Effects of target–substrate geometry and ambient gas pressure on FePtnanoparticles synthesized by pulsed laser deposition

J.J. Lin, L.S. Loh, P. Lee, T.L. Tan, S.V. Springham, R.S. Rawat *

NSSE, National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, Singapore 637616, Singapore

Applied Surface Science 255 (2009) 4372–4377

A R T I C L E I N F O

Article history:

Received 25 May 2008

Accepted 6 November 2008

Available online 17 November 2008

Keywords:

FePt

PLD

Backward plume deposition

Nanoparticle

MOKE

A B S T R A C T

FePt nanoparticles, in the forms of nanoparticle agglomerates and floccules-like nanoparticle networks,

can be synthesized by pulsed laser deposition (PLD) at different ambient gas pressures. Backward plume

deposition (BPD), as special target–substrate geometry, can achieve higher uniformity in terms of

agglomerate size and size distribution, compared to conventional PLD. Both as-deposited FePt

nanoparticles exhibit low Ku fcc phase and post-annealing at 600 8C is required for the phase transition to

high Ku fct phase. FePt nanoparticle agglomerates deposited by BPD were found to have better fct phase

crystallinity after annealing, which may be caused by the higher kinetic energy of backward moving

ablated species due to shorter travel distance.

� 2008 Elsevier B.V. All rights reserved.

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1. Introduction

FePt nanoparticles, containing a near-equal atomic percentageof Fe and Pt, are an important class of magnetic materials. Theyhave attracted intense research interests due to their distinctmagnetic properties and potential applications in ultra-highdensity magnetic data storage [1,2]. For example, chemicallyL10 ordered face-centered-tetragonal (fct) FePt nanoparticlescan remain ferromagnetic with high thermal and chemicalstability due to their high magnetocrystalline anisotropy [3](Ku > 6 � 107 erg/cm3) when particle size is reduced to fewnanometers (nm). It is expected that the data storage densitycan be enhanced to greater than 1 Tb/in.2, if uniform FePtnanoparticles with particle size of around 4 nm can be used tostore data in one bit per grain. As-deposited FePt nanoparticlesnormally exhibit chemically disordered face-centered-cubic (fcc)phase with small magnetocrystalline anisotropy, which require tobe converted to high Ku fct phase by post-annealing [4,5].Therefore, the synthesis of FePt nanoparticles is of scientific andtechnological interests.

Various deposition techniques have been successfullyemployed to synthesize magnetic nanoparticles, such as chemicalvapor deposition (CVD) [6,7], molecular beam expitaxy (MBE)[8,9], chemical methods [10,11], pulsed plasma focus [12] andpulsed laser deposition (PLD) [13–15]. Compared with other

* Corresponding author.

E-mail address: rajdeep.rawat@nie.edu.sg (R.S. Rawat).

0169-4332/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2008.11.017

growth methods, PLD has the most distinct advantage ofstoichiometry preservation [16], which meets the requirementof FePt nanoparticles to have near-equal atomic percentage toachieve highest magnetocrystalline anisotropy. As special target–substrate geometry of PLD technique, backward plume deposition(BPD) can synthesize uniform nanoparticles with high depositionrate and significantly less laser droplets [17,18]. In this paper, FePtnanoparticles were synthesized by both conventional PLD and BPDat different ambient gas pressures and investigated by variouscharacterization techniques. The experimental results and com-parisons between these two PLD techniques, in terms ofmorphological, structural and magnetic properties of FePtnanoparticles, are also presented.

