Magneto-optic surface plasmon resonance of Au/IrMn/Co/Au exchange biased layersystemsSebastian Kübler, Nicolas Müglich, and Arno Ehresmann Citation: Journal of Applied Physics 116, 064502 (2014); doi: 10.1063/1.4892563 View online: http://dx.doi.org/10.1063/1.4892563 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/116/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Magneto-optic surface plasmon resonance optimum layers: Simulations for biological relevant refractive indexchanges J. Appl. Phys. 112, 034505 (2012); 10.1063/1.4742130 Surface plasmon resonance and magneto-optical enhancement on Au–Co nanocomposite thin films J. Appl. Phys. 107, 103924 (2010); 10.1063/1.3428470 Evidence of localized surface plasmon enhanced magneto-optical effect in nanodisk array Appl. Phys. Lett. 96, 081915 (2010); 10.1063/1.3334726 IrMn as exchange-biasing material in systems with perpendicular magnetic anisotropy J. Appl. Phys. 97, 10K114 (2005); 10.1063/1.1855699 Observation of magneto-optical second-harmonic generation with surface plasmon excitation in ultrathinAu/Co/Au films Appl. Phys. Lett. 75, 190 (1999); 10.1063/1.124315

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Magneto-optic surface plasmon resonance of Au/IrMn/Co/Au exchangebiased layer systems

Sebastian K€ubler,a) Nicolas M€uglich, and Arno EhresmannInstitute of Physics and Center for Interdisciplinary Nanostructure Science and Technology (CINSaT),University of Kassel, Heinrich-Plett-St. 40, 34132 Kassel, Germany

(Received 25 April 2014; accepted 28 July 2014; published online 12 August 2014)

The magneto-optic surface plasmon resonance in transverse geometry of glass/Au/IrMn/Co/Au/

dielectric exchange biased thin film systems has been studied as a function of the Au-cap layer

thickness for the dielectric air and water. The results show that it is possible to achieve either an op-

timum steepness of the reflectivity as compared to a glass/Au/dielectric layer system or to maxi-

mize the magneto-optic activity. As the Au-cap layer thickness is increased, the magneto-optic

activity is reduced whereas the steepness of the reflectivity in the range of the resonance is raised.

The width of the reflectivity curves and the magneto-optic signals is broadened for thin Au-capping

layers. VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4892563]

I. INTRODUCTION

The binding behaviour of biomolecules is one of the

main topics in biochemical and pharmaceutical research.

The most common biosensors for bimolecular interaction

analysis (BIA) are based on the surface plasmon resonance

(SPR) effect.1,2 Surface plasmons (SPs) are collective charge

density oscillations at the interface between dielectric mate-

rials and metals. In most biosensor applications, SPs are

excited by linearly polarized light measuring either the

reflectivity as a function of the incident angle or the trans-

mission as function of the wavelength. The resonance angle

hR or wavelength kR depends on the refractive index n in

front of the metal layer (dielectric), and therefore, on the

concentration of biomolecules bound to the sensor surface.1,3

Different measurement configurations (e.g., based on prism-,

grating-, and waveguide couplers)4 and material systems

(e.g., Au, Ag, and Au/ZnO)4,5 have been studied in order to

increase the sensitivity of SPR based biosensors, since the

first commercial SPR based biosensors have been introduced

in the early 1990s. The ability of fabricating nano-structures

and particles with dimensions much smaller than the wave-

length of the exciting light enabled the study of localized

surface plasmon resonance effects leading to highly sensitive

LSPR biosensors.6,7 In recent years, the combination of

magneto-optic (MO) and plasmonic features in planar films

as well as in nanosystems has been established as an active

research field.8 A first investigation towards a magneto-

optic SPR (MOSPR) based biosensor with enhanced sensi-

tivity as compared to standard SPR based sensors used the

combination of the transverse magneto optic Kerr effect

(tMOKE) and the SPR effect in Kretschmann configuration

[Fig. 1(a)], where the MO activity is strongly enhanced due

to the evanescent field connected to SP excitation.9 The

tMOKE causes a change in reflectivity as a function of My

(magnetization perpendicular to the plane of incidence, see

coordinate system Fig. 1(a)) for linearly polarized light

(parallel to the plane of incidence). MOSPR based biosen-

sors most commonly employ the normalized reflectivity dif-

ference d

d ¼ Rþ � R�R0

(1)

