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Néel-type skyrmions and their current-induced motion in van der Waals ferromagnet-based heterostructures

TL;DR: In this article, the authors reported the experimental observation of Neel-type chiral magnetic skyrmions and their lattice formation in a vdW ferromagnet Fe3GeTe2 (FGT).
Abstract: Since the discovery of ferromagnetic two-dimensional (2D) van der Waals (vdW) crystals, significant interest on such 2D magnets has emerged, inspired by their appealing properties and integration with other 2D family for unique heterostructures In known 2D magnets, spin-orbit coupling (SOC) stabilizes perpendicular magnetic anisotropy (PMA) Such a strong SOC could also lift the chiral degeneracy, leading to the formation of topological magnetic textures such as skyrmions through the Dzyaloshinskii-Moriya interaction (DMI) Here, we report the experimental observation of Neel-type chiral magnetic skyrmions and their lattice (SkX) formation in a vdW ferromagnet Fe3GeTe2 (FGT) We demonstrate the ability to drive individual skyrmion by short current pulses along a vdW heterostructure, FGT/h-BN, as highly required for any skyrmion-based spintronic device Using first principle calculations supported by experiments, we unveil the origin of DMI being the interfaces with oxides, which then allows us to engineer vdW heterostructures for desired chiral states Our finding opens the door to topological spin textures in the 2D vdW magnet and their potential device application

Summary (3 min read)

Introduction

  • Since the discovery of ferromagnetic two-dimensional (2D) van der Waals (vdW) crystals, significant interest on such 2D magnets has emerged, inspired by their appealing physical properties and integration with other 2D family for unique heterostructures.
  • Here, the authors report the experimental observation of Néel-type chiral magnetic skyrmions and their lattice (SkX) formation in a vdW ferromagnet Fe3GeTe2 (FGT).
  • Using first principle calculations supported by experiments, the authors unveil the origin of DMI being the interfaces with oxides, which then allows us to engineer vdW heterostructures for desired chiral states.
  • The authors then examine the stability of SkX against thermal fluctuation and magnetic fields, which eventually constitutes an experimental phase diagram of the SkX state.

A. Crystal structure and domain configuration

  • Figure 1(a) schematically shows the crystal structures of monolayered FGT viewed from xy and yz planes and bilayered FGT exhibiting vdW bonding between monolayers.
  • Each FGT monolayer consists of a Fe3Ge covalently bonded slab and two Te layers placed above and underneath the Fe3Ge, and each layer is separated by a 2.95-Å vdW gap in multilayered stack [28].
  • It is noteworthy that Rxy measurements yield two distinct slopes (sharp and slanted slopes) in the temperature range 100 K T 180 K, and the slanted area becomes more prominent as temperature increases.
  • Using this slanted area, the authors can drive the magnetization into the multidomain state at low temperatures and near zero magnetic fields, as shown in Fig. 1(d).
  • The magnetization state of the FGT device was imaged by probing the intensity of transmitted circularly polarized x-ray at the Fe edge (L3 absorption edge), where x-ray magnetic circular dichroism (XMCD) provides contrasts corresponding to the out-of-plane magnetization.

B. Dynamic generation and stabilization of SkX

  • Having established that multidomain states can be readily stabilized and observed in FGT, the authors then examined the currentinduced generation of magnetic skyrmions, as summarized in Fig.
  • Thus, the spontaneous transition from the labyrinth random domain state to the skyrmionic state is triggered by both the external magnetic fields and strong current pulses.
  • The authors performed the same procedure at slightly lower temperature, 100 K, and the consistent transformation into multiple skyrmions is observed and the generated skyrmions remain stable at zero magnetic field, Bz = 0 mT [highlighted in a blue-boxed area in Fig. 3(a)].
  • After stabilizing the SkX state, the authors then plotted the experimental phase diagram of magnetic configurations in FGT, based on the real-space STXM measurements as summarized in Fig. 3(c).
  • The authors observed three magnetic configuration phases: (i) SkX, (ii) the coexistence of SkX and multidomains, and (iii) saturated ferromagnetic states, where the representative STXM images of each state are included in the right panel of Fig. 3(c).

