Topology constrained magnetic structure of Ni photonic crystals

Article abstract of DOI:10.1016/j.physb.2007.02.072

We report the small angle polarized neutron scattering study of nickel inverse opals, prepared by templating colloidal crystals made of polystyrene microspheres.

Full text of DOI:10.1016/j.physb.2007.02.072

ARTICLE IN PRESS
Physica B 397 (2007) 23–26
www.elsevier.com/locate/physb
Topology constrained magnetic structure of Ni photonic crystals
a,
Kirill S. Napolskii , Alexander Sinitskii , Sergei V. Grigoriev , Natalia A. Grigorieva ,
Ã
a
b
c
d
a
a
Helmut Eckerlebe , Andrei A. Eliseev , Alexei V. Lukashin , Yuri D. Tretyakov
a
a
Moscow State University, Moscow 119992, Russia
b
Petersburg Nuclear Physics Institute, Gatchina, St-Petersburg 188300, Russia
c
St-Petersburg State University, St-Petersburg 198504, Russia
d
GKSS Forschungszentrum, Geesthacht 21502, Germany
Abstract
We report the small angle polarized neutron scattering study of nickel inverse opals, prepared by templating colloidal crystals made of
polystyrene microspheres.
r 2007 Elsevier B.V. All rights reserved.
PACS: 42.70.Qs; 75.75.+a; 75.25.+z
Keywords: Small angle polarized neutron scattering (SAPNS); Photonic crystals; Inverse opals; Frustrated magnetism; Electrodeposition
1
. Introduction
laser wavelength is less than PC lattice period. Meanwhile,
important PCs for visible region possess a periodicity in a
range of 200–300 nm, making laser diffraction method not
applicable to their study. One can overcome this limitation
by using radiation of another type than laser one with a
wavelength short enough for recording diffraction patterns
from PCs with any periodicity.
Although small angle neutron scattering (SANS) is
extensively used for the study of spatially ordered
materials, to the best of our knowledge, it was never
applied to characterization of PCs. In this work we report
the preparation and small angle polarized neutron scatter-
ing (SAPNS) study of nickel inverse opals.
Photonic crystals (PCs), the materials with a periodic
modulation of dielectric constant, have recently attracted a
great attention due to their unusual optical properties and
promising applications [1]. Self-assembly of monodisperse
submicron microspheres provides a convenient tool for
synthesis of colloidal crystals, also referred to as artificial
opals, which are widely used for modeling of PC optical
properties. Another important class of PCs is inverse opals,
usually prepared by filling the voids of opal templates with
desirable materials and removing of the initial micro-
spheres to leave three-dimensionally ordered porous
frameworks [2]. The structure quality is a key parameter
for PCs, because only PCs well-ordered on a macroscale
could be used for fabrication of optoelectronic devices. The
local ordering of microspheres or spherical voids is usually
verified by scanning electron microscopy (SEM). Recently,
laser diffraction was demonstrated to be effective for
characterization of PC structure quality on a scale up to
several millimeters [3,4]. The significant limitation of this
method is that the diffraction can be observed only if the
2. Experimental part
The Ni PC films with an inverse opal structure were
prepared using a templating technique. Colloidal crystal
films made of 450 nm polystyrene microspheres were grown
onto polished Cu substrates by the vertical deposition
method [5].
Electrochemical crystallization of nickel in the voids
between the spheres was carried out in three-electrode cell
at room temperature. The counter electrode was a Pt wire
and the reference electrode was saturated Ag/AgCl
Ã
Corresponding author. Tel.: +7 495 939 52 48; fax: +7 495 939 0998.
E-mail address: napolsky@inorg.chem.msu.ru (K.S. Napolskii).
0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2007.02.072

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K.S. Napolskii et al. / Physica B 397 (2007) 23–26
24
electrode connected to the cell via a Luggin capillary; 0.1 M
NiCl , 0.6 M NiSO , 0.1 M H BO and 4 M C H OH
solution was used for potentiostatic Ni deposition at
2
4
3
3
2
5
E ¼ ꢀ0.85 V versus reference electrode. In order to obtain
d
free-standing Ni structure onto Cu substrate, the polystyr-
ene microspheres were dissolved in toluene.
SEM images of Ni PCs were recorded by LEO Supra 50
VP instrument. The structure and magnetic properties of
the samples have been studied by SAPNS with the SANS-2
instrument at the Geesthacht Neutron Facility (GeNF).
The beam of polarized neutrons with initial polarization
P ¼ 0.96, wavelength l ¼ 0.58 nm (Dl/l ¼ 0.1) and diver-
0
gence of 1.0 mrad was used. The scattered neutrons were
detected with a position-sensitive detector with 256 ꢁ 256
pixels. In SAPNS experiments the PC sample was oriented
perpendicular to the incident beam. An external magnetic
field (0 and 800 mT) was applied in the plane of the sample.
3
. Results and discussion
According to the SEM observations, all synthesized
samples possess an ordered porous structure with uniform
spherical pores (Fig. 1). Cross-section SEM images reveal a
face-centered cubic structure of spherical pores in the
Fig. 2. Typical experimental SAPNS diffraction pattern from Ni PC. The
square in the center marks a beam-stop, white circles represent theoretical
diffraction pattern.
Fig. 1. Low (a) and high (b) magnification SEM images of Ni inverse opals; (c) cross-section SEM image.

