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Materials Chemistry and Physics 144 (2014) 186e193

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Materials Chemistry and Physics

journal homepage: www.elsevier .com/locate/matchemphys

Pure magnetic hard fct FePt nanoparticles: Chemical synthesis,structural and magnetic properties correlations

L. Suber a,*, G. Marchegiani a, E.S. Olivetti b, F. Celegato b, M. Coïsson b, P. Tiberto b, P. Allia c,G. Barrera d, L. Pilloni e, L. Barba f, F. Padella e, P. Cossari g, A. Chiolerio h

a ISM-CNR, Area della Ricerca di Roma 1, Via Salaria km 29.300, 00015 Monterotondo Scalo, RM, Italyb INRIM, Electromagnetism Division, Strada delle Cacce 91, 10135 Torino, ItalycDISAT Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, ItalydDipartimento di Chimica, Università di Torino, Via Pietro Giuria 7, 10125 Torino, ItalyeUTTMAT-CHI, Via Anguillarese 10, 00123 S. Maria di Galeria, Roma, Italyf IC-CNR, Area Science Park, SS 14 Km 163.5 Basovizza, 34149 Trieste, Italyg IGAG-CNR, Area della Ricerca di Roma 1, Via Salaria km 29.300, 00015 Monterotondo Scalo, RM, Italyh Istituto Italiano di Tecnologia, Center for Space Human Robotics, Corso Trento 21, 10129 Torino, Italy

h i g h l i g h t s

� fct FePt nanoparticles for hard magnetic nanotechnology applications.� Influence of synthesis parameters on the precursor fcc FePt nanoparticle structure.� Easy hydrothermal method for preparing pure fct FePt nanoparticles.� Monitoring the role of temperature and time on the FePt fccefct phase transformation.� Correlation between FePt nanoparticle structural and magnetic properties.

a r t i c l e i n f o

Article history:Received 20 August 2013Received in revised form10 December 2013Accepted 29 December 2013

Keywords:A. Magnetic materialsA. NanostructuresB. Chemical synthesisC. Powder diffractionD. Magnetic properties

* Corresponding author.E-mail address: lorenza.suber@ism.cnr.it (L. Suber

0254-0584/$ e see front matter � 2014 Elsevier B.V.http://dx.doi.org/10.1016/j.matchemphys.2013.12.041

a b s t r a c t

FePt nanoparticles, containing a near-equal atomic percentage of Fe and Pt, with a face centeredtetragonal structure (fct), are challenging for potential applications in high performance permanentmagnets and high density data storage. In this study, we report on the chemical synthesis, carried outboth solvothermally and hydrothermally in autoclave reacting iron (III) acetylacetonate and platinum (II)acetylacetonate with tri- or tetra-ethylene glycol, these employed as solvents, reducers and particlesurface protecting agents as well. In both methods, a subsequent thermal treatment at high temperaturesis necessary to transform the magnetic soft face centered cubic (fcc) phase to the hard fct one. Organiclow-weight molecules, generally used to protect the nanoparticle surface and avoid particle aggregation,are decomposed by the thermal treatment resulting in particle aggregation and coalescence phenomena;on the contrary, in this case, a polymer matrix is formed as particle protecting agent and, by thermallytreating the hydrothermally prepared nanoparticles up to 750 �C for 1 h, the pure magnetic hard fctphase is obtained while preserving the nanostructure. A detailed study is carried out on FePt nano-particle structure (fcc and fct phases) and correlated to the magnetic properties of the system.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

The large uniaxial magnetic anisotropy energy of bulk FeePtalloy, roughly 6.6 MJ m�3, makes its nanostructured form attractivefor permanent magnets applied to the production of more efficient

).

All rights reserved.

motors and generators, actuators, inverters, accelerators or col-liders, ultra-high vacuum equipment, separation systems, etc.