(Rþ; R�: reflectivity for a relative magnetization

my ¼ My

Ms¼ 61, where Ms is the saturation magnetization of

the ferromagnetic layer; R0: reflectivity for my¼ 0) to

achieve an enhanced sensitivity. The enhancement of the

tMOKE has been investigated experimentally and theoreti-

cally on a variety of metallic layer combinations, e.g.,

Co/Au,9 Ag/Co/Ag,10,11 Au/Co/Au,12–15 and Au/Fe/Au.16

These investigations have shown that the thickness of the

FM material is critical due to the high absorbance of mag-

netic materials. For this reason exchange biased (EB) layer

systems, a combination of a ferromagnetic (FM) layer and

an antiferromagnetic (AFM) layer with a remanent magnet-

ization of the FM layer, have up to now not been considered.

However, these layer systems possess high potential for

SPR and/or MOSPR biosensing applications. They may be

patterned magnetically17,18 and, therefore, employed for the

transport of superparamagnetic particles as biomolecular

carriers. With this concept, the integration of (1) mixing by

micro magnetic stirrers, (2) biomolecule pre-concentration

procedures, and (3) signal enhancement by magnetic

markers into the sensor would be feasible.19,20

In the present paper, we investigated the SPR

and MOSPR characteristics in transverse geometry of

Au/IrMn/Co/Au EB layer systems theoretically and

experimentally employing a prism coupled set-up in

Kretschmann configuration [Fig. 1(a)]. Therefore, the

reflectivity R0(h), the slope R00ðhÞ ¼DR0ðhÞ

Dh , the difference

DRðhÞ ¼ RþðhÞ � R�ðhÞ, and the d-signal dðhÞ have been

studied as a function of the Au-cap layer thickness for the

dielectrics air and water. As a benchmark, the results are

compared to the ones of a 48 nm Au layer commonly used

in commercial SPR sensors.a)Electronic mail: kuebler@uni-kassel.de

0021-8979/2014/116(6)/064502/7/$30.00 VC 2014 AIP Publishing LLC116, 064502-1

JOURNAL OF APPLIED PHYSICS 116, 064502 (2014)

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II. THEORETICAL SECTION

The reflectivity of Au/IrMn/Co/Au four layer systems in

between a glass prism (BK7, n¼ 1.515), and the dielectric

(air: n¼ 1, water n¼ 1.33) has been calculated with a tested

code15 employing the transfer matrix formalism for magnetic

multilayers.21,22 The wavelength of the p-polarized (parallel

to the plane of incidence) light reflected through the glass

prism was set to k ¼ 635 nm. The following permittivity

values were used for calculation: �Au ¼ �11:00þ i1:98;�Co ¼ �11:43þ i18:15,9 and �IrMn ¼ �2:2ð60:2Þ þ i15:05

ð60:3Þ, the latter obtained from ellipsometry measurements

of a 10 nm IrMn film. The Voigt-constant Q of Co was con-

sidered as Q ¼ 0:043þ i0:007.23 For better comparison

with the experiments, the reflectivity curves RþðhÞ; R�ðhÞand R0ðhÞ have been normalized for the maximum of

RþðhÞ.The influences of surface- and interface roughness as

well as the layer thickness dependence of the optical con-

stants have been neglected. For reason of these approxima-

tions and the uncertainty of the used parameters, the results

of the calculations can only predict the general dependencies

of the SPR and MOSPR characteristics on the Au layer

thicknesses of Au/IrMn/Co/Au layer systems. Therefore, the

simulations are not capable to predict absolute experimental

values.

