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BEGIN:VEVENT
SUMMARY:Shape transformations of elastic ferromagnetic systems
DTSTART;VALUE=DATE-TIME:20190524T110000Z
DTEND;VALUE=DATE-TIME:20190524T111500Z
DTSTAMP;VALUE=DATE-TIME:20190422T044913Z
UID:indico-contribution-4-619@indico.knu.ua
DESCRIPTION:Speakers: Kostiantyn Yershov (Leibniz-Institut für Festkörpe
r- und Werkstoffforschung\, IFW Dresden)\nWe propose a minimal extension o
f the anisotropic Heisenberg model in order to describe flexible magnetic
systems with coupled magnetic and mechanical subsystems. The interaction b
etween the magnetic and mechanical subsystems is driven by uniaxial anisot
ropy with the easy-axis oriented along the tangential direction and by the
Dzyaloshinskii-Moriya interaction (DMI).\n\nFirstly\, we study flexible f
erromagnetic wires (rings) [1]. For elastic rings we show that magnetic su
bsystem can determine the equilibrium shape of the ferromagnet. Depending
on the magnetic and elastic parameters and the size of the system one can
obtain two different states: the onion state with the quasi-uniform magnet
ization is typical for small enough rings\, while the vortex state with th
e magnetization oriented tangential to the wire is preferable for large sy
stems.\n\nWe also show that the presence of DMI\, results in a spontaneous
deformation of a flexible magnetic ribbon. The final state is characteriz
ing by the geometrical chirality whose sign is determined by the sign of t
he DMI constant. Depending on the mechanical\, magnetic\, and geometric pa
rameters of the system one can obtain two different states: twisted-state
with zero curvature of the central line is typical for small DMI constants
and narrow ribbons\, while the DNA-like state with nonzero curvature of c
entral line is preferable for large DMI constants and wider ribbons. Using
in-house developed simulator we build phase diagram of equilibrium states
for flexible ribbon.\n\nThe theoretical results are verified by means of
numerical simulations.\n\n[1] Yu. Gaididei\, K. Yershov\, D. Sheka\, V. Kr
avchuk\, A. Saxena\, PRB 99 014404\, (2019).\n\nhttps://indico.knu.ua/even
t/4/contributions/619/
LOCATION:
URL:https://indico.knu.ua/event/4/contributions/619/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Dipole-exchange oscillations in transversely magnetized ferromagne
tic nanowires of elliptical cross-section
DTSTART;VALUE=DATE-TIME:20190524T111500Z
DTEND;VALUE=DATE-TIME:20190524T113000Z
DTSTAMP;VALUE=DATE-TIME:20190422T044913Z
UID:indico-contribution-4-614@indico.knu.ua
DESCRIPTION:Speakers: Maksym Popov (Faculty of Radio Physics\, Electronics
and Computer Systems)\nThe investigations of the nanosize magnetic partic
les and arrays continuously attract academic and technological interest. T
heir magnetic properties can be varied over a wide range by modifying the
nanoparticle shape\, size\, curvature and separation [1]. While static mag
netic properties of a single particle are mostly well understood\, their h
igh-frequency spin-wave dynamics still lacks comprehensive theoretical exp
lanation for some important cases [2]. In the given paper the effect of pa
rticle curvature on the dipole-exchange oscillations frequency is investig
ated \nWe have considered an infinitely long ferromagnetic curvilinear cyl
inder with elliptical cross-section given by semiaxes *a* and *b*\, made f
rom ferrite with magnetization $4\\pi M$ and biased with external magnetic
field *H*\, applied along cylinder semiaxis *a*. We introduced the modifi
ed elliptical coordinate system\, according to [3]\n\\begin{equation}\nz =
\\left(\\rho + \\frac{{c^2 }}{{4\\rho }}\\right)\\cos \\phi \, \\\, y =
\\left(\\rho - \\frac{{c^2 }}{{4\\rho }}\\right)\\sin \\phi\, \\\, \\phi
\\in [ - \\pi \,\\pi ]\, \\\,\n\\rho \\in \\left[\\frac{c}{2}\,\\infty \
\right)\,c=\\sqrt{a^2-b^2}\n\\end{equation}\nIn these coordinates the magn
etostatic potential inside the ferrite is given by $\\Psi _1 (\\rho \,\\p
hi ) = \\sum\\limits_{k = 1}^\\infty {\\left( {M_k U^k (\\rho \,\\phi ) +
N_k V^k (\\rho \,\\phi )} \\right)} $\, where $U(\\rho \,\\phi ) = \\lef
t( {\\rho + \\frac{{c^2 }}{{4\\rho }}} \\right)\\cos \\phi + \\frac{1}{{
\\sqrt { - \\mu } }} \\left( {\\rho - \\frac{{c^2 }}{{4\\rho }}} \\right
)\\sin \\phi $\, $V(\\rho \,\\phi ) = \\left( {\\rho + \\frac{{c^2 }}{{4\
\rho }}} \\right)\\cos \\phi - \\frac{1}{{\\sqrt { - \\mu } }}\\left( {\\
rho - \\frac{{c^2 }}{{4\\rho }}} \\right)\\sin \\phi$\, and $\\mu = \\fr
ac{{\\omega ^2 - \\gamma ^2 H_i (H_i + 4\\pi M)}}{{\\omega ^2 - \\gamma
^2 H_i ^2 }}$ - is the diagonal part of tensor magnetic permeability\, $H
_{i} = H - 4\\pi M \\cdot b/(a + b)$\, and $\\gamma$ is the gyromagnetic
ratio.\nAfter applying standard boundary conditions at the lateral surface
of the cylinder we get a pair of independent secular equations \n\\begin{
equation}\n{(1 - i\\sqrt { - \\mu } )\\left( {a + \\frac{{ib}}{{\\sqrt { -
\\mu } }}} \\right)^k + (1 + i\\sqrt { - \\mu } )\\left( {a - \\frac{{ib
}}{{\\sqrt { - \\mu } }}} \\right)^k } = 0\,\n\\end{equation}\n\\begin{equ
ation}\n{ - (i + \\sqrt { - \\mu } )\\left( {a + \\frac{{ib}}{{\\sqrt { -
\\mu } }}} \\right)^k + (i - \\sqrt { - \\mu } )\\left( {a - \\frac{{ib}}
{{\\sqrt { - \\mu } }}} \\right)^k } = 0 \\\,\\\,(1)\n\\end{equation}\nf
or the modes with symmetric and antisymmetric spatial distribution with re
spect to the *Z* axis. \nEqs. (1) are to be solved for $\\mu ^{(nk)}$\, an
d the spin modes eigenfrequencies $\\omega ^{(nk)}$ can then be retrieved
from the abovementioned expression for $\\mu $. Figure 1 demonstrates the
effect of cylinder cross-section aspect ratio on the normalized eigenfrequ
encies. All calculations were made for the YIG biased with external field
*H*=3 kOe. The aspect ratio *b/a* was taken as a variable parameter. A str
ong influence of sample’s cross-section curvature (from prolate cylinder
to oblate) is clearly visible. \nThe exchange interaction\, which plays c
rucial role for nanosized particle\, was accounted for by the substitution
$H_i \\to H_i + Dk^2 $\, where *D* is the exchange stiffness and *k* is
the spin-wave transversal wavenumber. The latter was extracted form the pe
ak value of the spatial Fourier transformation of the mode's dynamic magne
tization.\nFinally\, the presented theory was applied to explain the exper
imental results\, published in [4] for the dependence of the spin-mode fre
quencies on the intensity of the transversal magnetic field measured for t
he array of nickel nanowires with length *L*=175 nm and radius *R*=35 nm b
y Brillouin light scattering (BLS). The experimental values were found to
be in a good accordance with theoretical calculations. Also\, our theory c
onvincingly reproduces the characteristic fine structure of the BLS magnet
o-optical response. \n\n![FIG1][1a]\nFIG. 1\n \n[1] P. Toneguzzo\, G. Viau
\, F Fiévet\, Handbook of Advanced Magnetic Materials\, vol.3: Fabricatio
n and Processing\, Springer\, New York\, 2006\, p 217-266.\n[2] R. Arias\,
D.L. Mills\, Phys. Rev. B\, 63 (2001) 134439. \n[3] I.V. Zavislyak\, G.P.
Golovach\, M.A. Popov\, V.F.Romanyuk\, J. Comm. Tech. and Electronics. 51
\, (2006) 203-211.\n[4] A.A. Stashkevich\, Y. Roussigne\, P. Djemia et al.