C. Lorentz transmission electron microscopy (LTEM) study of SkX

  • To deeply understand magnetic configurations observed by STXM measurements, the authors first performed the LTEM measurement as summarized in Fig. 4 (see Methods for details [30], and Refs. [39–41] therein).
  • Note here that Fresnel-LTEM is useful to detect the in-plane components of Bloch-type spin spirals at defocused modes, whereas it cannot directly observe Néel-type magnetic configurations with zone-axis beam 104410-4 irradiation, due to the cancellation of magnetic inductions between electrons and symmetric in-plane magnetic moments with opposite directions projected by Néel-type spin textures [39,42,43].
  • Figure 4(a) first shows in-focus and defocused LTEM images of the FGT sample tilted about −20◦ along the x axis at zero field and 160 K, where dark/bright contrasts are only visible in defocused images.
  • To generate SkX, the authors then performed the field cooling (FC) of FGT with an oblique magnetic field of B = −40 mT (the oblique angle is 20◦ to the zone axis).

D. Current-driven motion of isolated skyrmions

  • To further highlight the potential of FGT-based 2D vdW heterostructures for skyrmion devices, the authors next demonstrate the current-driven motion of skyrmions in this material, as summarized in Fig.
  • Figure 5(a) shows a schematic image of the FGT track and electric contacts fabricated on the Si3N4 membrane for STXM measurements.
  • The current was applied 104410-6 along the +x direction, opposite to the electron flow along the −x direction as schematically indicated in each image.
  • It is first noteworthy that skyrmions move upon the application of current pulses, and the propagation direction is along the electron-flow direction (against current flow), where this directionality indicates that the skyrmion is driven by spin-transfer torques (STTs) arising within the FGT.

E. First principle calculation on Dzyaloshinskii-Moriya interaction (DMI) from O-FGT

  • With these experimental demonstrations of chiral skyrmions, their SkX state, and their current-driven motion in the FGT-based heterostructures, let us now discuss the physical origins of DMI in vdW FGT crystals.
  • The authors first examined the possible DMI sources from the FGT crystal symmetry.
  • For FGT crystal monolayer the calculated DMI, arising at both Fe/Te interfaces is of almost equal magnitude with opposite sign yielding negligible DMI as expected from the aforementioned crystal symmetry analysis.
  • In particular, the concentration of Te atoms at both interfaces rapidly decreases and vanishes upon oxidation, while Fe and Ge concentrations only fluctuate and recover their original values near the largest oxidation areas (oxygen peaks).
  • Nevertheless, the authors believe further systematic experimental studies probing the dependence of spin textures on the total FGT thickness, and/or the internal magnetization profile of skyrmions from top to bottom layers in FGT considering the role of van der Waals interactions could shed light into more precise tailoring of DMI and resulting magnetic textures in FGT crystal and heterostructures.

III. CONCLUSIONS

  • In summary, the authors observed Néel-type chiral magnetic skyrmions and their lattice phase stabilization in a vdW ferromagnet FGT using high resolution magnetic microscopy.
  • The authors examined the stability of SkX in FGT over a wide range of temperatures and magnetic fields, including its zero-field manifestation.
  • The authors also demonstrated current-driven motion of individual skyrmions in FGT, highlighting its potential for device applications.
  • The authors performed symmetry analysis and first principles calculations to unveil the origins of the emergent Neél-type spin textures, namely DMI at the oxidized interfaces of FGT, which also demonstrates the controllability of chiral states in vdW heterostructures by process and/or interfacial material engineering.

