Nuclear Instruments and Methods in Physics Research B 280 (2012) 123–130

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Nuclear Instruments and Methods in Physics Research B

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Effect of swift heavy ion irradiation on sputter deposited SiO2/Co/Pt/SiO2 multilayers

Rajan Walia a, J.C. Pivin b, Ravish Jain a, R. Jayaganthan c, Eckhard Pippel d, Fouran Singh e,Ramesh Chandra a,⇑a Nanoscience Laboratory, Institute Instrumentation Centre, Indian Institute of Technology Roorkee, Roorkee 247667, Indiab Centre Spectrometrie Nucleaire et de Spectrometrie de Masse (CSNSM), IN2P3-CNRS, Batiment 108, 91405 Orsay Campus, Francec Department of Metallurgical and Materials Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, Indiad Max-Planck-Institut für Mikrostrukturphysik Weinberg 2, D-06120 Halle/Saale, Germanye IUAC, P.O. Box 10502, Aruna Asaf Ali Marg, New Delhi 110067, India

a r t i c l e i n f o

Article history:Received 31 January 2012Received in revised form 9 March 2012Available online 28 March 2012

Keywords:Magnetron sputteringMultilayersMagnetic storageMagnetic measurement

0168-583X/$ - see front matter � 2012 Elsevier B.V.http://dx.doi.org/10.1016/j.nimb.2012.03.011

⇑ Corresponding author. Tel.: +91 1332 285743; faxE-mail address: ramesfic@iitr.ernet.in (R. Chandra)

a b s t r a c t

The present paper reports the structural and magnetic properties of sputter deposited (SiO2/Co/Pt/SiO2)n

multilayers deposited by magnetron sputtering on quartz substrates as a function of the number of layers(n = 1, 2, 3, 4) and the effect of swift heavy ions (120 MeV Ag9+) irradiation on them. The deposition ofmultilayers was performed under high vacuum on substrates held at 780 �C. All the deposited films werefound to contain stoichiometric (Co50Pt50) particles with L10 ordered structure and their coercivityincreases with the increasing number of layers. The formation of L10 ordered structure was evident bythe presence of (001), (110) super lattice peaks in XRD patterns and from selected area electron diffrac-tion pattern images in TEM. The coercivity of these films was measured using SQUID magnetometer. Irra-diation of these films induced an increase in coercivity together with a loss of chemical ordering in themetal phase. However, the easy axis of magnetization remained in the film plane contrary to the caseof FePt particles in silica matrix as previously reported.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

The growth and microstructure of FePt, CoPt or FePd thin filmswith the L10 ordered structure have received extensive attentiondue to their high anisotropy energy and potential applications inhigh density longitudinal recording media [1–5], magneto-opticalrecording media [6] and high energy magnets [7]. High-densitymagnetic recording requires media with higher anisotropy thancurrent conventional Co alloys media, which must be stable againstsuperparamagnetism. Therefore, higher anisotropy energy isneeded to retain thermal stability [8,9]. Nevertheless, the chemicalordering (to obtain L10 structure) in these intermetallic com-pounds require high temperature (650–750 �C) annealing whichleads to increase in grain size. By reducing the exchange couplingof the magnetic grains the noise level can be reduced in high den-sity recording media. It can be achieved by embedding these mag-netic grains in a non-magnetic matrix like SiO2, carbon, etc. Thesematrices not only limit the growth of the alloy particles during thethermal treatment required for the L10 chemical ordering of thestructure but also their contamination by atmospheric elements.Thus the encapsulation of the alloy particles provides an idealframework to study the magnetic properties of particles with

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limited sizes and little exchange coupling. However the synthesisof particles with stoichiometric composition ratio (Fe/Pt = 1/1,Co/Pt = 1/1) by thermochemical route or physical vapor depositiontechniques is not an easy task, especially if these particles need tobe embedded in a matrix. The effects of SiO2 addition on chemicalordering of CoPt thin films on MgO (001) single crystal substratehave been investigated by Yang et al. [10]. In their study the chem-ical ordering increases with SiO2 addition, showing maximum at10 vol.% SiO2, and decreases with further increase in SiO2.However, coercivity was found directly related to the degree ofchemical ordering.

