IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 21 (2010) 065602 (10pp) doi:10.1088/0957-4484/21/6/065602

Growth, structure and magneticproperties of FePt nanostructures onNaCl(001) and MgO(001)F Liscio1, D Makarov2, M Maret1,4, B Doisneau-Cottignies1,H Roussel3 and M Albrecht2

1 Science et Ingenierie des Materiaux et Procedes (SIMaP), INP-Grenoble/CNRS/UJF, BP 75,38402 Saint-Martin d’Heres, France2 Institute of Physics, Chemnitz University of Technology, D-09107 Chemnitz, Germany3 Consortium des Moyens Technologiques Communs, INP-Grenoble, BP 75,38402 Saint-Martin d’Heres, France

E-mail: mireille.maret@simap.grenoble-inp.fr

Received 1 July 2009, in final form 11 December 2009Published 8 January 2010Online at stacks.iop.org/Nano/21/065602

AbstractA comparison of the structural and magnetic properties of FePt nanostructures grown atdifferent temperatures on NaCl(001) and MgO(001) substrates is presented. A strong influenceof the deposition temperature on the epitaxial growth as well as on the size distribution of FePtnanostructures grown on NaCl substrates is observed. In spite of a large lattice mismatchbetween FePt and NaCl, a ‘cube-over-cube’ growth of nanostructures with a narrow sizedistribution was achieved at 520 K. Moreover, the growth of FePt nanostructures on NaCl(001)is not preceded by the formation of a wetting layer as observed on MgO(001). The higherdegree of L10 chemical ordering in FePt nanostructures grown on MgO(001) accompanied bythe absence of L10 variants with an in-plane tetragonal c-axis indicates that the tensile epitaxialstress induced by the MgO substrate is a key factor in the formation of the L10 phase with anout-of-plane c-axis. Superparamagnetic behavior is revealed for the FePt nanostructures grownon NaCl(001) due to their small size and relatively poor chemical order.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Hard magnetic nanoparticles have attracted much attentionas future ultrahigh density magnetic recording media [1–3].In this respect, the most promising alloy is FePt in thetetragonal L10 chemically ordered phase. Owing to its highuniaxial magnetic anisotropy (Ku ∼ 8 × 107 erg cm−3 [4])induced by chemical modulation along the tetragonal c-axis,the thermal stability criteria for magnetic recording media [5]can be satisfied for magnetic nanostructures with a size downto 3 nm, resulting in an enormous areal storage densityof about 50 Tbit in−2 [6]. However, one of the crucialparameters limiting the maximum achievable areal density isthe size distribution in the array of magnetic nanostructures.To control the size of the magnetic nanostructures and to

4 Author to whom any correspondence should be addressed.

induce the required (001) texture, deposition of FePt alloyson single crystalline MgO(001) [7–10] or on amorphoussubstrates using various seed layers such as Cr [11], Cu [10]or CrRu [12] is usually carried out. In this respect, recentinvestigations have focused on the growth of self-assembledmagnetic particles on NaCl(001). Owing to the low surfaceenergy of NaCl(001) substrates the deposition of thin metalfilms leads to the formation of small nanoparticles with a rathernarrow size distribution. Thus, NaCl(001) substrates havealready been used to grow assemblies of binary FePt [14, 15]and FePd [13, 14] as well as ternary FeCuPd [16], FeCoPd [17]and FeCoPt [18] alloy nanostructures. Moreover, the chemicalpassivity between FePt and NaCl has also been exploited byElkins et al [19] to prevent sintering between FePt grainsduring high temperature annealing using salt powders. Inaddition, the alignment of the tetragonal c-axis of FePt

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Nanotechnology 21 (2010) 065602 F Liscio et al

nanoparticles using ultrasound and chloroform suspension wasrecently demonstrated by Foglia et al [20]. Despite theperformance of extensive studies, the structural and magneticproperties of assemblies of FePt nanostructures grown onNaCl(001) are still unclear. For example, the growth conditionsfavorable for the formation of FePt nanostructures with asingle variant of the L10 phase with the c-axis pointing alongthe surface normal are still to be determined. In addition,the degree of L10 chemical order, which is one of the mostimportant parameters, has not yet been reported.