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PHYSICAL REVIEW B 103, 104410 (2021)
Editors’ Suggestion
Néel-type skyrmions and their current-induced motion in van der Waals
ferromagnet-based heterostructures
Tae-Eon Park ,
1
Licong Peng,
2
Jinghua Liang,
3
Ali Hallal,
4
Fehmi Sami Yasin,
2
Xichao Zhang,
5
Kyung Mee Song,
1
Sung Jong Kim,
1,6
Kwangsu Kim,
1,7
Markus Weigand,
8
Gisela Schütz,
9
Simone Finizio,
10
Jörg Raabe,
10
Karin Garcia,
11
Jing Xia,
5
Yan Zhou,
5
Motohiko Ezawa,
12
Xiaoxi Liu,
13
Joonyeon Chang,
1,14
Hyun Cheol Koo,
1,6
Young Duck Kim,
15
Mairbek Chshiev,
4
Albert Fert,
11,16
Hongxin Yang,
3,*
Xiuzhen Yu,
2,
and Seonghoon Woo
1,17,
1
Center for Spintronics, Korea Institute of Science and Technology, Seoul 02792, Korea
2
RIKEN Center for Emergent Matter Science, Wako 351-0198, Japan
3
Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China Center of Materials
Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
4
Université Grenoble Alpes, CEA, CNRS, Spintec, 38000 Grenoble, France
5
School of Science and Engineering, The Chinese University of Hong Kong, Shenzhen, Guangdong 518172, China
6
KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 02841, Korea
7
Department of Physics, University of Ulsan, Ulsan 44610, Korea
8
Helmholtz Center Berlin, Albert Einstein Straβe 15, 12489 Berlin, Germany
9
Max-Planck-Institut für Intelligente Systeme, 70569 Stuttgart, Germany
10
Swiss Light Source, Paul Scherrer Institut, 5232 Villigen, Switzerland
11
DIPC and University of the Basque Country, 2018 San Sebastian, Spain
12
Department of Applied Physics, University of Tokyo, Hongo 7-3-1, Tokyo 113-8656, Japan
13
Department of Electrical and Computer Engineering, Shinshu University, Wakasato 4-17-1, Nagano 380-8553, Japan
14
Department of Materials Science & Engineering, Yonsei University, Seoul 03722, Korea
15
Department of Physics, Kyung Hee University, Seoul 02447, Korea
16
Unité Mixte de Physique, CNRS, Thales, Université Paris-Sud, Université Paris-Saclay, Palaiseau 91767, France
17
IBM T.J. Watson Research Center, 1101Kitchawan Road, Yorktown Heights, New York 10598, USA
(Received 13 November 2019; revised 5 February 2021; accepted 8 February 2021; published 5 March 2021)
Since the discovery of ferromagnetic two-dimensional (2D) van der Waals (vdW) crystals, significant interest
on such 2D magnets has emerged, inspired by their appealing physical properties and integration with other 2D
family for unique heterostructures. In known 2D magnets, spin-orbit coupling (SOC) stabilizes perpendicular
magnetic anisotropy down to one or a few monolayers. Such a strong SOC could also lift the chiral degeneracy,
leading to the formation of topological magnetic textures such as skyrmions through the Dzyaloshinskii-Moriya
interaction (DMI). Here, we report the experimental observation of Néel-type chiral magnetic skyrmions and
their lattice (SkX) formation in a vdW ferromagnet Fe
3
GeTe
2
(FGT). We demonstrate the ability to drive an
individual skyrmion by short current pulses along a vdW heterostructure, FGT/h-BN, as highly required for
any skyrmion-based spintronic device. Using first principle calculations supported by experiments, we unveil
the origin of DMI being the interfaces with oxides, which then allows us to engineer vdW heterostructures for
desired chiral states. Our finding opens the door to topological spin textures in the 2D vdW magnet and their
potential device application.
DOI: 10.1103/PhysRevB.103.104410
I. INTRODUCTION
Two dimensional (2D) van der Waals (vdW) crystals
have been significantly highlighted as a unique material
platform, mainly due to their fascinating physical proper-
ties, low-cost fabrication, and high integrability to produce
appealing artificial heterostructures [1,2]. The recent addi-
tion of magnetic vdW crystals, where intrinsic long-range
*
Corresponding author: hongxin.yang@nimte.ac.cn
yu_x@riken.jp
shwoo@ibm.com
magnetic orders were observed in Cr
2
Ge
2
Te
6
and CrI
3
,of-
fered an additional building block to this platform, opening
another door to vdW magnet-based spintronics [310]. There-
fore, significant following interests have emerged and rapidly
demonstrated a few key elements for applications, includ-
ing the magnetoresistance (MR) effects [5,6], gate-tunable
room-temperature magnetism [7], and manipulation of mag-
netization switching by electrical current [11]. However,
there are still some challenges for the development of vdW
ferromagnet-based spintronics devices, such as rare candi-
dates of atomic-thickness vdW ferromagnets, poor stability
under ambient conditions, and low Curie temperature (usually
below room temperature).
2469-9950/2021/103(10)/104410(11) 104410-1 ©2021 American Physical Society