In the literature, different solvothermal methods are reportedfor the preparation of FeePt nanoparticles (NPs) [1e6]. The as-prepared particles feature a disordered face centered cubic (fcc)structure associated to a soft magnetic behavior. To obtain a hardmagnetic behavior, an annealing treatment at high temperatures(>600 �C) is necessary to transform the fcc structure into the facecentered tetragonal (fct) one. The thermal treatment can destroy

L. Suber et al. / Materials Chemistry and Physics 144 (2014) 186e193 187

the organic surfactant around the particle surface resulting inparticle aggregation and coalescence phenomena. It is thennecessary to set up a preparationmethod able to directly obtain theNPs with the fct structure or, alternatively, to hinder particle ag-gregation and coalescence phenomena.

Recently, some authors described the solvothermal preparationin high boiling solvents of FePt NPs having mixed fcc/fct structures[7e9]. Herewe present a further step in the synthesis procedure. Byemploying a hydrothermal method, we tailored the particle sizedown to the size limit to observe, after thermal treatment, pre-serving the nanostructure, only the pure face centered tetragonal(fct) L10 structure. A detailed analysis of the XRD data allowed us tocorrelate structural and magnetic properties.

In order to exploit the magnetic properties of the NPs preparedas described in the experimental section, we point out that inkjetprintingmay represent a new interesting technology to realize highresolution patterns of NPs suspended in either water or organicsolvents, as previously investigated by some of the authorsemploying other magnetic materials [10].

2. Experimental

2.1. Materials and methods

All chemicals were purchased and used as received withoutfurther purification. Triethylene glycol (Teg) 99%, tetraethyleneglycol (Tetraeg) 99% and oleic acid were obtained from Aldrich.HPLC grade Water, anhydrous ethanol and toluene were purchasedfrom Carlo Erba.

Pt(II) acetylacetonate [Pt(acac)2] and Fe(III) acetylacetonate[Fe(acac)3], Ag acetate, [Ag(ac)], Ni acetate, [Ni(ac)2], were pur-chased from SigmaeAldrich.

FePt NPs were prepared in tri- and tetra-ethylene glycolfollowing with small variations the method of Iwamoto et al. [7].The main parameters describing the reaction conditions of thestudied samples are reported in Table 1, together with informationabout the annealing treatments performed and structural param-eters calculated from diffraction data.

Fe(acac)3 and Pt(acac)2 (acac ¼ acetylacetonate, [CH3COCH-COCH3]) were used as metallic precursors. Poly (N-vinyl-2-pyrrolidone) (PVP) and triethylene (Teg) or tetraethylene glycol(Tetraeg) were adopted the first as stabilizer, whereas the second

Table 1Reaction conditions and structural parameters extracted from XRD data.

Sample Dopant (mg) Stabilizer(mg)

Solvent/reducera Temp (�C) Ti

T4 Ag(ac) (16) PVP (200) Teg 200 9T4-AT5 Ag(ac) (16) PVP (200) Tetraeg 280 9T5-AT6 Ag(ac) (16) PVP (200) Tetraeg 285T6-AT7 Ni(ac) (15) PVP (200) Tetraeg 280 9T7-AT8 Ag(ac) (16) PVP (20) Tetraeg 300T8-AT26 e PVP (7) Tetraeg 280 31T26-AT26-BT26-CH30 Ag(ac) (25) PVP (7) Tetraeg 200 4H30-A

Short forms: ac ¼ acetate, PVP ¼ polyvinylpyrrolidone, Teg ¼ tri-ethyleneglycol, Tetraega All the reactions were performed using 25 mL of solvent. The sample H30 was prepare

the temperature of the oven was set at 200 �C.

and the third as solvents, reducing agents and surface particleprotecting agents as well. The amounts of Fe(acac)3, Pt(acac)2 andTeg or Tetraeg were settled at 176.6 mg (0.5 mmol), 177.0 mg(0.45 mmol) and 25 mL, respectively. The amount of PVP waschanged in the range (200-5 mg) [11]. In a general procedure, theTeg or Tetraeg solution containing all the reagentswasmagneticallystirred under Ar flow for 1 h at 100 �C to eliminate traces of H2O andsubsequent heated up to the selected temperature (Table 1) in an Aratmosphere. A hydrothermal method was used for the preparationof sample H30. In this case, after the elimination of H2O traces, theflask was introduced in an autoclave provided with a gas inlet andheated up under Ar at the pressure of 2.2 bars in oven.

