Deterministic Generation and Guided Motion of Magnetic Skyrmions by Focused He+-Ion Irradiation [original]

Deterministic Generation and Guided Motion of Magnetic

Skyrmions by Focused He+‑Ion Irradiation

Lisa-Marie Kern, Bastian Pfau,*Victor Deinhart, Michael Schneider, Christopher Klose,

Kathinka Gerlinger, Steﬀen Wittrock, Dieter Engel, Ingo Will, Christian M. Gunther, Rein Lieﬀerink,

Johan H. Mentink, Sebastian Wintz, Markus Weigand, Meng-Jie Huang, Riccardo Battistelli,

Daniel Metternich, Felix Buttner, Katja Höﬂich, and Stefan Eisebitt

Cite This: Nano Lett. 2022, 22, 4028−4035

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sıSupporting Information

ABSTRACT: Magnetic skyrmions are quasiparticles with nontrivial top-

ology, envisioned to play a key role in next-generation data technology while

simultaneously attracting fundamental research interest due to their emerging

topological charge. In chiral magnetic multilayers, current-generated spin−

orbit torques or ultrafast laser excitation can be used to nucleate isolated

skyrmions on a picosecond time scale. Both methods, however, produce

randomly arranged skyrmions, which inherently limits the precision on the

location at which the skyrmions are nucleated. Here, we show that

nanopatterning of the anisotropy landscape with a He+-ion beam creates

well-deﬁned skyrmion nucleation sites, thereby transforming the skyrmion

localization into a deterministic process. This approach allows control of

individual skyrmion nucleation as well as guided skyrmion motion with nanometer-scale precision, which is pivotal for both future

fundamental studies of skyrmion dynamics and applications.

KEYWORDS: magnetic skyrmions, ion irradiation, current-induced and laser-induced dynamics, magnetic racetrack, soft X-ray imaging

Magnetic skyrmions are topological quasiparticles that can

be as small as a few nanometers.

1−3

During the past

decade, research on magnetic skyrmions attracted interest from

scientiﬁc

4,5

and industrial

research communities due to the

skyrmion’s fascinating properties emerging from its topological

charge. They can exist in thin ﬁlm materials with perpendicular

magnetic anisotropy (PMA) and are stabilized by stray ﬁelds

and the Dzyaloshinskii−Moriya interaction (DMI).

7−13

particular, skyrmions in chiral magnetic multilayer systems can

occur as isolated particle-like textures at room temperature and

in a low external magnetic ﬁeld regime down to zero ﬁeld.

13−15

Great advances have been reported in generating, annihilating,

and shifting skyrmions via spin−orbit torques (SOT) induced

by electric currents inside a suitable magnetic race-

track.

3,12,14,16−20

Moreover, recent studies have revealed that

faster and potentially more energy-eﬃcient skyrmion gen-

eration is feasible by replacing current pulses with femtosecond

laser pulses, allowing optical nucleation even with a single laser

pulse.

21−23

While the underlying mechanisms are diﬀerent, both

current-induced and optical nucleation suﬀer from a random-

ness in the spatial distribution of the skyrmions nucleated.

SOT magnetic switching and nucleation of a skyrmion are

mediated either by an applied symmetry-breaking in-plane

magnetic ﬁeld

24,25

or by a locally modiﬁed anisotropy at a

magnetic defect site in combination with DMI.

16,18

Natural

magnetic defects caused by structural and chemical inhomo-

geneity can act as such nucleation sites, but they also inﬂuence

the SOT-driven skyrmion motion.

14,26

Since the density and

distribution of these defects depend on the growth process of

the material, the localization of the skyrmions remains poorly

controllable. In the case of the optical nucleation, skyrmions

emerge from spin ﬂuctuations during a high-temperature phase

after the laser excitation. They ﬁnally appear randomly

distributed inside the illuminated area of the magnetic

ﬁlm.

23,27

However, a controllable, reproducible, and reliable

localization of the skyrmion nucleation is often requiredboth

for future applications in data technology

28,29

and for

fundamental research, e.g., to realize repetitive pump−probe

experiments on skyrmion dynamics.