TAE-EON PARK et al. PHYSICAL REVIEW B 103, 104410 (2021)
Whereas the long-range magnetic order is often suppressed
in vdW crystals due to thermal fluctuations given by the
Mermin-Wagner theorem [12], strong spin-orbit coupling
(SOC) in vdW magnets plays an essential role in stabiliz-
ing the perpendicular magnetic anisotropy (PMA) and thus
overcomes the thermal fluctuations down to a monolayer limit
[4,7]. In a material with such large SOC and broken inver-
sion symmetry, the antisymmetric exchange interaction, the
so called Dzyaloshinskii-Moriya interaction (DMI) [13,14],
can emerge and be strong enough to stabilize topological
magnetic configurations including skyrmions [15,16]. Recent
theoretical works have also discussed the emergence of DMI
in vdW magnets with various possible origins, e.g., crystal
symmetry or sample boundary, as well as resulting skyrmion
stabilization [1719]. Once established, by taking advantages
of other 2D crystals that are stackable and offer unique
electrical properties [1,2], this material platform could pro-
vide a different route towards skyrmion-based devices that
have been challenging with conventional metallic ferromag-
nets [20,21]. Recently, skyrmionlike spin textures have been
observed experimentally in exfoliated vdW ferromagnet ma-
terials (e.g., FGT and Cr
2
Ge
2
Te
6
) and their heterostructures
with the magnetic fields [2226]. However, both Bloch-type
and Néel-type skyrmions of the previous results have still been
a subject of controversy. In addition, experimental demon-
stration of their field-driven and current-driven dynamics in
vdW magnets and heterostructures has remained elusive so
far.
Here, we experimentally present the observation of Néel-
type skyrmions and their ordered crystal structures in a
vdW ferromagnetic Fe
3
GeTe
2
-based heterostructures (FGT
hereafter). Moreover, we demonstrate the ability to drive
skyrmions using nanoseconds current-pulses in 2D het-
erostructure based on FGT, i.e., FGT/h-BN. Among various
types of vdW magnets, FGT exhibits relatively high ferro-
magnetic transition temperature (T
c
), large PMA, and metallic
nature that enables efficient charge/spin transport suitable
for spintronic applications [7,27]. In this study, we utilize
high spatial resolution magnetic imaging techniques, scanning
transmission x-ray microscopy (STXM), Lorentz transmis-
sion electron microscopy (LTEM), and differential phase
contrast microscopy (DPCM) to directly observe magnetic
structures in the FGT-based heterostructures. We first show
the dynamic generation and stabilization of the skyrmion
crystal (SkX, also referred to as skyrmion lattice) state in the
FGT flake, where strong pulse-induced thermal fluctuations
transform magnetic domains into SkX. We then examine the
stability of SkX against thermal fluctuation and magnetic
fields, which eventually constitutes an experimental phase
diagram of the SkX state. We also present the static generation
of magnetic skyrmions and SkX using a tilting magnetic field,
where we simultaneously unveil the Néel-type chiral nature
of skyrmions stabilized in the SkX state by taking advantage
of in-plane magnetization sensitivity in LTEM measurements.
Moreover, we present the current-driven motion of skyrmions,
where we drive isolated individual skyrmions by short current
pulses along a FGT racetrack at speeds approaching a meter
per second. Finally, using first principle calculations corrob-
orated by additional DPCM measurements, we demonstrate
the presence of significant interfacial DMI at FGT interfaces
with oxidized layers, which then allows us to “engineer” the
chiral states in the vdW ferromagnetic heterostructure be-
tween Bloch and Néel types, by using selective fabrication
processes.
II. RESULTS
A. Crystal structure and domain configuration
Figure 1(a) schematically shows the crystal structures of
monolayered FGT viewed from xy and yz planes and bilay-
ered FGT exhibiting vdW bonding between monolayers. Each
FGT monolayer consists of a Fe
3
Ge covalently bonded slab
and two Te layers placed above and underneath the Fe
3
Ge, and
each layer is separated by a 2.95-Å vdW gap in multilayered
stack [28]. Within a Fe
3
Ge slab, two inequivalent Fe sites
exist, Fe
II
(the valence states of Fe
2+
) and Fe
III
(the valence
states of Fe
3+
), as indicated in Fig. 1(a). Overall, the reduced
bulk crystal symmetry in FGT is known to provide a mag-
netocrystalline anisotropy induced by strong SOC [29]. For
both electrical and transmission microscopy measurements on
the same sample, we fabricated Hall-bar-type FGT devices
on a 100-nm-thick Si
3
N
4
membrane using soft mechanical
exfoliation technique together with e-beam lithography and
liftoff (see Methods and Supplemental Material Fig. S1 [30]
for details). Figure 1(b) shows the cross-sectional view of
high-resolution transmission electron microscopy (HRTEM)
images of the device, where layered high-crystalline quality
FGT is observed [Fig. 1(b), inset]. Note that the FGT layer is
sandwiched by two oxidized FGT (O-FGT) due to the sample
fabrication under ambient condition, and 5-nm-thick Pt was
deposited ex situ as a capping material to prevent further
oxidation (see Methods and Supplemental Material Fig. S2
[30] for details). The magnetic hysteresis behaviors of the
FGT device were measured using the Hall resistance (R
xy
)
measurement, where external magnetic field was applied to
the out-of-plane direction at controlled temperatures ranging
100–220 K [Fig. 1(c)]. While the R
xy
consists of a normal
Hall resistance (R
N
) and an anomalous Hall resistance (R
AH
),
FGT films exhibit a large value of R
AH
in R
xy
, which roughly
scales with the magnetization (M
z
)[7]. Therefore, the square
hysteresis loops at 100 K in Fig. 1(c) corresponds to an out-of-
plane magnetic anisotropy, which persists up to 200 K (T
c
200 K). It is noteworthy that R
xy
measurements yield two dis-
tinct slopes (sharp and slanted slopes) in the temperature range
100 K T 180 K, and the slanted area becomes more
prominent as temperature increases. This area indicates the
presence of a multidomain state, where the initially nucleated
domains at sharp but incomplete switching propagate across
the film, originating from the reduced magnetic anisotropy of
FGT at higher temperature due to increased thermal fluctua-
tions. Using this slanted area, we can drive the magnetization
into the multidomain state at low temperatures and near zero
magnetic fields, as shown in Fig. 1(d). Red and blue curves
indicate down-to-up and up-to-down switching at 120 K,
respectively. For example, at a down-(up-)magnetization sat-
uration state, increasing (decreasing) magnetic field just
enough to generate multidomain states and then subsequently
reversing the field drives the overall magnetization into mul-
tidomain states near zero magnetic field. This technique was
employed to generate multidomains during STXM, which
104410-2