III. EXPERIMENTAL SECTION

The metallic thin film systems were deposited on 1 mm

BK7 glass substrates via RF sputter deposition under a base

pressure in the range of 10�4 Pa. The substrates were

cleaned ultrasonically in consecutive baths of acetone,

methanol, and isopropanol and were plasma cleaned for 10

min at 100 W prior to deposition. The metallic layers were

deposited with 0.6 nm/min for the Au layers, 1.8 nm/min for

the IrMn layers (Ir17Mn83 target), and 0.5 nm/min for the

Co layers. DektakVR

profile measurements with an accuracy

of 610% were used for the rate determination. For five ran-

dom samples, the reproducibility of the IrMn layers with a

constant nominal thickness t and the relative change of the

Au layer thicknesses were tested by X-ray fluorescence

spectroscopy (fluorescence counts divided by t) and varia-

tions of up to 66% have been obtained. After deposition,

the thin film systems were subsequently heated to 523 K for

30 min and field cooled with a cooling rate of vc¼ 8 K/min

in an applied magnetic field of 80 kA/m. The magnetic

characterization of the metallic layer systems was carried

out by vibrating sample magnetometer (VSM) measure-

ments. The root mean squared surface roughness (averaged

over 2 5� 5lm images per sample) was determined by

atomic force microscopy for two substrates and four ran-

domly selected field cooled samples.

FIG. 1. (a) Schematic setup and coordinate system for the experiments and calculations. For the calculations, the matching liquid and a possible influence of

the unidirectional anisotropy on the SP characteristics have been neglected. The dielectric and the glass layer have been considered being infinite. (b) dmax (col-

our scale) as a function of the Au-buffer and Au-cap layer thicknesses for the dielectric air (n¼ 1) and (c) for the dielectric water (n¼ 1.33). (d) R00;min (colour

scale) as a function of the Au-buffer and Au-cap layer thicknesses for the dielectric air and (e) for the dielectric water. The white markers in (b)–(e) denote the

layer thickness combinations considered for experiments.

064502-2 K€ubler, M€uglich, and Ehresmann J. Appl. Phys. 116, 064502 (2014)

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SPR and MOSPR measurements were performed with a

home built set-up in Kretschmann configuration employing

p-polarized radiation of a diode laser (k¼ 635 nm, ULN,

Coherent). In this setup, the sample is coupled by a refractive

index matching liquid (n¼ 1.517, AppliChem) to a plano-

convex cylindrical lens (BK7, custom made, Hellma Optics),

forming a semicylindrical prism along with the substrate

[Fig. 1(a)], and mounted on top of a motorized rotation stage

(8MR151–30, Standa) changing the angle of incidence in

steps of 0:05 �. The reflected light is focused on a transimpe-

dance amplified Si photodetector (PDA36A-EC, Thorlabs)

using a gain of 30 dB. The magnetization þ and � of the Co

layer is induced by an electro magnet generating magnetic

fields of Hþ¼ 50 kA/m and H� ¼ �50 kA=m. The unidirec-

tional anisotropy of the samples was aligned along Hþ. The

reflectivity R0(h) has been calculated from the average of the

measured reflectivities Rþ(h) and R�ðhÞ. All reflectivity

curves RþðhÞ; R�ðhÞ, and R0(h) have been normalized for

the maximum of Rþ(h) and smoothed with unweighted aver-

ages of 5 points. Double distilled, filtered, and degased water

was used for experiments with the dielectric water.

IV. RESULTS AND DISCUSSION

As the absorption of the magnetic layer system is high,

it is important to make it as thin as possible maintaining the

exchange bias. The thickness of the complete IrMn/Co EB

system was therefore reduced to 9 nm (IrMn6 nm=Co3 nm)

achieving stable EB fields of jHEBj ¼ 1963 kA=m on top of

an 8 nm and jHEBj ¼ 1962 kA=m on top of an 12 nm Au-

buffer with coercive fields of jHCj ¼ 962 kA=m and

jHCj ¼ 661 kA=m, respectively (see Fig. 2).