In this paper, we present a detailed study of the structuraland magnetic properties of FePt nanoparticles grown on singlecrystalline NaCl(001) substrates at different temperaturesunder ultrahigh vacuum (UHV) conditions. Moreover, thesimultaneous deposition of FePt alloys on NaCl(001) andMgO(001) substrates allows us to consider the influence of thesubstrate properties (lattice parameter, surface energy) on theformation of the L10 phase, the morphology and the magneticproperties of the FePt nanostructures.

2. Experimental technique

Co-deposition of Fe and Pt atoms simultaneously on freshlycleaved NaCl(001) and polished MgO(001) substrates wasperformed in a molecular beam epitaxy chamber equippedwith two independent e-guns loaded with Fe and Pt of puritybetter than 99.99%. The pressure during deposition did notexceed 5 × 10−9 mbar. In order to improve the quality ofthe substrate surfaces, thermal annealing (650 K for 2 h inUHV) was applied prior to co-deposition. The depositionof FePt alloys was performed at substrate temperatures (Ts)ranging from 420 to 620 K. The composition of the depositedalloys was controlled during growth using a quadrupole massspectrometer adjusting the ratio between Fe and Pt fluxes.The typical growth rates for Fe and Pt were 0.01 A s−1 and0.0134 A s−1, respectively. Finally, a 1 nm thick Cr protectivelayer was additionally deposited at room temperature.

The epitaxial growth was followed in situ using reflectionhigh energy electron diffraction (RHEED). The structureand the degree of L10 chemical ordering were determinedfrom x-ray diffraction (XRD) measurements carried out atthe seven-circle diffractometer of the CRG-BM02 beamline(ESRF, Grenoble) using a photon wavelength of 1.385 A.Complementary x-ray measurements were performed using aconventional four-circle diffractometer with Cu Kα radiation.

The morphology of FePt deposits on NaCl(001) wasinvestigated by transmission electron microscopy (TEM) inconventional and high resolution mode (HRTEM) using aJEOL 2010 microscope operating at 200 kV. For TEM studies,FePt deposits were first covered by a 8 nm-thick carbon layerand merged in distilled water. After the separation from theNaCl substrate, the carbon layer supporting the FePt particleswas mounted on a copper grid. In addition, Fourier transformsof HRTEM images of individual particles were performed inorder to identify the orientation of the FePt nanostructures.

Due to the low surface energy of NaCl(001)(230 erg cm−2) compared to that of MgO(001)(1200 erg cm−2), the condensation coefficients of metal atoms

on NaCl(001) are less than 1 [21] and differ substantially fordifferent metal atoms [22]. The condensation coefficient ofPt on NaCl(001) was previously determined by Rupprechteret al [21] based on a TEM study and found to equal 0.4 in atemperature range between 573 and 673 K. For the purpose ofthis work, a condensation coefficient of Fe atoms on NaCl(001)of about 0.35 was determined by analyzing a plane-view TEMimage taken on an assembly of Fe particles grown at 520 K(not shown).

Magnetic characterization was performed in out-of-planeand in-plane geometry using a superconducting quantuminterference device (SQUID) with a maximal field of ±7 T.The effect of thermal activation and magnetic relaxation wasstudied by measuring the zero-field and field cooled (ZFC/FC)curves.

3. Results and discussion

Figures 1(a) and (b) show the RHEED patterns taken beforedeposition along the [100] and [110] azimuths of a NaCl(001)surface. After deposition of a 0.3 nm-thick FePt alloy at520 K (figures 1(c) and (d)), besides the still visible substratestreaks the presence of broad diffraction spots indicates a three-dimensional island growth mode of the FePt alloy. The positionof these spots with respect to the substrate streaks reflectsthe epitaxial growth of FePt with the following epitaxialrelationships:

[100]NaCl(001)‖[100]FePt(001) and

[110]NaCl(001)‖[110]FePt(001)

corresponding to ‘cube-over-cube’ growth. The separationbetween the NaCl streaks and the FePt dots is in agreementwith the large lattice mismatch of about 30% betweenFePt(001) (a = 3.86 A and c = 3.71 A for the bulkL10 FePt) and NaCl(001) (a = 5.64 A). This suggeststhe growth of rather stress-free FePt islands. In addition,the weak superstructure spots between the strong fundamentalreflections along the [001] direction (figure 1(c)) are thesignature of chemical ordering [23, 24]. The indices of theobserved reflection spots are indicated in figures 1(e) and (f)and refer to the variant of the L10 phase with the tetragonalc-axis oriented perpendicular to the NaCl(001) surface.

