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Materials Characterization

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

The facile synthesis, crystallization behavior and magnetic property ofFeNiP amorphous nanoparticles

Jie Yuana,b, Cai-Fu Lia,d, Bo Yangc, Zhi-Quan Liua,b,d,⁎

a Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, Chinab School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, Chinac Key Laboratory for Anisotropy and Texture of Materials, Ministry of Education, School of Materials Science and Engineering, Northeastern University, Shenyang 110819,Chinad The Institute of Scientific and Industrial Research, Osaka University, Osaka 567-0047, Japan

A R T I C L E I N F O

Keywords:Amorphous FeNiP nanoparticlesSintering processCrystallization behaviorMagnetic property

A B S T R A C T

Amorphous FeNiP nanoparticles with homogeneous composition are synthesized by reduction of aqueous Fe2+

and Ni2+ with sodium borohydride (NaBH4) and hypophosphite at room temperature. These amorphous na-noparticles can be crystallized by heating up to 300 °C to form FeNi3 and Ni3P crystals. Ni phase appeared at thesintering temperature of 500 °C, and a special net structure was formed at 700 °C due to the joining and diffusionamong different nanoparticles. At a higher temperature of 800 °C, phase transformation happened and only pureNi nanospheres and monoclinic ((Ni, Fe)3[PO4]2) nanorods can be observed, which should be governed by thelong-range diffusion in the net structure. The amorphous FeNiP nanoparticles have excellent soft magneticproperty with Ms of 13.2 emu/g and Hc of 0.7 Oe. However, the sintered mixing nanoparticles show variedmagnetic properties due to the phase transformation and proportion change among different phases involved in.

1. Introduction

Amorphous materials with soft magnetic behavior are now availablein strip or bulk shape for a variety of alloys [1,2]. These amorphousmagnetic materials not only show enhanced soft magnetic properties[3–5], but also exhibit good mechanical strength and hardness [6,7],which are necessary to implement the magnetic performance. However,amorphous metals might become brittle upon sintering. In recent years,metal-metalloid based amorphous alloys have been investigated mostextensively and form a broad and versatile group of magnetic materials[8–11].

There are many methods to obtain FeNiP ternary amorphous ma-terial in literatures. As well known, rapid solidification is an effectiveand general technique to get metal glass, especially for the industrialproduction [12–14]. Electrodeposition is another way to obtain FeNiPamorphous materials, including not only amorphous film but alsoFeNiP ternary amorphous nanowires by using porous anodic aluminumoxide (AAO) templates [15,16]. In the case of electroless depositionmethods, FeNiP amorphous ternary alloy particles can be reduced fromthe solutions of their salts by chemical reduction method [17]. As forelectrodeposition glass FeNiP films, its morphology, composition con-trol, annealing, phase separation of crystallization process and mag-netic properties have been reported [17–19]. Winkler et al. [15] have

synthesized FeNiP amorphous nanowires with AAO templates and in-vestigated the phase stability and transformation from amorphous na-nowires into crystalline nanowires using in-situ transmission electronmicroscopy (TEM) technique, which provides important views into theevolution of complex magnetic structures in confinement. It was foundthat crystallization of amorphous FeNiP nanowires can lead to couplingexchange between spins and the multilayer structure of two differentalternating magnetic phases. But for FeNiP amorphous powders, mostresearchers only paid attention to the synthesis process which sig-nificantly affects the shapes, composition, size and the correspondingmagnetic property. Few studies focused on the process of crystallizationbehavior and the microstructure transition, which are important for themagnetic applications of the nanoparticles.

In this study, a facile way to synthesize amorphous FeNiP nano-particles is reported by reduction of aqueous Fe2+, Ni2+ and hypo-phosphite with sodium borohydride (NaBH4) at room temperature. Itsthermal stability and crystallization behavior at varied temperatureswere clarified, and the corresponding magnetic properties resulted frommicrostructural transformation was also discussed.