2. Experiments

FePt nanoparticles were synthesized by conventional PLD andBPD at various ambient gas pressures using 6000 shots at roomtemperature. Continuum Nd:YAG laser (532 nm, 10 Hz and 10 ns)was focused on equi-atomic FePt (99.99%, Kurt J. Lesker) targetdisc. The laser pulse energy was about 75 mJ and the laser focalspot size was adjusted to about 100 mm. Hence, the laser energydensity at the target surface was estimated to be around 955 J/cm2.Silicon substrates were mounted on the substrate holder at frontposition for conventional PLD and also on the center of targetsurface at back position for BPD. Target and substrate holders wererotated by magnetic motors to obtain homogeneous materialablation and uniform deposition across the entire substrates.Silicon substrates were cleaned sequentially in acetone, alcohol,

J.J. Lin et al. / Applied Surface Science 255 (2009) 4372–4377 4373

and distilled water for 10 min each using ultrasonic bath. Thedistance between laser irradiation area and substrate center forBPD is around 6 mm, which is about five times shorter than the onebetween target holder and substrate holder for conventional PLD.The detailed experimental set-up of PLD along with the specialtarget–substrate geometry of BPD is provided elsewhere [19]. ThePLD chamber was evacuated to the base pressure of better than5 � 10�5 mbar and then filled with argon gas to adjust the ambientgas pressure. Samples were consequently heated to the requiredtemperature at 60 8C/min in vacuum and maintained at requiredannealing temperature for 2 h before cooling down naturally.

The morphology of deposited samples was investigated by JEOLJSM-6700F field emission scanning electron microscope (FESEM).The elemental and compositional analyses were done usingelectron energy dispersive X-ray (EDX) spectrometer, which isattached with JEOL JSM-6700F FESEM. X-ray diffraction (XRD) wasperformed using SIEMENS D5000 diffractometer to determine thestructural properties and phase transition of FePt nanoparticles.The magnetic properties were characterized by LakeShore 7400vibrating sample magnetometer (VSM). Magneto-optical Kerreffect (MOKE) magnetometer was also used to investigate themagnetic properties. Self-developed MOKE magnetometer worksin longitudinal mode and can detects in-plane hysteresis behaviorsof ultra-thin metallic films with maximum applied field of 8000 Oeat room temperature.

Fig. 1. SEM images of samples deposited by conventional PLD at different ambie

3. Results and discussion

Fig. 1 shows the morphology of samples deposited byconventional PLD at different argon ambient gas pressures. Thereare three major morphological features: (i) smooth thin films,shown in Fig. 1(a), at low ambient gas pressure (0.01 mbar); (ii)nanoparticle agglomerates at slightly higher ambient gas pressure(0.5 mbar), refer Fig. 1(b); (iii) dispersive floccules-like nanopar-ticle networks at high ambient gas pressures of 1 and 10 mbar,shown in Fig. 1(c) and (d). The change in the morphologicalfeatures can be explained by the surface mobility of ablated speciesupon their arrival at substrate surface, which is governed by theirkinetic energy. The kinetic energy is related to the collision rate ofablated species and in turn can be tailored through depositionconditions, such as ambient gas pressure and substrate to targetdistance. As ambient gas pressure increases, the collision rateincreases and then causes the kinetic energy to decrease. There-fore, FePt nanoparticle agglomerates and nanoparticle networksare formed on silicon substrates, instead of smooth thin films,when deposition is done at relatively high ambient gas pressure,due to the decrease in the surface mobility of ablated species. Italso can be observed that, as shown in Fig. 1(c) and (d), the densityof floccules-like nanoparticle networks decreases as the ambientgas pressure increases. In addition to the decrease in surfacemobility, it maybe also related to the reduction in the amount of

nt gas pressures. From (a) to (d): 0.01 mbar, 0.5 mbar, 1 mbar and 10 mbar.

Fig. 2. XRD patterns of samples deposited by conventional PLD at 0.5 mbar and

annealed at various temperatures.

J.J. Lin et al. / Applied Surface Science 255 (2009) 4372–43774374

forward moving ablated species, since higher the ambient gaspressure greater will be the backscattering or lateral expansion ofablated species. The atomic percentage of Fe:Pt for the sampledeposited by conventional PLD at 0.5 mbar is determined to be48.7:51.3 using its EDX spectrum, which is close to 50:50 of targetmaterials and indicates the stoichiometry preservation in PLD.