Ion beam irradiation is a versatile means for synthesizing nano-materials or modifying their magnetic properties [11–14]. It is anexcellent tool to modify the structural and magnetic propertiesof intermetallic systems. An energetic ion (with energy E) passingthrough a solid can be slowed down either through elastic scatter-ing with the target atoms (nuclear energy loss or nuclear stoppingpower, Sn = �dE/dx) or through inelastic scattering with the targetelectrons (electronic energy loss or electronic stopping power,Se = �dE/dx). In the case of swift heavy ions–solid interactions Sn

can be neglected with respect to Se. In the Sn regime, collisions withtarget nuclei produce atomic displacements and recoils which maythen yield collision cascades. In the Se regime, energy is transferredto the lattice via electron–phonon coupling and the resulting rapidincrease in local temperature can potentially yield a molten ion

Ar Gas

Turbo Molecular Pump

Rotary Pump

Co

PtSiO2

Heater Cum substrate holder (780o C)

Quartz Substrate

RF PowerDC Power DC Power

Sputtering Gun

DC motor to rotate substrate

Fig. 1. Schematic view of sputtering chamber used for the deposition.

SiO2 buffer layer

SiO2 capping layer

Co layer

Pt layer

Quartz

Fig. 2. Scheme for the deposition of n = 1 case.

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0

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40

60

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100

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ized

Yie

ld

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Co PtSi

O Si

Co Pt

Fig. 3. Recorded RBS spectra for n = 2 film for determining the elemental compo-sition and thickness of thin film.

Table 1.Chemical composition and thickness of the CoPt layer in each film calculated by RBSspectra.

Sample Composition Equivalent thickness of CoPtphase summing all Co and Ptatoms per unit area (nm)

n = 1 Co50Pt50 42n = 2 Co51Pt49 82n = 3 Co52Pt48 120n = 4 Co56Pt44 150

124 R. Walia et al. / Nuclear Instruments and Methods in Physics Research B 280 (2012) 123–130

track of several nanometers in diameter [15]. However the effect ofswift heavy ion (SHI) irradiation on metallic particles embedded insilica matrix is different for different materials. The effect of SHI onmetal particles like elongation in the beam direction, reduction orgrowth in particle size also depends upon the size of the particlesembedded [16–20]. SHI irradiation can generate controlled defectssuch as point defects, clusters and columnar defects and modifythe local strain in the material and dissolve the clusters in the ma-trix [21–25]. As a consequence, SHI irradiation has been widelyused for modifications of physical properties of various materials[26]. In case of Co/Pt multilayers ion beam irradiation is useful tomodify their magnetic and structural properties. Chappert et al.[27] have shown that the magnetic anisotropy axis of Co/Pt multi-layers can be altered from perpendicular to in-plane by using He+

irradiation which causes atomic rearrangements on very shortrange only. Vieu et al. [28] have modified the magnetic propertiesof Pt/Co ultra-thin layers by focused Ga+ ion irradiation in theenergy range of 20–100 keV. Ion-beam synthesis of an orderedCoPt layer was achieved by Pt+ ion irradiation on Pt/Co bilayerfilms by Balaji et al. [29]. Higher ordering was found with higherdose of Pt+ ions. Abes et al. [30] have shown that it is possible tochange locally the magnetization easy axis of CoPt films with,4 � 1016 ions/cm2 He+ ions of 40 keV. Pivin et al. [31] have usedswift heavy ions producing high densities of electronic excitationand little ballistic motion to switch the easy magnetic axis fromin plane to out of plane in case of FePt particles embedded in silicafilms. The Present study has been focused to study the effect of

swift heavy ion irradiation (120 MeV Ag+9) on the structural andmagnetic properties of SiO2/Co/Pt/SiO2 multilayers.