2. Material and Method

Analytical grade reagents of ferrous sulfate (FeSO4·7H2O), nickel

https://doi.org/10.1016/j.matchar.2017.12.008Received 10 October 2017; Received in revised form 7 December 2017; Accepted 7 December 2017

⁎ Corresponding author at: School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China.E-mail addresses: jyuan14s@imr.ac.cn (J. Yuan), licf@alum.imr.ac.cn (C.-F. Li), yangb@atm.neu.edu.cn (B. Yang), zqliu@imr.ac.cn, zhiquanliu@yahoo.com (Z.-Q. Liu).

Materials Characterization 136 (2018) 94–99

Available online 09 December 20171044-5803/ © 2017 Elsevier Inc. All rights reserved.

T

sulfate (NiSO4·6H2O), ammonium chloride (NH4Cl), trisodium citrate(C6H5Na3O7·2H2O), sodium borohydride (NaBH4), sodium hypopho-sphite (NaH2PO2·H2O), sodium hydroxide (NaOH), and doubly distilledwater were used in all experiments. All the chemical reagents were usedas received without further purification.

In a typical synthesis process, 7.0 g FeSO4·7H2O, 1.6 g NiSO4·6H2O,4.0 g NH4Cl, 12.0 g C6H5Na3O7·2H2O, and 8.0 g NaH2PO2·H2O weredissolved in 200 mL doubly distilled water and magnetic stirred vig-orously for 30 min. Then the PH value of the solution was adjusted to9.0–9.2 using sodium hydroxide solution. Next, the prepared NaBH4

solutions with the molarity of 2 mol/L were dropped into the abovegreen solutions at a constant rate. And at the same time, mechanicalagitation is necessary. At the early stage, it is easy to find that the colorof the solution changes from green, light green to dark quickly. Afterone hour's reaction, the solutions became colorless and transparent, anda black solid product was deposited on the bottom of beaker, indicatingthe formation of particles. Then the product was separated by cen-trifugation and was washed by doubly distilled water for 5 times andthen by ethyl alcohol for 3 times. These black products were dried invacuum oven for 10-24 h at room temperature, which is the original as-prepared sample. These original samples were then annealed in a va-cuum furnace at a temperature range of 300–800 °C for 30 min to getsintered samples. The furnace vacuum is about 4 Pa and the heatingrate is 5 °C/min.

After synthesis, the phase identification of prepared nanoparticleswas conducted by a step scanning XRD system (Rigaku D/max-2400,Tokyo, Japan) with Cu Kα radiation (λ = 1.54178 Å) using scanning 2θangle from 20° to 90°. About 15–20 mg of the nanoparticles wereanalyzed using a thermogravimetry and differential thermal analysisanalyzer (TG-DTA, SETSYS Evolution18, SETARAM). The sample wasloaded into an aluminum oxide crucible which was then heated in theTG-DTA furnace from room temperature to 850 °C at a rate of 10 °C/min. A purified argon stream (30 mL/min) was filled into the furnacethroughout the programmed heating. For microstructural character-ization, the powders were suspended in ethyl alcohol by sonication inan ultrasonic water bath for 20 min. A drop of this well-dispersedsuspension was then applied on a piece of 200-mesh carbon-coatedcopper grid, followed by air-drying under ambient condition. Themorphology, composition and size of nanoparticles were characterizedby SEM (ZEISS, super 55) and TEM (JEOL, JEM-2100) equipped with anenergy dispersive X-ray spectroscopy (EDS) systems, as well as an FEITecnai F30 TEM equipped with a scanning high-angle annular dark-field (HAADF) detector and an energy dispersive X-ray (EDX) detector.In order to improve the electro conductivity of the nanoparticles, the Authin layer (about 10 nm) were sputtered on the samples before the SEMobservation. While the composite of the original amorphous nano-particles were measured, the accelerating voltage is 20 kV, the magni-fication times is 10,000 and the observational mode is InLens. Thestandardless analysis and XPP quantitative analysis (INCA Energy) wereused. As for the composites measurement by TEM, the acceleratingvoltage is 200 kV. The standardless analysis and Cliff Lorimer factorcorrection method were used. What's more, normalization was chosen.It is important that the angle of Tilt X of the sample is 10–15° to obtaingood viewpoint before the measurement. And selected area electrondiffraction (SAED) was used to identify the different crystal structures.The magnetic properties were investigated through vibration samplemagnetometer (Lakeshore VSM 7407). Before the VSM measurement,the Gaussmeter offset calibration, moment offset calibration and mo-ment gain calibration are needed. Moreover, the Ni standard sample(6.92 emu/5000 Oe) is used for the moment gain calibration. For hys-teresis loop measurement, samples were prepared by wrapping 20 mgnanoparticles into 3 mm× 3 mm (length and width) with parchmentpaper. And the magnetic properties including saturation magnetization(Ms) and coercivity (Hc) were determined from the loop.