NÉEL-TYPE SKYRMIONS AND THEIR PHYSICAL REVIEW B 103, 104410 (2021)
FIG. 1. Crystal structure and the Hall measurement of a van der Waals Fe
3
GeTe
2
. (a) An atomic structure of a Fe
3
GeTe
2
(FGT) monolayer
(left) and the structure of a FGT bilayer with an interlayer van der Waals (vdW) gap (right). Fe
III
and Fe
II
represent the two inequivalent Fe
sites in the +3and+2 valence states, respectively. (b) Cross-sectional high-resolution transmission electron microscopy (HRTEM) image of
the FGT with a Pt capping Hall-bar device fabricated on 100-nm-thick Si
3
N
4
membrane substrate (right, scale bar is 20 nm). Oxidized FGT is
indicated as O-FGT. The enlarged panel shows the high angle annular dark field (HAADF) image in scanning TEM mode of the red-dashed
highlighted area in (b) (left, scale bar is 1 nm). (c) Temperature dependent Hall resistance (R
xy
) as a function of applied out-of-plane magnetic
field B
z
. (d) The measured normalized Hall resistance (R
xy
/R
xy,sat
) as a function of magnetic field, B
z
, at 120 K, where red and blue dashed
rectangular boxes represent the areas of multidomains in the hysteresis loops (left). Bottom two hysteresis loops exhibit the magnetic field
sequences used for the generation of multidomains near zero fields from (i) B
z
saturation and (ii) +B
z
saturation, respectively. Black dashes
lines are included to show the original full saturation hysteresis loops.
require large moments and multidomain states for high con-
trast observations.
Figure 2(a) shows the schematic of STXM experimental
setup, where the temperature of cooling stage was con-
trolled in the range 100 K T 300 K using liquid nitrogen
(LN
2
) and heat exchanger. The scanning electron microscopy
(SEM) image of the measured FGT device with Hall-cross
geometry and the electrical circuit diagram is also included
in Fig. 2(a) (see Methods for details). The magnetization
state of the FGT device was imaged by probing the in-
tensity of transmitted circularly polarized x-ray at the Fe
edge (L
3
absorption edge), where x-ray magnetic circular
dichroism (XMCD) provides contrasts corresponding to the
out-of-plane magnetization. Figure 2(b) shows the magnetic
domain configurations in the FGT device as a function of
out-of-plane magnetic field B
z
at 120 K, which confirms
strong magnetic contrast observable in FGT from STXM
measurements. Note that the alternative field-sweep procedure
(B
z
=+200 mT →−60 mT 0 mT) described in Fig. 1(d)
was used to generate the initial magnetic configuration at
zero field, B
z
= 0 mT. The dark and bright contrasts in
STXM images correspond to downward (M
z
) and upward
(+M
z
) out-of-plane magnetization direction of Fe atoms in
FGT, respectively. With increasing out-of-plane field B
z
>
0, the up domains expand while the down domains shrink
into narrow domains, vanishing at the saturation field of
B
z
=+80 mT.
B. Dynamic generation and stabilization of SkX
Having established that multidomain states can be readily
stabilized and observed in FGT, we then examined the current-
induced generation of magnetic skyrmions, as summarized
in Fig. 3. In our previous study using conventional chiral
ferromagnetic multilayers, Pt/CoFeB/MgO, we demonstrated
that the application of bipolar pulses could transform labyrinth
domains with chiral domain walls into multiple skyrmions
[31], and the recent study by Lemesh et al. [32] unveiled
the mechanism to be current-induced thermal transformation
into skyrmions, because the energy barrier towards the global
skyrmionic ground state decreases with increasing tempera-
ture. To utilize the same technique on the FGT device, we
applied a burst of 100 bipolar pulses, where the pulse fre-
quency of 1 MHz, the peak-to-peak voltage of V
pp
= 2.96 V,
and the pulse width of 10 ns were used at B
z
=−40 mT and
120 K. As shown in Fig. 3(a), it is obvious that the bipolar
pulse injection transformed the labyrinth random domain state
into a multiple circular domain state, where these circular
domains turn out to be Néel-type chiral magnetic skyrmions
in Fig. 4. It is noted that the labyrinth random domain state
104410-3