Figure 2 presents the RHEED patterns taken along the[001] azimuth after deposition of 1 nm thick FePt films onNaCl(001) (a, c) and 3 nm thick FePt films on MgO(001) (b, d)at 520 and 620 K. Note that since the deposition of FePt onboth substrates was done simultaneously, the thinner depositon NaCl results from the lower condensation coefficientsof Fe and Pt atoms on NaCl as discussed previously. Asalready observed by Miyazaki et al [25], the growth of FePtalloys with equiatomic composition on MgO(001) occurs withthe same epitaxial relationships as those indicated above forNaCl(001). However, in contrast to the observations onNaCl(001) (figure 2(a)), the deposition of FePt on MgO(001) at520 K does not give rise to diffraction spots but produces ratherdiffuse streaks (figure 2(b)). The difference in the growth mode

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Figure 1. RHEED patterns taken along the [100] and [110] azimuths of a NaCl(001) substrate (a, b) before and (c, d) after deposition of a0.3 nm-thick FePt alloy on NaCl(001) at 520 K. (e), (f) The indices of the observed spots corresponding to the variant of the L10 phase with atetragonal c-axis oriented along the surface normal.

Figure 2. RHEED patterns taken along the [100] azimuth after deposition of FePt alloys at 520 and 620 K on (a, c) NaCl(001) and (b, d)MgO(001) substrates.

becomes less pronounced at high deposition temperatures of620 K (figures 2(c) and (d)). In this case, the RHEED patternsreveal diffraction spots for the deposits on both substrates,NaCl(001) and MgO(001). However, the diffraction spotscoming from the FePt alloy grown on a MgO(001) substrate aremuch smaller than those observed on NaCl(001), indicating thedifference in size of the nanostructures. FePt islands are thusexpected to be larger on MgO than on NaCl. This result can beattributed to the thicker deposit on MgO but, as shown later, itis essentially related to a different growth mode. In addition,figures 2(c) and (d) reveal a remarkable difference in positionof the superstructure reflections. The superstructure spots of

the L10 chemically ordered phase of the FePt alloy grownon the MgO(001) substrate are observed only along the [001]growth direction, indicating the existence of a single variantof the L10 phase with the c-axis perpendicular to the substratesurface (figure 2(d)). In contrast, superstructure reflections ofthe alloy deposited on the NaCl(001) substrate are additionallyobserved along the [010] direction in the film plane: solidand dashed circles in figure 2(c) mark the superstructure spotscorresponding to the variants of the L10 phase with the c-axisparallel to [010] and [100], respectively.

Figure 3 shows the superposition of the θ–2θ scan andthe rocking curve (RC) measured at the 002 reflection of FePt

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Figure 3. θ–2θ scans and rocking curves (RC) of the 002 reflectionof FePt grown at 620 K on (a) MgO(001) and (b) NaCl(001)(measurements performed using Cu Kα radiation).