At the end of the reactions, the mixtures were let spontaneouslycool down to room temperature where the atmosphere filled withthe Ar gas could be opened to an ambient environment. The sep-aration of the black products was obtained by addition of ethanol asflocculant followed by centrifugation. The products were purifiedby dispersion in ethyl acetate, precipitated by addition of ethanoland separated by way of centrifugation. The purification processwas repeated 3 times and finally the magnetic black precipitateswere dried under vacuum.

The thermal treatments were performed under a hydrostar H-5(mixture of 95% argon and 5% hydrogen) gas flow at the selectedtemperatures (Table 1).

2.2. Characterization techniques

The content of carbon, hydrogen and nitrogen in the materialswas determined using a Fisions 1108 elemental microanalyser atCNR-ISM. X-ray diffraction (XRD) patterns of the studied sampleswere obtained with the Co Ka radiation in pseudo-parallel beamconfiguration, with a glancing angle of incidence of 3�. Due to thesmall amount of powders available (w50 mg) a zero backgroundsample holder cut from a silicon single crystal was used to mini-mize the background signal and to avoid the occurrence of spuriouspeaks.

SEM images were obtained by way of a Leo 1530 FE-SEM or,alternatively, an Inspect-F FE-SEM. A few drops of sample disper-sion in hexane were deposited on a graphite substrate and thesolvent was let to evaporate at room temperature.

Room-temperature magnetic hysteresis loops were measuredon dried powders by a Vibrating Sample Magnetometer (VSM;

me (h) Annealingtemp (�C)

Annealingtime (min)

Crystallitesize (nm)

Orderingparameter S2

6 3 n.a.600 5 7 0.26

6 5 n.a.600 5 7 0.32

5 3 n.a.700 30 80 0.70

6 7 n.a.700 30 49 0.75

8 4 n.a.700 30 52 0.61

2 4 n.a.750 60 59 0.82750 30 24 0.66700 30 18 0.66

8 Nearly amorphous n.a.750 60 118 0.78

¼ tetra-ethylene glycol, n.a. ¼ not applicable.d in an autoclave heated in oven. The pressure inside the autoclave was 2.2 bars and

L. Suber et al. / Materials Chemistry and Physics 144 (2014) 186e193188

Lakeshore 7410) operating under a maximum field of 10 kOe.Magnetization curves as a function of temperature have been per-formed at higher fields exploiting a cryomagnet equipped with aVSM setup (Hmax ¼ 70 kOe).

Field-cooled (FC) and zero-field-cooled (ZFC) magnetic suscep-tibility curves were obtained by recording themagnetic response ofthe powders under a small fixed field (H ¼ 500 Oe) while contin-uously varying the measurement temperature from 5 to 265 K.

3. Results and discussion

3.1. Preparation, structural and morphological properties

3.1.1. As-prepared samplesSeveral experiments were carried out changing different pa-

rameters, namely: doping agent type and quantity, stabilizerquantity and reaction temperature, in order to study their influenceon the magnetic and structural properties of the produced nano-particles. The synthesis reaction conditions are reported in Table 1.The preparation temperature varied between 200 and 300 �C, thereaction time between 5 and 312 h. The use of tetraethylene glycolhaving a higher boiling point than triethylene glycol, allowedreaching higher reaction temperatures.

In the literature, it is reported that Ag or Ni atoms can enter inthe FePt crystalline lattice and destabilize the fcc structure [12e18].In the subsequent thermal annealing, they could leave the lattice,forming lattice vacancies. On turn, the vacancies, increasing the Feand Pt atoms mobility, enhance the kinetics for the structuretransformation favoring the formation of the ordered fct structureat lower annealing temperatures. Thus, two doping agents wereconsidered in this work, namely [Ag(ac)] and [Ni(ac)2], and theresulting nanoparticles were compared with the products obtainedwhen no dopant was used.