TAE-EON PARK et al. PHYSICAL REVIEW B 103, 104410 (2021)
FIG. 2. Microscopy imaging of domain structures using scanning transmission x-ray microscopy. (a) Schematic of scanning transmission
x-ray microscopy (STXM) experimental setup used for magnetic domain imaging and simultaneous electrical pulse injections. The inset shows
scanning electron microscopy (SEM) image of the measured device with Hall bar geometry. Scale bar, 4 μm. Two electrode pads on horizontal
x axis were used for electrical pulse applications, and oscilloscopes before and after device were used to verify the pulse profiles before and
after device, respectively. (b) Exemplary STXM images acquired as a function of increasing magnetic field from B
z
= 0mTtoB
z
= 80 mT at
120 K. Dark and bright contrast correspond to magnetization of Fe atoms oriented down (M
z
) and up (+M
z
), respectively. Scale bar, 1 μm.
is maintained when the strong current pulses are applied at
zero magnetic field. Thus, the spontaneous transition from the
labyrinth random domain state to the skyrmionic state is trig-
gered by both the external magnetic fields and strong current
pulses. We performed the same procedure at slightly lower
temperature, 100 K, and the consistent transformation into
multiple skyrmions is observed and the generated skyrmions
remain stable at zero magnetic field, B
z
= 0 mT [highlighted
in a blue-boxed area in Fig. 3(a)]. As was observed in ferro-
magnetic chiral multilayers, the thermal excitation induced by
the bipolar pulses may have opened a path towards the global
skyrmionics state [31,32]. We examined and observed the
consistent domain transformation in another sample capped
by graphite, as shown in Supplemental Material Fig. S3 [30].
This demonstration with graphite capping is significant, as it
excludes two possible contributions from Pt: (i) the spin-orbit
torques (SOTs) by transmitted spin current caused by the
spin-Hall effect (SHE) in Pt [33] and (ii) the DMI contribution
from the Pt/O-FGT interface. Additional Hall measurements
presented in Supplemental Material Fig. S4 [30] also con-
firm the negligible influence from capping materials on the
magnetic properties of the studied FGT structure. Further
analysis reveals that the average size of zero-field skyrmion
is 123 nm at 120 K, and the size decreases down to 80 nm
with increased density at 160 K (see Supplemental Material
Fig. S5 for details [30], and Refs. [3436] therein).
At such a disordered multiskyrmion state at 100 K, we
applied alternative positive and negative magnetic fields with
increasing magnitude up to B
z
80 mT with the step of
B
z
10 mT, as the application of static fields could anni-
hilate pinned weak skyrmions and rearrange them driven by
interskyrmion repulsive forces, leading to the stabilization of
the ordered skyrmion state [37,38]. Figure 3(b) shows the
zero-field magnetic configuration after the field sweep, and
surprisingly, the initial disordered magnetic skyrmions trans-
formed into ordered hexagonal SkX. The inset of Fig. 3(b)
presents the enlarged STXM images at a magnetic field,
B
z
=−80 mT, where the ordered SkX state is more clearly
observable (a few SkXs are highlighted with blue colors and
white lines for guide). As shown in the inset of the enlarged
image in Fig. 3(b), the presence of six spots in the fast-Fourier
transform (FFT) image also supports the presence of the or-
dered SkX state in our sample. The symmetry of SkX also
agrees with the symmetry observed in noncentrosymmetric
B20-type chiral magnets [15,16]. After stabilizing the SkX
state, we then plotted the experimental phase diagram of mag-
netic configurations in FGT, based on the real-space STXM
measurements as summarized in Fig. 3(c). We observed three
magnetic configuration phases: (i) SkX, (ii) the coexistence
of SkX and multidomains, and (iii) saturated ferromagnetic
states, where the representative STXM images of each state
are included in the right panel of Fig. 3(c). It should be noted
that, once generated, SkX in FGT can be stabilized at a wide
range of magnetic field and temperature. Moreover, the SkX
state remains stable at zero magnetic field. Together with the
recent discovery of gate-tunable room-temperature magneti-
zation in the same material [7], it might also be possible to
harness and manipulate magnetic skyrmions and their lattice
at room temperature and zero magnetic fields, which may
constitute a major step towards room-temperature skyrmion
applications based on vdW magnets.
C. Lorentz transmission electron microscopy (LTEM)
study of SkX
To deeply understand magnetic configurations observed by
STXM measurements, we first performed the LTEM measure-
ment as summarized in Fig. 4 (see Methods for details [30],
and Refs. [3941] therein). Note here that Fresnel-LTEM is
useful to detect the in-plane components of Bloch-type spin
spirals at defocused modes, whereas it cannot directly ob-
serve Néel-type magnetic configurations with zone-axis beam
104410-4