grown at 620 K on (a) MgO(001) and (b) NaCl(001). Note thatan additional peak observed on the right-hand side of the 002FePt reflection (figure 3(b)) corresponds to the 006 NaCl peakwhich originates from the contribution of the undesired λ/2wavelength not eliminated by the graphite monochromator. Itis worth emphasizing the observed difference in the shape ofthe rocking curves between the two deposits. Besides the broadcomponent of the RCs, a narrow peak is additionally observedfor the deposit on MgO(001). Its full width at half maximum(FWHM) of 0.2◦ is equal to the width of the rocking curvemeasured at the 002 reflection of the MgO(001) substrate,indicating that the growth of FePt islands on MgO(001)is preceded by the formation of a wetting layer. Such anarrow component was also observed in the rocking curveof the FePt deposit on NaCl at 520 K, but its integratedintensity with respect to that of the broad component is twotimes smaller, indicating that the wetting layer is favoredby an increase in the growth temperature. For that matter,such a layer was indeed not observed during the growth atroom temperature of Pd on MgO(001) from in situ grazingincidence diffraction measurements [26]. The width of thebroad component in the RCs, which is quite similar for thetwo substrates, comes from the FePt islands. It depends onboth the lateral coherent length, which is related to the lateralsize of monocrystalline islands and the mosaicity, i.e. thedisorientation of the [001] direction of particles with respectto the [001] direction of the substrate. The two effects can inprinciple be separated by measuring the RC for higher orderreflections. However, for our samples the 004 reflections couldnot be measured. Neglecting the mosaicity contribution, theFWHM of the RC (8◦) would lead to a typical lateral coherentlength of 1.4 nm, i.e. much smaller than the lateral size of FePtparticles grown on NaCl (determined by TEM, as presentedlater). Therefore to reproduce the observed broadening of theRC, some disorientation of the particles by a few degrees isexpected.

The analysis of the θ–2θ pattern (figure 3) allowsestimation of the normal coherence lengths, �002

⊥ , for thedeposits on both substrates (table 1). The larger value

Table 1. Structural parameters of FePt alloys grown simultaneouslyon MgO(001) and NaCl(001) substrates at 520 and 620 K. The erroron the last digit is given in parentheses.

Samples Ts (K) c (A) a (A) �002⊥ (A) S

FePt/MgO(001) 520 3.74(1) 3.84(3) 24(3) 0.45(5)620 3.74(1) 3.85(3) 22(3) 0.60(5)

FePt/NaCl(001) 520 3.830(5) 3.84(2) 60(2) 0.3(1)620 3.833(5) 3.86(2) 61(2) —

found for FePt nanostructures formed on NaCl(001) is a clearindication of the growth of higher islands on NaCl(001) thanon MgO(001), in spite of the lower deposited alloy thicknesson NaCl. Therefore, these XRD results combined with theRHEED observations suggest a Stranski–Krastanov growthmode for FePt deposited on MgO(001) and a Volmer–Webergrowth mode on NaCl(001). These different growth modes canbe explained by (i) the low surface energy of the NaCl substrateequal to 230 erg cm−2, i.e. five times smaller than the value forMgO (1200 erg cm−2), and (ii) the large lattice mismatch of30% induced by NaCl while the mismatch is only 8.5% withMgO (a = 4.215 A).

The θ–2θ scans at the 002 and 113 reflections of the FePtalloy grown on NaCl(001) are shown in figures 4(a) and (b).The positions of the 002 and 113 peaks allow extraction ofthe c and a lattice parameters of the tetragonal L10 phase(table 1). Note that only a rough estimate of the latticeparameters for the alloy grown at 620 K can be given owingto the superimposition of the 002 peak with the 200 peakcoming from several variants of the L10 phase (figure 2(c)).Due to the presence of a tail of the strong 002 NaCl peak,reliable 001 peaks for FePt could not be extracted. However,for the deposit at 520 K, complementarity x-ray experimentshave allowed the separation of a small 001 FePt peak. Themeasurements were performed slightly off the θ–2θ condition,leading to a strong damping of the intensity of the 002 NaClpeak. The ratio between the integrated intensities of the 001and 002 peaks, I001 and I002, taking into account the width ofthe rocking curves and corrected for absorption (Ahkl ), Lorentz(Lhkl ) and polarization (Phkl ) factors, allows the determinationof the long-range chemical order (LRO) parameter, S:

S =√

I001|F002|2 A002L002 P002

I002|F001|2 A001L001 P001(1)

where |F002|2 = 4[( fFe + fPt)2 + ( f ′′

Fe + f ′′Pt)

2] and |F001|2 =4[( fFe − fPt)