In the T4 up to T8 and H30 reaction mixtures, besides the Fe andPt precursors, silver acetate [Ag(ac)] or nickel acetate [Ni(ac)2] arepresent as dopants whereas polyvinylpyrrolidone (PVP) is intro-duced as particle stabilizer. The samples T4 up to T6, in particular,have been prepared using same reagents and quantities in order tostudy the effect of reaction temperature and time on crystal sizeand quality.

XRD patterns of all as-prepared samples display only the fcc FePtstructure. They show very broad peaks of low intensity, suggestingpoor crystallinity and/or very small crystal sizes, approaching the

Fig. 1. XRD patterns of sample T26 obtained without the addition of doping agents, as-prepared and after different annealing treatments. The pattern of the sample holder,which presents a few small peaks is also shown for comparison.

limit of detection of the XRD technique. In Figs. 1 and 2 the XRDpatterns of some representative as-prepared samples are reportedas an example.

The stabilizer (PVP) quantity was tuned from 200 to 7 mg. ThePVP quantity did not remarkably affect the crystallite size varyingbetween 2 and 7 nm. As regards the effect of dopant, [Ni(ac)2]seems to promote a better crystallization of the NPs in the as-prepared state. In fact, comparing the XRD patterns (not shown)of samples T5 and T7, synthesized with the same temperature andtime of reaction, the crystallite size is found to increase from 5 nmto 7 nm and reflections display higher intensity when [Ni(ac)2] isused as doping agent.

3.1.2. Annealed samplesThe poor crystallinity of the as-prepared samples and the

absence of the desired fct phase pointed out the need of post-synthesis annealing treatments. Selected samples were then heattreated, looking for the best conditions to obtain the hard fctmagnetic phase minimizing, at the same time, NPs coalescencephenomena. Different annealing conditions have been tested inorder to promote the desired phase transformation and crystallitegrowth (see Table 1, annealed samples are marked with a letterfollowing the sample name).

By comparing in an XRD pattern the intensity of a superlatticeline (i.e. one with mixed hkl) to the one of a fundamental line (i.e.one with hkl all odd or all even), one can get information about thedegree of long-range order of an ordered alloy [19]. The departurefrom perfect ordering can be expressed by means of the orderparameter S2 calculated from the ratio of XRD intensities of thesuperlattice and fundamental lines for experimental (exp) andtheoretical (theor) values: [7]

S2 ¼

hIð110Þ=Ið111Þ

iexph

Ið110Þ=Ið111Þitheor

(1)

where I(hkl) is the intensity of the (hkl) reflection. S¼ 1 in the case ofa perfectly ordered crystal, while S¼ 0when the atom arrangementis completely random. Values of S2 calculated according to eq. (1)for the annealed samples are reported in Table 1.

The crystallite size was calculated from XRD data using theScherrer equation, on the basis of the full width at half maximum of

Fig. 2. XRD patterns of selected samples obtained with Ag(ac) doping: a) as-preparedNPs; b) annealed NPs. The peak marked with a star is due to the single-crystal sampleholder used for the measurement (see Fig. 1).

L. Suber et al. / Materials Chemistry and Physics 144 (2014) 186e193 189

the (111) peak after subtraction of the instrumental contribution topeak broadening, evaluated by measuring in the same conditions acertified standard reference material (SRM660b from NIST). Theresulting values are also reported in Table 1.

Annealing treatments at temperatures below 700 �C turned outto be insufficient for the optimal development of the ordered hardmagnetic phase, both in the doped and undoped synthesis, since allsamples of this kind show very broad peaks in the XRD pattern,irrespectively of the synthesis conditions, although the incipientformation of the fct phase is evidenced by the appearance of a smallsuperlattice (110) peak.