3. Results and Discussion

3.1. Synthesis of FeNiP Amorphous Nanoparticles

Fig. 1 shows the synthesized FeNiP nanoparticles which have beencharacterized by SEM, XRD and TEM. The SEM morphology of theFeNiP nanoparticles is shown in Fig. 1(a) with some agglomeration. Theinserted size distribution profile indicates a mean nanoparticle size ofapproximately 70 nm. Fig. 1(b) shows the XRD pattern of the as-syn-thesized FeNiP nanoparticles, in which a large distribution of thescattered signal over a wide range of angle 2θ from 30° to 55° could beobserved, indicating the amorphous state of the nanoparticles. This wasverified by TEM observations as shown in Fig. 1(c), where the bottom-left inset is the SAED pattern of the FeNiP nanoparticles. This diffrac-tion pattern exhibits no crystalline reflections but a broad amorphoushalo, which means that there are no crystals in the sample. Determinedby SEM-EDS, the atomic percentage of FeNiP amorphous nanoparticlesis Fe17.3Ni74.0P8.7. It should be noticed that every FeNiP nanoparticlehas almost the same compositions according to individual measure-ment.

TG-DSC measurement of the FeNiP amorphous nanoparticles wascarried out as shown in Fig. 2 (exothermal up). The thermogram andthermo-gravimetric curve were obtained by heating 17.5 mg FeNiP inthe furnace from room temperature to 850 °C at a rate of 10 °C/min.The DSC curve shows a glass transition at a glass transition onsettemperature Tg of 95 °C as marked by a dark arrow. Two crystallinepeaks at temperatures of 405 °C and 472 °C are observed respectively.At the early stage of heating, the weight of the sample decreased due to

Fig. 1. The synthesized amorphous FeNiP nanoparticles characterized by (a) SEM withinserted size distribution plot, (b) XRD and (c) TEM with a inset of SAED pattern.

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the volatilization of the adsorbates on the surface of the nanoparticles.Later, the weight of the samples increased, which may result from theslightly oxidization (O-0.8 wt%) of the FeNiP nanoparticles, though apurified argon stream was passing through the furnace.

3.2. Crystallization of FeNiP Amorphous Nanoparticles

Sintering of amorphous nanoparticles was performed in a vacuumfurnace (≈4 Pa) at a heating rate of 5 °C/min. The samples were kept at300 °C, 500 °C, 700 °C, and 800 °C for 30 min respectively, followed byair cooling to room temperature. Fig. 3 shows the morphology of FeNiPnanoparticles sintered at different temperatures. Fig. 3(a) is the SEMimage of the FeNiP nanoparticles which are sintered at 300 °C. Com-pared with the original amorphous nanoparticles in Fig. 1(a), thesamples become seriously agglomerate and smooth due to surface effectat nanoscale. At the sintering temperature of 500 °C (Fig. 3(b)), thesurface of the nanoparticles became smoother and the whole particlesreunited heavily. The morphology of nanoparticles turned into netstructure while the sintering temperature is 700 °C, which is shown inFig. 3(c). The net structures were formed by the fusion of differentnanoparticles accompanying the crystallization. At 800 °C, the shape ofthe nanoparticles had a great change. There are two different