2 + ( f ′′Fe − f ′′

Pt)2] with fi = f 0

i + f ′i , i =

Fe or Pt. f 0i is the coherent scattering factor dependent

on the scattering vector q; f ′i and f ′′

i are the anomalousdispersion terms independent of q . Far from the absorptionedge, the contributions of the f ′′

i terms can be neglected forthe calculation of S. A value of the LRO parameter of 0.3 wascalculated according to equation (1) for the sample grown at520 K (table 1). It is important to note that the intensity ofthe 002 peak of the FePt alloy grown on NaCl(001) stronglydecreases with increasing deposition temperature from 520 to620 K (figures 4(a) and (b)). To get a better understanding ofthis effect, measurements of (111) pole figures were carried

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Figure 4. θ–2θ scans of the (a) 002 and (b) 113 reflections of FePt grown on NaCl(001) at 520 and 620 K (measurements performed at theESRF with λ = 1.385 A). (001) stereographic projections of the (111) poles measured for FePt grown on NaCl(001) at (c) 520 K and(d) 620 K (measurements performed using Cu Kα radiation).

out (figures 4(c) and (d)). Independently of the depositiontemperature, the pole figure measurements reveal the presenceof four 111 FePt reflections together with four 202 NaClreflections rotated by 45◦ in the (001) projection plane,confirming the cube-over-cube growth. For the deposit at520 K, the φ-angular broadening of the 111 reflections for χ

close to 54.74◦ leads to an in-plane mosaicity of about 6◦. Forthe deposit at 620 K, the 111 iso-intensity contours are threetimes weaker, indicating the degradation of the cube-over-cubegrowth mode. This result is consistent with the strong decreaseof the intensities of the 002 and 113 peaks found for the depositat 620 K.

The θ–2θ scans performed at the 001 and 002 reflectionsof the FePt alloy grown on MgO(001) are presented in figure 5.The substantial increase of the intensity of the 001 peak withthe growth temperature in comparison with the small reductionof the intensity of the 002 reflection suggests an enhancementof L10 chemical ordering. After subtracting the contribution ofthe 002 MgO peak to the 002 FePt peak which is significantlylarger for the deposit at 520 K, the ratios of the integratedintensities of the 001 and 002 FePt reflections lead to the LROparameters given in table 1. The reported values are close

to those measured in continuous FePt films by Farrow et al[27]. The c and a lattice parameters were extracted fromthe positions of the 002 and 113 peaks (not shown) and aresummarized in table 1. Note that the position of the 002 MgOreflection is close to the 002 peak of the FePt alloy, resulting ina larger error on the a lattice parameter. A tetragonal distortionof the FePt lattice is observed but it is substantially smallerthan that of the bulk L10 phase, in agreement with partialordering. The higher degree of LRO accompanied with a morepronounced tetragonality found in the FePt nanostructuresgrown on MgO(001) is firmly related to the substrate-inducedepitaxial stress. Even if surface disorder effects are expectedto be more important for the FePt deposit on NaCl due to alarger surface/volume ratio, they could partially explain thedifferent values of S but not the weak tetragonal distortion.Moreover, an effect of the different deposited thicknesses canbe ruled out. It is indeed well established that for such growthtemperatures the chemical ordering between Fe and Pt atomstakes place at the free growing surface. Due to negligible bulkdiffusion, chemical ordering cannot occur in the buried layers.For a constant deposition rate, an increase of the alloy thicknessand thus a longer deposition time will not enhance the averagechemical ordering.

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Figure 5. θ–2θ scans of the (a) 001 and (b) 002 reflections of FePt grown on MgO(001) at 520 and 620 K (measurements performed at theESRF with λ = 1.385 A).

Figure 6. (a) TEM plane-view micrograph of a 1 nm-thick FePt alloy grown on a NaCl(001) substrate at 420 K and (b) the correspondingSAED pattern.