Annealing treatments at temperature of 700 �C or higher provedto be much more effective in promoting the hard magnetic phaseformation. The effect of the annealing temperature on the forma-tion of the fct phase is represented in Fig. 1 where are reported theXRD patterns of sample T26, obtained with no dopant addition andannealed at different temperatures and/or for different times. As-prepared T26, like in the previous cases, is formed by the fccphase with poor crystallinity. Heat treatment at 700 �C for 30 minpromotes the fct phase formation, although the crystals are notoptimally developed, as indicated by the rather large peaks whichcause an incomplete resolution of some reflections (e.g. (200) and(002) or (220) and (202)). Crystallinity is improved when theannealing is performed at 750 �C (T26-B) and, as expected, the bestresult is obtained when the annealing duration is increased up to1 h (T26-A). In fact, via the latter treatment a doubling in crystalsize and a sensible increase (w24%) in ordering parameter wereattained, with respect to the shorter heat treatment.

By fitting the experimental dhkl values obtained from XRD pat-terns with the equation for a tetragonal lattice, the followingaverage values of lattice parameters were obtained:a ¼ 3.859 � 0.009 �A, c ¼ 3.721 � 0.007 �A, with c/a ratio of 0.9641.These values are consistent with the lattice parameters for nearequiatomic FePt bulk material with fct structure.

XRD patterns of selected as-prepared and annealed samples,obtained with the addition of [Ag(ac)] as doping agent, are shown.Whereas in the as-prepared samples only weak and extremelybroad reflections of the cubic FePt phase were detected, togetherwith traces of Ag segregation, after the post-synthesis heat treat-ment, peaks marking the fct ordered phase are clearly developed.Besides, an overall increase in peak intensity and sharpness isapparent in the annealed samples, due to an improved crystallinityand to larger size of the nanoparticles. Samples T6 and T8 displayedin Fig. 2 were prepared with the same amount, in the reactionmixture, of [Ag(ac)] as dopant; crystal Ag is barely detectable onlyin T8 pattern, revealing that in as-prepared samples Ag atoms aremainly dissolved into the fcc FePt structure; on the contrary, insamples T6-A and T8-A, the Ag reflections (the onesmarkedwith #)are clearly visible, indicating that, upon annealing, Ag segregatesfrom the lattice forming a different phase. The initial segregation ofAg in the as-prepared T8 sample can be possibly due to the higherreaction temperature and/or to the longer reaction time. SampleH30, prepared hydrothermally with a higher amount of Ag(ac),shows in fact Ag peaks more clearly visible even in the as-preparedsample, which in turn results nearly amorphous. The case of sampleH30 is also indicative of the importance of optimizing the prepa-ration method of the as-prepared sample. In fact, it plays the majorrole in the development of nanocrystals with good hard magneticproperties, since, even from nearly amorphous as-prepared NPs,large crystals with good structural ordering are obtained afterthermal treatment. Apparently in the hydrothermal synthesis, it isbeneficial to form very small, almost amorphous crystal nuclei inthe reaction mixture, in order to get pure fct nanocrystals after theannealing treatment, provided that it is performed at sufficientlyhigh temperature (see H30-A, Table 1).

By comparing XRD patterns of samples annealed at 700 �C (T26-Cin Fig. 1 and T6-A, T8-A in Fig. 2b) one can appreciate the beneficialeffect of the doping agent on the crystal quality of the producednanoparticles; all these samples have been annealed in the sameconditions (temperature ¼ 700 �C, time ¼ 30 min), the samplesobtained with the addition of [Ag(ac)] display sharper peaks andwell-separated reflections for the couples (200) and (002), (220) and(202), which instead appear completely unresolved in sample T26-Csynthesized with no foreign metal additives. This is true for sampleT7-A as well, prepared with [Ni(ac)2] and annealed at 700 �C for30 min, whose XRD pattern are not shown. Besides, also crystal sizeafter the annealing treatment is largely increased, by a factor of 3 ormore, with the use of the dopant [20].

Elemental analyses (C, H, N) were performed in order to esti-mate the organic component in the samples before and after theannealing process; the C weight % varied between 5.5 and 2.5 forthe as-prepared samples and 2.1 and 0.3 for the annealed ones.

Morphologic characterization on as-prepared and annealedsamples was performed by way of SEM and STEM measurementsand typical images are shown in Figs. 3 and 4.