Previous attempts to control the localization of skyrmions

include structural patterning of the magnetic ﬁlm with

notches

16,30,31

and discs

as well as the introduction of

defects

32,33

and nanopockets

to deﬁne positions for skyrmion

Received: February 17, 2022

Revised: May 2, 2022

Published: May 16, 2022

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nucleation. However, all of these methods include signiﬁcant

structural or even geometrical modiﬁcations of the magnetic

racetrack and render an unhampered motion of the skyrmion

after creation diﬃcult. Recent work also employed Ga+-ion

bombardment to create artiﬁcial defects as pinning sites for

skyrmions.

35,36

Due to the strong structural impact of these

heavy ions, it again remains an open question if a controlled

detachment of the skyrmion from the pinning site created in

this fashion is possible. In contrast, nanopatterning with a

focused He+-ion beam is suitable to prepare magnetic channels

for magnetic domain and skyrmion motion.

Light-ion

irradiation causes only minor structural reorganization on the

atomic scale, giving rise to an increased interfacial roughness

and intermixing of layerswithout aﬀecting the topogra-

phy.

38−44

As a consequence, magnetic properties such as

coercivity, PMA, and DMI can be controlled at the

nanoscale.

40,41,43,45,46

In this work, we use a focused He+-ion beam to control the

localization of current-induced and laser-induced magnetic

skyrmions in ion-irradiated areas with diﬀerent shapes and

sizes. In conjunction with the applied magnetic ﬁeld, we can

turn the nucleation into a fully deterministic process

guaranteeing skyrmions to be generated in irradiated sites

while simultaneously suppressing their creation in non-

irradiated regions. Furthermore, we demonstrate controlled

SOT-induced detachment from such a nucleation site and the

subsequent guided motion of a single skyrmion over several

micrometers along a linear path, precisely deﬁned by ion

irradiation, thereby eﬀectively diminishing the transverse

skyrmion drift due to the skyrmion Hall eﬀect. Our results

show the large potential of magnetic anisotropy patterning by

He+irradiation in multilayer structures for applied and

fundamental research on isolated skyrmions.

We prepared ferromagnetic multilayers with a nominal

composition of Ta(3 nm)/Pt(4 nm)/[Pt(2.5 nm/

Co60Fe25B15(0.95 nm)/MgO(1.4 nm)]15/Pt(2 nm) (see Sup-

porting Information for more details on sample fabrication).

The material system is well-known from previous experiments

on skyrmion nucleation and supports both SOT- and laser-

assisted nucleation.

16,23,27

He+-ion nanopatterning was carried

out with a ZEISS Orion Nanofab (acceleration voltage: 30 kV).

We locally irradiated the magnetic ﬁlm with He+-ion doses

between 25 and 400 ions/nm2in predeﬁned patterns with a

nominal resolution below 10 nm. However, the ion impact

diameter broadens to approximately 20 nm (full width at half-

maximum, fwhm) due to the ion collision cascade in our more

than 80 nm thick multilayer stack (see Supporting Information

for details). In Figure 1a, we present the magnetic hysteresis,

measured with Kerr microscopy on the pristine ﬁlm and the

irradiated regions as shown in Figure 1c. In the irradiated

regions, at doses below 50 ions/nm2, the ﬁeld HNallowing for

spontaneous nucleation increases with increasing dose. Above

50 ions/nm2, this dependence diminishes and no further

increase of HNis observed for ion doses above 100 ions/nm2

(see also Figure 1b). This behavior is in line with stopping and

range of ions in matter (SRIM) simulations for the parameters

in our experiment: The additional intermixing eﬀect caused by

the He+-ions decreases with increasing ion dose and saturates

for ion doses above 100 ions/nm2(see Supporting

Information). The characteristic out-of-plane hysteresis shape

and the presence of labyrinth-like domains conﬁrm that the

magnetic multilayer retains its perpendicular anisotropy and

supports magnetic domains in the nanometer range for all He+-

ion doses used here.

