Plasmonic Au/Co/Au nanosandwiches with enhanced magneto-optical activity
Juan B. González-Díaz1, Antonio García-Martín1, José M. García-Martín1, Alfonso Cebollada1, Gaspar
Armelles1*, Borja Sepúlveda2, Yury Alaverdyan2, and Mikael Käll2
1
Instituto de Microelectrónica de Madrid (IMM-CNM-CSIC), Isaac Newton 8, Tres Cantos 28760,
Madrid, (Spain)
2
Chalmers University of Technology, SE-412 96, Göteborg, (Sweden)
Keywords
Nanoparticles • Optics • Ferromagnetic materials • Plasmonics • Magneto-optical activity
Abstract
A nanostructure composed of Au/Co/Au nanodisks is fabricated. The behaviour of this system is
completely different to that of a continuous Au/Co/Au layer. There is a relative enhancement of the Kerr
rotation and ellipticity, as well as a strong dependence on the particle size. The origin of these properties
is the plasmonlike structure that is supported by the nanosandwiches.
E-mail: gaspar@imm.cnm.csic.es
Manuscript:
As is well known, localized surface plasmon resonances (LSPR’s) greatly influence the optical
properties of metallic nanostructures. The spectral location of the LSPR sensitively depends on the shape,
size and composition of the nanostructure, as well as on the optical properties of the surrounding
dielectric.[1] The latter effect has been used to develop different types of optical biosensors, where
biological reactions near the surface of the nanostructure can be monitored through the changes in the
frequency of the LSPR.[2-6] The induced electromagnetic field associated with the LSPR is highly
enhanced at the metal/dielectric interface, a phenomenon that is the basis for various types of surfaceenhanced spectroscopies, such as surface-enhanced Raman scattering.[7] Moreover, metallic nanoparticles
have also shown light-guiding capabilities at the nanometer scale, which makes them suitable to develop
nano-optic devices.[8] The overwhelming majority of LSPR studies have focused on nanoparticles made
of Au or Ag, as these metals present suitable optical constants for applications in the visible. However,
once the morphology and composition of a nanostructure have been fixed, it is difficult to change or
control the LSPR properties by external means, a property that would be desirable for the development of
active nanoplasmonic devices. One way to overcome this problem could be to embed the metal
nanostructure in an active medium, such as a liquid crystal,[9] which can be controlled by an external
electrostatic field, or a ferromagnetic garnet,[10,11] which can be affected by a magnetic field. An
alternative approach could be to let the controlling field act directly on the metallic nanostructure, for
instance, by using nanoparticles made of ferromagnetic metals. Such metals have strong magneto-optical
(MO) activity, i.e. their optical properties change markedly even if the applied magnetic field is weak.
1
Unfortunately, their high optical absorption results in a strong damping of any intrinsic LSPR, which
prevents the development of active plasmonic devices made solely of ferromagnetic metals. A promising
route forward could then be to combine ferromagnetic materials, which would endorse a strong MO
activity, with noble metals, which would be responsible for the plasmonic response. The large
enhancement and spatial localization of the electromagnetic field associated to the LSPR suggest that a
strong enhancement of the MO properties should be possible.[12] Several attempts to obtain this kind of
structures have been carried out by fabricating complex onion-like nanoparticles made of noble metals
and ferromagnetic materials, using different chemical synthesis methods.[13-16] These systems do exhibit
LSPRs, but no MO activity has so far been reported. On the other hand, in the case of continuous thin
films made of Au/Co/Au trilayers, it was found that well-defined propagating surface plasmon polaritons
and strong MO activity at low magnetic fields can be present simultaneously. [17] Moreover, such
composite structures also showed a magnetic-field induced non-reciprocal effect in the surface plasmon
polariton propagation,[18,19] i.e. forward or backward propagating surface plasmons exhibit different
wavevectors. A promising recent application of the composite Au/Co thin films is the new type of high
sensitivity “magneto-plasmonic” biosensor reported in Ref. 20. In this communication, we show that
strong magneto-plasmonic effects occur in nanosandwiches composed of stacked Au/Co/Au disks. The
Au/Co/Au nanosandwiches, prepared by a self-assembly process, exhibit simultaneous LSPR, magnetic
and MO properties. We show that the optical and magneto-optical properties are strongly linked to the
LSPR spectrum, which can be tuned by modifying the size of the nanoparticles. Moreover, there is a large
enhancement of the MO properties caused by the LSPR effect. To the best of our knowledge, this is the
first demonstration of a plasmonic nanostructure that can be controlled by an external magnetic field.