The NPs are dispersed in a polymer matrix resulting from thethermal degradation of the tri and tetraethylene glycols. It cannotbe removed even after several washings of the as-prepared samplesas can be observed in Fig. 3a, b, samples T4 and T5. The subsequentannealing treatment at 600 �C for 5 min does not sensibly modifythe morphology as can be seen in SEM images of samples T4-A, T5-A (Fig. 3c, d) and in SEM and STEM images of sample H30-A,thermally treated at 750 �C for 1 h (Fig. 4). SEM images of othersamples are shown in the Supplementary material.

It can be concluded that, because of the presence around theFePt nanoparticles of the polymeric matrix, the thermal annealingtreatment does not lead to any significant FePt particleeparticleaggregation.

3.2. Magnetic properties

Room-temperature hysteresis loop of as-prepared sample T6 isshown in Fig. 5a. As it can be observed, the as-prepared sampledisplays vanishingly small coercive field Hc and magnetizationremanence Mr values together with a slow approach to saturation.This behavior is in agreement with the sample microstructure asindicated by the XRD pattern of poorly crystalline fcc FePt (Fig. 2a).

On the contrary, the hysteresis curve of the annealed sample,T6-A, reported in the Fig. 5b is characterized by a very high Hc andMr value with respect to the as-prepared one. This behavior is to beascribed to the presence of the highly ordered fct phase as revealedby the XRD pattern (Fig. 2b) and structural data (Table 1). Hysteresiscurves measured at selected temperatures T for sample T6-A arereported in Fig. 5c. As it can be observed, there are no evidentchanges in the behavior with decreasing T indicating that theferromagnetic behavior is stable with temperature. The kinkoccurring at lower magnetic field marks the presence of a magneticphase characterized by a lower coercivity with respect to the highlyordered fct phase such as a less ordered L10 or a fcc phase possiblydue to the fraction of particles with smaller size. The stability of themagnetic behavior is confirmed by reporting the coercive fieldvalue as a function of temperature as shown in Fig. 5d where analmost linear increase of coercivity with reducing T is observed asexpected for ferromagnetic systems. The temperature behavior ofhysteresis loop in sample T8-A is reported in Fig. 6. Again, thepresence of two magnetic FePt phases (i.e. fct and fcc) marked by akink occurring at lower field with respect to Hc is observed. How-ever, in this case, the decreasing of temperature differently in-fluences the two phases, disentangling the magnetic interactionamong the two.

Fig. 3. a: SEM image of sample as-prepared, T4. b: SEM image of sample as-prepared, T5. c: SEM image after the annealing treatment, sampleT4-A. d: SEM image after the annealingtreatment, sample T5-A.

L. Suber et al. / Materials Chemistry and Physics 144 (2014) 186e193190

By using coercive field as the parameter exploited to assess theformation of fct phase, it can be concluded that suitable thermaltreatments induce the formation of the highly ordered fct phase.This behavior has been observed in all studied samples.

Samples annealed at different temperatures display differentmagnetic properties (i.e. Hc and magnetization remanence). Tostudy this effect, selected annealing conditions have been exploited

Fig. 4. SEM Image (Secondary Electrons) of Sample H30A. Inset: Bright Field STEMimage of a different portion of the same sample.

for sample T26 (A: Ta ¼ 750 �C for 60 min, B: Ta ¼ 750 �C for 30 min,C: Ta¼ 700 �C for 30min). The high-field hysteresis loops measuredat T ¼ 50 K are reported in Fig. 7.

It can be seen than the sample thermally treated at highertemperatures for longer time (T26-A) displays the highest value ofcoercive field (Hc ¼ 16.56 kOe) confirming, as expected, that theordering of the fct phase is improved by increasing the annealingtemperature and time. Again, a low-field kink is observed indi-cating a non-perfect coupling among the two magnetic phases. Acomparison of the low-temperature hysteresis properties of an as-prepared sample and the corresponding annealed one is reportedin Fig. 8 for sample H30.

