in the micromagnetic simulation. A much weaker high-frequency
component $m_{2}$ with $f_{2}\approx 5.4$ GHz is additionally observed in the
experiment, however, its origin needs to be further investigated. For magnetic
fields of $\mu_{0}H\approx 190-240$ mT the disordered B/SK lattice is present
in the upsweep of the magnetic field only. Correspondingly, experiment and
simulation show only in this case an oscillatory mode $m_{bsk}$ with
$f_{bsk}\approx 1.4$ GHz. This frequency is identified as a breathing mode of
bubbles and skyrmions [25] in the micromagnetic simulation in very good
agreement with previous studies on skyrmion dynamics [15, 13, 26]. We note
that micromagnetic simulations indeed predict a slightly lower frequency for
magnetic bubbles compared to skyrmions, however, we cannot resolve this
difference with our experimental frequency resolution. Last, in saturation, no
oscillatory spin dynamics are observed in experiment and simulation.

While the agreement between experiment and simulation is very good in general,
we note that small differences do exist in the magnetic field range of
$\mu_{0}H\approx 150-200$ mT for both field up- and downsweep. Furthermore,
there is an apparent phase shift in the oscillatory dynamics with magnetic
field in both experiment and simulation. For details on these observations we
refer to the Supplementary Information Section V.1.

###  II.3 Magnetic phase transformation in response to a strong optical
excitation

Having shown a clear correlation between spin texture and spin dynamics, we
now focus on the response of the spin texture to a strong non-equilibrium
excitation of the spin system. We repeat the experiments from above for a
strong laser excitation of
$F=$3\text{\,}\mathrm{mJ}\text{\,}{\mathrm{cm}}^{-2}$$, i.e., an order of
magnitude larger fluence than before, and we find significant changes in the
observed dynamics. A much stronger incoherent demagnetization of about 45%
occurs, i.e., the initial magnetic spin texture is strongly disturbed (see SI
Figs. S6a and b). For a detailed investigation, we again subtracted the
incoherent background (Fig. 4a and b) and performed a Fourier transform (Fig.
4c and d).

Figure 4: High-fluence coherent magnetization dynamics in time-resolved MOKE
in dependence of the applied oop magnetic field. High fluence coherent
magnetization dynamics for a) upsweep and b) downsweep of the magnetic field
with the corresponding Fourier spectrum c) and d), respectively. For both
directions of magnetic field sweep a single spin wave mode is observed within
$\mu_{0}H=$100\text{\,}$-$190\text{\,}\mathrm{mT}$$.

We now observe the stripe domain phase mode $m_{1}$ in a magnetic field range
$\mu_{0}H=0-$100\text{\,}\mathrm{mT}$$ with slightly lower frequency than
before. A second, higher-frequency mode $m_{2}$ is weakly present at
$f_{2}\approx$4.8\text{\,}\mathrm{GHz}$$. The mode previously identified to be
B/SK breathing now appears around $f_{bsk}\approx$1.2\text{\,}\mathrm{GHz}$$
in the magnetic field range $\mu_{0}H=100-$190\text{\,}\mathrm{mT}$$. In
contrast to the weak perturbation limit, the mode now exhibits a clear
frequency dependence, shifting from $f\approx$1.4\text{\,}\mathrm{GHz}$$ to
$f\approx$1.0\text{\,}\mathrm{GHz}$$ from low to high magnetic fields. Above
$\mu_{0}H=$190\text{\,}\mathrm{mT}$$ no oscillatory behavior is observed
anymore. Most surprisingly, however, we now observe the $m_{bsk}$ mode even in
the downsweep of the magnetic field (Fig. 4b and d), where it was previously
absent. This observation suggests that the strong laser excitation itself
might induce a B/SK lattice phase in the material. Indeed, skyrmion nucleation
from a saturated state by strong laser excitation was recently theoretically
predicted [27] and experimentally observed [18, 19], which could explain our
findings. However, our micromagnetic simulations strongly suggest that a
nucleation of skyrmions from saturation does not appear to be possible for the
Fe/Gd multilayers studied here.

To understand the observed behavior better, we performed LTEM measurements
before and after femtosecond laser excitation. These measurements indeed allow
us to identify the changes in dynamics from Fig. 3 to Fig. 4 as a laser-
induced transformation of the underlying spin texture. We will show in the
following that this transformation is not trivial, and not dominantly given by
a simple static laser-induced temperature shift of phase stability ranges [21]
together with laser-induced skyrmion nucleation from magnetic saturation [27,
18, 19].

Figure 5: Laser-induced transformation of magnetic textures in LTEM. LTEM
experimental data showing the magnetic texture before and after strong laser
excitation of $F=$6.4\text{\,}\mathrm{mJ}\text{\,}{\mathrm{cm}}^{-2}$$. A)
Unmodified stripe domain phase for low magnetic fields; B) transformation of a
mixed phase into a B/SK lattice phase; C) annihilation of bubbles and
skyrmions; D) unmodified saturated magnetic state; E) transformation of a
stripe state in the downsweep of the magnetic field into a mixed state (see SI
Fig. S9) and then further into a B/SK lattice state.