First, we present skyrmion nucleation in 2D-, 1D-, and 0D-

like ion-irradiated areas with homogeneous ion doses within

each pattern. We then turn our attention to the movement of

skyrmions in more complex irradiation proﬁles. Both the SOT

and laser-induced skyrmion nucleation were investigated in

samples patterned into a racetrack geometry

4,17

(Figure 2),

irradiated with He+-ions in diﬀerent shapes and doses. We

imaged the magnetic textures nucleated via scanning trans-

mission X-ray microscopy (STXM) and Fourier-transform

holography (FTH) in transmission, utilizing X-ray magnetic

circular dichroism (XMCD) to obtain sensitivity to the

magnetization perpendicular to the sample plane (mz) (see

Supporting Information for more experimental details).

For the nucleation experiments, we ﬁrst fully saturated our

magnetic ﬁlm in an external magnetic ﬁeld. In line with the

modiﬁcation of the hysteresis behavior for diﬀerent irradiation

doses, we reduced the magnetic ﬁeld to a value for which

spontaneous nucleation of stripe domains remains suppressed

Figure 1. Magnetic hysteresis of He+-ion irradiated multilayer. (a) Normalized out-of-plane magnetization mzas a function of magnetic ﬁeld Hzfor

variable He+-ion dose. (b) Nucleation ﬁeld HNas a function of applied He+-ion dose. (c) Kerr image of irradiated areas in the magnetic ﬁlm for

μ0Hz= 26 mT after saturation. Ion doses in ions/nm2. Scalebar corresponds to 10 μm.

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but skyrmion nucleation from an external stimulus is

possible.

16,27

At the outset, we conﬁrmed that our non-

irradiated Pt/CoFeB/MgO multilayer supports skyrmion

nucleation. As a reference for the unmodiﬁed magnetic ﬁlm

in Figure 2a and b, we present images of the skyrmion patterns

created by applying a single bipolar current pulse (jx=±1.1 ×

1012 A/m2) and a single infrared (IR) laser pulse (53 mJ/cm2),

respectively (see Supporting Information for pulse parame-

ters). For the nonirradiated magnetic ﬁlm, we observed a

random distribution of skyrmions in both cases. The reference

sample in Figure 2b was not patterned into a racetrack, and

skyrmions form in the entire area illuminated by the laser

where the laser ﬂuence overcomes the nucleation thresh-

old.

23,27

In samples with an ion irradiated 2D area covering half

the racetrack (c) and with a 1D line pattern (d), we applied a

single bipolar current pulse to create skyrmions ((c) jx=±1.3

×1012 A/m2, (d) jx=±7.7 ×1011 A/m2). To suppress

nucleation in nonirradiated regions, we increased the applied

ﬁeld compared to the nucleation in nonirradiated racetracks. In

this fashion, skyrmion nucleation only occurs in irradiated

regions. Within these extended 2D and 1D ion-irradiated areas,

the skyrmions preserve their spherical shape compared to the

pristine ﬁlm and appear almost homogeneously distributed. In

Figure 2e and f, we show the results from current-induced and

optical excitations in racetracks with predeﬁned 0D dot

patterns (100 nm diameter) by He+-ion irradiation ((e) jx=

±7.7 ×1011 A/m2, (f) 45 mJ/cm2). For appropriate external

ﬁelds, we observe skyrmion nucleation exclusively and exactly

in the predeﬁned dots.

In Figure 1 and Figure 2, we demonstrated that we can

employ the applied ﬁeld to eﬀectively suppress nucleation in

nonirradiated regions. Next, we discuss the inﬂuence of the

magnetic ﬁeld on the skyrmion formation inside the predeﬁned

regions. For He+patterned dots of 100 nm diameter, we

observed reliable nucleation at the dots by both laser pulses

and SOT pulses, as shown in Figure 3a and b, respectively.

