1Institute of Nanoscience and Nanotechnology, University of Kashan, Kashan, 87317, Iran2Department of Physics, Shahid Beheshti University, G. C., Evin, Tehran, 19839, Iran
3Department of Physics, Alzahra University, Tehran, 19938, Iran4Material Physics, Royal Institute of Technology (KTH), Kista, 16440, Sweden
5Department of Physics, University of Gothenburg, Gothenburg, 41296, Sweden
(received date: 29 October 2014 / accepted date: 19 January 2015 / published date: 10 May 2015)
1. INTRODUCTION
Surface plasmons (SPs) are defined as the quanta ofsurface charge density oscillations of the free electrons on ametal surface. The surface charge oscillations are naturallycoupled to electromagnetic waves at the interface of twomedia, one with positive and another with negative dielectricconstants, which explain their designation as surface plasmonpolaritons (SPPs). A media to support SPP resonance (SPPR)could be made with metal/dielectric materials in whichSPPR forms at their interfaces. Electromagnetic waves
become localized at the interface and its amplitude decaysdifferently in either side of metal or dielectric structures.[1,2]
Such wave localization suggests research opportunities inapplied science such as nano-bio-sensors,[3-10] nano-wave-guides,[11] Raman scattering,[12] and so forth. While aplasmonic material is combined with a magnetic material,both the plasmonic and the magnetic properties of theresulting magnetoplasmonic system become interrelated,which could be detectable via magneto-optical (MO)effects.[13-15] In addition, in the magnetic multilayer system,the MO activity can be greatly enhanced when the SPR isexcited as the light absorption is maximum, that is, theelectromagnetic field of the light is greatly enhanced at theMO active layer.[16] We note that this mechanism is differentto what have been already investigated for enhancement ofMO response.[17-22]
Surface plasmon polariton resonance (SPPR) can be originated from thesurface charge oscillation via light localization at the interface of ametal, for example, Au or Ag and a dielectric. Such localization can beimplemented to increase the magneto-optical (MO) activity of amagnetic medium while SPPR is fulfilled, which is known asmagnetoplasmonics. In this paper, a magnetoplasmonic bilayer of Au/NiFe (Py) sputter deposited on glass is demonstrated. Large enhancementin MO-Kerr effect (MOKE) response due to SPPR effect is observed atdifferent light incident angles. By measuring and analyzing the MOsignals from the sample with different thicknesses of Au and Py layers,the optimal thicknesses’ range is obtained with the largest MOKE. Thelarge MOKE intensity from ultra-soft magnetic Py layer with lowcoercivity and small saturation field suggests a weak magnetic field-sensitive MO-based element. Finally, different applications of suchstructures, for example, weak magnetic field sensors and magneticmultilevel memory elements are demonstrated.
MO-Kerr effect (MOKE) describes the polarizationchanges of the reflected light from a magnetic surface underthe external magnetic field. In transverse geometry, themagnetic field oriented in the plane of the sample and onereads the transverse MOKE as the polarized light interactswith in-plane component of the magnetization.
Historically, the first magnetoplasmonic effects havebeen seen in pioneering works.[23-25] There are manyreports on the magnetoplasmonic effects in multi-layersand particulate structures combining magnetic and non-magnetic materials.[26,27] For example, the combination ofmagnetic metals of Co, Ni, or Fe together with other non-magnetic metals such as Ag and Au with great SPPRresponse have so far shown striking magnetoplasmonicproperties.[25-29]
Although for SPPR purpose, many magnetic and non-magnetic structures have been studied so far, there is noreport on the combination of Au/NiFe with intense focus onmagnetic properties. When there is a strong SPPR effectthanks to the Au layer in this nanostructure, we show that theresult of the MOKE measurements conveys differentscientific applications, for example, sensors and memories.Why there is a motivation to implement the NiFe (Py) layerin the structure can be addressed as follows. The well-knownultra-soft Py ferromagnetic layer nominated as Permalloy(Ni81Fe19) has in-plane magnetization, zero magneto-crystallineanisotropy, low coercivity, low microwave damping, and soon dictating this material to be implemented in magneticelements proposed for weak magnetic field sensing appli-cations. In addition, the Ni nanostructure itself has shownnano-plasmonic properties.[31,32] To observe the SPPR effectin these media, different techniques of MOKE measurementscan be used. Those techniques can be the measurement ofthe MOKE effect at (I) different wavelength, (II) differentangles of incident light, and (III) different light power. In anytechnical ranges of wavelength, angles, or power, thestrength of SPPR can be seen as enhancement in the MOKEsignal. Here, we show that the MOKE signal increases in thePy layer with a linear and hysteresis-less-like response. Inthis paper, based on the low magnetic field saturation of Pylayer together with enlarged MOKE intensity via SPPReffect in the glass/Au/Py nano-structure, we introduce amultilevel reading magnetic element and a linear weakmagnetic field magnetoplasmonic sensor. The sample underthis study is a wedge-shaped Au and Py nano-layers made inthe form of thicknesses gradient in order to find themaximum MOKE intensity properly.