In the as-prepared sample, an ordinary evolution of the hys-teresis parameter is seen with decreasing temperature. The slowapproach to saturation is slightly reduced with decreasing Tpossibly indicating nanoparticle blocking with decreasing T. Asexpected, the annealed sample, H30-A, displays a higher coercivityvalue, nomagnetization kink indicating a very well ordered fct hardmagnetic phase and practically no changes in the hysteresis prop-erties with decreasing temperature.

The absence of any kink indicates that the sample is magneti-cally homogeneous, i.e. the NPs are structured with the ordered L10phase. As the thermal treatment was analogous to the one carriedout on sample T26 where sample T26-A showed the kink, weattribute the better performance of sample H30-A to the differentpreparation method, hydrothermal instead of solvothermal. Withthis method in fact (see Table 1 and Fig. 2), we obtain almostamorphous very small FePt NPs probably well dispersed in the

Fig. 5. a) Room temperature hysteresis loops (Hmax ¼ 10 kOe) of as-prepared (T6) and b) annealed (T6-A) samples. c) Hysteresis loops at different temperatures in the interval (5e300 K) and d) coercive field values vs temperature for sample T6-A.

L. Suber et al. / Materials Chemistry and Physics 144 (2014) 186e193 191

polymer matrix. Because of the very small size and almost amor-phous nature, they may be able, during the thermal treatment, toassemble in a homogeneous way, forming fct crystallites of about100 nm (Table 1).

The different hysteretic behavior of the two series, as-preparedand annealed, can be explained in terms of a markedly different

Fig. 6. Hysteresis loops at different temperatures in the interval (5e300 K) for sampleT8-A.

nanoparticle size involving a different fraction of magneticallyblocked nanoparticles at room temperature. Such a conclusion canbe supported by the FC/ZFC curves of the low-field magnetizationin the 5e300 K temperature range for the two samples H30 andH30-A (Fig. 9). In particular, H30 sample exhibits the standardbehavior of a nanoparticulate material, with a monotonically

Fig. 7. Hysteresis loops measured at 50 K of sample T26 annealed in differentconditions.

Fig. 8. Low temperature hysteresis loops of the as-prepared (H30) and annealed (H30-A) samples.

L. Suber et al. / Materials Chemistry and Physics 144 (2014) 186e193192

decreasing FC curve and a non-monotonic ZFC curve displaying abroad maximum at TB ¼ 70 K and merging with the FC curveimmediately above TB. These features are typical of individualnearly superparamagnetic nanoparticles undergoing low-temperature blocking. Conversely, sample H30-A displays almostsimilar ZFC/FC curves indicating the stable ferromagnetic characterof the magnetic phase with blocked magnetic entities.

4. Conclusions

Several chemical strategies were explored in this work in orderto prepare fct FePt NPs displaying optimized hard magnetic prop-erties useful for applications in technologies such as data storageand inkjet printing. Different post-synthesis annealing conditionshave been tested in order to promote the optimal hard phase for-mation preserving the nanostructure. The results have been dis-cussed on the basis of both magnetic properties (remanence andcoercive field) and structural parameters extracted from XRD data,such as crystallite size and ordering parameter. It was found that fccFePt NPs with crystallite sizes varying between 3 and 7 nm areeasily prepared by chemical reduction using Fe(acac)3 and Pt(acac)2as precursors, tri- or tetra-ethylene glycol both as solvent, reducer

Fig. 9. FC/ZFC curves of the low field magnetization in the 5e300 K tempera

and particle surface protecting agent and the polymer PVP as sta-bilizer. The transformation into the structurally ordered L10 phaseis achieved by way of an optimized thermal treatment at 700e750 �C for 30e60 min in flowing argon (containing 5% hydrogen).The crystallite size of the annealed samples, depending onannealing time and temperature, varies between 20 and 120 nmdemonstrating that, thanks to the protecting polymer matrix, thenanostructure is preserved even after thermal treatment up to750 �C for 1 h.

The lattice parameters obtained by fitting the experimental dhklvalues of XRD patterns with the equation for a tetragonal lattice areconsistent with the lattice parameters for near-equal atomic per-centage of Fe and Pt bulk material with fct structure.