In order to identify thickness combinations with opti-

mized MO and SPR features, the maximum value dmax of d(h)

and the minimum R00;min (corresponds to the maximum nega-

tive value of R00ðhÞ) have been calculated as a function of the

Au layer thicknesses of glass/Au-buffer/IrMn6 nm=Co3 nm/Au-

cap/dielectric layer systems for the dielectric air and the one

of water [Figs. 1(b)–1(e)]. The simulations for dmax [Figs. 1(b)

and 1(c)] clearly show a defined narrow band of thickness

combinations, where the MO activity is high (dmax� 0.5). As

the glass-dielectric refractive index difference is reduced by a

change from air to water this narrow area shifts to higher Au

layer thicknesses. These results are consistent with previous

calculations for Au/Co/Au three layer systems.15 The sensitiv-

ity g of refractive index sensors is defined as the change of the

output signal dY per refractive index change dn1

g ¼ dY

dn: (2)

For angle dependent reflectivity measurements Y¼R(h, n)

and dY :¼ dR can be expressed as

dR h; nð Þ ¼ @R

@hdhþ @R

@ndn: (3)

Therefore, the maximum accessible sensitivity for a distinct

metallic layer system depends on the slope of the reflectivity

R00ðhÞ ¼DR0ðhÞ

Dh for a constant n. The calculations show that

the absolute value of R00;min is maximized for Au-buffer layer

thicknesses of 5–10 nm and Au-cap layer thicknesses of

33–36 nm for the dielectric air [Fig. 1(d)]. For the dielectric

water, the slope is optimized for layer systems with a

7–13 nm Au-buffer layer and a 22–27 nm Au-capping layer

[Fig. 1(e)]. For these reasons, the MOSPR signals of the EB

thin film systems glass/Au8 nm=IrMn6 nm=Co3 nm=Au5–18 nm/

air, glass/Au12 nm=IrMn6 nm=Co3 nm=Au5–23 nm/-air, and -water

have been studied experimentally. Additionally MOSPR

measurements of a glass/Au8 nm=IrMn6 nm=Co3 nm=Au35 nm/air

and a glass/Au12 nm=IrMn6 nm=Co3 nm=Au25 nm/water system,

with parameters for high steepness of the reflectivity in the

range of the resonance, have been performed. The results of

the glass/Au8 nm=IrMn6 nm=Co3 nm=Au5–18 nm/air measurement

series are summarized in Figure 3.

The reflectivity R0(h) and its slope R00ðhÞ are compared to

the one of a glass/Au48 nm /air system with optimized SP exci-

tation properties used as a benchmark. It can be seen that the

minimum reflectivity decreases slightly from 5–7 nm Au-cap

FIG. 2. Hysteresis loops of (a) glass/Au8 nm=IrMn6 nm=Co3 nm=Au15 nm and (b) glass/Au12 nm=IrMn6 nm=Co3 nm=Au9 nm EB systems obtained by VSM measure-

ments. The given HEB and HC values have been determined employing an arctangent fit function (red line).

064502-3 K€ubler, M€uglich, and Ehresmann J. Appl. Phys. 116, 064502 (2014)

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layer thickness before it increases with the cap layer thickness

[Fig. 3(a)]. This behaviour is similar to the one of single layer

systems approaching a critical thickness with optimum SP ex-

citation conditions. Increasing the thickness further leads to

higher absorbance of the metal layer and therefore to an

increase of R0,min until the minimum vanishes completely.13

As expected from the calculations [Fig. 1(d)], the maximum

negative slope R00;min is observed for the EB system with the

thickest Au-cap layer of 35 nm. The slope R00ðhÞ of this sys-

tem is similar to the one of the pure Au layer [Fig. 3(b)] and

in good agreement with the simulations. This finding demon-

strates that this magnetic EB layer system in between two Au

layers achieves similar sensitivities as compared to standard

Au layers. In contrast to R00ðhÞ, both MO signals DR(h) and

d(h) show the maximum amplitude for the thinnest Au-cap

layer [Figs. 3(c) and 3(d)]. As the capping layer thickness

increases the evanescent field penetration into the Co layer is

decreased leading to a reduced MO activity. A significant

increase of the maximum of the differential signal d(h) at a

cap layer thickness of 7 nm as expected from the calculations

[Fig. 1(b)] is not observed. This is explained by the uncertain-

ties of the optical constants, the error of the layer thicknesses

as well as the surface- and interface roughness, which have

not been considered in the simulations. Therefore, the simula-

tions of dmax [Figs. 1(b) and 1(c)] have to be considered as an

estimate for optimum thickness combinations.