2. MODELING
As we know, the SPPR effect in a bilayer structure canoccur in a specific range of thicknesses. A theoreticalcalculations based on the model described below can
determine the optimal ranges of thicknesses. This paper willfocus on finding of the SPPR effect at a constant wavelength,but the incident angle differs. The modeling is based onKretschmann configuration[33] (Fig. 1); for example, theglass/Au/Py structure is placed in which it is closed by asemi-cylindrical prism and the light illuminates into thestructure from the glass side. The goal is to find the MOKEand reflectivity of the structure for optimal range of thicknessbefore any experiment.
Regarding the small quadratic contribution of the relativepermittivity and for simplification, we focus on linear MOproperties. For the transverse geometry, the permittivitytensor is defined as[34-36]
(1)
where is known as Voigt’s parameter and i is the
layer number. In the equation, for simplicity, we haveassumed isotropic refractive indices and
. The matrix formalism 4 × 4 is an extremely usefulform of the steady-state solution of Maxwell’s equationssubjected to a boundary condition for either of isotropic oranisotropic structures.[34-36] If Pi is the electric field at the bot-tom surface of the (i) layer, then the relation between electricfield in glass, PG, and that of infinite dielectric (air), PA, canbe given by
(2)
(3)
(4)
where the matrix Ai is the 4 × 4 medium boundary matrix of
εi( )
εxx
i( ) 0 0
0 εyy
i( ) εyz
i( )
0 εzy
i( ) εzz
i( )⎝ ⎠⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎛ ⎞
εxx
i( )1 0 0
0 1 i– Q
0 iQ 1⎝ ⎠⎜ ⎟⎜ ⎟⎜ ⎟⎛ ⎞
= =
Q iεyz
εxx
------=
εxx εyy εzz= =εyz εzy=
AGPG A1D
1P1
A1D
1A1
1–A1P1
= =
A1D
1A1
1–A2D
2P2
=
Πi 1=
3
AiDiAi
1–( )AAPA=
Fig. 1. Schematic view of the SPP excitation in the Kretschmann con-figuration for the wedge-shaped glass/Au/Py structure. The right insetshows the gradient direction for the thickness of each layer.
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the (i) layer which relates the tangential components of thelight electric and magnetic fields with the s and p compo-nents. The Di is a 4 × 4 medium propagation matrix whichrelates the s and p components of the electric field for thetwo surfaces of the (i) layer. We can rewrite equation (4)as
(5)
(6)
(7)
The transverse MOKE signal, that is, the intensity changesof the reflected light under the magnetic field is ST =
, where R(0) denotes thereflection amplitude without magnetization, which wasdetected and analyzed without the sample, and denotes the field-dependent reflection amplitude.[34]
Calculations are carried out in transverse configurationwith 632.8 nm (He-Ne Laser) and p-polarized light. Thecomplex refractive indices nprism = 1.47, nAu = 0.197 + 3.09 i,nPy = 2.18 + 3.73 i, nAir = 1, and MO constant Q = 0.0117 +0.0007 i are assumed for constituent layers.[36] As can beseen later, the strong SPPR response occurs for anglesbetween 42° and 56° with great MOKE intensity; we choosehere in the calculations that the incident angle to be 49°.
Figure 2 shows that the MOKE intensity is highlydependent on either of Py or Au thicknesses and has themaximum value for tAu = 11 nm and tPy = 9 nm (at incidentangle of 49°). We, in addition, checked that such maximumvalues can be achieved for other incident angles withdifferent thicknesses of Au and Py (not shown). Finally, allmodeling results suggest that the thicknesses for either of Py
or Au layers can be found in the range between 6 and 12 nmwhere significant SPPR effect and thereby large MOKEintensity can be achieved.
Therefore, based on the conclusion of the theoretical work,we fabricated double-side wedge-shaped layers with gradientthickness covering all necessary thickness ranges. Moreover,to highlight some beneficial points about the graded thicknesssample[37,38] used for this study, we refer to (i) a littlevariation in the thickness of each layer changes the MOKEintensity dramatically, (ii) fabrication of samples with fine-tuning of thicknesses is not easy since deposition techniqueshave thickness uncertainty, (iii) wedge-shaped sample orthickness of gradient layers can provide a wide range ofthicknesses in which one can easily find the right thicknessby scanning one sample only, and (iv) making only onedouble-wedge sample, as Au-graded thickness in one directionand Py-graded thickness in other direction (perpendicular)provides all thickness requirement in one sample for a widerange of angles.