Upon ultrafast and strong laser excitation, we distinguish five scenarios
depending on the field-dependent magnetic ground state of the system depicted
in Fig. 5 starting with the upsweep of the magnetic field: A) For low magnetic
fields, a pristine stripe domain phase remains nearly unchanged after laser
excitation. B) A former mixed stripe domain and B/SK state is turned into a
pure B/SK lattice phase, as also observed in the simulation, see SI Fig. S10.
C) Starting within the B/SK lattice phase, bubbles and skyrmions are
annihilated by the laser excitation, i.e., the system is driven into a fully
saturated state even by a single laser shot. Thereafter, the new saturated
magnetic state is not modified by further laser excitation. D) From
saturation, we did not observe any laser-induced creation of skyrmions without
irreversibly changing the sample properties, which is in contrast to
literature [27, 18, 19]. However, this observation concurs with the
micromagnetic simulations, where it was also not possible to nucleate
skyrmions from a saturated spin state. Note that close to saturation in the
downsweep of the magnetic field, a gradual transformation of the system
starting from a single isolated stripe domain appears possible (see SI Fig.
S7). E) For the downsweep, a plain stripe domain phase with low stripe density
is transformed into a mixed phase. Here, all stripe domains convert into
cylindrical spin textures after exposure to a single laser pulse and new
stripe domains nucleate in regions that are initially saturated (see SI Fig.
S9). Upon subsequent laser excitation the newly generated domains transform
into B/SKs. Hence, using ultrafast and strong laser excitation, a pure B/SK
lattice phase can be created. A schematic phase diagram of the magnetic
texture depending on magnetic field and the number of incident laser pulses
can be found in SI Fig. S8.

In comparison to literature, for a Co/Pt multilayer sample, Gerlinger and
coworkers [28] observed laser-induced skyrmion nucleation from both i)
magnetic saturation and ii) the stripe domain phase. Furthermore, they
observed iii) an annihilation of skyrmions and stripe domains close to
saturation. In our study, we obviously find similar evidence for processes ii)
and iii), but not for i), the laser-induced nucleation from saturation.

However, taking the predictions from micromagnetic simulations and the
observations from the LTEM measurements into account, we can now understand
the changes in the coherent oscillatory component that come along with the
strong excitation in Fig. 4: In the upsweep of the magnetic field, the former
B/SK lattice phase completely vanishes before the actual data recording starts
due to continuous excitation by laser pulses in the time-resolved MOKE
experiment. Hence, we observe no oscillatory component, but a signal
corresponding to magnetic saturation in the magnetic field range
$$190\text{\,}$<\mu_{0}H<$250\text{\,}\mathrm{mT}$$ (scenario C, Fig. 5). For
lower magnetic fields, the initially mixed stripe domain and B/SK phase is
laser-transformed into a pure B/SK lattice phase, leading to the apparent
shift of the $m_{bsk}$ mode to lower frequency at high fields,
$$150\text{\,}$<\mu_{0}H<$190\text{\,}\mathrm{mT}$$ (scenario B, Fig. 5). The
slightly higher frequency observed in Fig. 4c for magnetic fields of about
$$110\text{\,}$<\mu_{0}H<$150\text{\,}\mathrm{mT}$$ originates from a mixed
phase that forms upon laser excitation out of a pristine stripe domain phase
exhibiting none or only very few skyrmions, respectively. This means that no
final pure B/SK lattice phase is created within this field range (see also SI
Fig. S5). In the downsweep of the magnetic field, for high fields
($$150\text{\,}$<\mu_{0}H<$190\text{\,}\mathrm{mT}$$) a pure B/SK lattice
phase is formed out of a pure stripe phase (scenario E, Fig. 5), and, for
lower fields ($$100\text{\,}$<\mu_{0}H<$150\text{\,}\mathrm{mT}$$), a mixed
phase is the final state. All these mechanisms lead to the generally more
equal oscillatory behavior observed for the strong perturbation case for
magnetic field up- and downsweep. However, the absence of an initially mixed
phase in the downsweep hinders the generation of a B/SK lattice phase. For
equal magnetic fields we observe less bubbles and skyrmions being created in
the downsweep of the magnetic field resulting in a dominance of the stripe’s
oscillatory component in the generated mixed phase. Finally, the generally
lower frequencies of all modes can most likely be attributed to the transient
laser-induced change of macroscopic sample properties, e.g., sample
temperature, magnetization and anisotropies [21].