XRD patterns of samples deposited by conventional PLD at0.5 mbar and consequently annealed at different temperatures areshown in Fig. 2. The weak and broad peak at around 418, which is

Fig. 3. SEM images of samples deposited by BPD at different ambient ga

determined to be the fundamental (1 1 1) peak, indicates that as-deposited FePt nanoparticles exhibit fcc phase. After annealing at400 8C, the intensities of diffraction peaks of (1 1 1) at about 418and (2 0 0) at about 478 increase, indicating the improvedcrystallinity of fcc phase FePt. The crystallinity of fcc phase FePtimproved further as annealing temperature increases to 500 8C.The phase transition of FePt from magnetically soft fcc tomagnetically hard fct only occurs at the annealing temperatureof 600 8C, which is indicated by the presence of (0 0 1) diffractionpeak of fct phase at around 248. The reason is that, according toXRD Database@Socabin (03-065-9121 and 03-065-9122), theintensity ratio I(0 0 1)/I(1 1 1) and I(1 1 0)/I(1 1 1) for fcc phase FePtare very low (about 0.001) and they increase significantly to 0.33and 0.27, respectively, for fct phase FePt. Therefore, the presence of(0 0 1) peak at about 248 with intensity ratio I(0 0 1)/I(1 1 1) of about0.18 can be used to determine the phase transition of FePt. Inaddition, the splitting of fundamental (1 1 1) and (2 0 0) peaks alsoindicates the phase transition, since the lattice constants of fccphase are different from those of fct phase. It also can be observedthat the full width at half maximum (FWHM) of diffraction peaksdecreases as the annealing temperature increases, indicating theincrease in crystallite size. The mean crystallite size can beestimated using Scherrer formula: t = 0.9l/(Bcos uB), wherein t isthe mean crystallite size, l is the radiation wavelength, uB is theBragg angle, and B is the width of the corresponding peak at halfmaximum [20]. The mean crystallite size of samples deposited by

s pressures. From (a) to (d): 1 mbar, 5 mbar, 10 mbar and 15 mbar.

Fig. 4. XRD patterns of samples deposited by BPD at 1 mbar and annealed at various

temperatures.

J.J. Lin et al. / Applied Surface Science 255 (2009) 4372–4377 4375

conventional PLD and consequently annealed at 400 8C, 500 8C and600 8C are estimated to be about 4.8 nm, 5.2 nm, 15.4 nm and21 nm, respectively. It can be concluded that higher the annealingtemperature greater will be the increase in crystallite size.

The morphology of samples deposited by BPD at different argonambient gas pressures is shown in Fig. 3. It has been reported thatablated species can move backward and deposit on target surfacedue to backscattering and lateral expansion at relatively highambient gas pressure [21,22]. At ambient gas pressure of 1 mbar,FePt nanoparticle agglomerates are observed on the sampledeposited by BPD. It also can be seen that FePt nanoparticleagglomerates deposited by BPD have higher uniformity, in terms ofagglomerate size and size distribution, compared with samplesgrown by conventional PLD. In addition, laser droplets, which are

Fig. 5. VSM hysteresis loops of samples deposited by conventional PLD at 0.5 m

known as one of the major drawbacks of PLD technique, aresignificantly reduced in BPD. Its atomic ratio of Fe:Pt is estimatedto be about 46:54, which is still close to 50:50 of target materialand indicates that BPD also has stoichiometry preservation. As theambient gas pressure increases, the collision rate of ablated speciesincreases, which leads to the increase in the amount of backwardmoving ablated species and decrease in their kinetic energy.Therefore, the surface mobility of backward moving ablatedspecies at higher ambient gas pressure is not enough for them todiffuse and form uniform nanoparticle agglomerates and theyrather form floccules-like nanoparticle networks, as shown inFig. 3(b)–(d). Other possible reason attributed for this morpholo-gical change is the high deposition rate of BPD [17]. The depositionrate may exceed the migration rate of ablated species, leading tothe formation of less compact and disordered nanoparticlenetworks. Furthermore, it also can be seen that higher the ambientgas pressure more will be FePt nanoparticle networks on samplesdeposited by BPD, whereas, less FePt nanoparticle networks can befound on samples deposited by conventional PLD at higherambient gas pressure.