\, Phys. Rev. B\, 80 (2009) 144406.\n\n\n [1a]: https://indico.knu.ua/eve
nt/4/images/117-Popov_Fig1_sm.png "Fig. 1"\n\nhttps://indico.knu.ua/event/
4/contributions/614/
LOCATION:
URL:https://indico.knu.ua/event/4/contributions/614/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Local and nonlocal interactions in curved ferromagnets
DTSTART;VALUE=DATE-TIME:20190524T080000Z
DTEND;VALUE=DATE-TIME:20190524T083000Z
DTSTAMP;VALUE=DATE-TIME:20190422T044913Z
UID:indico-contribution-4-618@indico.knu.ua
DESCRIPTION:Speakers: Oleksandr Pylypovskyi (Taras Shevchenko National Uni
versity of Kyiv)\nMagnetostatics and engineered anisotropy following the g
eometry of a magnetic nanoobject result in the coupling of a magnetic text
ure with geometry and topology of magnetic nanoparticles. It opens a novel
ways for developing of novel devices\, utilizing the third dimension at n
anoscale [1]. \n\nIn this talk\, we discuss the influence of both\, local
and nonlocal interactions on the state of a ferromagnetic curvilinear shel
l. In the main order of the shell thickness\, magnetostatics can be reduce
d to a local anisotropy [2]. Depending on the sample geometry\, it takes t
he form of easy-tangential anisotropy in nanowires\, biaxial anisotropy in
curvilinear ribbons and easy-surface one in infinite shells [3]. We propo
se a framework allowing to distinguish effects of curvature from spurious
effects of curvilinear reference frame. Local curvatures of thin shells pu
t forth additional anisotropy and Dzyaloshinskii-Moria interaction of inte
rfacial symmetry. Magnetostatic energy results in new nonlocal anisotoropi
c and chiral energy terms in addition to the shape anisotropy in corrugate
d and curvilinear films. These interactions emerge in shells with nonzero
mean curvature. In addition new chiral interaction appears due to the inte
raction surface and volume magnetostatic charges: it can exist both in fla
t but rough films and shells. We classify effects of curvature-induced chi
ral and anisotropic terms in exchange and magnetostatic energies on equili
brium states of magnetization by shell types according to mean and Gauss c
urvatures.\n\n[1] R. Streubel\, P. Fischer\, F. Kronast\, V. P. Kravchuk\,
D. D. Sheka\, Y. Gaididei\, O. G. Schmidt\, D. Makarov\, J. Phys. D\, 49\
, 363001 (2016)\n[2] V. Slastikov\, Math. Models Methods Appl. Sci. 15\, 1
469 (2005)\; G. Di Fratta\, arXiv:1609.08040 \n[3] Y. Gaididei\, A. Gouss
ev\, V. P. Kravchuk\, O. V. Pylypovskyi\, J. M. Robbins\, D. Sheka\, V. Sl
astikov\, S. Vasylkevych\, J. Phys. A. 50\, 385401 (2017)\n\nhttps://indic
o.knu.ua/event/4/contributions/618/
LOCATION:
URL:https://indico.knu.ua/event/4/contributions/618/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Flexomagnetic response of multiferroics
DTSTART;VALUE=DATE-TIME:20190523T134500Z
DTEND;VALUE=DATE-TIME:20190523T141500Z
DTSTAMP;VALUE=DATE-TIME:20190422T044913Z
UID:indico-contribution-4-625@indico.knu.ua
DESCRIPTION:Speakers: Anna Morozovska (Institute of Physics\, National Aca
demy of Sciences of Ukraine)\nUsing direct matrix method we established th
e structure\, including the number of nonzero and independent elements\, o
f the poorly studied flexomagnetic coupling tensor for all 90 magnetic cla
sses. We used the point symmetry of the unit cell\, its magnetic symmetry\
, previously known permutation symmetry\, as well as we establish previous
ly unexplored permutation symmetry of the flexomagnetic tensor. For severa
l symmetries\, which correspond to the most important for applications mul
tiferroics with nonzero flexomagnetic effect\, such as $\\mathrm{BiRFeO_3}
$ ($\\mathrm{R=La\, Pr\, Eu}$)\, $\\mathrm{Sr_{1-x}Ba_xMnO_3}$\, $\\mathrm
{LiMPO_4}$ ($\\mathrm{M = Fe\, Co\, Ni}$)\, $\\mathrm{TbMn_2O_5}$\, $\\mat
hrm{YMnO_3}$\, $\\mathrm{InMnO_3}$ and $\\mathrm{RMn_{1−x}Ga_xO_3}$ ($\\
mathrm{R=Ho\, Y}$)\, we visualize the effective flexomagnetic response of
the bended magnetoelectric plate\, analyze its anisotropy and angular depe
ndences. We discuss how the established symmetry can simplify the problem
of the flexomagnetic constants determination from experiments\, opening th
e way for its novel applications.\n\nhttps://indico.knu.ua/event/4/contrib
utions/625/
LOCATION:
URL:https://indico.knu.ua/event/4/contributions/625/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Curvature effects on the properties of the magnetization in ferrom
agnetic nanoparticles: from torus to saddle
DTSTART;VALUE=DATE-TIME:20190523T123000Z
DTEND;VALUE=DATE-TIME:20190523T130000Z
DTSTAMP;VALUE=DATE-TIME:20190422T044913Z
UID:indico-contribution-4-630@indico.knu.ua
DESCRIPTION:Speakers: Vagson Carvalho-Santos (Departamento de Física\, Un
iversidade Federal de Viçosa)\nNanomagnetism has received a lot of attent
ion in last decades due to the possibility to use nanomagnets in random ac
cess memory\, data storage\, spintronic and magnonic devices\, and cancer
therapy. These potential applications demand the understanding on the magn
etization properties of magnetic nanoparticles\, as from experimental as f
rom theoretical point of view. Several works have reported the production
and characterization of nanomagnets with different shapes and sizes such a
s rolled-up magnetic membranes\, paraboloidal magnetic caps\, spherical an
d conical nanoparticles\, modulated nanomagnets\, and cylindrical nanoring
s\, nanodots\, nanowires and nanotubes. The production of magnetic nanopar
ticles with different shapes promotes the description on how geometry infl
uences their magnetic properties in a very important issue. Therefore\, th
e influence of the curvature on the properties of nanosized ferromagnetic
systems is a very prominent research area and is under constant investigat
ion. Among main theoretical results in this area\, one can cite the appear
ing of curvature-induced chiral effects\, which are responsible for a symm
etry breaking of a domain wall motion in magnetic helices\, for stabilizin
g magnetization configurations with topological protection\, and for an as
ymmetry regarding the sign of the wave vector in the dispersion relation o
f spin-wave propagation in ferromagnetic nanotubes. In addition\, curvatur
e afects the domain wall stability and dynamics when it is displacing alon
g curved ferromagnetic nanowires. \n\nIn this talk\, we will present the c
ontributions that our research group has developed in this area. In partic
ular\, we will present the analysis of the non-linear sigma model in curve
d surfaces. In particular\, we will present the influence of curvature in
the charcteristic length of solitons in geometries with different curvatur
e properties such as torus\, paraboloid and hyperboloid. After\, some resu
lts describing the magnetic groundstate of ferromagnetic nanotori will be
presented. Finally\, from the analysis of the remanent magnetization confi
guration in a ferromagnetic torus with large aspect ratio\, we will presen
t our recent results on the description of influence of the curvature on t
he winding number of vortices when they are stable configurations in geome
tries with positive and negative curvatures. In this context\, we will pre
sent the obtention of a vortex-antivortex pair in geometries with variable
curvature. Moreover\, we will show that geometries with positive curvatur
e can support vortices\, while geometries with negative curvature support
antivortices as the minimum energy configuration.\n\nhttps://indico.knu.ua
/event/4/contributions/630/
LOCATION:
URL:https://indico.knu.ua/event/4/contributions/630/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Micromagnetic studies of ferromagnetic nanotubes
DTSTART;VALUE=DATE-TIME:20190523T110000Z
DTEND;VALUE=DATE-TIME:20190523T113000Z
DTSTAMP;VALUE=DATE-TIME:20190422T044913Z
UID:indico-contribution-4-623@indico.knu.ua
DESCRIPTION:Speakers: Pedro Landeros (Departamento de Física\, Universida
d Técnica Federico Santa María)\nA review of recent theoretical research
in ferromagnetic nanotubes is presented. The focus is on the basic physic
al behavior that emerges from the micromagnetic theory\, from which intere
sting properties appear [1-6]. Depending on the size parameters\, magnetic
material\, external driving agents\, and proper experimental conditions\,
particular properties are expected\, where one can highlight (i) an almos
t uniform equilibrium state with vortex domains at the tube ends [2]\, (ii
) ultra-fast and chiral domain-wall dynamics [4-5]\, (iii) ferromagnetic n
anorings/nanotubes for magnetic hyperthermia applications [6]\, where the
flux-closure vortex state may be useful to avoid particle agglomeration\,
while reasonable high values of the specific absorption rate (SAR) are exp
ected. Most of these properties arises from the interplay between the exch
ange and the magnetostatic energy in the curved ferromagnetic structure. T
he magnetostatic energy also induces chiral features\, which early reporte
d in the motion of a vortex domain wall [4]. \n\n[1] P. Landeros\, S. Alle
nde\, J. Escrig\, E. Salcedo\, D. Altbir\, and E. E. Vogel\, Reversal mode
s in magnetic nanotubes\, Appl. Phys. Lett. 90\, 102501 (2007). \n[2] P. L
anderos\, O. J. Suarez\, A. Cuchillo\, and P. Vargas\, Equilibrium states
and vortex domain wall nucleation in ferromagnetic nanotubes\, Phys. Rev.