Closing the discussion, three further points are noteworthy: First, an
asymmetry in the system response exists for the upsweep and downsweep of the
magnetic field in the field regions where no B/SKs are observed in static
studies. This asymmetry increases with a stronger perturbation. This is
evident from the different shifts of magnetic oscillations for the upsweep and
downsweep of the magnetic field in Fig. 3a and c as well as Fig. 4a and b,
which even shows a frequency shift between the up and down field sweeps (cf.
Fig. 4c and d). However, SQUID-VSM and LTEM data from Fig. 1 do not suggest
any specific macroscopic difference in the low-field range for both field
sweep directions, except possibly in domain wall density. Currently, we do not
have an explanation for this behavior, but aim to investigate this further in
the future. Second, in contrast to Refs. [18, 19, 28], skyrmion creation in
our case is only possible if there is an initially existing chiral spin
texture. Even further, the nucleation of skyrmions does not only have a lower
but also an upper fluence threshold. Above a certain fluence, a mixed phase is
formed in the region of maximum excitation that is surrounded by a pure
skyrmion phase due to the Gaussian intensity distribution of the laser pulse,
see Fig. S12 in the SI. Last, we want to note that no influence of spin
texture on the incoherent magnetization dynamics was observed, neither on the
sub-ps timescale nor during the remagnetization.

##  III Conclusion

In summary, we studied the response of the spin-textured material Fe/Gd to
weak and strong ultrafast laser excitation. In the weak excitation limit,
unique coherent spin dynamics evolve that allowed us to directly identify the
magnetic stripe domain phase, the bubble/skyrmion lattice phase, as well as
magnetic saturation. In the strong excitation limit, we observed significant
changes in the dynamic magnetic phase diagram, which we attributed to a laser-
induced transformation of the spin texture as verified by micromagnetics and
LTEM measurements. Indeed, from the stripe domain state, we created both a
bubble/skyrmion lattice and a mixed phase of bubbles, skyrmions and stripe
domains, depending on the external magnetic field. These results highlight
that ultrafast optical control of spin wave resonances is possible by optical
transformation of the spin textures in the material. Moreover, these rich spin
physics are observed at room temperature and at quite moderate magnetic fields
and laser fluence, making the Fe/Gd multilayer system an attractive model
system for fundamental studies of spin texture-based physics, as well as
potential application in spin-electronics in the future.

##  IV Methods

###  IV.1 Sample preparation and characterization

[Fe(0.35 nm)/Gd(0.40 nm)]160 multilayer samples were prepared on both
thermally-oxidized Si(100) substrates, as well as on 30-nm thick Si3N4
membranes required for LTEM imaging. The films were deposited at room
temperature by dc magnetron sputtering in one run. Nevertheless, slightly
varying magnetic properties were obtained due to different growth conditions
during deposition on the two different substrates. The sputter process was
carried out using an Ar working pressure of 3.5 µbar in an ultra-high vacuum
chamber. For all samples, a 5-nm-thick Pt seed layer and a 5-nm-thick Si3N4
capping layer were used to protect the films from oxidation. The thickness of
the layers was controlled by a calibrated quartz balance during layer
deposition.
The magnetic properties of the samples were analyzed by Superconducting
Quantum Interference Device-Vibrating Sample Magnetometry (SQUID-VSM). $M$-$H$
hysteresis loops were measured both in out-of-plane and in-plane configuration
at room temperature. The magnetic spin textures were imaged by Lorentz
Transmission Electron Microscopy (LTEM) at room temperature using a JEOL
NEOARM-200F system operated at $200\text{\,}\mathrm{keV}$ beam energy in the
Fresnel mode with an underfocus of 2 mm. Images are recorded with a Gatan
OneView camera in external out-of-plane magnetic fields.

###  IV.2 Time-resolved magneto-optical Kerr spectroscopy (TR-MOKE)

Transient reflectivity $\Delta R(t)$ and magnetization dynamics $\Delta M(t)$
were measured simultaneously using a bichromatic pump-probe setup.
Sub-$\text{\,}\mathrm{ps}$ laser pulses are generated by a fiber amplifier
system operating at a repetition rate of $50\text{\,}\mathrm{kHz}$. A pulse
duration of less than $40\text{\,}\mathrm{fs}$ is achieved at the sample
position via spectral broadening in a gas-filled hollow core fiber and
subsequent compression with chirped mirrors [29]. In the setup, the
fundamental light pulses with central wavelength of $1030\text{\,}\mathrm{nm}$
and their second harmonic at $515\text{\,}\mathrm{nm}$ are used as pump and
probe pulses, respectively. External out-of-plane magnetic fields up to
$\mu_{0}H=$0.7\text{\,}\mathrm{T}$$ can be applied by mounting the sample in-
between the poles of a variable-gap electromagnet. The data is measured by a
balanced bridge detector and collected by a $250\text{\,}\mathrm{MHz}$
digitizer card enabling single pulse detection. Using a mechanical chopper to
improve signal quality, both the pumped and the unpumped signals can be
recorded, allowing for maximum scope of data post processing. Data was
acquired in polar Kerr geometry, where the magnetic field was applied in oop
direction. All $M(t)$ data shown were recorded as the difference of $M(t)$
traces for two opposite magnetic fields to remove nonmagnetic signal
components, taking care to set the correct spin texture by approaching the
current magnetic field either from below or above.