The fabrication of the sandwich nanostructures from sputtered Au/Co/Au trilayer films was performed
using colloidal lithography (CL).[21,22] In Figure 1 we show an AFM image corresponding to the sample
obtained using polystyrene spheres of 76 nm diameter. As can be observed, the resulting nanoparticles
have the shape of truncated cones due to the leftovers of the polystyrene spheres. The plasmonic and
magneto-optic properties of the fabricated nanosandwiches are analyzed by conventional UV-vis
extinction spectroscopy and magneto-optic Kerr spectroscopy, respectively. The magneto-optic Kerr
effect consists in the change of the reflectivity of the magnetic material when a magnetic field is applied.
In particular, we employ a polar Kerr configuration in which the magnetic field is perpendicular to the
nanosandwiches. In this configuration, the reflected light by the magnetized sample suffers a rotation of
the polarization plane and change in the ellipticity state with respect to the incident linearly polarized
light.
In Figure 2 we present the extinction spectra for samples of Au/Co/Au nanoparticles of 60, 76 and
110 nm diameter. We compare these spectra to that of a sample containing Au nanodisks of 60 nm
diameter and 32 nm height. As can be observed, the extinction spectra of all the samples are commonly
characterized by a peak associated to the LSPR. The effects induced by the Co can be extracted from the
comparison of the Au/Co/Au and Au 60 nm diameter disks; these are a broadening and a shift to higher
energies of the LSPR peak. Both shifts and broadenings of the LSPR peak induced by Co with respect to
pure Au have previously been observed in core-shell nanoparticles made of ferromagnetic and noble
metals. Depending on the system, the peak is blue- (Ref. 16) or red-shifted (Refs. 13,14), which suggests
that it depends on the actual structure of the nanoparticles. However, the broadening of the LSPR peak
has always been observed, and has been ascribed to the high absorption of Co. On the other hand, the
extinction peak of the Au/Co/Au nanoparticles shifts towards lower energy as we increase the disk
diameter. This behavior is similar to the one observed in pure noble metal nanoparticles, reflecting its
2
plasmonic origin. This is best viewed in the inset of Figure 2, where the energetic positions of the
absorption peaks for the Au/Co/Au nanoparticles is depicted.
The effect of incorporating Co into the Au nanoparticles is not only to induce a shift and a broadening of
the LSPR, but it also introduces MO activity absent in pure Au. This effect has not been reported in any
of the composite systems so far developed. As an example, Figure 3 shows the effect that a magnetic field
applied perpendicular to the sample plane has on the polarization state of the reflected light for the
Au/Co/Au nanoparticles of 60 nm diameter. In particular, we present the magnetic field dependence of
the ellipticity (hysteresis loop of the Kerr ellipticity) at normal incidence, obtained using the experimental
set-up described in Ref. 23. It is worth noticing that no magnetic field induced changes of the polarization
state of the reflected light was observed for the pure Au nanoparticles sample, as expected. As can be
observed, as we increase the magnetic field, the Kerr ellipticity increases and reaches a saturation value
for a magnetic field of around 12000 Oersted. This value corresponds to the magnetic field needed to
saturate the magnetization of the Co layer within the nanoparticles. The value of the ellipticity at
saturation depends on the wavelength of the incident light, as is shown in Figure 4a, where we present its
wavelength dependence compared to that of a continuous Au/Co/Au layer with identical Au and Co
thickness. As can be observed in the nanosandwiches sample, the Kerr ellipticity spectrum presents a
peak at the same energy region of the extinction peak. Moreover, as we increase the size of the
nanoparticles, the position of the maximum of the Kerr ellipticity is red-shifted, as the extinction peak
does (see Figure 4a). Therefore, we can associate the peak of the ellipticity spectrum to the LSPR of the
Au/Co/Au nanoparticles.