The morphology of FePt nanoparticles grown onNaCl(001) was examined using transmission electron mi-croscopy after dissolution of the NaCl substrates. The thinprotective layer of Cr, which is polycrystalline, gives rise toa poor electronic contrast and its contribution mixed with thatof the amorphous carbon film does not hinder the visualizationof the FePt nanoparticles. Figure 6(a) shows a TEM imageof the FePt deposit grown at 420 K. A dense assembly ofnanoparticles with an average lateral size of about 3 nm isfound in agreement with the Volmer–Weber growth mode asalready suggested by the XRD study. To access the orientationof the FePt nanoparticles, selected-area electron diffraction(SAED) measurements were performed. The 200 and 220reflections (indicated with thick solid circles in figure 6(b))correspond to [001]-orientated particles. It is important toemphasize the difference in shape and intensity of the 220and 200 spots. While the 220 reflections are presented asarcs of a circle revealing a slight deviation from the perfectcube-over-cube growth, the 200 spots are more intense andare slightly shifted towards the center with respect to theposition of the 220 peaks. The latter can be attributed tothe superimposition of the 002 reflections of FePt and 220reflections coming from NaCl, which is recrystallized afterthe dissolution in water. Moreover, weak 111 spots can be

identified on a diffuse 111 ring. The positions of the 111 spotswith respect to those of the [001]-oriented particles allow us toidentify: (i) particles in the 〈110〉 zone axis with the peculiarepitaxial relationships: [100] NaCl(001)‖[110] FePt(110)

and [010] NaCl(001)‖[110] FePt(110), giving rise to thereflections indicated with dotted circles; (ii) particles likely inthe 〈112〉 zone axis leading to the appearance of reflectionsindicated with thin solid circles (figure 6(b)).

The increase of the deposition temperature to 520 Kresults in an assembly of well-separated FePt nanoparticlesof larger lateral size (figure 7(a)). The size distribution iscentered at 10 nm with a FWHM of 4 nm. The majorityof nanoparticles exhibit a square-based shape confirming thepreferential 001 growth direction. A HRTEM image of a[001]-oriented FePt nanoparticle with the lattice fringes of the(200) and (220) planes is shown in figure 7(b). The mainedge directions of the particles are 〈110〉, with truncationsat the corners along the 〈100〉 directions. Similar to theobservations on Au/MgO(001) [28], Pt/MgO(001) [29], andPd/MgO(001) [30], the FePt nanoparticles adopt a truncatedcubo-octahedral shape with large {111} and small {110}side-wall facets. The Fourier transform of the HRTEMimage reveals the 110 superlattice reflections (figure 7(c))

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Figure 7. (a) TEM plane-view micrograph of a 1 nm-thick FePt alloy deposited at 520 K on a NaCl(001) substrate. (b) HRTEM image and(c) FFT of a 001-oriented L10 ordered FePt nanostructure.

Figure 8. (a) TEM plane-view micrograph of a 1 nm thick FePt alloy grown on a NaCl(001) substrate at 620 K. (b), (d) HRTEM image andcorresponding FFT of a L10 ordered nanostructure of rectangular shape with the tetragonal axis in the plane. (c), (e) HRTEM image andcorresponding FFT of a 110-oriented nanostructure of triangular shape.

characteristic for a L10 ordered particle with the c-axis orientedperpendicularly to the substrate surface.

Further increase of the deposition temperature to 620 Kleads to the coalescence of FePt nanoparticles (figure 8(a)).The statistical analysis of the TEM image shows a broaderlateral size distribution centered at 17 nm with a widthof 9 nm. In addition to the square shaped 001-orientednanostructures, particles with rectangular as well as triangularshapes are observed (figures 8(b) and (c)). In this respect,the rectangular shape stems from the merging of two square-shaped particles. Its Fourier transform (figure 8(d)) revealsthe 001 and 002 reflections which are characteristic for L10

ordered particles with an in-plane c-axis, as already observedby RHEED (figure 2(c)). In the case of the triangular-shaped particles (figure 8(c)) the presence of lattice fringescoming from (111) and (200) planes specific for a particlein the 〈110〉 zone axis is confirmed by the Fourier transform

of the HRTEM image (figure 8(e)). Note that this growthorientation could not be detected by XRD since the 220reflection overlaps with the strong 004 reflection of NaCl. Thepresence of 110-oriented nanostructures is in agreement withthe degradation of the cube-over-cube growth mode as revealedby the strong decrease of the intensity of the 002 peak ofFePt with increasing deposition temperature from 520 to 620 K(figure 4(a)).

