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Scripta Materialia 58 (2008) 635–638

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He+ ion irradiation-induced disordering in L10-FePd thin films:Ion fluence dependence

Andras Kovacs,a,*,1 Daniel G. Merkel,b Ferenc Tancziko,b Svetoslav Stankov,c

Yoshihiko Hirotsua and Laszlo Bottyanb

aThe Institute of Scientific and Industrial Research, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, JapanbKFKI Research Institute for Particle and Nuclear Physics, PO Box 49, H 1525, Budapest, Hungary

cEuropean Synchrotron Radiation Facility, BP 220, 38043 Grenoble CEDEX 9, France

Received 16 October 2007; revised 19 November 2007; accepted 20 November 2007

Highly ordered, epitaxial, isotope-periodic [natFePd/57FePd] multilayers were prepared by molecular beam epitaxy and irradiatedwith 130 keV He+ ions to fluences of up to 1.49 � 1016 atoms cm�2. Electron microscopy showed a gradual accumulation of defectsand disordering in irradiated samples. A significant decrease in the long-range order parameter was observed by X-ray diffractionanalysis. However, the structural changes remained local, as nuclear resonant X-ray reflectometry confirmed that the isotopemultilayer maintained good periodicity.� 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Molecular beam epitaxy; Ion irradiation; Transmission electron microscopy; Magnetic thin film

Ion irradiation of L10 (tetragonal, CuAu (I) type)structure has been applied to modify the structuraland magnetic properties of these materials [1,2]. TheL10 structure is attractive as a material for perpendicu-lar magnetic recording media due to its high magneto-crystalline anisotropy energy of about 107 erg cm�3 [3].The high anisotropy energy is generated by the alternatepacking of elements along the tetragonal c-axis. He+ ionirradiation can be used to decrease the L10 orderingtemperature of FePt nanoparticles [4] and to inducethe direct ordering of L10 phase at low temperaturesby either direct beam heating [5] or by performing theirradiation at elevated substrate temperatures [6]. Inboth cases, the penetrating He+ ions introduce anappropriate amount of energy into the structure, whichincreases the number of vacancies and interstitials inirradiated films, leading to higher diffusivity of theatoms during annealing and consequently to the forma-tion of the L10-ordered phase at low temperature. He+

ion irradiation was reported to be capable of producingthe L10 structure at low temperature, while it can alsocause disordering of the formed L10 structure. Thus,

1359-6462/$ - see front matter � 2007 Acta Materialia Inc. Published by Eldoi:10.1016/j.scriptamat.2007.11.025

* Corresponding author. E-mail: kovacs.andras22@gmail.com1 On leave from Research Institute for Technical Physics and Materials

Science, P.O. Box 49, 1525, Budapest, Hungary.

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investigation of He+ ion irradiation of the ordered L10

structure is of both technological and scientificimportance.

In the present study, the He+ ion irradiation-inducedmicrostructural changes of highly ordered L10-FePdthin films were investigated. Isotope-periodic multilayersof natFePd and 57FePd were prepared to investigate theintermixing effects upon irradiation by nuclear reflec-tometry. Here, as-prepared sample were characterizedand compared with those irradiated with different iondoses based on electron microscopy observations andX-ray diffraction measurements.

L10-FePd layers were prepared on MgO(001) single-crystalline substrates by molecular beam epitaxy in avacuum of 10�9 mbar. [natFePd(3 nm)/57FePd(2 nm)]10

isotope-periodic multilayers were grown on a Pd bufferlayer 15 nm thick and a Cr seed layer 3 nm thick at623 K at a growth rate of 0.005 nm s�1. The composi-tion of the film was Fe47Pd53, verified by energy disper-sive X-ray spectroscopy and Rutherford backscatteringmeasurements. A Pd capping layer 1 nm thick was evap-orated on the film to prevent the sample from oxidation.As-prepared As-prepared samples were irradiated withlow, medium and high fluences (1 � 1014, 3.7 � 1015

and 1.5 � 1016 atoms cm�2) of 130 keV He+ ions, typi-cally with 0.1 lA cm�2. Structural analysis of as-pre-pared and ion-irradiated samples was performed by

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. (2008), doi:10.1016/j.scriptamat.2007.11.025