B 79\, 024404 (2009).\n[3] P. Landeros\, P. R. Guzmán\, R. Soto-Garrido\,
and J. Escrig\, Magnetostatic fields in tubular nanostructures\, J. Phys.
D: Appl. Phys. 42\, 225002 (2009).\n[4] P. Landeros and Á. S. Núñez\,
Domain wall motion on magnetic nanotubes\, J. Appl. Phys. 108\, 033917 (20
10).\n[5] J. A. Otálora\, J. A. López-López\, P. Vargas\, and P. Lander
os\, Chirality switching and propagation control of a vortex domain wall i
n ferromagnetic nanotubes\, Appl. Phys. Lett. 100\, 072407 (2012).\n[6] D.
F. Gutierrez-Guzman\, L. I. Lizardi\, J. A. Otálora\, and P. Landeros\,
Hyperthermia in low aspect-ratio magnetic nanotubes for biomedical applica
tions\, Appl. Phys. Lett. 110\, 133702 (2017).\n\nhttps://indico.knu.ua/ev
ent/4/contributions/623/
LOCATION:
URL:https://indico.knu.ua/event/4/contributions/623/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Antiferromagnetism for crystals with curved atomic planes: atomic
dislocations create spin half-vortices (disclinations)
DTSTART;VALUE=DATE-TIME:20190524T070000Z
DTEND;VALUE=DATE-TIME:20190524T073000Z
DTSTAMP;VALUE=DATE-TIME:20190422T044913Z
UID:indico-contribution-4-622@indico.knu.ua
DESCRIPTION:Speakers: Boris Ivanov (Institute of Magnetism)\nThe standard
spin state of antiferromagnets (AFM) contains two magnetic sublattices wit
h equivalent spins $s_1$ and $s_2$\, magnetized in the opposite direction
such that $s_1+s_2=0$ in the ground state. For their description\, so-ca
lled Neel vector $l= (s_1-s_2)/|s_1-s_2|$ is used. This kind of ordering
is sensitive to crystal lattice dislocations\, which destroy the sublattic
e structure of a perfect antiferromagnet such that $s_1\\to s_2$ and $s_
2\\to s_1$ after a passing around the dislocation along a simple closed co
unter. Lattice dislocations can be treated as a destruction of the “idea
l” geometry and even topology of the atomic planes in a crystal\, may gi
ve rise to an inhomogeneous spin distribution [1\, 2]. Both edge dislocati
on (atomic plane\, broken along some line) and screw dislocation (atomic p
lane of the form of the Riemann surface for the function $w=z^\\nu$ with i
rrational $\\nu$) are the origin of the singular line in the field of the
Neel vector $l$. This line is common to the disclinations in the field of
vector-director $n$ for nematic liquid crystals: the vector $l$ changes i
ts sing when passing around the AFM disclination line. Spin disclinations
were observed in thin films of chromium [3]. For a layered antiferromagnet
with a screw dislocation\, spin disclination have a macroscopic-sized fer
romagnetic core [4]. \nThe review of the static and dynamic properties of
the spin non-uniform states caused by different non-ideal topology of the
atomic planes of the crystal lattices is done. Single atomic dislocation
produces spin disclination\, whereas a “compensated” system of disloca
tions\, closed dislocation loop in 3-dimensional (3D) AFM or pair of dislo
cations loop in 2D AFM produces localized non-uniformity (singular droplet
soliton) of the Neel vector. For easy-plane AFM with the form of these so
litons is spherical or circular in 3D or 2D cases\, accordingly\, but it t
ransforms to the ellipsoidal in the presence of in-plane anisotropy. The d
ynamical solitons with the precession of the Neel vector (e.g.\, excited b
y the spin transfer torque) are also possible. \n\n[1] I. E. Dzyaloshinsk
y\, JETP Lett. 25\, 98 (1977)\n[2] A. S. Kovalev and A. M. Kosevich\, Sov.
J. Low Temp. Phys. 3\, 125 (1977)\n[3] M. Kleiber\, M. Bode\, R. Ravlic\,
and R. Wiesendanger\, Phys. Rev. Lett. 85\, 4606 (2000)\n\nhttps://indico
.knu.ua/event/4/contributions/622/
LOCATION:
URL:https://indico.knu.ua/event/4/contributions/622/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Effect of Curvature on Topological Defects in Chiral Magnets and S
oft Matter
DTSTART;VALUE=DATE-TIME:20190523T064500Z
DTEND;VALUE=DATE-TIME:20190523T071500Z
DTSTAMP;VALUE=DATE-TIME:20190422T044913Z
UID:indico-contribution-4-633@indico.knu.ua
DESCRIPTION:Speakers: Avadh Saxena (Los Alamos National Lab)\nThe interpla
y of geometry and topology underlies many novel and intriguing properties
of a variety of soft and hard materials including biological vesicles\, ne
matic liquid crystals and chiral magnets. These materials harbor a gamut
of topological defects ranging from domain walls\, disclinations\, soliton
s\, vortices\, skyrmions and merons to monopoles\, Dirac strings\, hopfion
s and boojums among many others. I will illustrate this rich interplay wi
th three distinct physical examples. (i) Either the change in the underly
ing curved manifold or the variation of the Dzyloshinskii-Moriya interacti
on (DMI) with curvature in magnetic systems. (ii) Controlled motion of li
quid crystal skyrmions near curved boundaries using the Q-tensor (as oppos
ed to director) based free energy where the twist acts as the analogue of
DMI. (iii) Deformation of biological membranes and vesicles using Canham-H
elfrich free energy and Bogomol’nyi decomposition technique to determine
equilibrium shapes. Finally\, I will briefly describe specific applicati
ons of these ideas in spintronics\, memory devices as well as drug deliver
y systems and active matter.\n\nhttps://indico.knu.ua/event/4/contribution
s/633/
LOCATION:
URL:https://indico.knu.ua/event/4/contributions/633/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Shape influence on statics and linear dynamics of antiferromagneti
c textures in curvilinear quasi-one-dimensional spin chains
DTSTART;VALUE=DATE-TIME:20190524T120000Z
DTEND;VALUE=DATE-TIME:20190524T121500Z
DTSTAMP;VALUE=DATE-TIME:20190422T044913Z
UID:indico-contribution-4-632@indico.knu.ua
DESCRIPTION:Speakers: Denys Kononenko (Taras Shevchenko National Universit
y of Kyiv)\nModern developments in nanotechnology allow production of arti
ficial curvilinear nanostructures [1] as well as modification of natural n
anosized objects with curved geometry [2]. Such objects attract considerab
le interest for both theoretical and experimental investigation due to the
ir outstanding physical properties. Also\, in the last few years interest
in antiferromagnets (AFM) has increased due to their advantages over ferro
magnets\, namely higher characteristic frequencies and absence of macrosco
pic magnetization. \nHowever\, there is still a need for fundamental inves
tigation of AFM curvilinear systems.\n \nOur work focuses on the investig
ation of equilibrium states and linear dynamics of AFM textures in curvili
near AFM one-dimensional spin chains. We show\, that the magnetic dipole-d
ipole interaction results in a hard-tangential anisotropy in a spin chain.