Skyrmions nucleate at all dots, and exclusively there, over a

broad range of ﬁelds applied. Similar to skyrmions in

homogeneous media, the skyrmion diameter continuously

decreases with increasing magnetic ﬁeld. The skyrmion size is,

thus, not ﬁxed to the size of the He+irradiated area. In

contrast, for the skyrmion nucleation at He+patterned dots of

300 nm diameter (Figure 3c), we observe the generation of

noncompact magnetization textures. At low magnetic ﬁeld, a

single bipolar current pulse creates a variety of almost closed or

fully closed ring-shaped domains (so-called skyrmionia

target skyrmions

) surrounding the irradiated dot. Increasing

the magnetic ﬁeld mostly leads to the formation of a single

circular skyrmion per irradiated dot. At even higher applied

ﬁelds, several magnetic skyrmions (mostly three) stabilize

inside the He+-ion irradiated areas. We infer that the most

suitable choice for controlled nucleation of individual sky-

rmions is to rely on He+-ion irradiated dots of a size

comparable to or smaller than the skyrmion diameter

supported by the material at a particular ﬁeld. Nevertheless,

larger He+dots provide an interesting route to create and

investigate topological textures beyond charge-unity sky-

rmions.

In Figure 3d, we present the skyrmion diameter on the array

of 100 nm sized He+-ion irradiated dots for diﬀerent applied

ion doses in a range from 25 to 350 ions/nm2and as a function

of the external ﬁeld. The current and optical excitation

amplitudes were chosen such that, for the lowest magnetic ﬁeld

investigated, all predeﬁned dots were ﬁlled with skyrmions

while nucleation in nonirradiated regions remains suppressed.

The excitation setting per sample was kept constant while

varying the magnetic ﬁeld. The skyrmion diameter was

determined from the full width at half-maximum (fwhm) of

the central autocorrelation peak of the STXM images as fwhm/

2to account for the skyrmions’self-convolution. Each data

point thus corresponds to the average diameter of several

skyrmions recorded in one STXM image. In Figure 3d, we

observe a decrease in skyrmion diameter with increasing ﬁeld

in the direction opposite to the net perpendicular magnet-

ization of the skyrmion from about 155 nm at 25 mT to 75 nm

at 52 mT. We attribute the large scatter to a spatial variation of

anisotropy and DMI and a variation of the location of the

irradiated regions within the narrowed racetrack aﬀecting stray

ﬁelds.

Within this variance, there is no apparent systematic

dependence of skyrmion diameter on ion dose.

For many applications, the subsequent detachment of a

skyrmion from its predeﬁned nucleation site will be important.

In the following, we will thus discuss the possibility for

detachment and guided motion after deterministic nucleation.

Aﬁrst detachment experiment is presented in Figure 4,

performed via FTH-based imaging with a ﬁeld of view (FOV)

of 1.2 μm diameter. We started by applying a single bipolar

current pulse (jx=±1.4 ×1012 A/m2) to nucleate skyrmions

on a He+-ion irradiated dot array with 60 nm dot diameter. We

ﬁrst imaged the nucleation from saturation ﬁve times to

demonstrate a reliable skyrmion formation in the irradiated

dot. While the skyrmion nucleated at the central dot is of

interest (Figure 4a), a second skyrmion is nucleated at a

neighboring dot visible in our FOV. In order to detach and

move the central skyrmion via SOT to the left, we applied a

Figure 2. Localized current-induced and optical skyrmion nucleation.

(a−b) Stochastic nucleation in the pristine material either by a

current pulse (a, current density jx=±1.1 ×1012A/m2,μ0Hz=26

mT) or by an optical laser pulse (b, peak ﬂuence 53 mJ/cm2,μ0Hz=

40 mT). In (c−f), a scheme of the structure indicating the He+

irradiated regions (purple) has been overlaid with images of the out-

of-plane magnetization obtained by STXM. (c−d) Current-induced

nucleation in an irradiated area (c, 100 ions/nm2dose, jx=±1.3 ×

1012A/m2,μ0Hz= 27 mT) and on lines (d, 200 nm width, 350 ions/

nm2dose, jx=±7.7 ×1011A/m2,μ0Hz= 40 mT). (e) Current-

induced nucleation in a dot array of dose 350 ions/nm2(jxas in (d),

μ0Hz= 47 mT). (f) Optical nucleation in a dot array of dose 350

ions/nm2(peak ﬂuence 45 mJ/cm2,μ0Hz= 51.5 mT). (a−f)

Skyrmions are created from a single current or laser pulse. Scalebars

correspond to 500 nm.