The magnetic properties of FePt nanostructures grown onNaCl(001) at 520 K are summarized in figure 9. In agreementwith the small size and relatively low degree of chemical order,the in-plane and out-of-plane hysteresis loops measured at300 K are nearly closed with a negligibly small remanence aswell as coercive field (HC < 50 Oe) (figures 9(a) and (b)). Theout-of-plane hysteresis loop opens at lower temperatures andthe coercivity increases substantially to about 3 kOe measuredat 10 K, indicating a blocked state of superparamagnetic

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Figure 9. Measurements for 1 nm thick FePt alloy grown at 520 K ona NaCl(001) substrate. (a) Out-of-plane and (b) in-plane hysteresisloops measured at 10 K (filled circles) and 300 K (open circles).(c) ZFC/FC performed with a magnetic field of 200 Oe applied in thefilm plane. The inset in (c) reveals the temperature derivative of thedifference between the ZFC and FC magnetization curves.

particles. The similar shape of the in-plane hysteresis loopmeasured at 10 K suggests that the nanostructures possessrandomly oriented easy axes of magnetization. A similarobservation was reported by Sato et al [13] on FePd alloysgrown on NaCl(001) substrates. Note that owing to thepresence of a Cr capping layer a slight broadening of thehysteresis loop (enhancement of the coercive field) caused bythe free rotatable uncompensated spins at the interface betweenthe ferromagnetic FePt layer and antiferromagnetic Cr layermight occur [31].

The effect of thermal activation and magnetic relaxationwas studied by measuring zero-field cooled and field cooled(ZFC/FC) curves using a small magnetic field of 200 Oeapplied in the film plane (figure 9(c)). The behavior of theZFC/FC curves is typical for an ensemble of noninteractingsuperparamagnetic particles, revealing a broad energy barrierdistribution (distribution of blocking temperatures). Sucha broad distribution might originate from the particle size

distribution accompanied with a variation of the magneticproperties, such as magnetic anisotropy in the assembly of FePtnanostructures. The mean value as well as the distribution ofblocking temperatures (TB) was estimated using the methoddeveloped by Wohlfarth [32], where the distribution of TB canbe approximated by the first derivative on temperature of thedifference between the magnetizations, MFC and MZFC, in FCand ZFC curves, respectively [33–35]:

f (TB) = d(MFC − MZFC)

dT. (2)

In this case, the maximum of the derivative corresponds tothe mean blocking temperature giving a value of TB = 247 K(inset in figure 9(c)). The FWHM of the derivative of about170 K reveals the wide distribution of the energy barriers in thesystem. The blocking temperature is related to the magneticactivation volume, Vact, responsible for the magnetizationreversal process in the nanoparticles. Assuming a thermallyactivated magnetization reversal process with a switching ratefollowing the Arrhenius law, the activation volume can beroughly estimated using the expression

K Vact = kBTB ln(τ/τ0) (3)

with K being the mean anisotropy constant of FePtnanoparticles, τ is the time constant related to the measurementtime (typically 100 s) and τ0 ≈ 10−10 s is assumed as theattempt frequency. A mean value of the magnetic anisotropyconstant, K , of 0.76 × 107 erg cm−3 was considered accordingto the data available in the literature [36]. It is importantto mention that the shape anisotropy contribution, Kshape =2π N M2

S ∼ 1.5 × 106 erg cm−3, using a demagnetizingfactor N of about 0.3 and a saturation magnetization MS of880 emu cm−3, is substantially smaller. The extraction ofthe activation volume using equation (3) results in a value ofabout 110 nm3. Such a volume is smaller than the physicalvolume of an individual FePt nanostructure with a truncatedcubo-octahedral shape (235 nm3) as observed by TEM, andtaking as a height value the perpendicular coherent lengthdetermined by XRD. This finding suggests that only the coreof the FePt particles is chemically ordered due to surfacedisorder effects. Therefore, an inhomogeneous magnetizationreversal process rather than a coherent switching within theindividual nanostructure is expected. The latter is in agreementwith data reported by Jones et al [37] where the incoherentmagnetization reversal was observed for particles with a sizeof 15 nm.

Moreover, the FC data are well described using thetheoretical M(T ) dependence [38, 39]:

(1 − MFC/M(0 K)) ∝ (exp(/T ) − 1) (4)

with being an adjustable parameter related to the Curietemperature. The fitting (not shown) gives a -value of(750 ± 40) K, which is close to the Curie temperature foundby Rong et al [40] for FePt nanoparticles with a size of about10 nm.