Similar to samples deposited by conventional PLD, FePtnanoparticles synthesized by BPD exhibit low Ku fcc phase withcrystallite size of about 4 nm, indicated by the weak and broad(1 1 1) peak at about 418, as shown in Fig. 4. After annealing at400 8C, the intensity of (1 1 1) peak increases with a decrease in itsFWHM, indicating the improved crystallinity of fcc phase andincrease in crystallite size to about 7.6 nm. As the annealingtemperature increases further to 500 8C, the intensity of (1 1 1)peak increases further and peak splitting can be observed. Thesplitting of fundamental (1 1 1) peak indicates the partial phasetransition to high Ku fct phase, since its lattice constants aredifferent from those of fcc phase. The crystallite size increases

bar: (a) as-deposited and annealed at (b) 400 8C, (c) 500 8C and (d) 600 8C.

Fig. 7. MOKE hysteresis loops of samples deposited by BPD at 1 mbar: (a) as-

deposited and annealed at (b) 500 8C and (c) 600 8C.

J.J. Lin et al. / Applied Surface Science 255 (2009) 4372–43774376

further to about 17.1 nm. However, the phase transition occurs atthe annealing temperature of 600 8C, which is indicated by theappearance of (0 0 1) peak at about 248, as shown in Fig. 4.Compared with 0.18 of samples deposited by conventional PLD(refer Fig. 2), (0 0 1) peak of samples deposited by BPD has higherpeak intensity ratio I(0 0 1)/I(1 1 1) of 0.31, after annealing at 600 8C.It may be related to the higher kinetic energy of backward movingablated species due to shorter travel distance. The crystallite size ofthe sample annealed at 600 8C is estimated to be about 35.9 nm.

The VSM hysteresis loops of samples deposited by conventionalPLD at 0.5 mbar and consequently annealed at various tempera-tures are shown in Fig. 5. As-deposited FePt nanoparticles showsmall coercivity value of 67 Oe and are magnetically soft, referFig. 5(a), which is in line with XRD result (refer Fig. 2) that the as-deposited sample is in low Ku fcc phase. As shown in Fig. 5(b) and(c), the coercivity increases to 231 Oe and 427 Oe after annealing at400 8C and 500 8C, respectively, which can be related to theimproved crystallinity of fcc phase FePt and possible partial phasetransition to fct phase. By increasing annealing temperaturefurther to 600 8C, the coercivity value drastically increases from67 Oe of as-deposited sample to 938 Oe, as shown in Fig. 5(d). It isattributed to the phase transition of FePt from low Ku fcc to high Ku

fct after annealing. Weak kinks can be observed in the hysteresisloop of the sample annealed at 600 8C, indicating the existence ofcomposite phase consisting of magnetically hard fct and magne-tically soft fcc phases. The kinks in the hysteresis curves originatefrom the exchange coupling between the grains of the magneti-cally hard and soft phases. The higher the percentage ofmagnetically soft fcc phase, the higher will be the exchangeinteraction between the two phases increasing the magnitude ofthe kinks. The remanence ratio S, which is defined by S = Mr/Ms,where Mr is the remanence magnetization and Ms is the saturationmagnetization, is estimated to be 0.45 for as-deposited sample and0.73 for the one annealed at 600 8C. For the materials withrandomly oriented nanoparticles undergoing coherent magneticmoment rotations without interaction, the remanence ratio isequal to 0.5. The remanence ratio decreases when the nanopar-ticles exhibit magnetostatic interaction and increases when theyexhibit exchange-coupled interaction. The intergranular exchangecoupling is one of the major sources of media noise in magneticdata storage media, and it need to be minimized to enhance thesignal-to-noise ratio. The remanence ratio of 0.73 indicates that themajor intergranular interaction is exchange coupling rather thanmagnetostatic after the sample has achieved fct phase uponannealing at 600 8C. The high exchange coupling effects areattributed to the grain growth and agglomeration during hightemperature annealing, which allows FePt nanoparticles to adoptmulti-domains instead of single domain for single nanoparticles.The increase in crystallite size, indicated by XRD results shown in

Fig. 6. Coercivity of samples deposited by conventional PLD at 0.5 mbar (dot line)

and BPD at 1 mbar (solid line) and annealed at different temperatures.