In Figure 4b the polar Kerr rotation spectrum is also shown: this spectrum has an S-shape structure
that is, once again, located in the energy region of the extinction peak. For comparison, we present in the
same figure the polar Kerr rotation and ellipticity spectra of the continuous trilayer system with identical
Au and Co thickness. As can be observed, these spectra are completely different, which pinpoints the
effect of the LSPR in the MO properties of the Au/Co/Au nanoparticles. Furthermore, although the
amount of Co in the continuous layer is nearly 5 times higher than that of the nanoparticle layer, the
magnitude of the MO effects are similar, which indicates that the LSPR present in the nanoparticles
induce a large enhancement of their MO activity.
To understand the origin of the observed MO effects and their correlation with the LSPR excitations,
we have calculated the Polar Kerr ellipticity and rotation spectra for an array of Au/Co/Au nanoparticles
with identical dimensions as the experimentally studied. This is done using a scattering matrix formalism
adapted to treat materials with MO activity,[24] which allows an exact solution of the wave propagation in
ordered arrays. It takes into account all the interactions between the different nanoparticles and substrate
effects, allowing us to calculate the full reflectivity Jones matrix r:
 rss rsp 


(1)
r

 ps rpp 
where s and p represent the two polarizations. From this matrix we can obtain the complex Kerr rotation
that is defined as:
r
    i  ps
(2)
rpp
,being  the Kerr rotation and  the Kerr ellipticity.
The nanoparticles are modeled as sandwiches of three disks, whose diameters correspond to those of
the polystyrene spheres, and the thickness of each disk to that of the experimental Au and Co layers. We
have considered two types of geometries for the disks arrangement: triangular and square. The distance
3
between the disks (lattice parameter) is determined by the surface coverage of the polystyrene spheres
(Figure 4 presents a sketch of the arrangement). The optical properties of the Au and Co were obtained
from Ref. 19, and the MO constants of Co were extracted from the experimental results of the Polar Kerr
rotation and ellipticity spectra of the continuous Au/Co/Au film. In figure 4c and d we present the
theoretical results for the 60nm and 110 nm Au/Co/Au sample. As can be observed, both types of
arrangements give similar results. They reproduce both the evolution observed in the MO spectra when
we change from a continuous to a nanoparticle layer (i.e: the appearance of both the S-shaped structure in
the polar Kerr rotation and the peak in the ellipticity spectra, as well as the enhancement of the MO
activity) and the red-shift of the spectra when we increase the disk diameter. On the other hand, the values
of the theoretical spectra are higher than the experimental ones. This effect may be attributed to an
overestimation of the amount of Co inside the nanoparticles. This reduction could be due to the existence
of a non ferromagnetic layer on the peripheral surface of the nanoparticles, since Co is laterally exposed
to air and, therefore, could be slightly oxidized. We have also calculated the absorption spectrum of the
disks layer and, in Figure 4e, we present the theoretical position of the absorption and ellipticity peaks for
the two arrangements as a function of the disk diameter. The theoretical result reproduces the trend
observed in the experimental results: a red-shift of both peaks as we increase the size of the nanoparticles,
and a red-shift of the position of the ellipticity peak with respect to the absorption peak. Therefore, the
observed structure in the Polar Kerr rotation and ellipticity spectra is indeed related with the LSPR of the
Au/Co/Au nanoparticles. The red-shift of the position with respect to the absorption peak is due to the
different wavelength dependence of the optical and MO constants.
In summary, large scale active nanoparticles exhibiting optical properties that can be controlled by an
external magnetic field were obtained by colloidal lithography from Au/Co/Au thin films. The optical and
magneto-optical properties are governed by the LSPR’s of the nanoparticles and can be tuned by
modifying the size or shape of the nanoparticles. In addition, the LSPR enhancement of the
electromagnetic field in the cobalt layer induces a large increase of the MO effects. These effects open the
possibility to develop new types of active optical devices based on LSPR’s. The subwavelength size and
the strong localization of the electromagnetic field in the nanoparticles make them as well promising
candidates for the development of high sensitivity magneto-plasmonic nanosensors with multiplexing
capabilities.