2. Experimental

(SiO2/Co/Pt/SiO2)n=1,2,3,4. multilayers were deposited in custombuilt DC/RF magnetron sputtering system by sputtering the threedifferent targets viz. SiO2 (purity 99.98%), Co (purity 99.99%), Pt(purity 99.95%) mounted on three different sputter sources. Theschematic diagram of sputtering chamber is shown in Fig. 1. Depo-sitions were carried out on quartz substrates held at a temperatureof 780 �C in Ar atmosphere at a pressure of 10 mTorr. A DC motorwas used to rotate the substrate holder cum heater to bring it be-neath different targets. The target to substrate distance was fixedat 4.5 cm. Prior to deposition of these multilayer samples, sputter-ing parameters were optimized to obtain the required thickness foreach target separately. RF power of 150 W was used to sputter theinsulating target of SiO2 while a DC power of 15 and 25 W was ap-plied to sputter the Co and Pt targets, respectively. The followingscheme (Fig. 2) was adopted for deposition of multilayers (forn = 1): (i) first deposition of silica for 10 min, (ii) of cobalt for3 min, (iii) of platinum 1 min, (iv) and as a final step silica wasdeposited for 5 min as a capping layer to avoid oxidation. Forn = 2, 3 and 4 the above scheme was repeated for two, three andfour times, respectively. Depositions parameters were kept thesame for all the depositions as those were optimized to obtainCo50Pt50 composition in the films. The thickness and compositionof each sample was measured by Rutherford Back Spectroscopy(RBS) with 2.4 MeV He2+ ions at normal incidence.

Glancing angle X-ray diffraction (GAXRD) (Bruker AXS, D8 ad-vance model) was used to study the formation of crystalline phasesand particle sizes. X-ray diffraction patterns were recorded by

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Fig. 4. GAXRD pattern (a) and (b) shows the presence of (200) and (002) peaks present in each sample which confirms the L10 structure of CoPt particles for pristine andirradiated films, respectively. (c) shows the difference in intensities before and after irradiation for n = 1 case.

R. Walia et al. / Nuclear Instruments and Methods in Physics Research B 280 (2012) 123–130 125

using Ni-filtered CuKa radiation (40 mA, 40 kV) with step size of0.02� in a h–2h scattering geometry with time taken per step as2 s with tube angle 2�. Transmission electron microscopy (TEM)investigation was carried out using a TEM (FEI, TECNAI G2) oper-ated at 200 kV. Selected area electron diffraction (SAED) was usedto cross check the nature of crystalline phases present in the sam-ples. Both plane view as well as cross sectional view of sampleswere prepared for TEM investigation. Plane view of the TEM sam-ple was prepared by cutting a disc of 3 mm diameter from the sam-ple. The disc was thinned by grinding it (from the substrate side)on different SiC grinding papers of various grain sizes followedby a dimpling process which created a dimple at the center byreducing its thickness down to 25 lm in the middle and then final-ly was thinned (<100 nm) by ion milling process in ion polishingsystem so that electron beam could pass through it in TEM .To pre-pare the cross sectional view samples two rectangular pieces of4 � 5 mm were cut from the sample and four pieces of same sizewere cut from silicon wafer using ultrasonic cutter. These pieceswere glued together face to face using an epoxy to form a stack.

The pieces having the surface or interface of interest were keptin the middle of the stack. A cylindrical core with a diameter of2.3 mm and having a length of 4 mm was then obtained from thisstack with the help of an ultrasonic cutting tool. The 2.3 mm diam-eter cylinder was now glued inside the 3.0 mm diameter coppertube using epoxy. After curing the epoxy cement the tube contain-ing the specimen cylinder was sliced into a series of almost 500 lmthick discs using a diamond saw. The disc now obtained was trea-ted as plan view sample and steps taken for preparation of planview TEM sample were repeated to reach to the final cross sec-tional TEM sample.

The magnetic properties of the films were studied using aSQUID (Quantum Design, MPMS Evercool) magnetometer. Hyster-esis loops were recorded at room temperature and 5 K, with a mag-netic field up to 20 k Oe.

The pristine (unirradiated) films were irradiated at room tem-perature and at normal incidence with 120 MeV Ag+9 ions at a flu-ence of 1 � 1013 ions/cm2 in the 15 UD Tandem Pelletronaccelerator at the Inter University Accelerator Center, New Delhi,

126 R. Walia et al. / Nuclear Instruments and Methods in Physics Research B 280 (2012) 123–130

India. The irradiation was performed in a high vacuum chamberwith a base pressure of 2.8 � 10�6 Torr. The beam current was keptat 1 pnA (particle nano ampere) during irradiation. The ion beamwas uniformly scanned over 1 � 1 cm2 area using an electromag-netic scanner.