In our study we consider two sublattice collinear AFM with slowly varyin
g AFM textures in the frame of generalized sigma-model approach.\n\nWe app
ly the approach [3\, 4]\, recently developed for ferromagnets\, to describ
e equilibrium AFM textures in helix-shaped and ring-shaped spin chains. Th
e binormal distribution of the Neel vector field corresponds to the ground
state of an AFM ring-shaped spin chain whereas AFM helix-shaped spin chai
n has two equilibrium states: (i) quasi-homogeneous and (ii) quasi-binorma
l. The state (i) appears for helices with curvature less than torsion and
state (ii) vice versa. The software package used to determine stability re
gions of both ground states was SLaSi simulator [5]. \nWe also describe li
near excitations of the equilibrium states in helix-shaped and ring-shaped
spin chains. We show that magnon spectrum in the AFM ring-shaped spin cha
in is discrete and each frequency state is four times degenerated. Magnon
spectrum of helix-shaped AFM spin chain is characterized by four dispersio
n branches.\n\n\n[1] R. Streubel\, P. Fischer\, F. Kronast\, V. P. Kravchu
k\, D. D. Sheka\, Y. Gaididei\, O. G. Schmidt and D. Makarov\, J. Phys. D
\, **49**\, 363001\, (2016). \n[2] K. Mizoguchi\, S. Tanaka\, M. Ojima\, S
. Sano\, M. Nagatori\, H. Sakamoto\, \nY. Yonezawa\, Y. Aoki\, H. Sato\, K
. Furukawa\, T. Nakamura\, Journal of the Physical Society of Japan\, **76
**\, 043801\, (2007).\n[3] Y. Gaididei\, V. P. Kravchuk\, D. D. Sheka\, Ph
ys. Rev. Lett.\, **112**\, 257203\, (2014)\; D. D. Sheka\, V. P. Kravchuk\
, Y. Gaididei\, J. Phys. A\, **48**\, 125202\, (2015).\n[4] D. D. Sheka\,
V. P. Kravchuk\, K. V. Yershov and Y. Gaididei\, Phys. Rev. B\, **92**\,
054417\, (2015)\; O. V. Pylypovskyi\, V. P. Kravchuk\, D. D. Sheka\, D. M
akarov\, O. G. Schmidt and Y. Gaididei\, Phys. Rev. Lett.\, **114**\, 1972
04\, (2015)\; V. P. Kravchuk\, U. K. Rößler\, O. M. Volkov\, D. D. Sheka
\, J. van den Brink\, D. Makarov\, H. Fuchs\, H. Fangohr and Y. Gaididei\,
Phys. Rev. B\, **94**\, 144402\, (2016).\n[5] O. V. Pylypovskyi\, D. D. S
heka\, V. P. Kravchuk\, Y. Gaididei\, Journal of Magnetism and Magnetic Ma
terials\, **361**\, 201 – 205\, (2014).\n\nhttps://indico.knu.ua/event/4
/contributions/632/
LOCATION:
URL:https://indico.knu.ua/event/4/contributions/632/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Band Structure of the Spectrum of the Curvature-Induced One-Dimens
ional Magnonic Crystal
DTSTART;VALUE=DATE-TIME:20190524T114500Z
DTEND;VALUE=DATE-TIME:20190524T120000Z
DTSTAMP;VALUE=DATE-TIME:20190422T044913Z
UID:indico-contribution-4-631@indico.knu.ua
DESCRIPTION:Speakers: Anastasiia Korniienko (Taras Shevchenko National Uni
versity of Kyiv)\nMagnonic crystals are the artificial periodic structures
which can be produced by spatial variations of magnetic parameters such a
s saturation magnetization\, exchange constant or geometrical parameters [
1]. They are promising for the control of magnon currents\, all-magnon dat
a processing [2] and for logic gates realization [3].\n\nIn the current st
udy we consider ferromagnetic wave-shaped nanowire with strong easy-tangen
tial anisotropy and propose new mechanism of magnonic crystal formation d
ue to the periodic curvature. The ground state of magnetization and spin-w
ave spectrum of this structure can be described using recently developed a
pproach [4] for arbitrary shaped wires. Curvature effects result in two ge
ometry-driven effective interactions: the Dzyaloshinskii-Moriya one and ad
ditional anisotropy. These interactions lead to the deviation of magnetiza
tion from tangential distribution. Analytically and numerically we calcula
te the ground state of magnetization in a wide range of curvatures. \n\nTh
e periodic structure of the magnetization ground state plays a role of per
iodic potential for magnon excitations. The spin-wave band structure is ca
lculated numerically and limit cases of large and small curvatures are sol
ved analytically. With increasing of the curvature\, the width of the main
gap asymptotically goes to zero by the law $\\Delta\\Omega_0\\propto(\\ka
ppa\\ell)^{-1}$\, with $\\kappa$ being the curvature amplitude and $\\ell
$ being the magnetic length. The widths of the all gaps except the first o
ne decrease with increasing of curvature\, the width of the first gap $\\D
elta\\Omega_1$ goes to a constant value $\\Delta\\Omega_1 \\asymp \\frac{3
}{4}$(in units of frequency of uniform ferromagnetic resonance for straigh
t wire). The latter effect can be of high importance for engineering of ma
gnon waveguides based on the magnetic molecular chains. \n\n\n[1] S. O. De
mokritov and A. N. Slavin (eds.)\, Magnonics: From Fundamentals to Applica
tions (Topics in Applied Physics)\, Springer Berlin Heidelberg\, Berlin Ne
w York (2013)\n[2] A. V. Chumak\, V. I. Vasyuchka\, A. A. Serga\, and B. H
illebrands\, Nat. Phys.\, 11\, 453 (2015)\n[3] T. Schneider\, A. A. Serga\
, B. Leven\, B. Hillebrands\, R. Stamps\, and M. Kostylev\, Appl. Phys. Le
tt.\,92\, 022505 (2008)\n[4] D. D. Sheka\, V. P. Kravchuk and Y.Gaididei\,
J. of Phys. A: Math. and Theor.\,48\, 125202 (2015)\n\nhttps://indico.knu
.ua/event/4/contributions/631/
LOCATION:
URL:https://indico.knu.ua/event/4/contributions/631/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Role of the shape anisotropy on the magnetization reversal in curv
ed surfaces
DTSTART;VALUE=DATE-TIME:20190524T113000Z
DTEND;VALUE=DATE-TIME:20190524T114500Z
DTSTAMP;VALUE=DATE-TIME:20190422T044913Z
UID:indico-contribution-4-629@indico.knu.ua
DESCRIPTION:Speakers: Sebastián Castillo (Universidad Autónoma de Chile)
\nIn the last years\, a strong interest has been focused on the properties
of curved magnetic structures. An important aspect of these systems is th
e possibility of controlling the magnetic behavior at the nanoscale by cha
nging the curvature. This interest has been reinforced by the development
of a theory that allows to calculate the exchange energy of nano magnets w
ith arbitrary shapes\, which has been used to study the magnetic propertie
s of Möbius stripes\, helical wires and DWs in a paraboloidal local bend
of nanowires. \n\nIn this work\, we studied the magnetization reversal pro
cess as a function of curvature\, in nanotubes with a truncated tubular cr
oss-section. Using a theoretical formalism\, we obtain a phase diagram for
the DW mechanism as a function of curvature (transversal and vortex-like
domain walls). In the ultra thin limit\, our results can be interpreted as
suming that curvature generates an effective anisotropy in the structure.\
n\nhttps://indico.knu.ua/event/4/contributions/629/
LOCATION:
URL:https://indico.knu.ua/event/4/contributions/629/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Propagation of Volume Spin Wave through an Antiferromagnet/Ferroma
gnet Interface of Finite Thickness
DTSTART;VALUE=DATE-TIME:20190524T091500Z
DTEND;VALUE=DATE-TIME:20190524T093000Z
DTSTAMP;VALUE=DATE-TIME:20190422T044913Z
UID:indico-contribution-4-628@indico.knu.ua
DESCRIPTION:Speakers: Oksana Busel (National Technical University of Ukrai
ne “Igor Sikorsky Kyiv Polytechnic Institute”)\nAbridged general form
of the boundary conditions at an interface between antiferromagnetic (AFM)
and ferromagnetic (FM) materials have been obtained in the continuous med
ium approximation [1]\, taking into account the fact that the interface is
a composite material with finite thickness $\\delta$ which is much less t
han the length of the spin wave (SW) $\\lambda_{\\text{sw}}$ and penetrati
on depth $\\lambda$ into second material [2]. Three order parameters have
been considered inside an interface of finite thickness with magnetization
s of both sublattices $\\mathbf{M}_{1}$ and $\\mathbf{M}_{2}$ of AFM\, and
magnetization $\\mathbf{M}$ of FM. Using these boundary conditions\, the
excitation of a volume SW has been considered in FM when SW in AFM falls o
nto this interface. The uniform and non-uniform exchange between all order
parameters have been taken into account in the interface energy as the co
ordinate dependencies of the magnetic parameters characterizing two-sublat
tice AFM\, FM\, and the interface region on the coordinate along the axis
perpendicular to the interface in the energy. Interconnection between the
magnitudes of the amplitudes of the incident spin waves in both sublattice
s and the reflected spin waves in both sublattices of the two-sublattice A
FM have been derived. All possible variants of the energy transfer to the
second material in the current system via the Poynting vector have been co
nsidered and the dependency between the amplitudes of the incident\, trans
ferred and the reflected SWs in the positive cases has been demonstrated [
3]. The amplitude coefficients of the scattering and the phases of transmi
ssion and reflection of SW through the AFM/FM interface have been derived
in general case.\n\n[1] O. P. Busel\, O. Yu. Gorobets and Yu. I. Gorobets\
, “Boundary conditions at the interface of finite thickness between ferr
omagnetic and antiferromagnetic materials”\, J. Magn. Magn. Mater. **462
**\, 226-229 (2018)\n[2] V. V. Kruglyak\, O. Yu. Gorobets\, Yu. I. Gorobet
s\, and A. N. Kuchko\, “Magnetization boundary conditions at a ferromagn
etic interface of finite thickness”\, J. Phys.: Condens. Matter **26**\,
406001 (2014).\n[3] A. I. Akhiezer\, V. G. Bar’yakhtar\, and S. V. Pele
tminskii\, “Spin waves“ (North-Holland\, Amsterdam\, 1968).\n\nhttps:/
/indico.knu.ua/event/4/contributions/628/
LOCATION:
URL:https://indico.knu.ua/event/4/contributions/628/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Quantitative MFM on superconducting flux quanta
DTSTART;VALUE=DATE-TIME:20190523T133000Z
DTEND;VALUE=DATE-TIME:20190523T134500Z