3. EXPERIMENTS
To fabricate a high-quality structure with good adhesion tothe substrate (glass), first substrate was cleaned in a liquid-based solution in an ultrasound bath for 15 min and thendried out. Au and Py layers were subsequently deposited atroom temperature on the clean glass using a confocalmagnetron sputtering system in a chamber with a basepressure below 2 × 10−8 Torr. All layers were deposited inthe presence of Ar working gas pressure of 5 mTorr. Thedeposition rates for Py and Au were 0.8 Ås−1 and 1 Ås−1,respectively. To form a wedge-shaped layer, an obliquedeposition technique was used. The thicknesses of Py andAu wedge layers were varied from 6 to 13 nm and 6 to17 nm, respectively, with the thickness variation of %15/cm.
For MOKE measurement, the sample was opticallyinterfaced to a half-cylinder prism, via placing the glasssubstrate in contact with the prism and leaving the Py layerin contact with air, as shown in Fig. 1. To make a suitableoptical connection between prism and the glass, transparentoil with a refraction index of 1.3 was used. The optical andMOKE response of the sample were investigated using a p-oriented polarized He-Ne laser (λ = 632.8 nm). The prismwas mounted on a goniometer allowing illumination in totalinternal reflection with variable incidence angle, θ, withangular resolution of 10−2 degrees. A photo-detector wasused to detect intensity variations in the reflected radiation.The MOKE response was measured by applying themagnetic field in the plane of the sample and perpendicularto the incidence plane, as the transverse MOKE (TMOKE).The sample was moved along x and y orientations to checkthe suitable thicknesses and find the maximum MOKEintensity resulting from the SPPR effect.
PG AG
1–
Πi 1=
3
AiDiAi
1–( )AAPA MPA≡=
rpp M 4 3,( )/M 3 3,( )=
Rpp rpp=2
RPPΔ /RPP R+H( ) R H–( )–[ ]/R
0( )=
R±H( )
Fig. 2. Transverse MOKE signal in Prism/Au/Py structure versus Auand Py thicknesses, at 49° incident angle.
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4. RESULTS AND DISCUSSIONS
4.1 Reflectivity measurements and modeling
Results of reflectivity measurements at different incidentangles for four different sampling of Py with thicknesses of6.5, 8.3, 9.7, and 11.9 nm while Au thickness was 11 nm areshown in Fig. 3a-d. Same measurements for constant6.25 nm thick Py layer and variable Au thicknesses of 8, 9.3,11, and 12.6 nm are shown in Fig. 3e-h. Besides, in the sameplots, results of our modeling for the same measurements areincluded. All figures represent the total internal reflectionaround the incident angle of 43°, the maximum point, as wellas the SPPR effect around the incident angles of 49°, theminimum point. Interestingly, modeling and experimentalresults for reflectivity in all plots are very well consistent.The clearly observed minimum value in the reflectivity
represents a strong coupling between light and plasmonoscillations, and as consequence, there is a strong effect ofSPPR. Such a range will be suitable for the MOKE study.
4.2 MOKE measurements and calculations
Angular dependence of the MOKE based on the calculationshown in Fig. 4a suggests that a significant MOKE responsecan be observed in the angle ranging from 45° to 53°.Experimental results of the MOKE response measured atdifferent angles are summarized in Fig. 4b. As angle increasesfrom 45°, the MOKE intensity increases to a maximumvalue due to SPPR effect and then decreases. There is about5 times enhancement in the MOKE response measured at52° compared to that measured at 45°. The discrepancy inthe angle range here between theory and experiment mightbe from the difference in reflectance from the structure,thickness uncertainty, and roughness. Experimental resultsshown in Fig. 4b will be interpreted in more detail in the nextsection.
Fig. 3. The angular dependence of the reflectivity of Prism/Au/Pystructure, left and right axes are theoretical and experimental reflec-tivity, respectively. In (a-d) the Au thickness was fixed at 11 nm andPy thickness was varied and for (e-h) the Py thickness was fixed at6.25 nm and Au thickness was varied.
Fig. 4. (a) Angular dependence of MOKE response from the struc-ture determined by theory and (b) experimental evaluation of MOKEmagnetization loop of the sample at different angles.