Fig. 2, may also promote the high exchange coupling effects. Thehigh exchange coupling effects maybe also related to themorphological and structural properties of deposited samples,which are the agglomerates or networks of FePt nanoparticles.Fig. 6 shows the coercivity of samples deposited by conventionalPLD at 0.5 mbar and BPD at 1 mbar and consequently annealed atvarious temperatures. Similar to samples deposited by conven-tional PLD, the coercivity of FePt nanoparticles grown by BPD isenhanced after annealing at 400 8C and 500 8C due to improvedcrystallinity of low Ku fcc phase and partial phase transition to highKu fct phase (refer Fig. 4). However, the significant enhancement incoercivity happens at the annealing temperature of 600 8C, whereFePt nanoparticles are converted to high Ku fct phase and in turnhave high magnetocrystalline anisotropy. It can be observed thatcoercivity of FePt nanoparticles deposited by BPD is slightly higherthan that synthesized by conventional PLD due to better crystal-linity of fct phase indicated by higher intensity ratio of (0 0 1) peakto fundamental (1 1 1) peak (refer Figs. 2 and 4).

The MOKE hysteresis loops of the samples deposited by BPD at1 mbar and consequently annealed at 500 8C and 600 8C are shownin Fig. 7. After annealing at 500 8C, the coercivity increases from415 Oe of as-deposited sample to 1570 Oe, which is due to the partialphase transition of FePt from magnetically soft fcc to magneticallyhard fct (refer Fig. 4). The coercivity increases further to 2510 Oeafter annealing at higher temperature of 600 8C due to improvedcrystallinity of fct phase FePt. The remanence ratios of samplesbefore and after annealing at 600 8C are 0.49 and 0.71, respectively,indicating that the intergranular interaction changes from magne-tostatic to exchange coupling after annealing. For the sampleannealed at 600 8C, the coercivity value, 2510 Oe, measured byMOKE magnetometer is higher than 1149 Oe measured by VSM(refer Fig. 6). MOKE magnetometer only measures the surface layerof deposited thin films, which is typically about 10–20 nm thick dueto the short laser penetration depth, whereas, VSM measuresmagnetic moments of whole sample. Hence, for MOKE magnet-ometer, shape anisotropy becomes more significant and in turngives rise to the higher coercivity.

4. Conclusion

FePt nanoparticles, in the forms of nanoparticle agglomeratesand nanoparticle networks, can be synthesized by conventionalPLD and BPD at different ambient gas pressures. FePt nanoparticleagglomerates synthesized by BPD are found to have higheruniformity, in terms of agglomerate size and size distribution,compared to those deposited by conventional PLD. Furthermore,

J.J. Lin et al. / Applied Surface Science 255 (2009) 4372–4377 4377

less laser droplets can be observed on samples synthesized by BPD.In both cases, as-deposited FePt nanoparticles exhibit low Ku fccphase and post-annealing at 600 8C is required for the phasetransition to high Ku fct phase. FePt nanoparticle agglomeratesdeposited by BPD are found to have better crystallinity afterannealing, since backward moving ablated species have higherkinetic energy due to shorter travel distance. It in turn contributesto the higher coercivity of FePt nanoparticle agglomeratesdeposited by BPD. However, in both cases, the high remanenceratio indicates that the intergranular interaction changes frommagnetostatic to exchange coupling after high temperatureannealing due to thermal annealing effects (grain growth andagglomeration) and the increase in crystallite size.

Acknowledgements

The authors are grateful to the National Institute of Education/Nanyang Technological University, Singapore, for providing theAcRF grant RI 17/03/RSR and SEP grant RP 13/06/RSR. One of us, JJL,would like to thank NIE/NTU for providing the research scholarship.

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