3. Results and discussion

RBS allows the quantitative determination of the composition offilms with thicknesses up to a few micrometers and depth profilingof individual elements, with a high sensitivity provided that theyare not in solution in a much heavier matrix. The elemental com-position was estimated by fitting the RBS spectra (solid line inthe Fig. 3) using RUMP software and the concentration of ‘O’ lightatoms was estimated from the simulation giving a good fit of Pt, Coand Si signals. RBS result for ‘‘n = 2’’ case is shown in Fig. 3. It isclear from the fitting of RBS spectra that this sample containstwo times the sequence schemed in Fig. 2. These RBS results alsoshow that Co diffuses into the Pt layer (or vice versa) to form CoPtparticles, because depositions were made at high temperature(780 �C). The thicknesses of capping, spacer and buffer layer of sil-ica in each sample are found to be 50, 50 and 100 nm, respectively(from RBS analysis), neglecting the apparent mixing on some rangeof thickness due to the balling up of the metal phase. About 5% ofsilica is found at the mean depth of CoPt particles and 20% of metalphase at the mean depth of spacing silica layers. The thicknesses oflayers and compositions of CoPt phase are summarized in Table 1.

In order to identify the crystal structure of the CoPt alloyformed due to deposition at high temperature (780 �C), XRD mea-surements were carried out at grazing incidence (of about 2�). The

Fig. 5. (a) and (b) Plane view and diffraction pattern of pristine and irradiated n = 2 film,respectively (e) high resolution image of dark area observed in cross sectional n = 2 prist(g) HAADF image for cross sectional view of n = 2 irradiated film (h) high resolution HA

results of XRD are shown in Fig. 4(a) and (b). In the recorded XRDpatterns characteristic peaks of L10 ordered CoPt i.e. (200) and(002) are clearly observed in addition to the superlattice peaks(001) and (110) in all samples, pristine as well as irradiated. Thisindicates the alloy phase is well ordered in our samples. The crys-tallite sizes were calculated by using Scherrer formula [32] whichis given by:

t ¼ 0:9kBcos h

ð1Þ

Where B ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiB2

obs � B2ins

qð2Þ

‘t’ is the crystallite size, Bobs is the observed full-width at half max-imum (FWHM) of a Bragg peak in radian and Bins is the instrumentalbroadening, k is the wavelength of X-ray (1.54 Å for the Cu target),and h is the Bragg angle. Instrumental broadening ðB2

ins ¼ 0:01Þ wassubtracted from observed FWHM according to Eq. (2), before calcu-lating the crystallite size. The accuracy of the crystallite size calcu-lated by Scherrer formula depends upon the accurate measurementof FWHM. The broadening in the peak is primarily due to instru-mental broadening and effect of strain. We have removed instru-mental broadening in our calculation which means possible errorin the calculation is <20% [33]. Since crystallite sizes are in the rangeof 15–20 nm the error in these calculations would be ±3–4 nm. Dif-fraction peaks in XRD pattern show a higher intensity for all peaksin the pristine films according to number of counts noted by thedetector indicating that, some Co and Pt are dissolved in silica orform an amorphous alloy after irradiation. The difference in inten-sities for (111) peak before and after the irradiation has beenshown in Fig. 4(c) for n = 1 film. The reason why no peak is observed

, respectively (c) and (d) cross sectional view of pristine and irradiated n = 2 filmine (f) high resolution image of dark area observed in cross sectional n = 2 irradiatedADF image of bright area in cross sectional n = 2 irradiated film.

Fig. 5 (continued)

Table 2Crystallite size calculated before and after irradiation for all samples.

Sample Particle size for pristinesamples along (111) peak (nm)

Particle size for irradiatedsamples along (111) peak (nm)

n = 1 16.2 14.8n = 2 17.3 15.7n = 3 18.2 16.5n = 4 19.4 17.8

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from SiO2 and quartz substrate is that both the SiO2 layer as well asquartz substrate used is amorphous. Moreover in GAXRD, X-raystrikes the sample at glancing angle (2� in our case) to removethe effect of substrate and the diffraction pattern obtained is fromdeposited film only. TEM images of plane-view of specimen andselected area electron diffraction patterns of the structure of filmswith n = 2 as deposited (pristine) and after irradiation with1013 ions/cm2 are shown in Fig. 5. TEM images of plane view clearlyshow the formation of CoPt particles (corresponding to dark spots)of average size approximately 50 nm. The observed size discrepancybetween XRD and TEM analysis can be ascribed to the fact that XRDgives the mean size of the coherently diffracting domains not cut by