DTSTAMP;VALUE=DATE-TIME:20190422T044913Z
UID:indico-contribution-4-627@indico.knu.ua
DESCRIPTION:Speakers: Anna Maria Cucolo (University of Salerno)\nThe quant
itative interpretation of MFM data is still a hot topic in the community o
f scanning probe microscopists. Indeed\, the tip magnetic properties are a
priori not known and quantitative MFM investigations may only be achieved
through phenomenological pictures of the tip-sample interaction\, mainly
based on the so-called *point-probe approximation*. In such a model the ti
p magnetization distribution is sketched as a single magnetic monopole and
the measured MFM signal is used to determine the unknown magnetic charge
as well as its position within the real tip’s cone.\n\nHere we propose a
magnetic characterization of the tip\, starting from MFM measurements on
*superconducting vortices*\, which appear in type-II superconductors in th
e mixed state\, i.e. in magnetic fields $H_{c1} \n\nhttps://indico.knu.ua
/event/4/contributions/627/
LOCATION:
URL:https://indico.knu.ua/event/4/contributions/627/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Topology and transport in nanostructures with curved geometries
DTSTART;VALUE=DATE-TIME:20190523T061500Z
DTEND;VALUE=DATE-TIME:20190523T064500Z
DTSTAMP;VALUE=DATE-TIME:20190422T044913Z
UID:indico-contribution-4-626@indico.knu.ua
DESCRIPTION:Speakers: Carmine Ortix ( Institute for Theoretical Physics\,
Utrecht University & Physics Department “E. R. Caianiello”\, Universi
ty of Salerno)\nNanostructures are material structures with size in the ra
nge of one to a few hundreds of nanometers in at least one spatial dimensi
on. They include zero-dimensional quantum dots\, one-dimensional quantum w
ires and two-dimensional quantum wells. Apart from these conventional mate
rial geometries\, recent advances in nanostructuring techniques have enabl
ed the synthesis of complex nanoarchitectures: constructs of one- or two-d
imensional nanostructures assembled in curved geometries\, such as nanotub
es and nanohelices. \n\nIn this talk\, I will first show how the very fund
amental quantum mechanical properties of the charge carriers in these nano
materials are strongly affected by the curved background in which they liv
e. Then I will discuss examples of unique curvature-induced topological an
d transport properties\, including the ballistic anisotropic magnetoresist
ance effect in non-magnetic and spin-orbit free materials rolled-up into t
ubes\, and the generation of topological insulating phases in corrugated n
anowires with Rashba spin-orbit interaction.\n\nhttps://indico.knu.ua/eve
nt/4/contributions/626/
LOCATION:
URL:https://indico.knu.ua/event/4/contributions/626/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Curvature induced asymmetric dispersion in nanotubes: handy spin w
aves in handy tubes
DTSTART;VALUE=DATE-TIME:20190524T060000Z
DTEND;VALUE=DATE-TIME:20190524T063000Z
DTSTAMP;VALUE=DATE-TIME:20190422T044913Z
UID:indico-contribution-4-624@indico.knu.ua
DESCRIPTION:Speakers: Attila Kákay (Helmholtz-Zentrum Dresden - Rossendor
f)\nSpin-wave propagation in ferromagnetic nanotubes is fundamentally diff
erent than in flat thin films as shown recently [1]. The dispersion relati
on is asymmetric regarding the sign of the wave vector. As a consequence\,
spin waves traveling in opposite directions have different wavelength. Th
is purely curvature induced effect originates from the dipole-dipole inter
action\, namely from the dynamics dipolar volume charges. Such non-recipr
ocal spin-wave propagation [2] is known for flat thin films with interfaci
al Dzyalonshiinsky-Moriya interaction or for crystals with a special symme
try (C$_{\\text{nv}}$) and bulk Dzyalonshiinsky-Moriya interaction. Here\,
we will discuss in a nanotube with circular cross section the effect of t
he individual terms of the divergence on the spin-wave dispersion\, throug
h results obtained by finite element micromagnetic simulations. The diverg
ence terms in the cylindrical coordinate system together with the term dep
ending on the mean curvature leads to different asymmetries of the dispers
ion. We show\, that spin waves in nanotubes have chirality or handedness.
If the magnetisation in the nanotube is helical\, it is also shown that th
e cylindrical waveguide will also have handedness. This scenario can be co
mpared to that well known from optics: a circular polarised light travelin
g in a polariser media. We will emphasise the importance of the mean curva
ture and show that by tailoring it the asymmetry of the dispersion relatio
n can be tuned or even suppressed. It can be concluded that the curvature
induced magnetochiral effect with magnetostatic origin can be switched on
and off by the manipulation of the surface curvature. \n\n[1] J.A. Otálo
ra\, et. al.\, Phys. Rev. Lett. 117\, 227203 (2016). \n[2] K. Zakeri\, et.
al.\, Phys. Rev. Lett. 104\, 137203 (2010).\n\nhttps://indico.knu.ua/even
t/4/contributions/624/
LOCATION:
URL:https://indico.knu.ua/event/4/contributions/624/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Magnetism in curved geometries: A challenge for PEEM and other mi
croscopies
DTSTART;VALUE=DATE-TIME:20190523T091000Z
DTEND;VALUE=DATE-TIME:20190523T094000Z
DTSTAMP;VALUE=DATE-TIME:20190422T044913Z
UID:indico-contribution-4-620@indico.knu.ua
DESCRIPTION:Speakers: Florian Kronast (Helmholtz-Zentrum Berlin)\nAt the n
anoscale materials exhibit novel properties that are relevant to a wide va
riety of applications. Geometry can become a design parameter for new elec
tronic\, plasmonic or magnetic functionalities. Transforming planar magnet
ic films to curved objects can introduce magnetochiral properties\, simila
r to the Dzyaloshinskii-Moria interaction. Objects such as hollow cylinder
s or half spheres can feature faster domain wall motion or the formation o
f topological spin configurations\, such as skyrmions.\n\nExperimental evi
dence and visualization of curvature induced effects sets a particular cha
llenge for magnetic microscopies such as XPEEM (X-ray photoemission electr
on microscopy). The relevant length scales are close to resolution limits
in the nanometer regime and two- or three-dimensional curved magnetic surf
aces add further complexity to magnetic imaging.\n\nThis paper will review
recent activities using XPEEM to study magnetism in curved geometries. By
a combination of element-specific magnetic contrast and high lateral reso
lution XPEEM offers a unique toolbox for magnetic nanoscale science. Attem
pts have been made to expand the technique towards the third dimension or
magnetic tomography exploiting the magnetic shadow contrast\, trying to cl
ose a gap in the length scale of magnetic tomographies. An outlook to the
near future will show how XPEEM might profit from a new generation of imag
ing electron detectors with superior dynamic range\, highest quantum effic
iencies and intrinsic time-resolution. Enabling the technique for studies
of magnetization dynamics in curved geometries.\n\nhttps://indico.knu.ua/e
vent/4/contributions/620/
LOCATION:
URL:https://indico.knu.ua/event/4/contributions/620/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Arrays of nanostructures on polyimide substrate. Heterogenous stra
in effect on magnetic properties
DTSTART;VALUE=DATE-TIME:20190524T063000Z
DTEND;VALUE=DATE-TIME:20190524T070000Z
DTSTAMP;VALUE=DATE-TIME:20190422T044913Z
UID:indico-contribution-4-617@indico.knu.ua
DESCRIPTION:Speakers: Damien Faurie (LSPM-CNRS\, France)\nMagnetic nanostr
uctures deposited on flexible substrates are of increasing interest for fl
exible magnetoelectronic applications. In this context\, it is crucial to
study the links between strain fields and magnetic behavior. In this study
\, a large area (5 mm × 5 mm) of ferromagnetic nanostructures (nano
wires and antidots) have been deposited on top of a polyimide substrate us
ing interference lithography and sputtering processes [1\,2]. \nIn order t
o characterize the strain effect on magnetic properties\, we have develope
d an in situ technique that combines microtensile tests and ferromagnetic
resonance. From the shift of resonance spectra as function of applied stra
in\, it is possible to estimate the magneto-mechanical properties. The mag
netic resonance frequencies have been measured as function of macroscopic
strain applied to the polyimide substrates. We have shown that the resonan
ce shift due to macroscopic strain depends on the kind of system: the effe
ct is stronger for continuous thin film and lower for nanowires. The nanos
tructuration (nanowires\, nano-antidots) induces strain relaxation whose a
mplitude depend on nanostructures geometrical features [3]. \nModelling co
mbining micromagnetism and solid mechanics\, describing the strain heterog
eneities in such systems\, has explained the experimental results. We show
that the nanostructuration affects the strain distribution and its mean v
alue in magnetic nanostructures. Therefore\, for a given macroscopic strai
n applied to the substrate\, the strain-induced shift of the magnetic reso
nance field depends on the nanostructures geometry. Especially\, the magne
tomechanical response depends on the tensile strain direction as referred
to the nanostructure orientation.\n\n[1] F. Zighem\, D. Faurie\, M. Belmeg
uenai\, A. Garcia-Sanchez. P. Lupo\, A.O. Adeyeye\, “Large area periodic
ferromagnetic nanowires deposited onto a polymer substrate”\, Applied P
hysics Letters. 111\, 052408 (2017)\n[2] S. Merabtine\, F. Zighem\, D. Fau
rie\, A. Garcia-Sanchez\, P. Lupo\, A.O. Adeyeye\, "Multicracking and Magn
etic Behavior of Ni80Fe20 Nanowires Deposited onto a Polymer Substrate"\,
Nano letters. 18 (5)\, 3199-3202 (2018).\n[3] N. Challab\, F. Zighem\, D.