morphologies, one is the spherical particles which have inerratic crystalfacet, and the other is rod-like as shown in the Fig. 3(d). The inset is thehigh magnification image of these two different shapes. It's obvious thatthe crystalline structure of the FeNiP powders changed as the sinteringtemperature increased. Fig. 4 shows the XRD patterns of different FeNiPpowders sintered at various temperatures. While the annealing tem-perature is 300 °C, the amorphous FeNiP nanoparticles have crystal-lized as shown in Fig. 4(a). The existing peaks can be easily assigned tothe (111), (200) and (220) reflections of FeNi3 (JCPDS No. 38-0419).While the annealing temperature increased to 500 °C and 700 °C, thepeaks in XRD profiles are matched with FeNi3 (JCPDS No. 38-0419),Ni3P (JCPDS No. 34-0501) and Ni (JCPDS No. 04-0850) as shown inFig. 4(b) and (c). At the annealing temperature of 800 °C, all strongpeaks come from Ni (JCPDS No. 04-0850), while only some tiny peaks(marked by star) can be identified as (Ni,Fe)3[PO4]2 [20]. Moreover,

Fig. 2. TG-DSC measurements of amorphous FeNiP nanoparticles with a heat flow from25 °C to 850 °C at a rate of 10 °C/min. The curves are pointed to the corresponding Y axisby the circles on the curves with arrows.

Fig. 3. SEM images of sintered FeNiP powders after heatingamorphous nanoparticles at (a) 300 °C, (b) 500 °C, (c)700 °C, and (d) 800 °C with inserted high magnificationimage.

Fig. 4. The XRD pattern of FeNiP powders sintered at different temperatures of (a)300 °C, (b) 500 °C, (c) 700 °C, and (d) 800 °C.

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none of FeNi3 or Ni3P peak existed in Fig. 4(d), which indicates acomplete phase transformation at 800 °C. As for FeNiP amorphousmaterials, the crystallization process is mainly achieved through thediffusion of nonmetallic P atoms and the local adjustment of Fe/Niatoms [21]. Moreover, the mass fraction of the nonmetallic element canaffect the position of the crystallization peak [22]. It is no doubt thatthe P element have a vital effect on the crystallization process. Ac-cording to the Figs. 2 and 4, it indicates that the crystallization peak(405 °C) is related to the phase transformation of the FeNi3, Ni3P andslight Ni. Next, there will be a further Ni(Fe)/P element diffusion withthe heating process. And the crystallization peak (472 °C) should bemostly related to the further formation of Ni phase from the FeNi3 andNi3P.

To clarify the phase transformation occurred at 800 °C, detailedmicrostructural characterization were carried out as shown in Fig. 5.

The high magnification image of the sintered FeNiP samples is shown inFig. 5(a). It's clear that there are two kinds of crystals in the sample, bigspherical particles and small nanorods. Both of them are agglomerateaccording to their magnetic dipolar interaction or van der Walls in-teraction, which might also be related to the sintering process. Twotypical spherical particles are shown in Fig. 5(b), which have inerraticfacets. The composition of these big spherical particles was determinedas 100% Ni (at.%) by EDS systems in TEM on different single sphericalparticles (more than five particles). This was verified by the corre-sponding SAED pattern in Fig. 5(c), which was taken from the leftparticle of Fig. 5(b) and can only be indexed with pure Ni crystalstructure in [211] zone axis. Fig. 5(d) is the typical morphology of ananorod with an atomic composition of O61.4Fe17.4Ni20.8P0.4. Comparedto the original HAADF image, the TEM-EDX mapping demonstrates ahomogeneous distribution of Fe, Ni, P and O elements from the left tothe right as shown in Fig. 5(e). Fig. 5(f), (g) and (h) are a series of tiltedSAED patterns of the single nanorod in Fig. 5(d). These three diffractionpatterns can be indexed as (Ni,Fe)3[PO4]2 in [−754], [−145] and[−522] zone axis, respectively. Calculated with the (Ni,Fe)3[PO4]2 unitcell [20], the angle between the [−754] and [−145] zone axis is 26.9°which is close to the experimental tilting angle of 26.4°, and the theo-retical angle between [−145] and [−522] zone axis is 38.1° which isalmost equal to the experimental tilting angle of 37.8°. Therefore, theformation of (Ni,Fe)3[PO4]2 with monoclinic structure can be verifiedfirmly, which is consistent with the identification of star-marked tinypeaks in Fig. 4(d). According to XRD and TEM investigations as shownin Figs. 4 and 5, the crystallization behavior or the involved phasetransformation can be summarized as follows:

Amorphous FeNiP → Ni + FeNi3 + Ni3P → Ni + (Ni,Fe)3[PO4]2.Based on the above observation of the sintered nanoparticles, we

proposed a mechanism to explain the sintering behavior of the amor-phous FeNiP samples. Fig. 6 shows the schematic illustration of thecrystallization process of amorphous FeNiP nanoparticles. It is easy tofind that the smooth and agglomerated sintered nanoparticles(Fig. 6(b)), net structure (Fig. 6(c)) and pure Ni nanospheres withmonoclinic nanorod (Fig. 6(d)) can be obtained while the sinteredtemperature ranged from 300 °C to 800 °C. As the annealing processtook place, it's no doubt that the nanoparticles will become smooth andagglomerate compared with the original samples which is consistentwith other sintering experiment [23] as shown in Fig. 3(a). The nano-particles have crystallized at 300 °C for 30 min which is verified byFig. 4(a). Once the crystallization behavior occurs, long-rang diffusionand rearrangement of different species of atoms will be necessitated forprecipitation of crystalline phases, and then the unique amorphousstructure will be destroyed. Although the sintering morphology of the300 °C and 500 °C is similar, the degree of the diffusion of differentspecies of atoms is different. Because the Ni phase (Fig. 4(b)) appearedcompared with the sintered samples at 300 °C for 30 min. When thesintering temperature is 700 °C, a typical net structure can be observedwhich formed by the nanoparticles connecting with each other at anatomic level as shown in Fig. 6(c). In a typical sintering process ofnanoparticles, it is common that the net forms between neighboringparticles to reduce the surface energy [23–26]. And it is clear that the

Fig. 5. Precise characterization of sintered FeNiP powders at 800 °C. (a) SEM image withtwo kinds of morphologies, (b) the typical morphology of Ni nanospheres with (c) thecorresponding SAED pattern, (d) the morphology of (Ni,Fe)3[PO4]2 nanorod with (e) theHAADF micrograph and EDX mapping of Fe, Ni, P, O. (f), (g) and (h) are the series tiltingSAED patterns of a single nanorod in different orientation.

Fig. 6. Schematic illustration of the crystallization process of amorphous FeNiP nanoparticles.

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net structure is vital because it can offer a passageway which eliminatesthe barrier for the element diffusion. With complete long-range diffu-sion, Ni nanoparticles and (Ni,Fe)3[PO4]2 nanorod can be obtained asshown in Fig. 6(d). It is no doubt that high temperature can supplymore energy which also contributes to the element diffusion.

3.3. Magnetic Properties of Different FeNiP Nanoparticles

Magnetic properties of the original amorphous FeNiP and sinterednanoparticles at different temperatures were characterized by vibratingsample magnetometry (VSM). Fig. 7 shows the hysteresis loops andmagnetic data of these nanoparticles measured at room temperature(300 K). The hysteresis loop of the original amorphous FeNiP nano-particles is shown in Fig. 7(a), from which the corresponding Ms and Hcare obtained as 13.2 emu/g and 0.7 Oe, respectively. And the inset