networks of dislocations [34]. While in TEM aggregates of thesecoherent domains are observed. TEM observations take into accountthe grains which are the region of a polycrystalline material withthe same crystallographic orientation and same structure. In mostof the cases grains and coherently diffracting domains do not coin-cide. In these materials the structure usually consists of substruc-tures, grains which are separated by high angle boundaries andsubgrains/dislocation cells separated by low angle grain boundaries.Spatial orientation of these substructures differs by typically 1–2�which does not result in any visible contrast in TEM [34]. However,such a small difference in orientation is sufficient enough for break-ing down the coherent scattering. Consequently, there is a phaseshift between the X-rays diffracted from different substructures in-side a single grain [35]. This is the reason for noticed difference inparticle size calculated from XRD and TEM. Electron diffraction pat-terns were recorded for n = 2 pristine as well as irradiated sample.Fig. 5 shows a random orientation of the nanocrystals with a L10

structure in both the cases. The formation of L10 structure is evidentfrom the rings in SAED pattern with diameters matching closely thelattice spacing of the (111), (200), (220) and (311) planes inthe CoPt fct lattice [36]. However in SAED pattern of irradiatedsample in Fig. 5(b) more continuous rings are observed while in

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128 R. Walia et al. / Nuclear Instruments and Methods in Physics Research B 280 (2012) 123–130

pristine sample discrete spot pattern is observed. It is known that inelectron diffraction pattern, finer particles give more continuousrings while larger particles give spot patterns [37]. Thus both XRDresults and SAED patterns indicate that particle size reduces uponirradiation. However from XRD results (given in Table 2) it can beseen that reduction in crystallite size is not much. This small reduc-tion in particle size is less likely to create the observed difference inthe SAED patterns hence these more continuous rings might haveoccurred because of any other experimental factor such as probablylarger area selected for SAED pattern of irradiated sample.

In addition to the plane view of the samples, cross sectionalview of sample n = 2 pristine and irradiated both have been takento clearly observe the effect of irradiation. Cross sectional view ofthe samples have been shown in Fig. 5(c) and (d). In these crosssectional images areas of almost homogeneous contrast are obser-vable. Selecting a subarea inside this area we find some interfer-ence fringes indicating a same crystalline arrangement andproducing a pattern of a crystal lattice in Fourier transform shownwith the Fig. 5(e) and (f), respectively. But upon recording the dif-fraction on same area we obtain as one of a polycrystalline (sameas in Fig. 5(a) and (b)). So each homogeneous area in Fig. 5(c)and (d) is made of several coherent domains, which strictly speak-ing means several crystals. In fact when looking carefully in eachyellow square of Fig. 5(e) and (f) one sees that there are several do-mains, as if a family of crystal planes are all parallel within thesquare but the orientation of the lattice within this particularplanes are locally tilted. It is like drawing a square on a sheet of pa-per with a dot at center of the square, and then tilting the squareseveral times within the sheet of paper. We obtain several squareswhich are all parallel to the sheet of paper but rotated. They giveconstructive interference perpendicular to the sheet of paper butare nevertheless different crystals. Fig. 5(g) and (h) show the high

angle annular dark field (HAADF) images of n = 2 irradiated sample.HAADF imaging technique is highly sensitive to variations in theatomic number of atoms in the sample (Z-contrast images). Inthese images of n = 2 irradiated sample it can be seen that Z-con-trast is coming from variation of atomic number in CoPt particles(High Z is bright) and substrate and silica layers are dark (low Z).Fig. 5(h) shows the HAADF image of n = 2 irradiated sample on5 nm scale which shows orientation of crystals.