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K.S. Napolskii et al. / Physica B 397 (2007) 23–26
25
nickel framework (Fig. 1c). The average center-to-
center distance between close-packed voids is 450 nm,
suggesting no structure shrinkage during the fabrication
process.
Typical SAPNS diffraction pattern is shown in Fig. 2,
demonstrating several clearly resolved sets of hexagonally
arranged reflections. In similar laser diffraction experi-
ments, the patterns were interpreted considering each layer
of close-packed spherical voids in the PC as an individual
two-dimensional (2D) diffraction grating [4]. We can
introduce a 2D basis shown in Fig. 1b . For the consi-
dered geometry the interplanar spacings ðdh k 0Þ can be
calculated as
1
1
1
1
6000
4000
2000
0000
H = 0 mT
H = 26 mT
H = 67 mT
H = 135 mT
8
6
4
2
000
000
000
000
0
-
-
-
-
2000
4000
6000
8000
pffiffiffiffiffi
3
a
0.02
0.03
0.04
0.05
0.06
0.07
0.08
dh k 0 ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi .
q, nm-1
2
2
2
h þ hk þ k
Using a ¼ 450 nm, we have found the interplanar distances
1
0000
000
6000
for the first diffraction maxima ({1 0 0}, {1 1 0}, etc.) and
calculated the corresponding values of q -vectors ðq
8
¼
h k 0
2
p=dh k 0Þ, which were used for plotting the theoretical
diffraction pattern (white circles in Fig. 2). It can be seen
that the calculated positions for the most intense {1 0 0}
and {1 1 0} reflections perfectly match the experimental
maxima. The other diffraction reflections are not so well-
resolved onto the diffuse scattering background. Mean-
while, high-order diffraction maxima can be seen in q-
dependences of the neutron intensity I(q) for defined
directions. For instance, q-dependence in [1 1 0]-direction
4
2
000
000
0
-
-
2000
4000
H = 10 mT
H = 40 mT
H = 80 mT
H = 135 mT
-6000
-8000
ꢀ
1
demonstrates diffraction peaks at q
ꢂ 0:028 nm and
1
1 0
ꢀ
1
q2 2 0 ꢂ 0:056 nm (Fig. 3). This result differs from the
common laser diffraction experiments, in which only
sixfold diffraction patterns are usually observed [3,4].
It is worth noting that the patterns, typical of single
0.02
0.03
0.04
0.05
0.06
0.07
0.08
-1
q, nm
Fig. 4. q-Dependence of the nuclear–magnetic interference (a) and
magnetic contribution to the scattering I (q) (b).
H
crystals, were recorded using a large beam spot area (about
2
cm ), suggesting the well-ordering of Ni PCs on a
1
macroscale.
The increase in a magnetic field results in decrease in the
small-angle scattering from the multidomain structure
ꢀ
1
(
qo0.023 nm ; magnetic domain size more than 270 nm)
constrained by elementary blocks of Ni PC, while intensity
of Bragg peaks increases.
According to the polarization-dependent part of scatter-
(110)
ꢀ
+
1
00000
ing DI(q) ¼ (I (q)ꢀI (q)), nuclear and magnetic structures
are well-correlated (Fig. 4a). The two types of contribu-
tions to the interference scattering, the diffuse small angle
scattering and Bragg reflection, with domination of the
second one are clearly seen.
1
0000
The pure magnetic contribution to the scattering, also
referred to as field-induced scattering, was extracted as
(220)
I (q) ¼ I(q, H)ꢀI(q, 0) (Fig. 4b). In addition to the
H
1
000
decreasing of the small-angle scattering described above,
the magnetic reflections are clearly observed. It testifies to
transition from multi- to single-domain magnetic structure
at H450 mT. The nature of the several additional
magnetic reflections (indicated by arrows) with rather
small intensities and positions not connected to the Bragg
peaks demands the future investigation.
H = 0 mT
H = 680 mT
0
.02
0.03
0.04
0.05
q, nm-1
0.06
0.07
0.08
Fig. 3. SAPNS intensity profile for Ni PC in [1 1 0] direction.

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4
. Conclusions
References
[
[
1] T.F. Krauss, R.M. De La Rue, Prog. Quant. Electron. 23 (1999) 51.
2] A. Stein, Micropor. Mesopor. Mater. 44–45 (2001) 227;
C. Lopez, Adv. Mater. 15 (2003) 1679.
3] Yu.A. Vlasov, et al., Nature 414 (2001) 289;
J.F. Galisteo-Lopez, et al., Phys. Rev. B 68 (2003) 115109;
It was first demonstrated that SAPNS technique is
effective for the study of features of the nuclear and
magnetic structures of PCs. Magnetic inverse opals can be
promising model objects for the investigation of highly
frustrated magnetism and artificial spin ice effects.
´
[
[
[
´
B.G. Prevo, O.D. Velev, Langmuir 20 (2004) 2099.
4] R.M. Amos, et al., Phys. Rev. E 61 (2000) 2929;
A. Sinitskii, et al., Phys. Lett. A (2007), doi:10.1016/j.physleta.
Acknowledgment
2
007.02.075.
5] P. Jiang, et al., Chem. Mater. 11 (1999) 2132;
A. Sinitskii, et al., Mendeleev Commun. 17 (2007) 4.
This work is supported by RFBR (Grant nos. 06-03-
3052, 06-03-08157 and 05-03-32778) and Federal Target
3
Science and Engineering Programme. The authors thank
A.G. Veresov for SEM studies.