Faurie\, M. Haboussi\, M. Belmeguenai\, P. Lupo\, A.O. Adeyeye\, "Local st
iffness effect on ferromagnetic response of nanowires arrays in stretchabl
e systems"\, Physica Status Solidi – Rapid Research Letters. 13\, 197001
5 (2019)\n\nhttps://indico.knu.ua/event/4/contributions/617/
LOCATION:
URL:https://indico.knu.ua/event/4/contributions/617/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Effects of curvature in low-dimensional ferromagnets
DTSTART;VALUE=DATE-TIME:20190523T071500Z
DTEND;VALUE=DATE-TIME:20190523T074500Z
DTSTAMP;VALUE=DATE-TIME:20190422T044913Z
UID:indico-contribution-4-616@indico.knu.ua
DESCRIPTION:Speakers: Volodymyr Kravchuk (Leibniz-Institut für Festkörpe
r- und Werkstoffforschung\, IFW Dresden)\nCurvilinear geometry of magnetic
films and wires enriches physics of their magnetic subsystems with a numb
er of new effects. For example\, magnon dispersion relation for a helix-sh
aped wire demonstrates a linear frequency shift typical for magnets with i
ntrinsic Dzyaloshinskii-Moriya interaction (DMI) [1]\; magnetic skyrmion c
an be stabilized on a spherical shell which is free of any intrinsic DMI [
2]\; localized curvilinear defect of magnetic film can create a strong pin
ning potential for a magnetic skyrmion and induce a discrete ladder of ene
rgy levels in the pinned skyrmion [3]. These and many other effects can be
explained within the general framework [4\,5]\, which introduces a number
of curvature induced interactions. The latter are effectively generated b
y the common energy terms comprising spatial derivatives\, e.g. in presenc
e of the curvature the exchange interaction generates the effective DMI an
d anisotropy\, the intrinsic DMI generates effective anisotropy. \n Topolo
gical magnetic solitons (domain walls\, skyrmions\, vortices) in curviline
ar geometries are of special interest because of the high potential for th
e spintronics applications. In addition to the curvature induced pinning [
3\, 6] and driving [7]\, the topological magnetic solitons in curvilinear
systems demonstrate strong effects of chirality symmetry breaking\, e.g. c
oupling of chiralities in spin and physical spaces for a domain wall on a
Möbius strip [8]\, chirality-polarity coupling for a vortex on a spherica
l shell [9]. \n An inverse effect of the influence of the magnetic subsyst
em on the shape of an elastic magnet [10] opens a new area in curvilinear
magnetism. The possibility to control the shape and mechanics of the magne
t by means of its magnetization creates a promising tool for nanorobotics.
\n\n[1] D. Sheka et al.\, PRB 92\, 054417 (2015).\n[2] V. Kravchuk et al.\
, PRB 94\, 144402 (2016).\n[3] V. Kravchuk et al.\, PRL 120\, 067201 (2018
).\n[4] Yu. Gaididei et al.\, PRL 112\, 257203 (2014).\n[5] D. Sheka et al
.\, J. Phys. A 48\, 125202 (2015).\n[6] K. Yershov et al.\, PRB 92\, 10441
2 (2015).\n[7] K. Yershov et al.\, PRB 98\, 060409(R) (2018).\n[8] O. Pyl
ypovskyi et al.\, PRL 114\, 197204 (2015).\n[9] V. Kravchuk et al.\, PRB 8
5\, 144433 (2012).\n[10] Yu. Gaididei et al.\, PRB 99\, 014404 (2019).\n\n
https://indico.knu.ua/event/4/contributions/616/
LOCATION:
URL:https://indico.knu.ua/event/4/contributions/616/
END:VEVENT
BEGIN:VEVENT
SUMMARY:3D reconstruction of magnetic textures in nanomagnets by electron
holographic tomography
DTSTART;VALUE=DATE-TIME:20190523T113000Z
DTEND;VALUE=DATE-TIME:20190523T120000Z
DTSTAMP;VALUE=DATE-TIME:20190422T044913Z
UID:indico-contribution-4-615@indico.knu.ua
DESCRIPTION:Speakers: Axel Lubk (IFW Dresden\, Germany)\nThe fundamental u
nderstanding of nanomagnetic phenomena such as domain walls in nanowires r
equires quantitative magnetization mapping techniques resolving textures
down to the nanometer scale in 3D. In our contribution we report on the de
velopment of a tomographic reconstruction technique for all Cartesian comp
onents of the magnetic induction with a spatial resolution of slightly bel
ow 10 nm. To that end we combine off-axis electron holography (EH) and dua
l tilt-axis tomography in the transmission electron microscope (TEM). Elec
tron holography allows to reconstruct projections of the magnetic flux den
sity by an evaluation of the Aharonov-Bohm phase. The combination with tom
ography then facilitates the reconstruction of the 3D distribution of the
Cartesian component parallel to the respective tilt axis from a tilt serie
s of projections. Employing standard tomographic specimen holders only two
perpendicular tilt axis and hence Cartesian components are currently acce
ssible. The third component is evaluated by numerically integrating $\\mat
hrm{div} \\vec{B} = 0$. Going one step further\, we derive the 3D magnetiz
ation distribution and other magnetic quantities such as the magnetization
current from the reconstructed 3D magnetic flux density by invoking analy
tic and numerical micromagnetics. \n\nUsing this approach we reconstruct t
he remnant magnetic configuration of an electro-deposited Co/Cu multilayer
ed nanowire (NW). We particularly reveal several characteristic magnetizat
ion states (vortex states or homogeneously magnetized) of the individual C
o disk and correlate them to the structural and chemical composition of th
e NW. We discuss pertaining limits (e.g.\, resolution) and give an outlook
to further instrumental development (such as 3-axis tomography holders).
The powerful approach presented here is widely applicable to a broad range
of 3D magnetic nanostructures and may help to elucidate a wide range of n
anomagnetic configurations.\n\nhttps://indico.knu.ua/event/4/contributions
/615/
LOCATION:
URL:https://indico.knu.ua/event/4/contributions/615/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Experimental observation of exchange-driven chiral effects in para
bolic nanostripes
DTSTART;VALUE=DATE-TIME:20190524T083000Z
DTEND;VALUE=DATE-TIME:20190524T090000Z
DTSTAMP;VALUE=DATE-TIME:20190422T044913Z
UID:indico-contribution-4-613@indico.knu.ua
DESCRIPTION:Speakers: Oleksii Volkov (Helmholtz-Zentrum Dresden-Rossendorf
)\nBroken magnetic symmetry is a key aspect in condensed matter physics an
d in particular in magnetism. It results in the appearance of chiral effec
ts\, e.g. topological Hall effect [1] and non-collinear magnetic textures
including chiral domain walls and skyrmions [2\,3]. These chiral structure
s are in the heart of novel concepts for magnonics [4]\, antiferromagnetic
spintronics [5]\, spin-orbitronics [6] and oxitronics [7]. \n \nThe main
origin of the chiral symmetry breaking and thus for the magnetochiral effe
cts in magnetic materials is associated to an antisymmetric exchange inter
action\, the intrinsic Dzaloshinskii-Moriya interaction (DMI). At present\
, tailoring of DMI is done rather conventionally by optimizing materials\,
either doping a bulk single crystal or adjusting interface properties of
thin films and multilayers. A viable alternative to the conventional mater
ial screening approach can be the exploration of the interplay between geo
metry and topology. This interplay is of fundamental interest throughout m
any disciplines in condensed matter physics\, including thin layers of sup
erconductors [8] and superfluids [9]\, nematic liquid crystals [10]\, cell
membranes [11]\, semiconductors [12]. In the emergent field of curvilinea
r magnetism chiral effects are associated to the geometrically broken inve
rsion symmetries [13]. Those appear in curvilinear architectures of even c
onventional materials. There are numerous exciting theoretical predictions
of exchange- and magnetostatically-driven curvature effects\, which do no
t rely on any specific modification of the intrinsic magnetic properties\,