###  IV.3 Lorentz Transmission Electron Microscopy with in-situ optical
excitation

To obtain a better understanding of the spin dynamics observed in TR-MOKE in
the strong excitation limit, we studied a sample on a thin commercial Si3N4
membrane by LTEM at the Göttingen Ultrafast Transmission Electron Microscope,
allowing for in-situ optical excitation [30, 17, 31]. We recorded energy-
filtered images using a CEOS CEFID equipped with a TVIPS XF416 at a
$10\text{\,}\mathrm{eV}$ window-width, an acceleration voltage of
$200\text{\,}\mathrm{keV}$ and a $6\text{\,}\mathrm{mm}$ defocus. In these
experiments a laser source with a wavelength of
$750\text{\,}\mathrm{n}\mathrm{m}$ and a pulse width of around
$300\text{\,}\mathrm{fs}$ excites the sample under a near normal angle of
incidence with a defined number of laser pulses starting from single shot. At
various oop magnetic fields, micrographs were acquired before, after a single
and after multiple excitations to observe changes of the samples magnetic
texture. In order to obtain reproducible magnetic textures before laser
excitation, we fully saturated the sample with an oop field before approaching
a specific field magnitude.

In addition to the somewhat different laser conditions, the LTEM sample on the
thin TEM membrane has different thermal couplings and strain compared to the
MOKE sample on the rigid Si substrate. This also leads to slightly higher
magnetic field ranges, where the stripe phase, B/SK lattice phase, and
magnetic saturation are present. Due to these differences in experimental
conditions, a quantitative fluence matching between TR-MOKE and LTEM
experiments was not attempted. However, the LTEM results, at a fluence about a
factor of 2 higher than in TR-MOKE, allow to fully understand the the changes
to the magnetic texture by laser excitation and allow to perfectly explain the
observations in the TR-MOKE measurements. To obtain the fluences in the LTEM
measurements from the laser pulse energy, a $1/e^{2}$ beam diameter of
$60\text{\,}\mathrm{\SIUnitSymbolMicro m}$ was used, estimated from the
maximum region of switched magnetic texture upon laser excitation.

###  IV.4 Static micromagnetic simulations

We use magnum.np [32], a GPU-enhanced micromagnetic simulation software, to
perform large-scale micromagnetic simulations. We simulate the Fe/Gd
multilayers as a 3D ferromagnet with low saturation magnetization
$M_{s}=$340\text{\,}\mathrm{k}\mathrm{A}\mathrm{/}\mathrm{m}$$ and low
perpendicular magnetic anisotropy
$K_{u}=$40\text{\,}\mathrm{k}\mathrm{J}\mathrm{/}\mathrm{m}^{3}$$. The
exchange constant is chosen
$A_{\mathrm{ex}}=$6\text{\,}\mathrm{p}\mathrm{J}\mathrm{/}\mathrm{m}$$. Our
finite-difference micromagnetic solver discretizes the simulation box into
$512\times 512\times 10$ cuboids, each with a constant volume of $10\times
10\times 11.2\mathrm{nm}^{3}$. Note that all simulations are performed without
any thermal fluctuations. Basically, the system is simulated at
$T=$0\text{\,}\mathrm{K}$$, but with effective magnetic parameters that were
measured at room temperature. Similar modeling was used in experimental
studies on Fe/Gd-based multilayers [21, 22]. To obtain the $M$-$H$ hysteresis
loops shown in Fig. 1, we start from a cellwise random magnetization state. We
relax the structure in zero field by solving the Landau-Lifshitz-Gilbert (LLG)
equation numerically [33] at high damping $\alpha=1.0$, where we include the
demagnetization, exchange, anisotropy and Zeeman energies in the calculation
of the effective field. We obtain the stripe domain structure as our new
initial state. Once the magnetic system is relaxed with respect to its energy,
we apply an oop field along the $z$-direction. We increase the magnetic field
by $2\text{\,}\mathrm{m}\mathrm{T}$ after each relaxation step in which we
solve the LLG for $10\text{\,}\mathrm{n}\mathrm{s}$. The complete hysteresis
is then simulated by raising and lowering the field between
$\pm$250\text{\,}\mathrm{m}\mathrm{T}$$. Magnetization states at each field
are saved to be used as input for dynamic simulations to simulate the
resonance of the spin objects.

