Comprehensive structural and magnetic properties of iron oxide nanoparticles synthesized through chemical routes

Journal of Alloys and Compounds xxx (xxxx) xxx

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Comprehensive structural and magnetic properties of iron oxide nanoparticles synthesized through chemical routes Anamika Ghosh a, b, Veeturi Srinivas b, Ramaprabhu Sundara a, * a b

Alternative Energy Nanotechnology Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai, 600036, India Department of Physics, Indian Institute of Technology Madras, Chennai, 600036, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 August 2019 Received in revised form 5 November 2019 Accepted 5 November 2019 Available online xxx

Magnetite (Fe3O4) is one of the most widely explored ceramic materials for the applications in various industrial and biomedical levels. Applications towards specific field require tuning of its properties, which can be modulated by controlling particle size as well as different synthesis parameters and conditions. In present work, we have chosen six different chemical synthesis routes and performed a comparative study on variations in structural and physical properties. Present results show that all the synthesis methods provide particles of nanometer range, but the average size of the particle and particle size distribution is different for each method. Present analyses of room temperature and low temperature magnetic data confirm the possibility of presence of superparamagnetic state in 6e8 nm particles. Moreover, the interaction effects are dominant above blocking temperature. From the magnetization data it is also shown how the exchange term evolves as the particle size increases. Additionally, high temperature magnetic measurements are also carried out to compare size dependent magnetic response towards increasing temperature. Since the finite size effects dominate in this range of particles, we believe present study can provide a guideline to choose particular synthesis method for specific application. © 2019 Elsevier B.V. All rights reserved.

Keywords: Superparamagnetic Size effect Exchange interaction High temperature magnetization Anisotropy constant ISP model

1. Introduction Spinel ferrites nanoparticles (NPs) have been the subject of interest for the past few decades because of their remarkable magnetic properties particularly in the high-frequency region. Spinel ferrites with structural formula AB2O4, where A represents divalent cation (Feþ2) distributed in tetrahedral site and B represents trivalent cation (Feþ3) in octahedral site coordinated by oxygen [1,2]. Properties of ferrites are solely dependent on the cationic distribution among tetrahedral and octahedral sites. However, size effects play an important role in modifying the physical and magnetic properties that enables us to use them for a wide range of potential applications, such as, electronic circuits, power delivering devices, electromagnetic interference suppression, and in biomedicine [1,3e7]. Among various magnetic nanoparticles (MNP), typically iron oxides nanoparticles (IONPs) which form in three natural types, such as, hematite (a-Fe2O3), maghemite (g-

* Corresponding author. E-mail addresses: [email protected] (A. Ghosh), [email protected] (V. Srinivas), [email protected] (R. Sundara).

Fe2O3), and magnetite (Fe3O4) are physically and chemically stable, biocompatible and environmentally safe [8], thus presenting unique characteristics for clinical applications. As the particle size is reduced it undergoes multidomain state to a stable single domain state below a critical diameter, Dc, and then it undergoes stable to unstable single domain called superparamagnetic (SPM) particles [8]. Stable single domain particles show high coercivity and remanence which is highly desirable for memory storage application while SPM particles show zero coercivity and remanence suitable for various biomedical applications. In fact, IONPs reach smaller sizes (about 10e20 nm for iron oxide), superparamagnetic properties become evident, so that the particles reach a better performance for most of the aforementioned applications [8]. Despite the growing body of evidence attesting their biomedical usefulness, superparamagnetic IONPs are still in early stage of clinical investigation, with studies pointing out to the need for their improvement prior to their commercialization. As mentioned earlier the functions of IONPs directly related to size, shape, coating and stability which in turn depend on synthesis methods. Surface effect of nanoparticles arises from the uncompensated surface spin or canted spin that lead reduction in magnetization compared to its