but allow to create non-collinear magnetic textures in a controlled manne
r by tailoring local curvatures and shapes [14\,15]. Until now the predict
ed chiral effects due to curvatures remained a neat theoretical abstractio
n. \n \nHere\, I review the very first experimental confirmation of the ex
istence of the curvature-induced chiral interaction with exchange origin i
n a conventional soft ferromagnetic material. It is experimentally explore
d the theoretical predictions\, that the magnetisation reversal of flat pa
rabolic stripes shows a two step process. At the first switching event\, a
domain wall pinned by the curvature induced exchange-driven DMI is expell
ed leading to a magnetisation state homogeneous along the parabola's long
axis. Measuring the depinning field enables to quantify the effective exch
ange-driven DMI interaction constant. The magnitude of the effect can be t
uned by the parabola's curvature. It is found that the strength of the exc
hange-induced DMI interaction for the experimentally realised geometries i
s remarkably strong\, namely $\\approx 0.4~$mJ/m$^2$\, compared the surfac
e induced DMI. The presented study legitimates the predictive power of ful
l-scale micromagnetic simulations to design the properties of ferromagnets
through their geometry\, thus stabilising chiral textures.\n\n[1] N. Naga
osa\, et al.\, Nature Nanotech. 8\, 899 (2013)\n[2] U. K. Rößler\, et al
.\, Nature 442\, 797 (2006)\n[3] A. Fert\, et al.\, Nature Rev. Mat. 2\, 1
7031 (2017)\n[4] A. V. Chumak\, et al.\, Nature Physics 11\, 453 (2015)\n[
5] T. Jungwirth\, et al.\, Nature Nanotech. 11\, 231 (2016)\n[6] I. M. Mir
on\, et al.\, Nature 476\, 189 (2011)\n[7] V. Garcia\, et al.\, Nature 460
\, 81 (2009)\n[8] J. Tempere\, et al.\, Phys. Rev. B 79\, 134516 (2009)\n[
9] H. Kuratsuji\, Phys. Rev. E 85\, 031150 (2012)\n[10] T. Lopez-Leon\, et
al.\, Nature Physics 7\, 391 (2011)\n[11] H. T. McMahon\, et al.\, Nature
438\, 590 (2005)\n[12] C. Ortix\, Phys. Rev. B 91\, 245412 (2015)\n[13] Y
. Gaididei\, et al.\, Phys. Rev. Lett. 112\, 257203 (2014)\n[14] J. A. Ot
álora\, et al.\, Phys. Rev. Lett. 117\, 227203 (2016)\n[15] V. P. Kravchu
k\, et al.\, Phys. Rev. Lett. 120\, 067201 (2018)\n\nhttps://indico.knu.ua
/event/4/contributions/613/
LOCATION:
URL:https://indico.knu.ua/event/4/contributions/613/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Magnetochiral effects in the dynamics of micromagnetic structures
on curved surfaces: finite element modeling
DTSTART;VALUE=DATE-TIME:20190523T081000Z
DTEND;VALUE=DATE-TIME:20190523T084000Z
DTSTAMP;VALUE=DATE-TIME:20190422T044913Z
UID:indico-contribution-4-611@indico.knu.ua
DESCRIPTION:Speakers: Riccardo Hertel (Université de Strasbourg\, CNRS\,
IPCMS)\nMicromagnetic simulations have reached a high degree of reliabilit
y and accuracy over the past years\, which allows to ascribe to them predi
ctive power for the magnetic properties of nanostructures. To investigate
magnetic processes unfolding as a result of surface curvature\, simulation
s based on finite elements are particularly suited due to their ability to
approximate arbitrary geometries. A few years ago\, such simulations have
revealed extraordinary features regarding the dynamics of vortex-type dom
ain walls in cylindrical soft-magnetic nanotubes with domains oriented alo
ng the symmetry axis [1]\, [2]. The simulations predicted that in these sy
stems the stability and the mobility of field-driven domain walls depend o
n their chirality\, defined by combining the circulation of the vortex in
the domain wall with the propagation direction along the tube axis. In par
ticular\, domain walls with suitable chirality can attain very high dynami
c stability and thereby reach velocities which are significantly larger th
an the limit that is usually reached by the Walker breakdown [2]. Simulati
ons\, supported by analytic studies\, also revealed a further type of chir
al asymmetry (or non-reciprocity) regarding the propagation of spin waves
in magnetic nanotubes with azimuthal magnetization [3]\, [4]. \n\nBoth eff
ects\, the spin-wave non-reciprocity and the stability of domain walls wit
h specific chirality\, are known phenomena in the case of flat thin films
of magnetic material with intrinsic Dzyaloshinskii-Moriya interaction (DMI
). The observation of these magnetochiral effects in ordinary ferromagneti
c materials with curved geometries suggests an analogy between surface cur
vature and intrinsic DMI [5]. Such a link was also discussed in a number o
f analytic and numerical studies [6]\, [7]. In this talk I will review som
e of these previous findings and present further simulation studies on the
dynamics of micromagetic structures on curved surfaces\, in ferromagnetic
materials with and without intrinsic DMI.\n\n[1] M. Yan\, C. Andreas\, A.
Kákay\, F. García-Sánchez\, and R. Hertel\, “Chiral symmetry breakin
g and pair-creation mediated Walker breakdown in magnetic nanotubes\,” A
ppl. Phys. Lett. 100\, 25\, 252401 (2012)\n[2] M. Yan\, C. Andreas\, A. K
ákay\, F. García-Sánchez\, and R. Hertel\, “Fast domain wall dynamics
in magnetic nanotubes: Suppression of Walker breakdown and Cherenkov-like
spin wave emission\,” Appl. Phys. Lett. 99\, 12\, 122505 (2011)\n[3] J.
A. Otálora\, M. Yan\, H. Schultheiss\, R. Hertel\, and A. Kákay\, “Cu
rvature-Induced Asymmetric Spin-Wave Dispersion\,” Phys. Rev. Lett.\, 11
7\, 22\, 227203 (2016)\n[4] J. A. Otálora\, M. Yan\, H. Schultheiss\, R.
Hertel\, and A. Kákay\, “Asymmetric spin-wave dispersion in ferromagnet
ic nanotubes induced by surface curvature\,” Phys. Rev. B\, 95\, 18\, 1
84415 (2017)\n[5] R. Hertel\, “Curvature-Induced Magnetochirality\,” S
PIN\, 3\, 3\, 1340009 (2013)\n[6] Y. Gaididei\, V. P. Kravchuk\, and D. D.
Sheka\, “Curvature Effects in Thin Magnetic Shells\,” Phys. Rev. Lett
. 112\, 25\, 257203 (2014).\n[7] V. P. Kravchuk et al.\, “Topologically
stable magnetization states on a spherical shell: Curvature-stabilized sky
rmions\, Phys. Rev. B. 94\, 14\, 144402 (2016)\n\nhttps://indico.knu.ua/ev
ent/4/contributions/611/
LOCATION:
URL:https://indico.knu.ua/event/4/contributions/611/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Fabrication of 3D nanoarchitectures by focused particle beams
DTSTART;VALUE=DATE-TIME:20190523T120000Z
DTEND;VALUE=DATE-TIME:20190523T123000Z
DTSTAMP;VALUE=DATE-TIME:20190422T044913Z
UID:indico-contribution-4-610@indico.knu.ua
DESCRIPTION:Speakers: Oleksandr Dobrovolskiy (Physikalisches Institut Goet
he University Frankfurt am Main)\nExtending 2D structures into the third d
imension has become a general trend in various areas\, including photonics
\, plasmonics and magnetics. This approach provides a means to modify conv
entional and to launch novel functionalities by tailoring vector potential
s inducing anisotropic and chiral effects. Recently\, there has been signi
ficant progress [1] in the fabrication of free-standing ferromagnetic and
superconducting nanostructures by focused particle direct-write techniques
\, as in part reviewed in [2]. In this respect\, 3D shell structures such
as framed tubes\, spheres\, Swiss rolls and helices are especially interes
ting as they offer unprecedented prospects for nanomagnetism [3] and super
conductivity [$4$] because of topology and geometry-controlled effects. Na
mely\, in magnetism\, curvilinear geometry brings about two exchange drive
n interactions - effective anisotropy and antisymmetric vector exchange\,
i.e. an effective Dzyaloshinskii-Moriya interaction. In addition\, another
magneto-chiral contribution emerges due to the dipole-dipole interaction.