636 A. Kovacs et al. / Scripta Materialia 58 (2008) 635–638

transmission electron microscopy (TEM) observation ofcross-sectional samples. The TEM samples wereprepared by the conventional method, i.e. face-to-facegluing, slice cutting, mechanical polishing from bothsides and finally Ar+ ion-milling at low energy. Thelong-range order parameter and lattice parameters ofnon-irradiated and irradiated samples were determinedby X-ray diffraction (XRD) measurements using Bragg–Brentano geometry and Cu Ka radiation. The grazingincidence nuclear resonant X-ray reflectometry (GIN-RR) experiments were performed at the ID18 beam lineof the European Synchrotron Radiation Facility usinghybrid (24 � 8 + 1 filling) mode. The beam was mono-chromatized by a Si(111) and then a Si(4 22)/Si(1222)double channel-cut monochromator to around the14.4 keV Mossbauer transition energy (k = 0.86025 A)of 57Fe. Delayed (nuclear resonant) counts followingthe synchrotron pulse were time integrated and plottedas a function of the angle of incidence to give a nuclearresonant reflectivity curve.

The non-irradiated sample was investigated by cross-sectional TEM analysis using bright-field (BF) anddark-field (DF) TEM images and selected-area electrondiffraction (SAED) patterns. Figure 1 shows the contin-uous layer structure of L10-FePd film with Pd buffer andCr seed layer. The exclusive appearance of {001} typesuperlattice reflection in the SAED pattern (Fig. 1a in-set) indicated the perfect epitaxy of the L10 film withthe c-axis parallel to the surface normal. The epitaxialrelationship of L10-FePd film on MgO was determinedas [001]FePdk[001]MgO, (010)FePdk(010)MgO. DF-TEMimages (Fig. 1b) taken with (001) superlattice reflectionwere used to visualize the structure of the L10-orderedlayer. The dark lines in the bright L10 layer were relatedto the defects in the sample (anti-phase boundaries,stacking faults, dislocations). As TEM is not sensitiveto the isotope-periodicity of the structure, the 57Fe-and natFe-containing layers could not be separated inthe TEM images. The bottom and top thin dark layers

Figure 1. TEM analysis of isotope-periodic non-irradiated L10-FePdthin film. BF-TEM (a) and SAED pattern (inset) indicate thecontinuous layer structure and the good epitaxy of the film. The L10

layer has a bright contrast in the DF-TEM image (b) taken with the(001) reflection of the L10 phase.

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matched with the Pd buffer and cap layers. Both inter-faces were sharp. The long-range order parameter ofthe non-irradiated sample was determined to be 0.87from the XRD measurements, whereas the latticeparameter along the crystallographic c direction was0.367 nm.

Figure 2 shows the BF-TEM and DF-TEM images ofHe+ ion-irradiated samples with low (Fig. 2a and b),medium (Fig. 2c and d) and high doses of irradiation(Fig. 2e and f). The applied low-energy implantationfeatured a low collision cross-section to avoid defectinteractions and a small energy transfer to minimizerecoil displacement. However, the beam energy was suf-ficient for the He+ ions to pass through the entiremultilayer film, �70 nm thick, leaving a rather homoge-neous defect distribution in the film to relax and anenhanced defect accumulation at a depth of �700 nmin the MgO substrate (according to the BF-TEM; datanot shown). According to SRIM [7] simulations, eachincoming ion generates an average of 7.3 displacements,of which 7.1 generate vacancies and an equal number ofinterstitials. DF-TEM images (Fig. 2b, d and f) and cor-responding BF-TEM images (Fig. 2a, c and e) show theeffect of the increase in the number of ions transmittedthrough the film. Again, the DF-TEM images weretaken with the (00 1) reflection of the L10 structure. Itis clear that the penetrating He+ ions generated a largernumber of defects in the layer even at a dose as low as1 � 1014 atoms cm�2 (Fig. 2b) in comparison with theas-deposited sample (Fig. 1b). A decrease in the areaof the ordered regions was seen as the ion dose increased(Fig. 2d and f). According to SRIM simulations, theHe+ irradiation at these energies does not induce colli-sion cascades and displacement thermal spikes in theL10-FePd structure. As a primary effect, Frenkel pairswere generated due to atomic displacement and replace-ment collisions that reproduce anti-site effects. Thestructural modifications were extremely local, as eventhe nano-beam diffraction and high-resolution TEManalysis could not provide better detail of the characterof the disordered areas in ion-irradiated samples. Ionirradiation increased the interface roughness by inter-mixing the Pd buffer and cap layers with the adjacentFePd layers. This was seen in DF-TEM images of sam-ples irradiated at higher doses (Fig. 2d and f). The esti-mated thickness of the Pd buffer layer increased from15.2 nm in the as-deposited film to 16.3, 18.2 and18.5 nm in the low-, medium- and high-dose ion-irradi-ated films, respectively.