Replacing the 8 nm Au-buffer layer [Fig. 3] by a 12 nm

Au-buffer layer [Fig. 4] results in an increase of R0,min by a

factor of about 2 for measurements with the same cap-layer

thickness and dielectric (air). The lowest R0,min value of 0.06

is observed for the thinnest Au-cap layer (5 nm) and

increases almost linearly with the cap layer thickness. The

maximum negative slope R00;min and reflectivity difference

DRmax [Fig. 4(b)] are not affected significantly by the change

of the buffer layer thickness. The maximum of d(h) is

slightly reduced and decays exponentially with the capping

layer thickness [Fig. 4(d)] due to the exponential decrease of

the evanescent field demonstrating that the enhancement of

the tMOKE is dominated by the thickness of the capping

layer. The results for the measurements with air as dielectric

reveal the general dependence of R0ðhÞ; DRðhÞ, and d(h) on

the Au-cap layer thickness above a critical thickness: (1) The

width of the resonance is reduced and (2) the amplitude

decreases with increasing thickness of the Au-cap layer.

FIG. 3. (a) Experimentally determined reflectivity R0ðhÞ, (b) slope R00ðhÞ, (c) reflectivity difference DRðhÞ, and (d) MOSPR-signal dðhÞ of glass/

Au8 nm=IrMn6 nm=Co3 nm=Aux nm/air EB systems. The capping layer thicknesses x are given in the corresponding legend of each graph. As a benchmark R0ðhÞand R

00ðhÞ of a glass/Au48 nm/air (magenta dotted–48 nm Au) system are shown. Additionally, the calculated R

00ðhÞ curves are depicted in (b).

064502-4 K€ubler, M€uglich, and Ehresmann J. Appl. Phys. 116, 064502 (2014)

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The low minimum reflectivity values predicted by the

calculations cannot be achieved in experiment [Fig. 5(a)].

Reducing the nominal thicknesses t of all layers by the

uncertainty of the deposition rate of 10% improves the agree-

ment between the simulation and the experimental data. The

uncertainty for the IrMn permittivity values has only a small

influence. Therefore, the deviations are mainly caused by

damping and scattering effects due to the surface- and inter-

face roughness of the deposited layers. This assumption is

supported by the root mean squared surface roughness (rms)

of 1.6 6 0.8 nm for the layer systems (averaged over 4 ran-

dom samples) and of 1.3 6 0.4 nm (averaged over 2 random

samples) for the substrates. A similar broadening of the

angular reflectivity [Fig. 5(a)] and shift of the angular posi-

tion of the reflectivity minimum h(R0,min [Fig. 5(b)] (as com-

pared to the simulation) has been observed in literature for

comparable surface roughness values.24–26 As high dmax

values can be achieved by a compromise between minimiz-

ing R0(h(dmax)) and maximizing DR(h(dmax)), a difference of

one order of magnitude is observed between the measured

and calculated dmax [Fig. 4(d)]. Even larger deviations due to

surface- and interface roughness effects have been reported

for Ag/Co/Ag trilayer systems.10

For thick Au-cap layers, dmax is dominated by the optical

contribution (low R0ðhðdmaxÞÞ) due to the reduction of the

Fresnel coefficient rpp8 resulting in a small angular differ-

ence of hðR0;minÞ and hðDRmaxÞ [Fig. 5(b)]. As the cap layer

thickness is decreased the EM field within the Co layer

increases. Therefore, dmax is dominated by the MO contribu-

tion (high DRðhðdmaxÞÞ) resulting in a large angular differ-

ence of hðR0;minÞ and hðDRmaxÞ [Fig. 5(b)].