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sequence of single unipolar current pulses of reduced current

density (jx= 4.3 ×1011 A/m2) as compared to the nucleating

bipolar pulse and recorded an image after every pulse (Figure

4b−g). With the ﬁrst unipolar pulse, the central skyrmion

mostly deforms which we interpret as being part of the

depinning from the nucleation site. Once detached, the

skyrmion can move freely back and forth. After three unipolar

current pulses (Figure 4b−d), we reversed the current

direction and the skyrmion moves in the opposite direction

(Figure 4e−g). Important in the context of device applications,

the magnetic ﬁeld is kept constant during the entire sequence

including nucleation, detachment, and motion. Tracking the

position of the skyrmion in the x- and y-direction (Figure 4h),

we observe almost uniform motion steps along the current

direction with an average velocity of vx= 11(3) m/s. The

typical inclination of the motion path owing to the skyrmion

Hall eﬀect

19,20

directly proves the topological nature of the

magnetic textures createdthey are skyrmions.

Figure 3. Magnetic ﬁeld impact on skyrmion diameter and nucleation reliability. (a−c) STXM images after skyrmion nucleation in magnetic

racetracks with He+patterned dot arrays (350 ions/nm2dose). Nucleation method (symbols on the left), applied ﬁeld (above each image), and

irradiated dot diameter (right) are indicated. Scalebars correspond to 500 nm. (d) Skyrmion diameter dsk as a function of the applied ﬁeld in

magnetic magnetic racetracks with He+patterned arrays of dots with 100 nm diameter. Data points with red color shades and squared markers

correspond to laser-induced nucleation, and those with blue shades and round markers correspond to current-induced nucleation.

Figure 4. Controlled detachment from a He+-ion irradiated dot. (a−g) X-ray holography images of a skyrmion detaching from and moving over its

nucleation site due to single current pulses at constant applied ﬁeld of μ0Hz= 36 mT. (a) Current-induced nucleation at predeﬁned dots (50 ions/

nm2dose, jx=±1.4 ×1012 A/m2), (b−g) detachment and free motion (jx= 4.3 ×1011 A/m2). The orange circle marks the position of the

predeﬁned nucleation site for the skyrmion of interest. A movie of the series is available as Supporting Information. The skyrmion at the edge of the

ﬁeld of view is presumably moved out of the short racetrack (2 μm length) in the course of the motion to the left and then pinned in a region with

lower current density. Scalebar corresponds to 500 nm. (h) Distance traveled in x- and y-direction as indicated by the arrows.

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Next, we address the need to guide the SOT-driven

skyrmion motion within the racetrack in a fashion compatible

with the deterministic nucleation and subsequent detachment

demonstrated so far. To this end, we prepare a He+irradiation

pattern of dots (60 nm diameter, 50 ions/nm2), linked via 250

nm long connecting lines (25 ions/nm2), as illustrated in

Figure 5a. A single dot on the channel near the center of the

racetrack was produced with a higher dose of 100 ions/nm2to

act as a nucleation site. At this location, a single skyrmion

nucleates from a single bipolar current pulse (jx=±1.2 ×1012

A/m2) after saturation (not shown). Within the irradiated

channel, we moved the skyrmion with unipolar current pulses

(jx= 7.7 ×1011 A/m2) at an average velocity of vx= 2.5(8) m/

s. We are able to propagate the skyrmion back and forth over

several micrometers along a straight line, covering the entire

length of the magnetic racetrack. For nucleation and SOT-

driven motion, a constant magnetic ﬁeld of 39 mT was applied.

Figure 5b illustrates the guided motion with a selection of eight

(i−viii) consecutive STXM images of the motion series. The

full series including the nucleation at the central dot and all 24

motion steps is provided as a movie in the Supporting

Information. The skyrmion moves strictly along the guiding

channel; deviations in y-direction are smaller than the

skyrmion diameter of about 125 nm. Note that the skyrmion

also passes across its previous nucleation site without being

trapped when moving back and forth. Our approach also

directly supports the controlled synchronized motion of a train

of skyrmions as we present in Figure S2 of the Supporting

Information. The demonstration of this shift operation on a

whole bit pattern represents a crucial step toward proposed

memory applications of skyrmions.