To quantify the structural changes in irradiated films,the long-range order parameter (S) and alloy chemicalcoherence length (f) were determined from XRD mea-surements. The result indicated marked reductions inboth S and f for the L10-ordered structure along the sur-face normal, as shown in Figure 3. S decreased from0.87 to 0.55 with an increase in the ion dose from 0 to1.49 � 1016 atoms cm�2. In addition, the 001 peak ofthe L10 structure in the XRD spectrum shifted to lowerangles with increasing ion dose, indicating an increase inthe lattice parameter. After irradiation at the high dose,the c lattice parameter was 0.37 nm, whereas that of thePd buffer layer (a = 0.389 nm) remained unchangedafter irradiation.

. (2008), doi:10.1016/j.scriptamat.2007.11.025

Figure 2. BF- and DF-TEM images of ion-irradiated samples with low (a, b), medium (c, d) and high doses (e, f). DF-TEM images (b, d, f) weretaken with the (001) reflection of the L10 phase. The increase of dark contrast areas in the L10 layer indicates the increase in disordering.

Figure 4. Nuclear resonant X-ray reflectivity of the non-irradiated (a),medium- (b) and high-dose (c) ion-irradiated films. The second peaksof the spectra are the isotope Bragg peaks corresponding to the isotopeperiod of 5 nm of the multilayer. The maintained high intensity of theisotope Bragg peaks indicates that the isotope periodic layer structureis also maintained upon He+ irradiation.

Figure 3. Decrease of the long-range order parameter S and chemicalcoherence length f upon He+ ion irradiation.

A. Kovacs et al. / Scripta Materialia 58 (2008) 635–638 637

The GINNR and conversion electron Mossbauerspectroscopy (CEMS) measurements of isotope-periodicL10 multilayer were used to analyze the changes in theatomic environments in ion-irradiated samples. Analysisof the hyperfine field in CEMS showed that the L10 struc-ture was partly transformed to a disordered and to a par-tially ordered Fe-rich phase on irradiation. The amountof the L10-ordered phase decreased with increasing iondose, in good agreement with the TEM and XRD data.The details of CEMS results will be published elsewhere.The GINNR results of the ion-irradiated samples areshown in Figure. 4. The GINNR spectra of non-irradi-ated and irradiated samples consist of two peaks: theone at lower angle is the total reflection peak [8,9], a fea-ture of such spectra that is independent of the layer struc-ture, whereas the higher-angle peak is the Bragg peakoriginating from the isotope periodicity of the57Fe47Pd53/natFe47Pd53 layers. The slight decrease in thisBragg peak indicates that there is very little atomicmovement along the crystallographic c-direction causinginterface mixing between the isotope layers.

In summary, we demonstrated that 130 keV He+ ionirradiation at doses from 1 � 1014 to 1.49 � 1016

atoms cm�2 of highly ordered L10-FePd isotope-peri-

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odic multilayer thin films results in a significant decreasein the L10-ordered fraction. TEM analysis revealed anincrease in the defect concentration and a decrease inthe area of the ordered regions with increasing ion dose.Quantitative evaluation of the XRD spectra supportedthe TEM observations, i.e. the long-range order param-eter of L10 phase and alloy chemical coherence lengthdrop sharply on He+ ion irradiation. In particular,low-dose (<1015 atoms cm�2) irradiation has a moderateeffect on L10 structure, whereas at higher doses(>1015 atoms cm�2) the disordering is pronounced. Asthe He+ irradiation did not induce collision cascadesand displacement thermal spikes in L10-FePd structure,the large concentration of Frenkel pairs and their strainare considered primary effects in the L10 disorderingprocess.

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The authors indebted to Dr. M. Major for his con-tribution to optimization of the sample preparation, andto Dr. A. Nemeth and Mr. J. Waizinger for the He+

implantation at the NIK facility of KFKI RMKI, Buda-pest, and Dr. Zs. E. Horvath for performing the XRDscans. Financial support by the Grant-in-Aid for Scien-tific Research (S) (No. 16106008) from the Ministry ofEducation, Culture, Sports, Science and Technology,Japan and by the Hungarian National Fund (OTKA)and National Office for Research and Technology ofHungary under contracts K62272 and NAP-VEN-EUS’05 are gratefully acknowledged.

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