Magnetic measurements were done by SQUID Magnetometerwith sample surfaces set parallel as well as perpendicular to theapplied field. The results are shown in Fig. 6 and Fig. 7 for pristineand irradiated samples, respectively. All samples are found to beferromagnetic at room temperature and saturation magnetizationvalues normalized to the volume of CoPt phase (calculated fromRBS spectra with an atomic density of 8.5 atoms/cm3) are higherthan values reported in literature for the bulk phase, i.e.800 emu/cm3. Values of coercive field obtained for pristine andirradiated samples in both directions are given in Table 3 and 4,respectively. It is clear from these tables that coercive fields in-crease after irradiation both at 300 K as well as at 5 K. In n = 4 casethe composition (given in Table 1) of the sample is not good whichmay be the cause of lower coercivity of this sample. To clearly ob-serve the effect of irradiation on coercive fields, M–H loops re-corded at 5 K for n = 1 pristine as well as irradiated have beenplotted in Fig. 7(e). It can be seen clearly from Fig. 7(e) that thereis increment in the coercive field of irradiated film after irradiationby swift heavy ions beams. M–H loops recorded in parallel direc-tion are of square shape while in perpendicular direction theyare more sheared. Looking at the shapes of hysteresis loops whichare more squared parallel to the surface, it can be concluded thatmagnetic easy axis lies in plane both in pristine as well as in irra-diated samples in contrast to FePt particles where SHI switched the

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R. Walia et al. / Nuclear Instruments and Methods in Physics Research B 280 (2012) 123–130 129

easy magnetic axis from in plane to out of plane reported by Pivinet al. [31]. Nevertheless the coercive field increases both paralleland perpendicular to the surface after irradiation. This incrementin coercive field in the irradiated samples may be attributed to(i) a decrease in the particle size after irradiation [38] (ii) a morerandom orientation of the particles decreasing the dipolar polari-zation [39–40] (iii) an effect of magnetostriction [31]. Howeverthe later effect should induce a stronger change of coercive fieldparallel or perpendicular to the surface, depending on the sign of

the magnetostriction coefficient. In case of FePt particles wellseparated from each other a perpendicular magnetization has beenobserved after irradiation and the effect has been ascribed to a pla-nar compression of the particles, assuming that the magnetostric-tion coefficient is positive in case of a tension [31]. Themagnetostriction effect for FePt particles with a much less volumefraction could be evidenced clearly by using spectra of electronspin resonance (ESR). But in case of presently studied films, ESRspectra (not shown) showed very broad ferromagnetic resonances

Table 3Values of coercive field (HC) for all the pristine films both in parallel as well as perpendicular direction.

Sample(pristine) HC (||) at 300 K (Oe) HC (||) at 5 K (Oe) HC (\) at 300 K (Oe) HC (\) at 5 K (Oe)

n = 1 800 1100 1050 1350n = 2 1700 2300 2600 n.mn = 3 2100 3200 n.m n.mn = 4 600 900 850 1150

Note: n.m. – not measured.

Table 4Values of coercive field (HC) for all the irradiated films both in parallel as well as perpendicular direction.

Sample(irradiated) HC (||) at 300 K (Oe) HC(||) at 5 K (Oe) HC (\) at 300 K (Oe) HC (\) at 5 K (Oe)

n = 1 1000 1500 n.m n.mn = 2 2000 2700 2300 3500n = 3 2700 3700 2900 3700n = 4 650 950 750 1150

130 R. Walia et al. / Nuclear Instruments and Methods in Physics Research B 280 (2012) 123–130

spreading over several thousand Oe and values of internal fieldcould not be defined. So in the present study the reasons for incre-ment in coercivity are more likely to be reduction in particle sizeand/or dipole–dipole interaction among CoPt paticles.

4. Conclusions

In conclusion, we have shown that the as-grown multilayeredSiO2/Co/Pt/SiO2 films exhibit a high degree order with parallel easymagnetization axis and high coercivity with a maximum of 0.21 T(n = 3 case). Both pristine and irradiated films are found to be L10 or-dered. Swift ion beam Irradiation of multilayers dissolves somemetallic phase and lesser crystallite size has been found in irradiatedfilms. The magnetic easy axis remains in the film even after the irra-diation which means SHI fails to switch the easy magnetic axis fromin plane to out of plane in case of presently studied CoPt particles incontrast to previously studied FePt particles by Pivin et al. [31].SQUID measurements show the coercive fields increase in bothdirection after the irradiation which could be explained on the basisof particle size reduction after the irradiation and/or random orien-tation of particles resulting in reduced dipolar polarization.

Acknowledgments

One of the authors, Rajan Walia is thankful to AICTE (All IndiaCouncil for Technical Education) for awarding fellowship for thiswork. We are thankful to IUAC staff for providing the stable beamand RW is thankful to Dr. Vinod kumar for his help in the irradia-tion experiment.

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