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bulk counterpart [8]. Hence, properties of Fe3O4 nanoparticles can be tuned and remains to be a challenge for researchers. Therefore, first of all it is important to identify a viable synthesis process to control the size, stoichiometry, structural morphology, monodispersity. Although both physical and chemical methods have been used to synthesize MNPs, chemical synthesis methods are expected to provide NPs with narrow size distribution. Further, chemical methods are simpler and cost effective. It is also known that low temperature synthesis is ideal to generate particles with smaller size, which inhibit the grain growth due to thermal effects. Several chemical methods such as coprecipitation [9], hydrothermal [10], solvothermal [11], thermal decomposition [12], microemulsion [13], polyol [14], microwave [15] have been explored for synthesizing magnetic nanostructures by different research groups. These reports present a confusing picture as regards to the different physical and magnetic properties. We believe a comparative study of in-depth structural and magnetic property can give a better insight on the discrepancies observed by the researchers. Although few literature [16e22] has focused on size dependent property of Fe3O4 NPs synthesized in chemical route and the interaction effect by dispersing NPs in nonmagnetic media, but to the best of our knowledge comparative study of physical properties of Fe3O4 NPs synthesized through different techniques reported for the first time. In our present work we have employed six different chemical routes to synthesize Fe3O4 NPs and compared structural, thermal and magnetic properties in order to probe the mechanism governing the properties of the same. 2. Experimental section 2.1. Materials Iron chloride (FeCl3, 6H2O), Iron sulphate (FeSO4, 7H2O), Iron nitrate (Fe (NO3)3, 9H2O), sodium hydroxide (NaOH), were purchased from Merck. Ammonium hydroxide (NH4OH) was purchased from Rankem. All chemicals used were of high analytical grade. Millipore water has been used to prepare all the solutions. 2.2. Synthesis of Fe3O4 NPs Iron oxide nanoparticles were synthesized by six different chemical routes: a. Hydrothermal b. Coprecipitation c. Sonochemical d. Wet chemical e. Microwave f. Reverse chemical precipitation. a. Hydrothermal: Iron salt solution was prepared by mixing iron chloride (FeCl3, 6H2O) and iron sulphate (FeSO4, 7H2O) in 2:1 ratio in 50 mL water. Later, ammonium hydroxide (NH4OH) solution was added to the iron salt solution slowly with vigorous stirring. Then, solution was transferred to a 100 mL Teflon lined autoclave and kept at 130  C for 3 h. Further the precipitate was collected and thoroughly filtered with DI water and dried at 60  C [23]. The as prepared sample is named as sample A. b. Coprecipitation: FeCl3, 6H2O and FeSO4, 7H2O were taken into 2:1 ratio and mixed with DI water. NH4OH solution was then added to the iron salt solution dropwise with vigorous stirring. Solution was heated at 90  C for 1 h and precipitate was collected and washed with DI water to achieve neutral pH. Precipitate was dried and brownish black Fe3O4 NPs were obtained [24]. Obtained sample is termed as sample B. c. Sonochemical: Appropriate amounts of FeCl3, 6H2O and FeSO4, 7H2O were dispersed in DI water in an ultrasonic bath. Then NH4OH aqueous solution was added slowly and solution was kept in ultrasonic bath at 60  C for 1 h [25]. The precipitate was collected and washed thoroughly with DI water to remove

additional functional groups. Dried sample is named as sample C. d. Wet chemical: Two different aqueous solutions were prepared by mixing FeCl3, 6H2O in DI water (solution 1) and mixing sodium borohydride (NaBH4) in DI water (solution 2). Further solution 1 was added to solution 2 dropwise and obtained precipitate was collected, filtered and dried to obtain to blackish Fe3O4 powder [26]. Sample is named as sample D. e. Microwave heating: In this method, iron salt solution was prepared by mixing iron nitrate (Fe(NO3)3, 9H2O) and FeSO4, 7H2O in DI water as mentioned above. Further aqueous NH4OH was added dropwise and resultant solution was placed in microwave oven at 100 W power for 2.5 min. The obtained sample was washed and dried [27]. Prepared sample is termed as sample E. f. Reverse chemical precipitation: In this method, iron salt solution was prepared by above mentioned method for sample A (solution 1). Highly concentrated basic solution 2 was prepared by dissolving sodium hydroxide (NaOH) in DI water. Further, solution 1 was added dropwise to solution 2 with rapid stirring and mixed solution was kept in room temperature for 1 h. After 1 h, precipitate settled down and collected after thoroughly washing with DI water. The washed sample was dried and brownish black powder was obtained [28]. The obtained powder was termed as sample F. 3. Characterization Structural properties and phase analysis data was recorded on Bruker D8 and Rigaku X-ray diffractometer with Cu-Ka radiation (1.5406 Å). Further in-depth phase transformation study with laser radiation effect were performed using HORIBA JOBIN YVON HR 800 UV Raman spectrometer with excitation wavelength 632 nm (HeeNe laser) and 600 lines grating with 60e120 s exposure time. Morphological studies, elemental composition and SAED pattern were carried out using Inspect F scanning electron microscopy (FESEM) (30 kV), TECNAI 20 transmission electron microscopy TEM (200 kV). Thermal properties were measured using SDT Q600 TGADTA. High temperature magnetic measurements (300Ke900K) were carried out by mounting sample with ceramic paste in presence of constant field of 500 Oe. Temperature dependent magnetization measurements (5e300 K) were done in quantum design MPMS SQUID VSM from 7 T to 7 T. Zero field cooled/field cooled (ZFC/FC) magnetization measurements were carried out by applying constant field of 100 Oe. 4. Result and discussion 4.1. Structural, microscopic and thermal property analysis Fig. 1 shows XRD of the as prepared Fe3O4 samples synthesized in six different chemical routes. Six prominent peaks (022), (113), (004), (224), (115), (044) correspond to cubic spinel phase of Fe3O4 with space group (Fd-3m) which is congruous with standard data (ICSD 98-011-1281). Crystallite size was further calculated using Scherrer formula and crystallite size of as prepared samples was in the range of 6e20 nm. Broadening of peaks also confirms the nanocrystalline nature. Rietveld refinement was carried out for all samples using EXPGUI-GSAS software and the lattice parameters for all samples found to be between 8.3519 Å to 8.3718 Å which smaller than the bulk magnetite value (8.39 Å). Reduction of lattice parameter or unit cell volume is mainly attributed to presence of large number of surface atoms for nanoparticles. However, no direct correlation can be drawn between lattice constant and size of NPs because lattice constant depends on the periodicity of the