Soft magnetic properties in as-synthesized FePt fcc structuredNPs and magnetic hardening in the thermal treated fct structuredNPs were observed. The samples prepared hydrothermally showed,after thermal treatment, a complete transformation to the mag-netic hard fct phase, whereas in the samples prepared with thesolvothermal method, a residual fcc phase was still observed.Among the various control parameter studied, temperature ofannealing, together with the presence of metallic doping agents inthe reaction mixture, have been found to play a major role in the

ture interval for the as-prepared (H30) and annealed (H30-A) samples.

L. Suber et al. / Materials Chemistry and Physics 144 (2014) 186e193 193

development of good crystal quality and pronounced hard mag-netic properties, while the initial temperature of the synthesis ofnanocrystals turned out to be a less significant parameter to drivethe crystallization of the desired magnetic phase.

Acknowledgments

Financial support from the MANUNET ERA-Net project ANA-TEMA (An iNnovative mAnufacturing TEchnology for Magnets)PODGPRE000101, is gratefully acknowledged.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.matchemphys.2013.12.041

References

[1] S. Sun, Adv. Mater. 18 (2006) 393e403.[2] S. Sun, C.B. Murray, D. Weller, L. Folks, A. Moser, Science 287 (2000) 1989e

1992.[3] M. Chen, J.P. Liu, S. Sun, J. Am. Chem. Soc. 126 (2004) 8394e8395.[4] G. Marchegiani, P. Imperatori, A. Mari, L. Pilloni, A. Chiolerio, P. Allia, P. Tiberto,

L. Suber, Ultrason. Sonochem. 12 (2012) 877e882.

[5] L. Suber, D. Peddis, in: C. Kumar (Ed.), Magnetic Nanomaterials, Wiley VSH,Weinheim, 2009.

[6] L. Suber, P. Martino, Nanoparticles Featuring Electromagnetic Properties,Research Signpost, Kerala, 2012.

[7] T. Iwamoto, K. Matsumoto, T. Matsushita, M. Inokuchi, N. Toshima, J. ColloidInterface Sci. 336 (2009) 336879e336888.

[8] C. Liu, X. Wu, T. Klemmer, N. Shukla, X. Yang, D. Weller, J. Phys. Chem. B 108(2004) 6121e6123.

[9] H-Loc Nguyen, L.E.M. Howard, G.W. Stinton, S.R. Giblin, B.K. Tanner, I. Terry,A.K. Hughes, I.M. Ross, A. Serres, J.S.O. Evans, Chem. Mater. (2006) 186414e186424.

[10] P. Tiberto, G. Barrera, F. Celegato, M. Coïsson, A. Chiolerio, P. Martino,P. Pandolfi, P. Allia, Eur. Phys. J. B 86 (2013) 173e180.

[11] T. Iwamoto, K. Matsumoto, Y. Kitamoto, N. Toshima, J. Colloid Interface Sci.308 (2007) 564e567.

[12] Z. Jia, S. Kang, S. Shi, D.E. Nikles, J.W. Harrell, J. Appl. Phys. 97 (2005) 10J310.[13] S. Kang, J.W. Harrell, D.E. Nikles, Nano Lett. 2 (2002) 1033.[14] S. Wang, S. Kang, D.E. Nikles, J.W. Harrell, X.W. Wu, J. Magn. Magn. Mater. 266

(2003) 49.[15] S. Kang, D.E. Nikles, J.W. Harrell, J. Appl. Phys. 93 (2003) 7178.[16] X. Sun, S. Kang, J.W. Harrell, D.E. Nikles, Z.R. Dai, J. Li, Z.L. Wang, J. Appl. Phys.

93 (2003) 7337.[17] H. Kura, T. Sato, J. Appl. Phys. 96 (2004) 5771.[18] T. Maeda, T. Kai, A. Kikitsu, T. Nagase, J. Akiyama, Appl. Phys. Lett. 80 (2002)

2147.[19] B.D. Cullity, Elements of X-Ray Diffraction, Addison-Wesley, London (U.K.),

1978, p. 383.[20] PDF Reference Pattern 00-043-1359, JCPDS-International Centre for Diffrac-

tion Data, 1999.