Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons and Peter Grünberg Institute, Forschungszentrum Jülich, Jülich, Germany.
Department of Physics, KTH-Royal Institute of Technology, Stockholm, Sweden.
M.N. Miheev Institute of Metal Physics of Ural Branch of Russian
Academy of Sciences, Ekaterinburg, Russia.
Ural Federal University, Ekaterinburg, Russia.
National Center for Electron Microscopy in Beijing, School of
Materials Science and Engineering, Tsinghua University, Beijing, China.
The Anhui Key Laboratory of Condensed Matter Physics at Extreme Conditions,
High Magnetic Field Laboratory and University of Science and Technology of China, Chinese Academy of Science (CAS), Hefei, Anhui Province, China.
Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Jiangsu Province, China.
Institute of Physics, Chinese Academy of
Sciences, Beijing, China.
Peter Grünberg Institute and Institute for Advanced Simulation, Forschungszentrum Jülich and JARA, Jülich, Germany.
*e-mail: firstname.lastname@example.org; email@example.com
Chiral magnetic skyrmions
are nanoscale vortex-like spin
textures that form in the presence of an applied magnetic
field in ferromagnets that support the Dzyaloshinskii–Moriya
interaction (DMI) because of strong spin–orbit coupling and
broken inversion symmetry of the crystal
. In sharp contrast
to other systems
that allow for the formation of a variety
of two-dimensional (2D) skyrmions, in chiral magnets the
presence of the DMI commonly prevents the stability and
coexistence of topological excitations of different types
Recently, a new type of localized particle-like object—the
chiral bobber (ChB)—was predicted theoretically in such
. However, its existence has not yet been verified
experimentally. Here, we report the direct observation of
ChBs in thin films of B20-type FeGe by means of quantita-
tive off-axis electron holography (EH). We identify the part of
the temperature–magnetic field phase diagram in which ChBs
exist and distinguish two mechanisms for their nucleation.
Furthermore, we show that ChBs are able to coexist with sky-
rmions over a wide range of parameters, which suggests their
possible practical applications in novel magnetic solid-state
memory devices, in which a stream of binary data bits can be
encoded by a sequence of skyrmions and bobbers.
In non-centrosymmetric crystals with a strong spin–orbit inter-
, such as B20-type MnSi (ref.
), FeGe (ref.
), β -Mn-type Co–Zn–Mn alloys
, the competition between the ferromagnetic
exchange interaction and the DMI results in a homochiral helical
spin spiral ground state. The equilibrium period of such a spin spi
= 4π A/D, is governed by the ratio of the exchange stiffness A
and the DMI constant D (ref.
). In a bulk sample, the spin spiral
usually appears as a multidomain state, with a set of spiral k-vector
directions across domains
. In the presence of an external mag-
netic field, B
, such a multidomain spiral state transforms into a
monodomain conical state, with the magnetization M tilted towards
the direction of B
and with k || B
. The conical state persists as the
lowest energy state over the entire range of applied magnetic fields
up to a critical value of B
/2AM, above which it converges to a
saturated ferromagnetic state.
In a thin film of thickness L ~ L
, the system undergoes a field-
induced transition into a regular lattice of topologically non-trivial
vortex-like spin textures—skyrmion tubes (SkTs) (Fig. 1a). Owing
to the effect of a chiral surface twist, the energy of a skyrmion lat
tice becomes lower than that of the conical phase over a wide range
). It has been shown both experimentally
that the range of magnetic fields for which skyrmions
are the ground state depends on the film thickness, and above a
threshold value the skyrmions become metastable. The mechanism
for skyrmion stabilization in isotropic chiral magnets, including
B20-type crystals with commonly weak cubic anisotropy, is differ
ent from that in ultrathin films and multilayers that possess strong
out-of-plane magnetocrystalline anisotropy and in which the DMI
appears due to a broken inversion symmetry at interfaces
Although it has been assumed that the magnetic skyrmion is a
unique localized spin texture in chiral magnets, it was recently pre
dicted theoretically that, for film thicknesses that are larger than the
equilibrium period L ≳ L
and magnetic fields B
skyrmions may coexist with another type of localized particle-like
object, the ChB
(Fig. 1a). The energy barriers that protect sky-
rmions and ChBs are of the same order of magnitude
and the two
objects are potentially ideal candidates for use as ‘1’ and ‘0’ bit car
riers (Fig. 1b). The potential advantages of using such an approach
for binary data encoding
, when compared to the recently pro-
posed skyrmion-based racetrack memory
, are discussed in detail
in Supplementary Section 1.
Here, we report the experimental observation of ChBs in thin
plates of B20-type FeGe by means of off-axis EH. EH provides quan
titative measurement of the phase shift φ of the incident electron
beam in a transmission electron microscope (TEM) with a high
spatial resolution. The phase shift originates from the interaction
of the electrons with the projected in-plane component of the mag
netic induction within and around the sample
. As a ChB has a
finite penetration depth and occupies a much smaller volume of
the sample than a SkT, the phase shift at the position of a ChB is
expected to be significantly weaker than that at a SkT (Fig.1c–e). A
quantitative comparison between the experimentally measured and
theoretically calculated phase-shift difference Δ φ, which is defined
in Fig. 1e, allows ChBs and SkTs to be identified and distinguished
Owing to the interaction of localized objects, the shapes of SkT
and ChB can be slightly distorted and the penetration depth of
Experimental observation of chiral magnetic
bobbers in B20-type FeGe
, Filipp N. Rybakov
, Aleksandr B. Borisov
, Dongsheng Song
, Zi-An Li
, Haifeng Du
*, Nikolai S. Kiselev
*, Jan Caron
, András Kovács
, Yuheng Zhang
, Stefan Blügel
and Rafal E. Dunin-Borkowski
© 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
NATURE NANOTECHNOLOGY | VOL 13 | JUNE 2018 | 451–455 | www.nature.com/naturenanotechnology