NÉEL-TYPE SKYRMIONS AND THEIR PHYSICAL REVIEW B 103, 104410 (2021)
FIG. 3. Generation and stabilization of magnetic skyrmion lattice phase. (a) The two images on the left side were acquired at B
z
=−40 mT
at 120 and 100 K after the initial saturation at B
z
=+200 mT, respectively, where initial labyrinth domain states were stabilized. The right two
images at B
z
=−40 mT were acquired after the application of bipolar pulse bursts at 120 and 100 K, respectively, and the other two images
at B
z
= 0 mT were acquired after removing magnetic fields. Scale bar, 1 μm. (b) Representative STXM image of skyrmion crystal (SkX)
stabilized over the whole FGT device at B
z
= 0mT and T = 100 K. Scale bar, 2 μm. For clarity, the enlarged image of SkX was obtained
at B
z
=−80 mT. Scale bar, 1 μm. The hexagonal white lines are drawn to guide eye for the ordered SkX, and inset shows the fast-Fourier
transform (FFT) image. The right schematic represents the exemplary magnetic configuration of SkX found in chiral magnets for comparison.
Note that skyrmion polarity in (b) (M
z
core) is different from (a) (+M
z
core), as the initial field-sweep procedure of reversed field direction
was used before the pulse application: B
z
=−200 mT →+40 mT. (c) Experimental phase diagram of magnetic configurations as a function of
temperature and magnetic field. Experimentally measured positions are marked with open circles, and star symbols correspond to exemplary
images shown on the right side of the phase diagram. Three representative images show each magnetic configuration state: SkX, SkX +
multidomains, and saturated ferromagnet (FM). Scale bar, 1 μm. Black dashed lines in phase diagram are guide to the eyes to indicate the
phase boundaries.
irradiation, due to the cancellation of magnetic inductions
between electrons and symmetric in-plane magnetic moments
with opposite directions projected by Néel-type spin textures
[39,42,43]. However, when samples are tilted away from the
zone axis, the projected configurations of up-down magnetic
domains should contribute to the LTEM contrasts at defocused
modes, therefore, Néel-type magnetic configurations can be
observed [38,39,42,43].
Figure 4(a) first shows in-focus and defocused LTEM im-
ages of the FGT sample tilted about 20
along the x axis
at zero field and 160 K, where dark/bright contrasts are only
visible in defocused images. Moreover, as shown in the red-
boxed areas in the left and right images in Fig. 4(a), under-
and over-focused LTEM images exhibit the labyrinth domain
structures with reserved domain wall contrasts, indicating a
multidomain state in the FGT flake at zero field [4244].
To generate SkX, we then performed the field cooling (FC)
of FGT with an oblique magnetic field of B =−40 mT (the
oblique angle is 20
to the zone axis). Figure 4(b) shows
LTEM images observed at 160 K. Noticeably, the FC gen-
erated quasistatic (metastable) Néel-type chiral SkX state
in the FGT crystal, which is in good agreement with the
104410-5

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References
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Journal ArticleDOI
TL;DR: A simple derivation of a simple GGA is presented, in which all parameters (other than those in LSD) are fundamental constants, and only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked.
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146,533 citations

Journal ArticleDOI
TL;DR: An efficient scheme for calculating the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set is presented and the application of Pulay's DIIS method to the iterative diagonalization of large matrices will be discussed.
Abstract: We present an efficient scheme for calculating the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrices will be discussed. Our approach is stable, reliable, and minimizes the number of order ${\mathit{N}}_{\mathrm{atoms}}^{3}$ operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special ``metric'' and a special ``preconditioning'' optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent and self-consistent calculations. It will be shown that the number of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order ${\mathit{N}}_{\mathrm{atoms}}^{2}$ scaling is found for systems containing up to 1000 electrons. If we take into account that the number of k points can be decreased linearly with the system size, the overall scaling can approach ${\mathit{N}}_{\mathrm{atoms}}$. We have implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable. \textcopyright{} 1996 The American Physical Society.