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Results of calculation and measurement for thicknessdependence of the MOKE effect are shown in Fig. 5a-c.Here, thickness for the Py layer was 7.3 nm while that for Aulayer was 9.35, 11, and 12.65 nm in Fig. 5a-c, respectively.As can be seen, there is a dramatic angular-dependent MOKEresponse, and as the Au thickness increases, the MOKEresponse increases between all three samples as shown, forcomparison, in Fig. 5d. If the Au layer is too thin, the SPPwill be strongly damped because of radiation damping intothe glass. If the metal film is too thick, the SPP can no longerbe efficiently excited due to absorption/damping.[39]
One can conclude that with increasing the Au thickness,the MOKE response increases and the peak position reducesto lower angles.[40] Between these three samples, themaximum MOKE response can be seen around angle of 50°for glass/Au (12.65 nm)/Py (7.3 nm) with twice MOKEresponse than that achieved from other samples. Looking atthe MOKE response from samples at low fields, beforesample saturates magnetically, one would expect to observethe minor loop of the magnetization. Although, we note thatthe applied field should be lowered that much in order totake a linear MOKE response versus field sweep. Figure 6ashows an example of MOKE response for glass/Au (11 nm)/Py (7.3 nm) measured at angle of 51°. As can be seen, theMOKE measurement for this field interval (±3 Oe) showslinear response without hysteresis effect. Such behavior wassimilarly observed for all range of thicknesses.
The major difference of MOKE responses at differentangles is the whole amplitude of the MOKE signal taken in afield sweep. Therefore, the slope of MOKE response changesand should vary at different angles, as shown in the Fig. 6b.The larger MOKE signal occurs where the SPPR effect ispronounced similar to those MOKE effects measured in the
larger field interval, for example, as shown in Fig. 4b.
4.3 Proposal for application in magnetic tags, sensors,
and memories
Results suggest some technological applications based onMOKE measurements. As can be seen in the inset of Fig. 4band also in Fig. 6b, there is a dramatic variation of MOKEresponse while field is swapped at different angles. One candetermine a range of angles and thicknesses in which thevoltage over the photo-detector changes significantly thanksto the magnetoplasmonic effect. By applying a small magneticfield, the MOKE signal changes in either positive ornegative field directions. This behavior can be implementedin microfluidic channels carrying magnetic particles as high-sensitive magnetic beads stray field detectors and magneticbiosensors.[41,42] Here, it is noted that the effect of magneto-plasmonics affects the MOKE sensitivity at different angles.Although, analogue to angular-dependent magnetoplasmonics,one would be able to vary the wavelength or power as othercapabilities to make useful devices operating based on
Fig. 5. Angular-dependent MOKE response for prism/Au (tAu)/Py(7.3 nm) with tAu = 9.35, 11, and 12.65 nm for (a)-(c), respectively,and (d) compares all MOKE responses for the three samples.
Fig. 6. (a) The MOKE response of sample in a field sweep, measuredat different angles and (b) the MOKE changes or MOKE sensitivity,the slope of plots in (a), as a function of angle.
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different sensing mechanisms, for example, wavelength,power, angle, and so on.
Moreover, the angular-dependent study suggests theapplication of such materials for magnetic recording media.Because of soft magnetic properties of the Py layer, theexternal field for encoding the information on the structuredoes not have to be large. In addition, the magnetizationobtained from the MOKE shows 100% remanence (Fig. 4b).Provided making such elements thermally stable, it maysuggest a suitable candidate for magnetic recording media.Importantly, while sample is saturated, as there is differentvoltage level over the photo-detector, different informationcan be read at different angles. This may suggest a memorydevice with different information readable from one pointwhich is detectable at different angles as differentdistinguishable MOKE intensity is shown in Fig. 7. Thisfigure represents a stepwise MOKE response taken from onepoint of the device (H > 10 Oe) with angle, as either thesample or detector rotates for a few degrees. Such rotationcorresponds to the time frame while the system (sample ordetector) is rotating by a stepper motor with an arbitraryspeed per minutes. Here, one can claim that by taking 1°rotation into account, one would be able to read up to sixdistinguishable MOKE levels as shown in Fig. 7. Moreover,via angle variation, one could be able to design an angular(and per say the wavelength or power)-dependent memristorelement based on magnetoplasmonic effect. It could be evenmore applicable while information to be recorded in onepoint with different field steps, as suggested earlier,[43,44]
magnetoplasmonically increases the reading points to builddense memories and memristors.
5. CONCLUSIONS
The SPPR effect on the MOKE response from the
magnetoplasmonic structure made of glass/Au/Py withthickness gradient is demonstrated. We have shown dramaticangular and thickness-dependent reflectivity and MOKEresponse originated from the magnetoplasmonic effect in thebilayer. When the thickness of either of the Au or Py layersis fixed, an optimal thickness of aother layer exists, wherethe largest increase in MOKE signal can be acheived. Basedon the MOKE responses, we suggest magnetic tags andhigh-sensitive magnetic field sensors could be fabricated formagnetic beads detection and bio-sensing elements thanks tothe soft magnetic properties and low field saturation of Pylayer. A magnetoplasmonic memory element is proposedbased on the angular-dependent MOKE response. Similardesign could be furthermore implemented to fabricate thewavelength and light power-dependent memory elements.Our results may be further more extended for application indesigning of future magnetoplasmonic elements.
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