###  IV.5 Dynamic micromagnetic simulations

With the magnetization states obtained from the static simulations, we now
simulate the intrinsic response of the spin textures. Commonly, ultrafast
laser demagnetization can be described micromagnetically by including a
stochastic temperature-dependent term to the LLG, where the temperature in the
magnetic system is calculated from the excitation energy via the two-
temperature model [34]. However, in our study we are not interested in the
demagnetization process itself, but rather in the excitation of the magnetic
system and how this relaxes into its equilibrium state. Thus, we employ a
similar approach as applied for simulating ferromagnetic resonance (FMR) [35].
That is, we assume that the laser pulse heats up the system and alters the
temperature-dependent magnetic parameters. Excitation of the system occurs in
the system trying to return to its original energetic minimum. Therefore, we
assume that during pumping $M_{s}$ is reduced with an excitation amplitude
between $5\%$ and $25\%$. $K_{u}$ and $A_{\mathrm{ex}}$ scale with $M_{s}^{2}$
[36]. We start from the magnetization states from the zero-temperature
hysteresis at a defined field. We solve the LLG for
$1\text{\,}\mathrm{n}\mathrm{s}$ at moderate damping $\alpha=0.1$ to get the
magnetization near its new energetic minimum for the new set of magnetic
parameters. We then abruptly set the initial magnetic parameters back and
reduce the damping to $\alpha=0.02$. The simulation time is
$10\text{\,}\mathrm{n}\mathrm{s}$. The magnetization starts to oscillate back
into the original or new ground state, based on the strength of the
excitation. Total resonances can then be captured by first recording the
averaged magnetization components for every $\Delta
t=$1\text{\,}\mathrm{p}\mathrm{s}$$ and performing an FFT. To probe the
influence of a stroboscopic measurement technique, we repeat the excitation up
to ten times and record the magnetization state after each excitation, where
changes in the magnetization states can be observed for high excitation
amplitudes. The resonance frequencies of the stripe domain and the skyrmion
are separated by averaging the magnetization in a smaller simulation box. For
the skyrmion phase we average $m_{z}$ for a single skyrmion and record it for
each time step $\Delta t=$10\text{\,}\mathrm{p}\mathrm{s}$$. For the stripe
domain we first find an appropriately small section where only one stripe
domain is stable and has almost parallel domain walls. We then sampled the
magnetization along a line that is $500\text{\,}\mathrm{n}\mathrm{m}$ long.
The magnetization is then averaged for each component at the same time step as
used for the skyrmions.

###  IV.6 Data availability

The data that support the findings of this study are available from the
authors upon reasonable request.

###  IV.7 Code availability

Micromagnetic simulations were performed with the open source micromagnetic
simulator magnum.np [32]. Simulations scripts to reproduce the results can be
requested from corresponding authors.

###  IV.8 Acknowledgements

S.K. and C.A. gratefully acknowledge the Austrian Science Fund (FWF) for
support through Grant No. P34671 (Vladimir). S.K. and D.Su. acknowledge the
Austrian Science Fund (FWF) for support through Grant No. I 6267 (CHIRALSPIN).
T.S. and M.A. gratefully acknowledge funding from Deutsche
Forschungsgemeinschaft (DFG, German Research Foundation) grant no. 507821284.
D.St., S.M. and T.T. acknowledge funding by the DFG, grant no. 217133147/SFB
1073, project A02. The computational results presented have been achieved in
part using the Vienna Scientific Cluster (VSC).

###  IV.9 Author contributions

D.St., M.A., and S.M. conceived the project. T.T. performed the time-resolved
Kerr effect studies and analyzed the time-resolved magnetism data with
supervision from D.St. Sample growth, static SQUID-VSM and LTEM analysis were
performed by T.S. LTEM with femtosecond laser excitation was conceived by
C.R., measured by M.M. and T.T., and evaluated by T.T. S.K., F.B., and C.A.
wrote and improved the micromagnetic code. S.K. performed the micromagnetic
simulations under the supervision of C.A. and D.Su. The simulations were
evaluated by S.K. and T.T. with support from D.St. T.T., D.St. and S.M. wrote
the paper with input from all authors.

###  IV.10 Corresponding authors

Correspondence to Daniel Steil or Stefan Mathias

###  IV.11 Competing interests

The authors declare no competing interests.

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##  V Supplementary

###  V.1 Weak perturbative regime: Extended discussion of the spin dynamics of
stripe domains, bubbles, and skyrmions

This section provides an extended comparison of the experimental results and
micromagnetic simulations in the weak perturbation limit. We also provide
further information on the different observed breathing modes by comparison of
experiment and simulation.

####  V.1.1 Field-dependent differences in coherent spin dynamics between
experiment and simulation

For magnetic fields $<$150\text{\,}\mathrm{mT}$$ the results obtained from
modelling and experiment in Fig. 3 are in very good agreement. The mode
$m_{1}$ with frequency $f_{1}\approx$2.3\text{\,}\mathrm{GHz}$$ is observed in
both cases. However, the higher-frequency mode $m_{2}$ as well as the opposite
phase shift observed in experiment for up- and downsweep of the magnetic field
are not reproduced by the simulation. We attribute these differences to sample
properties not accounted for in the simulation, e.g., sample inhomogeneities
and defects as well as the fact that the simulation is a strongly discretized
situation.