shows the details about the hysteresis loop. It indicates that theamorphous nanoparticles are soft magnetic materials with very low Hc.Fig. 7(b) illustrates hysteresis loops of the nanoparticles sintered at300 °C, 500 °C, 700 °C and 800 °C. It's evident that all the sintered na-noparticles are ferromagnetic materials with a typical hysteresis loop.And the magnetic data including Ms and Hc are plotted in Fig. 7(c) as afunction of sintering temperature. In order to clarify the influencefactors of the magnetic property, the mass fractions of different phasesin the sintered samples at different sintering temperatures were calcu-lated by the JADE 6.0 using the Whole Pattern Fitting and RietveldRefinement method as shown in Table 1. Although the fitting might notbe rigorous considering the correlation factor (especially the RIRvalue), the change trend of the proportion can still be obtained inFig. 7(c). The Ms and Hc increased at the sintering temperature of300 °C, because the amorphous materials start to crystallize and gen-erate new phase of FeNi3 and Ni3P in the samples as shown in Fig. 4(a).According to Fig. 2, the sintering samples may have some amorphousFeNiP while the annealing temperature is 300 °C. It is easy to find thatthe magnetization and coercivity of these sintered nanoparticles areeffected by the mixture of different phase [27]. Besides the amorphousparticles, the crystalline phase is mainly FeNi3 as shown in Table 1(300 °C). The Ms and Hc of the FeNi3 nanoparticles are as high as110 emu/g and 100 Oe respectively [28]. When FeNi3 is formed withinamorphous nanoparticles, the Ms and Hc of the sample increased be-cause of the “Mixed effects”. While the temperature is 500 °C as shownin Fig. 7(c) and Table 1, the appearance of Ni phase and the proportionof Ni/FeNi3/Ni3P (37.0/44.2/18.8 wt%) result in the decrease of Msand the increase of the Hc. Compared with the magnetic property of thesamples sintered at 500 °C, the Ms of the samples sintered at 700 °C issimilar due to the same kind of phases (Ni, FeNi3 and Ni3P) in themixture and the alike proportion of Ni (41.1 wt%–37.0 wt%), FeNi3(39.7 wt%–44.2 wt%) and Ni3P (19.2 wt%–18.8 wt%). As the sinteringtemperature rose to 800 °C, the Ms and Hc changed with the incrementof Ni phase because of the excellent ferromagnetic property of Ni(54.2 emu/g, 19.4 Oe) [29]. Therefore, due to the new phase formationand its proportion change, it is no doubt that the Ms and Hc of thesample also change as shown in Fig. 7, which is more complex com-pared with a single phase sample.

4. Conclusion

A facile chemistry method was proposed to synthesize amorphousFeNiP nanoparticles by reduction of aqueous Fe2+ and Ni2+ with so-dium borohydride (NaBH4) and hypophosphite at room temperature forone hour. The amorphous nanoparticle has a mean size of about 70 nmwith a homogeneous composition around Fe17.3Ni74P8.7 (at.%). Its Msand Hc were measured as 13.2 emu/g and 0.7 Oe respectively, showingan excellent soft magnetic property.

Phase transformation was found during the crystallization ofamorphous FeNiP nanoparticles within a sintering temperature from300 °C to 800 °C. FeNi3 and Ni3P first appeared at 300 °C and then Nipresented at 500 °C. These nanoparticles would shrink and connect witheach other to form a net frame structure for better long-range diffusion.

Fig. 7. Magnetic properties of different FeNiP powders measured at the room tempera-ture. (a) The hysteresis loop of amorphous FeNiP nanoparticles (Inset is the enlarge viewof the hysteresis loop) and (b) those of sintered nanoparticles at 300 °C, 500 °C, 700 °C,800 °C, and (c) the dependency of Ms and Hc on sintering temperature.

Table 1The mass fractions of the sintered samples at different sintering temperatures. (Ignoreamorphous phase at 300 °C).

Temperature(°C)

25 °C 300 °C 500 °C 700 °C 800 °C

Phase AmorphousFeNiP

FeNi3/Ni3P

Ni/FeNi3/Ni3P

Ni/FeNi3/Ni3P

Ni/(Ni,Fe)3[PO4]2

Mass fraction(wt%)

100% 89.4%/10.6%

37.0%/44.2%/18.8%

41.1%/39.7%/19.2%

81.7%/18.3%

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Finally, a phase separation into pure Ni nanospheres and monoclinic(Ni,Fe)3[PO4]2 nanorods took place at a higher sintering temperature of800 °C. The involved crystallization behavior can be summarized as:amorphous FeNiP → Ni + FeNi3 + Ni3P→ Ni + (Ni,Fe)3[PO4]2. As amixture of different phases, the sintered FeNiP powders have variedmagnetic properties at different temperature, because the proportion ofindividual species will change due to involved phase transformation.This may allow adjusting of its magnetic properties by temperaturecontrol.

Acknowledgement

We gratefully acknowledge the financial support from the NationalKey Research and Development Program of China (Grant No.2017YFB0305501) as well as the National Natural Science Foundationof China (Grants No. 51601033), and the Osaka University VisitingScholar Program (Grant No. J135104902).

Conflicts of Interest

None.

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