In the case of superconducting nanostructures\, the combination of low-di
mensionality with a curvilinear geometry allows in principle for the obser
vation of topology-driven effects\, such as unconventional phase slips\, r
eversible and irreversible switching\, fractional flux-flow instabilities\
, and the Berezinskii-Kosterlitz-Thouless transition. \n\nIn my talk\, I w
ill introduce focused ion and electron beam-induced deposition\, FIBID and
FEBID\, respectively\, as direct-write techniques suitable for the fabric
ation of free-standing 3D nanoarchitectures with a resolution in the sub-n
m range vertically and 10-20 nm laterally. Next\, I will dwell on selected
proof-of-concept experiments\, technological limitations\, and future pro
spects of using 3D hybrid ferromagnet / superconductor structures with pr
e-defined shape and curvature in such research areas as microwave engineer
ing [5]\, superconducting spintronics [6]\, magnon fluxonics [7]\, as well
as graded-refractive-index spin-wave nano-optics [8].\n\nThis work was co
nducted within the framework of the Actions CM1301 "Chemistry for electron
-induced nanofabrication" (CELINA) and CA16218 "Nanoscale coherent hybrid
devices for superconducting Quantum technologies" (NANOCOHYBRI) of the Eur
opean Cooperation in Science and Technology (e-COST).\n\n[1] L. Keller et
al.\, Direct-write of free-form building blocks for artificial magnetic 3D
lattices\, Sci. Rep. 8 (2018) 6160.\n[2] M. Huth\, F. Porrati\, O. V. Dob
rovolskiy\, FEBID meets materials science\, Microelectron. Engin. 185-186
(2018) 9.\n[3] R. Streubel et al.\, Magnetism in curved geometries\, J. Ph
ys. D: Appl. Phys. 49 (2016) 363001.\n[4] S. Lösch et al.\, Microwave rad
iation detection with an ultra-thin and free standing superconducting niob
ium nano-helix\, [ACS Nano (2019)][1a]. \n[5] O. V. Dobrovolskiy et al.\,
Microwave emission from superconducting vortices in Mo/Si superlattices\,
Nat. Comms. 9\, 4927 (2018).\n[6] K.-R. Jeon et al.\, Enhanced spin pumpin
g into superconductors provides evidence for superconducting pure spin cur
rents\, Nat. Mater. 17 (2018) 499. \n[7] O. V. Dobrovolskiy et al.\, Magno
n-Fluxon interaction in a ferromagnet/superconductor heterostructure\, [Na
t. Phys. (2019)][2a].\n[8] C. S. Davies et al. Towards graded-index magnon
ics Phys. Rev. B 92 (2015) 020408.\n\n\n [1a]: https://pubs.acs.org/doi/a
bs/10.1021/acsnano.8b07280\n [2a]: https://doi.org/10.1038/s41567-019-042
8-5\n\nhttps://indico.knu.ua/event/4/contributions/610/
LOCATION:
URL:https://indico.knu.ua/event/4/contributions/610/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Symmetries shape the currents
DTSTART;VALUE=DATE-TIME:20190524T121500Z
DTEND;VALUE=DATE-TIME:20190524T124500Z
DTSTAMP;VALUE=DATE-TIME:20190422T044913Z
UID:indico-contribution-4-609@indico.knu.ua
DESCRIPTION:Speakers: Niurka Rodriguez-Quintero (University of Seville)\nD
ynamical systems often contain oscillatory forces or depend on periodic po
tentials. Time or space periodicity is reflected in the properties of thes
e systems through a dependence on the parameters of their periodic terms [
1\,2]. In this talk it is shown that *simple symmetry considerations* dete
rmine how their properties depend functionally on the amplitudes and the p
hases of the periodic terms\, regardless of whether they are classical or
quantum\, stochastic or deterministic\, dissipative or non-dissipative [3]
. It is shown that\, by using this formalism\, a small set of measurement
s is enough to obtain the functional form for a wide range of parameters.\
n\n[1] J. A. Cuesta\, N. R. Quintero\, and R. Alvarez-Nodarse\, Phys. Rev.
X 3\, 041014 (2013).\n[2] Niurka R. Quintero\, J. Cuesta and R. Alvarez-N
odarse\, Phys. Rev. E 81\, 030102(R) (2010).\n[3] J. Casado-Pascual\, J. A
. Cuesta\, N. R. Quintero\, and R. Alvarez-Nodarse\, Phys. Rev. E 91\, 022
905 (2015).\n\nhttps://indico.knu.ua/event/4/contributions/609/
LOCATION:
URL:https://indico.knu.ua/event/4/contributions/609/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Dynamics of magnetic domain wall motion in cylindrical nanowires
DTSTART;VALUE=DATE-TIME:20190523T084000Z
DTEND;VALUE=DATE-TIME:20190523T091000Z
DTSTAMP;VALUE=DATE-TIME:20190422T044913Z
UID:indico-contribution-4-607@indico.knu.ua
DESCRIPTION:Speakers: Olivier Fruchart (Univ. Grenoble Alpes\, CNRS\, CEA)
\nThe investigation of magnetization dynamics underpinning domain wall mot
ion in one-dimensional conduits\, has been a very active research topic ov
er the past two decades. Reports first considered motion under magnetic fi
eld\, later under spin-polarized current. Experiments have been performed
nearly exclusively on thin-film conduits\, for ease of fabrication and ins
pection. This allowed a clear understanding of the physics at play\, and w
ent up to demonstrators of a flat race-track memory. In parallel\, **theor
y has predicted peculiarities in cylindrical nanowires\, with new topologi
es of domain walls**. In particular\, one expects that the so-called Bloch
-point wall is not subject to Walker-type instabilities\, and should allow
one to reach the magnonic regime for domain wall motion.\n\nI will presen
t emerging results of magnetization dynamics in such systems. These have b
een performed based on nanowires made of electroplated soft magnetic mater
ials\, with diameter in the range 100-150nm.\n\n**Motion under magnetic fi
eld** may give rise to the dynamic change of topology of domain walls\, fr
om Bloch-point type to transverse type\, in contradiction with the theoret
ical predictions. FIG1 illustrates this\, featuring a tail-to-tail DW in i
ts thin section. The experiments are partly reproduced by micromagnetic si
mulations. These results partly put at stake the existing knowledge on suc
h walls.\n\n![FIG1][1]\nFIG1. *XMCD-PEEM views under zero magnetic field o
f the shadow of a 140 nm-diameter Fe20Ni80 nanowire. Arrows stand for magn
etization in the domains\, and the vertical dotted line indicates the diam
eter modulation. a)-b) and c)-d) are two sequences initialized with a BPW
followed by the application of a quasistatic field with strength 12 mT and
24 mT\, respectively\, as indicated in the final image. e) follows d)\, a
fter application of -16 mT.*\n\n**Motion under electric current** highligh
ts the crucial effect of the OErsted field\, an ingredient which had been
overlooked in most theoretical works so far. We show that the Oersted fiel
d stabilizes Bloch-point domain walls\, and selects the sign of their azim
uthal circulation. We moved such walls under current\, at a speed exceedin
g 500m/s (See FIG2). Experimental results are reproduced quantitatively us
ing micromagnetic simulations.\n\n\n![FIG2][2]\nFIG2. *Domain wall speed v
ersus density of the electric current flowing in the wire\, monitored with
either XMCD-PEEM or MFM. Error bars are largely related to the rounded sh
ape of the electric pulses\, related to the limited bandwidth of the measu
rement setups*\n\n\n [1]: https://indico.knu.ua/event/4/images/111-FRUCHA
RT_FIG1_sm.png "XMCD-PEEM views under zero magnetic field of the shadow of
a 140 nm-diameter Fe20Ni80 nanowire"\n [2]: https://indico.knu.ua/event/
4/images/113-FRUCHART_FIG2_sm.png "Domain wall speed versus density of the
electric current flowing in the wire\, monitored with either XMCD-PEEM or
MFM. Error bars are largely related to the rounded shape of the electric
pulses\, related to the limited bandwidth of the measurement setups"\n\nht
tps://indico.knu.ua/event/4/contributions/607/
LOCATION:
URL:https://indico.knu.ua/event/4/contributions/607/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Deposition of homogeneous magnetic layers on 3D non-conductive mic
rostructures
DTSTART;VALUE=DATE-TIME:20190524T090000Z
DTEND;VALUE=DATE-TIME:20190524T091500Z
DTSTAMP;VALUE=DATE-TIME:20190422T044913Z
UID:indico-contribution-4-608@indico.knu.ua
DESCRIPTION:Speakers: Petai Pip (Empa / Paul Scherrer Institut / ETH Züri
ch)\nIn comparison to planar structures\, three-dimensional magnetic micro
- and nanostructures offer greater degrees of freedom\, showing novel prop
erties such as magneto chirality effects [1]\, enhanced domain wall dynami
cs [2] and curvature-induced anisotropy [3]. Exploiting such effects can l
ead to great benefits for new applications like high-density storage devic
es\, as well as artificial spin systems that provide the possibility of cr
eating and tailoring specific physical properties.\n\nWhile new insights i
nto realizing the investigation of 3D magnetic systems are highly desired\
, the fabrication of three-dimensional systems is still challenging due to
the lack of reliable methods for the deposition of magnetic thin films on
high gradient surfaces.\n\nHere\, we present the electroless deposition o
f NiFe on a 3D-printed\, non-conductive microstructure.\n\nLow coercivity
and low magnetocrystalline anisotropy are well-known properties of permall
oy\, making it the ideal material for the study of the influence of three
dimensionality on the magnetic properties of a system. In contrast to wide
ly used sputtering\, which leads to significant shadowing effects [4]\, wi
th electroless deposition we can achieve the deposition of homogeneous lay
ers\, covering the whole framework (fig.1). This new technique represents
an important step towards the experimental realisation of 3D magnetic nano
structures with tailored properties.\n\n![FIG1][1a]\nFIG. 1\n\n[1] Fernán
dez-Pacheco\, A.\, et al.\, Three-dimensional nanomagnetism. Nature Commun
ications\, 2017. 8: p. 15756.\n[2] Yan\, M.\, et al.\, Fast domain wall dy
namics in magnetic nanotubes: Suppression of Walker breakdown and Cherenko
v-like spin wave emission. Applied Physics Letters\, 2011. 99(12): p. 1225
05.\n[3] Robert\, S.\, et al.\, Magnetism in curved geometries. Journal of
Physics D: Applied Physics\, 2016. 49(36): p. 363001.\n[4] Donnelly\, C.\
, et al.\, Element-Specific X-Ray Phase Tomography of 3D Structures at the
Nanoscale. Physical Review Letters\, 2015. 114(11): p. 115501.\n\n\n [1a
]: https://indico.knu.ua/event/4/images/115-PetaiPip_Fig1_sm.png "FIG1"\n\
nhttps://indico.knu.ua/event/4/contributions/608/
LOCATION:
URL:https://indico.knu.ua/event/4/contributions/608/
END:VEVENT
END:VCALENDAR