For the dielectric water, the reflectivity minimum is

observed at an incident angle h between 70 � and 74 � for the

glass/Au12 nm=IrMn6 nm=Co3 nm=Auy nm EB systems [Figs.

6(a) and 6(b)]. The lowest value for R0;min is observed for a

capping layer thickness of 17 nm [Fig. 6(b)], however, no

clear correlation between the cap layer thicknesses and R0;min

is observed. Nevertheless, all R0;min values obtained are

between 0.01 and 0.08, which are in the range of the results

for the glass/Au8 nm=IrMn6 nm=Co3 nm/Au-cap/air systems

[Fig. 3(a)] optimized for the dielectric air. Furthermore,

R0;min observed for glass/Au48 nm/water is 0.05. The

FIG. 4. (a) R0, (b) DR, and (c) d (colour scale) as a function of the Au-cap layer thickness y and incident angle h for glass/Au12 nm=IrMn6 nm=Co3 nm=Auy nm/air

EB systems. (d) Corresponding experimental (red squares) and theoretical (pale red dashed) dmax values as a function of the Au-cap layer thickness. For com-

parison, the results of the glass/Au8 nm=IrMn6 nm=Co3 nm=Au5�18 nm/air measurement series (black dots–8 nm Au-buffer) are shown. Experimental values refer

to the left and simulated ones to the right y-axis.

064502-5 K€ubler, M€uglich, and Ehresmann J. Appl. Phys. 116, 064502 (2014)

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FIG. 5. (a) Measured (black) and calculated (red dashed) reflectivity R0ðhÞ of a glass/Au8 nm=IrMn6 nm=Co3 nm=Au7 nm/air system. For the calculation of the dot-

ted blue curve, all nominal layer thicknesses t have been reduced by the uncertainty of the deposition rate determination of 10%. (b) Experimentally and theoret-

ically determined angular positions hðR0;minÞ; hðR0;maxÞ; hðR00;minÞ; hðDRmaxÞ, and hðdmaxÞ as a function of the Au-cap layer thickness of glass/

Au12 nm=IrMn6 nm=Co3 nm/Au-cap/air systems.

FIG. 6. (a) R0ðhÞ (colour scale) as a function of the Au-cap layer thickness, (b) R0ðhÞ for selected Au-cap layer thicknesses, (c) DRðhÞ (colour scale) as a func-

tion of the Au-cap layer thickness, and (d) dðhÞ for selected Au-cap layer thicknesses of glass/Au12 nm=IrMn6 nm=Co3 nm=Auy nm/water EB systems. As a bench-

mark R0ðhÞ for glass/Au48 nm/water is shown in (b).

064502-6 K€ubler, M€uglich, and Ehresmann J. Appl. Phys. 116, 064502 (2014)

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maximum negative change of the reflectivity per angle R00;min

measured for the systems glass/Au48 nm/water and glass/

Au12 nm=IrMn6 nm=Co3 nm= Au25 nm/water is �0.3. This find-

ing demonstrates that a comparable sensitivity in intensity

modulated SPR applications can be achieved with these sys-

tems. The amplitudes of the MO signals DRðhÞ [Fig. 6(c)]

and dðhÞ [Fig. 6(d)] are significantly increased as compared

to all measurements presented for the dielectric air. The larg-

est dmax of 0.03 observed for the thinnest Au-cap layer thick-

ness of 5 nm is in the range of the values reported for Au/Co/

Au three layer systems.9,14

V. CONCLUSION

We have demonstrated that a combination of EB thin

film systems with Au layers can be used in SPR based

biosensors. The MO activity of the optimized EB systems

is comparable to the one of Au/Co/Au three layer systems

employed for MOSPR based biosensors with enhanced

sensitivity as compared to standard SPR sensors. The

present results are fundamental to a possible integration

of magnetically patterned EB layer systems for magnetic

particle transport as biomolecular carriers and/or active

micro stirrers.

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