Acknowledgemetns:
Financial support from the Spanish Ministry of Science and Education (NAN2004-09195-C04-01 and
MAT2005-05524-C02-01), and UE (NoE-Phoremost) are acknowledged.
4
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Experimental Section
Nanosandwich fabrication: Metal trilayer films composed of 6 nm of Au, 10 nm of Co and 16 nm of Au
are sputtered on glass substrates. The top metal surface is first coated with a layer of poly(diallyldimethylammonium chloride) (MW 200 000–350 000, Aldrich), which makes the surface positively charged and
facilitates electrostatic adsorption of negatively charged sulfate latex spheres (Interfacial Dynamics
Corporation, U.S.A.). The surface is then rinsed with Milli-Q water, dried with nitrogen, and exposed to a
5
0.2 wt% aqueous solution of latex spheres. After rinsing again with Milli-Q water and drying with
nitrogen, the top metal surface becomes patterned with a short-range ordered array of latex spheres. The
particle array is then used as a mask for directed Ar ion beam etching (Oxford model 300 Ion Beam
Etching System, 500 V, 200 mA, 240 s).[22] The diameter of the latex spheres (60, 76 or 110 nm) directly
controls the diameter of the nanosandwiches. The equilibrium coverage, defining the average distance
between the spheres on the surface, can be controlled by electrostatic repulsion through the surface charge
of the spheres. In this study, we used surface coverages between 20 and 30%, while the total area covered
by nanosandwiches was at least 1 cm2.
Figures
Figure 1. Three and two dimensional AFM images of the Au/Co/Au nanoparticle sample obtained with
76 nm diameter polystyrene spheres.
6
2.0
60 Au
60 Au/Co/Au
75 Au/Co/Au
110 Au/Co/Au
Energy (eV)
Absorption (a.u.)
2.5
2.24
2.20
2.16
2.12
2.08
Au/Co/Au
60
75
90 105
Diameter (nm)
1.5
1.0
0.5
1.6
2.0
2.4
2.8
3.2
Energy (eV)
Figure 2. Extinction spectra of the different samples studied. In the inset we present the energy position
of the extinction peaks as a function of the polystyrene sphere diameter.
Kerr Ellipticity (a.u.)
H
1.0
0.5
sat.
0.0
-0.5
-1.0
-15000
-7500
0
7500
15000
Magnetic Field (Oe.)
Figure 3. Polar Kerr loop of the Au/Co/Au nanoparticle sample obtained with the 60 nm diameter
polystyrene spheres. A sketch of the experimental configuration is also presented.
7
Experiment
Theory
Kerr Rot. (º) Kerr Ellip. (º)
Experiment
(a)
Theory
(c)
0.2
Continuous layer
060nm disks
TR
110nm disks
TR
0.0
(b)
SQ
SQ
Continuous layer
060nm disks
110nm disks
(d)
0.0
-0.2
1.8
2.4
3.0
1.8
Energy (eV)
2.4
(e)
2.2
Peak position (eV)
3.0
2.0
1.8
Absorption
Theory SQ
Theory TR
Kerr Ellip
Theory SQ
Theory TR
1.6
60
70
80
90
100
110
Disk Diameter
Figure 4. (a,b): Experimental polar Kerr ellipticity and rotation spectra of the 60 nm (circles) and 110
nm (squares) diameter Au/Co/Au sample, and experimental polar Kerr ellipticity and rotation spectra of
the continuous Au/Co/Au layer (triangles) (c,d): Theoretical polar Kerr ellipticity and rotation spectra for
two different disk arrangement: 60nm diameter disks in triangular (thin continuous line) and square (thin
doted line) lattices; 110nm diameter disks in triangular (thick continuous line) and square (thick doted
line) lattices. For comparison, the experimental spectra of the continuous Au/Co/Au layer are also
presented (thinner continuous line). (e): Position of the absorption and Kerr ellipticity peaks as a function
of the nanoparticle size. Dots: experimental position. Lines: theoretical position in triangular lattice
(continuous) or in square lattice (dots). In the inset, a sketch of the theoretical disk arrangement is
presented.
8