Furthermore, in the magnetic field range
$$150\text{\,}\mathrm{mT}$<\mu_{0}H<$200\text{\,}\mathrm{mT}$$ a sharp
transition exists between the two modes $m_{1}$ and $m_{bsk}$ in the
experiment, while in the simulation this transition appears to be more
continuous. Additionally, a transition area for mode $m_{1}$ is also present
for the downsweep of the magnetic field in the simulation. We explain this
difference to the experiment with the high amount of cylindrical-like spin
objects in the simulated ground state of the mixed phase for both magnetic
field up- and downsweep, see micromagnetic simulation data in Fig. 1b, image
III. In contrast, according to the LTEM data, the ground state in the
experiment is mainly governed by stripe domains. Due to the strong spatial
discretization within the simulation, as well as the absence of defects likely
acting as nucleation sites for domain walls in the actual sample, the
evolution of spin texture, especially starting from a formerly saturated
state, can be expected to deviate from the experiment.

####  V.1.2 Breathing mode of a single skyrmion

A snapshot of the spin texture is collected for each time step of the
micromagnetic simulation. We select one skyrmion
($\mu_{0}H=$200\text{\,}\mathrm{mT}$$, upsweep) to demonstrate the time
evolution of the skyrmion breathing mode for a single period in Fig. S1.

Fig. S1: Breathing mode of a single skyrmion. One period of the breathing mode
of a single skyrmion obtained from micromagnetic simulations. The black circle
indicates the initial skyrmion size as a guide to the eye.

####  V.1.3 Comparison of stripes, bubble/skyrmion breathing modes in
simulation and experiment

The width of a stripe domain and the size of B/SKs change periodically as a
response to the excitation. This breathing mode causes a periodic oscillation
of the magnetization component along the oop direction, which can be observed
both in our TR-MOKE experiment and in micromagnetic simulations as depicted in
Fig. S2. Note that the experimental data corresponds to the integrated
collective spin response over the probed sample area, whereas the simulation
(solid lines) shows the integrated $M_{z}$-component of the magnetization in a
small area containing a single skyrmion or a single stripe domain. Especially
at early times, deviations are to be expected, because the excitation in
experiment and the micromagnetic simulations are different (see Methods
section). Furthermore, in the stripe breathing the higher frequency mode
$m_{2}$ is present in the experiment, but not in the simulation. Overall,
considering these differences, experiment and theory match closely.

Fig. S2: Breathing modes of stripes and B/SK. Low-fluence coherent
magnetization dynamics within the stripe and B/SK phase that can be linked to
the collective breathing modes of stripes and skyrmions, respectively. Data
points are experimental data corresponding to the collective response of the
$M_{z}$-component of the spin system in the probed sample area, solid lines
depict the $M_{z}$-component of the magnetization from the micromagnetic
simulations for an area of $300\text{\,}$x$300\text{\,}\mathrm{nm}$2
containing a single skyrmion (bottom) or
$300\text{\,}$x$500\text{\,}\mathrm{nm}$2 for part of a stripe domain (top).

####  V.1.4 Phase and frequency shifts in oscillatory spin dynamics

In general, the spin wave dynamics observed in Fig. 3 and Fig. 4 exhibit a
field-dependent phase and/or frequency shift both in the stripe domain and
B/SK lattice phases. The expected direction of these shifts is currently not
well understood by the authors, but is believed to partially result from the
density of domain walls that will influence the spin wave phase [37]. We
discuss below two particular examples from Fig. 3 and outline our current
understanding of the observed effect.

##### B/SK lattice phase shift—weak perturbation limit

The frequency of $m_{bsk}$ does not visibly depend on the oop magnetic field
in experiment and simulation. However, we experimentally observe a shift of
the oscillation onset by $\Delta t\approx$100\text{\,}\mathrm{ps}$$ with
increasing magnetic field and a phase shift is also visible in the simulation.
Here, this phase shift interestingly appears together with the magnetic field-
dependent vanishing of bubbles and shrinking of skyrmions. As depicted in Fig.
S3, the micromagnetic simulation predicts a drop in both size and number of
cylindrical spin objects with respect to an increasing applied magnetic field.
As a result, the amount of Bloch-like domain walls is reduced. However, the
phase of a spin wave is modified when transmitting through a Bloch-like domain
wall [37]. Since there are fewer domain walls at higher magnetic fields, the
phase shift must change, which we attribute to the observed phase shift of the
$m_{bsk}$ mode.

Fig. S3: Micromagnetic simulation of field-dependent B/SK evolution. Evolution
of the size of cylindrical spin objects with magnetic field in the
micromagnetic simulation. The direction of $M$ is given by the color code.

Experimentally, we find that skyrmions are more stable with respect to an
increasing magnetic field than bubbles. As depicted in Fig. S4, the bubbles
continuously vanish with increasing field while the skyrmions remain almost
unaffected. As a result, within a specific magnetic field range, only
skyrmions are present. Again, the density of domain walls is reduced,
supporting the above argument.

Fig. S4: LTEM measurement of field-dependent B/SK evolution. Experimentally
measured spin texture in dependence of the magnetic oop field for upsweep of
the magnetic field. Bubbles are observed to be less stable than skyrmions.

##### Coherent magnetization dynamics within the mixed phase

Fig. S5: Coherent magnetization dynamics in the mixed phase state. Low-fluence
coherent magnetization dynamics within the mixed phase with experimental data
as red squares. The dynamics are modelled (solid line) by a superposition of
the coherent dynamics within stripe domain and B/SK lattice phase as depicted
in Fig. S2

.