81,985 citations

Journal ArticleDOI
TL;DR: A detailed description and comparison of algorithms for performing ab-initio quantum-mechanical calculations using pseudopotentials and a plane-wave basis set is presented in this article. But this is not a comparison of our algorithm with the one presented in this paper.

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TL;DR: In this paper, the authors present an ab initio quantum-mechanical molecular-dynamics calculations based on the calculation of the electronic ground state and of the Hellmann-Feynman forces in the local density approximation.
Abstract: We present ab initio quantum-mechanical molecular-dynamics calculations based on the calculation of the electronic ground state and of the Hellmann-Feynman forces in the local-density approximation at each molecular-dynamics step. This is possible using conjugate-gradient techniques for energy minimization, and predicting the wave functions for new ionic positions using subspace alignment. This approach avoids the instabilities inherent in quantum-mechanical molecular-dynamics calculations for metals based on the use of a fictitious Newtonian dynamics for the electronic degrees of freedom. This method gives perfect control of the adiabaticity and allows us to perform simulations over several picoseconds.

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Abstract: We present ab initio quantum-mechanical molecular-dynamics simulations of the liquid-metal--amorphous-semiconductor transition in Ge. Our simulations are based on (a) finite-temperature density-functional theory of the one-electron states, (b) exact energy minimization and hence calculation of the exact Hellmann-Feynman forces after each molecular-dynamics step using preconditioned conjugate-gradient techniques, (c) accurate nonlocal pseudopotentials, and (d) Nos\'e dynamics for generating a canonical ensemble. This method gives perfect control of the adiabaticity of the electron-ion ensemble and allows us to perform simulations over more than 30 ps. The computer-generated ensemble describes the structural, dynamic, and electronic properties of liquid and amorphous Ge in very good agreement with experiment. The simulation allows us to study in detail the changes in the structure-property relationship through the metal-semiconductor transition. We report a detailed analysis of the local structural properties and their changes induced by an annealing process. The geometrical, bonding, and spectral properties of defects in the disordered tetrahedral network are investigated and compared with experiment.

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Related Papers (5)
Frequently Asked Questions (9)
Q1. What have the authors contributed in "Néel-type skyrmions and their current-induced motion in van der waals ferromagnet-based heterostructures" ?

Park et al. this paper observed Néel-type chiral magnetic skyrmions and their lattice phase stabilization in vdW ferromagnet FGT using high resolution magnetic microscopy. 

The possibility to achieve and electrically manipulate magnetic skyrmions in vdW magnets marks a significant advance in vdW magnet-based spintronics. Along with the large potential of skyrmions for future spintronic devices to store, process, and transmit data with extremely low power cost, this work will pave a route towards vdW magnet-based topological magnetism and skyrmion electronics. Their results can support a further understanding of the fielddriven and current-driven dynamics of skyrmions in 2D vdW materials and provide guidelines for the design of magnetic devices based on 2D materials. 

due to the reflection symmetry of the system, the DMI contributions induced at the top and bottom FeIII sublayers are cancelledwith each other and the net DMI in the whole FGT structure vanishes. 

For the O-substitution case, the authors find that the single crystal monolayer DMI is nonmonotonic as a function of oxygen concentration, being weakly anticlockwise (respectively strongly clockwise) for low (respectively high) concentrations104410-7[Figs. 

The current-driven motion of skyrmion shows the potential of using skyrmions in FGT for functional device applications, such as the racetrack-type memory [20], where skyrmions act as moveable information carriers. 

Regarding the DMI in the bulk O-substituted FGT structures, very importantly, the authors found additional DMI contributions arising from the proximity of the pure FGT cell with the oxidized layer O-FGT. 

As shown in Fig. 5(c), the average skyrmion velocity was measured to be ∼1 m/s at a current density Je = 1.4 × 1011 A/m2, below which no skyrmion motion is observed. 

In fact, even in the case of only the substitution scenario present, the overall net clockwise DMI will be present due to oxidation region asymmetry. 

The DPCM measurement and analysis present that the observed magnetic textures are Bloch type, in good agreement with previous LTEM results observed in a FGT flake without oxidized interfaces [23].