Similar to the discussion above, ZFC/FC measurementsperformed on the sample grown at 620 K show as well a

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Figure 10. Measurements for 1 nm-thick FePt alloy grown at 620 Kon a NaCl(001) substrate. (a) ZFC/FC performed with a magneticfield of 200 Oe applied perpendicular to the film plane.(b) Out-of-plane and (c) in-plane hysteresis loops measured at 10 K(filled circles) and 300 K (open circles). Note that the horizontal axesin (b) and (c) have a different scale.

broad distribution of blocking temperatures (figure 10(a)). Theanalysis of the ZFC/FC data revealed a blocking temperatureof 238 K with a FWHM of 50 K. A rough estimation ofthe activation volume assuming the same anisotropy constantas for the sample prepared at 520 K leads to a similarvalue as before, suggesting again an inhomogeneous reversalprocess as expected. The presence of a small decay at lowtemperatures observed on the FC curve could be attributedto a spin-glass-like behavior. In this respect, such behaviorwas observed by Perumal et al [41] on FePt alloys grown onMgO(001) substrates. However, the performed investigation isnot sufficient to draw a final conclusion and an extended studyhas to be performed which is outside the scope of this work.

The measurements at 300 K reveal closed hysteresisloops (figures 10(b) and (c)) as expected for an ensemble ofsuperparamagnetic particles. However, the out-of-plane and in-plane hysteresis loops measured at 10 K show different shapes(figures 10(b) and (c)) with a substantially higher remanencevalue for the hysteresis loop measured in the out-of-planedirection, indicating the stabilization of a preferential out-of-

plane easy axis of magnetization. The value of saturationmagnetization estimated from the hysteresis loops is about1000 emu cm−3. This value is higher than the MS found forthe sample grown at 520 K which might be related to the grainsize effect as already reported by Rong et al [40].

The magnetic study performed in out-of-plane geometryon a 3 nm thick FePt alloy deposited on MgO(001) at 620 Kreveals a typical easy axis with an out-of-plane coercivity ofabout 8.1 kOe (not shown). This finding is in agreementwith (i) the relatively high degree of long-range chemicalorder (table 1) and (ii) the dense assembly of FePt particlesresulting in the formation of a magnetically exchange coupledcontinuous layer rather than an array of exchange decouplednanostructures as observed for the deposits on NaCl(001).

4. Conclusions

The structural and magnetic properties of FePt alloyssimultaneously grown on NaCl(001) and MgO(001) surfaceswere investigated. X-ray diffraction measurements haveallowed us to highlight the effect of the substrate properties onthe growth mode and the degree of L10 long-range chemicalordering of FePt alloys. A strong influence of the depositiontemperature on the size as well as on the orientation ofnanostructures on NaCl(001) is found. Thus, the depositionof a 1 nm thick FePt film at 520 K on NaCl(001) results inan assembly of FePt nanostructures with a truncated cubo-octahedron shape, an average lateral size of about 10 nmand a relatively low degree of chemical ordering. Thecomparison of the structural properties of FePt alloys grown onNaCl(001) and MgO(001) allows us to emphasize the role ofthe epitaxial stress induced by the substrate on the formationof the L10 chemical ordering. The magnetic measurementsperformed on the assembly of particles formed on NaCl(001)show a superparamagnetic behavior with a preferential out-of-plane easy axis of magnetization for deposits grown at620 K.

Acknowledgments

The authors thank H Tolentino and M De Santis for theirhelp during sample preparation in the SUV32 set-up atthe ESRF; M Audier (LMGP, Grenoble) for enlighteningdiscussion about TEM observations; J P Simon (SIMAP,Grenoble) and the CRG-BM02 team (N Boudet, J F Berar andB Caillot) for technical support. Allocation of beamtime onthe French CRG-BM02 beamline at the ESRF is gratefullyacknowledged. This work was supported in part bythe Deutscher Akademischer Austauschdienst and Frenchpartenariat Hubert Curien (Procope).

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