Considering the upsweep of the magnetic field in the experiment, we observe a
non-monotonous behavior of the observed oscillation shift within the stripe
domain phase, e.g., the oscillations shift to the right for magnetic fields
$\mu_{0}H>$150\text{\,}\mathrm{mT}$$ (see Fig. 3a). We attribute this behavior
to the coexistence of stripes and B/SKs within this field range. As a result,
the coherent dynamics can be modelled as a superposition of the stripe domain
and B/SK breathing mode. In Fig. S5 we exemplarily constructed the coherent
dynamics for $\mu_{0}H=$193\text{\,}\mathrm{mT}$$ by using the breathing modes
shown in Fig. S2 with a B/SK to stripe domain ratio of $1.4\text{\,}$. For the
strong excitation regime given in Fig. 4c) and d), this effect manifests in a
resolvable frequency shift in the mixed phase region between
$$110\text{\,}$<\mu_{0}H<$150\text{\,}\mathrm{mT}$$.

###  V.2 Strong perturbative regime: Extended discussion of the laser-induced
transformation of spin textures

####  V.2.1 Magnetization dynamics: Strong optical excitation

Figure S6 depicts the magnetization dynamics for a laser excitation of
$F=$3\text{\,}\mathrm{mJ}\text{\,}{\mathrm{cm}}^{-2}$$. In comparison to the
weak excitation shown in Fig. 2, we observe a much stronger incoherent
demagnetization of about 45%, which is indicative for the strong optically-
induced perturbation of the sample.

Fig. S6: Field-dependent TR-MOKE for strong optical excitation. Out-of-plane
magnetization dynamics in dependence of the applied oop magnetic field for
upsweep a) and downsweep b) of the magnetic field in the high fluence regime.
The colors of the curves represent the attribution to different static spin
textures: Stripe domains (blue), bubble/skyrmion lattice (green), and magnetic
saturation (red).

####  V.2.2 Skyrmion nucleation from an isolated chiral domain wall

The process of skyrmion nucleation starting from an almost fully saturated
state in the downsweep of the magnetic field is depicted in Fig. S7 a-i. The
LTEM images were significantly post-processed to obtain better visibility of
the magnetic texture. In more detail, the contours of chiral spin objects
associated with black and white LTEM contrast were extracted by running a
computer vision algorithm. Small contours that are referred to image noise
were disregarded. The remaining contours were then filled and framed with
color to increase the visibility of the spin objects. The color indicates the
underlying chirality of the spin object, with blue objects and red objects
originating from black and white LTEM contrast, respectively.

Fig. S7: Skyrmion nucleation from an isolated chiral domain wall. From a) to
i): Evolution of spin texture in dependence of the number of laser pulses with
fluence of $6.4\text{\,}\mathrm{mJ}\text{\,}{\mathrm{cm}}^{-2}$. The magnetic
field is chosen to be at the transition from saturation to the stripe domain
phase during a downsweep of the magnetic field. Note that the LTEM images were
significantly post-processed to enhance the visibility of magnetic skyrmions
and their chirality.

For the chosen magnetic field of $\mu_{0}H=$210\text{\,}\mathrm{mT}$$ in the
downsweep of the magnetic field only a single stripe domain is present. This
stripe domain transforms into a local skyrmion lattice upon excitation by the
first laser pulse. However, multiple shots are required to nucleate a new
chiral stripe domain pointing towards a rather stochastical nature of this
process. During the following five shots, we observe a subsequent growth of
the generated skyrmion phase that remains almost unchanged after ten
additional shots. Interestingly, skyrmions nucleate directly after single shot
excitation, however, only in a well defined area adjacent to already existing
skyrmions. We assume that this area is preferred for chiral stripe domain
nucleation upon a slight decrease of the magnetic field, i.e., the local spin
texture is not completely saturated. Laser-induced demagnetization then
results in a strong perturbation of this pre-stripe state. However, since the
chirality of the pre-stripe can be seen as an additional energy barrier, we
assume that chirality is conserved upon excitation. Instead, the spins along
the stripe domains are affected and cause local inhomogeneity and a partition
of the stripe domains in various parts (cf. Fig. S11). This spin alignment
then leads to a preferred forming of a skyrmion phase during the relaxation
process. Therefore, the chirality of all generated skyrmions is identical.

####  V.2.3 Optically-induced phase transformations: Schematic phase diagram

Fig. S8: Spin texture phase diagram. Schematic phase diagram of the spin
texture in dependence of magnetic field and number of laser pulses for up- a)
and downsweep b) of the magnetic field for a fixed fluence of
$6.4\text{\,}\mathrm{mJ}\text{\,}{\mathrm{cm}}^{-2}$. Stars correspond to the
LTEM data depicted in Fig. 5. The magnetic field ranges correspond to the spin
textures for the sample on the membrane, which are higher than for the MOKE
sample on a Si substrate, see also Fig. 1

.