Our ﬁndings prove that He+-ion irradiation represents an

excellent tool to reliably and precisely localize current-induced

and optical nucleation of isolated skyrmions in a magnetic

multilayer material. On the one hand, the magnetic

modiﬁcation is eﬀective enough to promote nucleation in

irradiated areas with full reliability while nucleation in

nonirradiated areas can be suppressed completely. On the

other hand, the modiﬁcation is gentle enough that the

magnetic racetrack remains topographically intact and the

magnetic properties in the irradiated regions are preserved in

the sense that stable nanometer-scale skyrmions are formed,

with a size and shape that we cannot distinguish from

skyrmions in nonirradiated areas of the sample. The latter

ﬁnding strongly suggests that a local pinning potential possibly

created by ion irradiation can only play a minor role for the

localization of the nucleation.

Instead, recent work reported that ion irradiation can

signiﬁcantly enhance the eﬃciency of SOT-driven magnet-

ization switching in heterostructures containing a single

ferromagnetic layer.

43,44,50

The reasons were found in a

reduced PMA of the irradiated structure

43,50

and an increased

spin-Hall angle from the Pt probably due to an increased defect

density in this layer.

In addition, intermixing at the Co−Pt

interfaces can lead to a higher spin transparency

and again to

an increased spin-Hall angle.

We expect that similar eﬀects

are key also for the easier SOT nucleation of skyrmions in He+

irradiated areas of our material even though it is much thicker

compared to material systems used in refs 43,44, and 50. Our

SRIM simulations conﬁrm a mostly homogeneous ion

penetration along the depth direction of our multilayer (see

Supporting Information). Consequently, the more eﬃcient

SOT switching allows us to act against a signiﬁcantly higher

applied ﬁeld compared to the SOT in nonirradiated regions. In

the case of laser-induced skyrmion nucleation, we also

identiﬁed the reduced PMA in the He+irradiated areas as

main origin for the preferred skyrmion nucleation. We required

a reduced laser ﬂuence to nucleate skyrmions in the ion-

irradiated dots. We modeled this dependence in atomistic

simulations of the high-temperature ﬂuctuation phase which

leads to the formation of skyrmions

in model systems with

diﬀerent values of the PMA. For the low-PMA material, the

simulations predict a lower excitation threshold for skyrmion

nucleation (see Supporting Information). The laser ﬂuence,

thus, provides a handle to trigger the skyrmion nucleation

exclusively in the predeﬁned regions.

In the second part of our work, we demonstrated the

localized nucleation of a skyrmion combined with its

detachment from the site and subsequent guided motion in a

narrow ion-irradiated channel. Such a controlled detachment

from the nucleation site and the unimpeded skyrmion motion

across it are only possible if the pinning potential formed by

the ion irradiation is suﬃciently shallow and tunablean

important advantage of light ions over the heavier ions such as

Ga+that are routinely used for topographic modiﬁcations on

the same length scale. Our method to create skyrmion

Figure 5. Guided skyrmion motion. (a) Schematic irradiation layout

showing the nucleation dot (dark purple), the guiding channel (light

purple) with intermediate medium-dose dots. (b) STXM images of a

motion series at constant ﬁeld of μ0Hz= 39 mT. After nucleation with

a single bipolar pulse at the nucleation dot (jx=±1.2 ×1012 A/m2),

the skyrmion is imaged along the guiding chain in 24 steps. Eight

representative consecutive images are shown (i−viii). In order to

increase the distance moved between two images, ten consecutive

unipolar current pulses (jx= 7.7 ×1011 A/m2) were applied before

recording the next image. The skyrmion moves back and forth along

the entire ﬁeld of view with no systematic deviation from the

irradiated channel, i.e., with suppressed skyrmion Hall eﬀect. The

orange-shaded circles mark previous positions. Scalebars correspond

to 500 nm.

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