Figure S8 shows a schematic phase diagram of the magnetic texture depending on
both the magnetic field and the number of incident laser pulses. Depending on
the magnetic state of the system, we observe different scenarios upon laser
excitation including unchanged magnetic texture (A,D), B/SK creation from
stripe domains (B,E), and B/SK annihilation (C).

In case of scenario E (see also Fig. 5), laser excitation leads to a gradual
transformation of a pristine stripe domain phase in case of the downsweep of
the magnetic field, as depicted in Fig. S9. Upon excitation by a single laser
pulse the initial stripes transform into bubbles and skyrmions, respectively.
In addition to that, we observe the creation of stripe domains in formerly
saturated regions. These generated stripe domains transform in turn into
bubbles/skyrmions, finally yielding a pure bubble/skyrmion lattice phase in
the laser excited region.

Fig. S9: Transformation of stripe state into B/SK lattice state via a mixed
state. Left: A stripe state in the downsweep of the magnetic field transforms
into a mixed state upon excitation by a single laser pulse with fluence of
$6.4\text{\,}\mathrm{mJ}\text{\,}{\mathrm{cm}}^{-2}$. Right: Excitation by
multiple laser pulses leads to a B/SK lattice state.

####  V.2.4 Skyrmion nucleation in the micromagnetic simulations

In Fig. S10 the simulated spin texture is depicted for three magnetic field
regimes considering multiple subsequent excitations with an $M_{S}$ reduction
amplitude of $25\text{\,}\mathrm{\char 37\relax}$.

Fig. S10: Skyrmion nucleation in the micromagnetic simulations. Spin texture
obtained from micromagnetic simulations after laser excitation for different
magnetic field regimes. For high magnetic fields
($>$150\text{\,}\mathrm{mT}$$) we observe the transformation of stripe domains
into skyrmions upon subsequent excitation, which is absent in case of low
magnetic fields ($50\text{\,}\mathrm{mT}$).

For low magnetic fields (here 50 mT), the spin texture, predominantly
consisting out of stripe domains, is stable, i.e., it remains unaffected by
laser excitation. Increasing the magnetic field to 150 mT transforms a part of
the stripe domains into skyrmions, leading to a mixed phase ground state.
Subsequent excitations transform additional stripe domains to skyrmions. A
stable equilibrium between stripe domains and skyrmions emerges after few
excitation cycles. Thus, a metastable state is created during the very first
laser excitations. As a consequence, it is not possible to capture this
process with our stroboscopic pump-probe measurement scheme. For slightly
higher magnetic fields ($160\text{\,}\mathrm{mT}$) the amount of stripe
domains in the ground state is further reduced, facilitating complete
transformation of the remaining stripes into skyrmions upon additional
excitation.

We find that this kind of skyrmion nucleation is mediated by the induced
motion of the stripe domains and therefore strongly depends on the excitation
amplitude. The process of skyrmion nucleation from a stripe domain is depicted
in Fig. S11. Note that no skyrmion nucleation is observed in micromagnetic
simulation upon excitation with an $M_{S}$ reduction amplitude of
$5\text{\,}\mathrm{\char 37\relax}$.

Fig. S11: Temporal evolution of stripe to skyrmion transformation. Time
evolution of skyrmion nucleation from stripe domains from micromagnetic
simulations. We observe the transformation of stripe domains into skyrmions
upon subsequent excitation for high magnetic fields
($>$150\text{\,}\mathrm{mT}$$).

####  V.2.5 Fluence-dependence of B/SK nucleation from LTEM studies

Multiple laser excitation of a pristine stripe phase imaged by LTEM is shown
in Fig. S12. Upon the first excitation with a fluence of
$12.8\text{\,}\mathrm{mJ}\text{\,}{\mathrm{cm}}^{-2}$ the stripe phase is
transformed. Interestingly, a mixed phase appears in the region of highest
excitation, while a pure skyrmion phase emerges in its surrounding. Further
excitation with the same pulse energy does not macroscopically change the spin
texture. However, we observe that the chirality of the skyrmions at the edge
of the mixed and the pure skyrmion phase can be inverted by subsequent laser
excitation (see offset image zooms). For subsequent weaker excitation, the
central mixed phase is transformed into a pure B/SK lattice phase as well. We
conclude that the laser pulse has to induce a specific amount of
demagnetization in order to nucleate the B/SK lattice phase thereafter. Above
a certain threshold of pulse energy this process gets more improbable. Our
explanation is that due to the strong demagnetization both the spin texture
and the underlying chirality completely vanish. As a result, during the
recovery process, the formation of skyrmions is suppressed and single
skyrmions only nucleate if by chance the local spin alignment is correct.

Fig. S12: Fluence-dependence of spin texture transformation. Spin texture in
dependence of the number of laser pulses. During a downsweep of the magnetic
field, the field was set to $195\text{\,}\mathrm{mT}$. Note that the LTEM
images were significantly post-processed to enhance the visibility of magnetic
skyrmions and their chirality. For details on post-processing see the
description of Fig. S7.