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Fig. 1. Rietveld refinement of XRD patterns for sample synthesized in six different chemical methods-sample A(a), sample B (b), sample C (c), sample D (d), sample E (e), sample F (f).

crystal structure. Change in lattice parameter also leads to change in mean radii of tetrahedral (A) and octahedral (B) site. Mean radii of both sites are calculated by using following equations [29].

pffiffiffi RA ¼ a 3ðu  0:25Þ  RO

(1)

RB ¼ að0:625  uÞ  RO

(2)

a is lattice parameter evaluated from Rietveld fitting of XRD pattern, u is oxygen parameter considering origin at tetrahedral site (u43m symmetry) and RO is ionic radius of oxygen (1.32 Å). Oxygen positional parameters (u) obtained from Rietveld fit is summarized in Table S1 and u is measured taking origin at octahedral site (u3m symmetry) [30]. Further bond lengths dAO, dBO, dAB were calculated using following equations [31].

dAO ¼

pffiffiffi 3 a 8

(3)

1 dBO ¼ a 4 dAB ¼

(4)

pffiffiffiffiffiffi 11 a 8

(5)

Change in u leads to change in tetrahedral and octahedral radii, which indicates that movement of oxygen ion in order to compensate the radii of A and B sites without breaking symmetry. Additionally, deviation in bond lengths dAO, dBO, and dAB for different samples tune cationic distribution and modulate structural and physical properties. Although significant changes in bond

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lengths are not observed (Table 1), the crystallite sizes vary by~65%. As reported by earlier authors all synthesis processes discussed in the present work show single phase spinel structure, though the crystallite sizes vary. It was believed that the synthesis temperature is responsible and hence low temperature synthesis processes were suggested to synthesize smaller particles. However, from the present study it may be suggested that apart from temperature, nucleating agents (synthesis method) also determines the crystallite size and its growth. The obtained structural parameters have shown deviations, which clearly indicate the structural variation with synthesis routes and particle sizes. Simulated structure (Fig. S1) of sample A, sample E, and sample F by the parameters obtained from Rietveld refinement, which clearly indicate the structural distortion between differently synthesized samples. Fitted and calculated parameters are summarized in Table 1. FESEM (Fig. S2) shows agglomerated nature of particles which could also be due to dipolar interactions between the magnetic particles. Elemental analysis was carried out by electron dispersive spectroscopy (EDS) which confirms the presence of iron (Fe) and oxygen (O) in the synthesized samples. No trace of impurity was observed which is in accordance with XRD. Guided by XRD results we have recorded TEM images of typical three samples A, E and F whose crystallite sizes are significantly different. Fig. 2 reveals spherical morphology of particles with some agglomeration. Visual examination of the micrographs suggests that sample F has smallest size among the samples synthesized in the present investigation. Particle size distribution was evaluated by Image J software and distribution was fitted using lognormal function. Resulting histograms are depicted in the inset of the figure. The estimated average diameters are 12 ± 1.2 nm, 6.9 ± 1.9 nm, and 3.6 ± 1.3 nm for A, E and F samples respectively, which appear to be smaller than calculated crystallite size from XRD. The discrepancy in both the measured values is due to noisier XRD data as all samples lies in nano-regime. Hence, an average fit to XRD profile gives approximated value of crystallite size which can be a guideline to observe the trend of changing particle size with synthesis routes. However, TEM particle distribution gives more accurate estimation than XRD. The ring patterned SAED of sample A was indexed with (022), (113), (004), (224), (115), (044) which confirms the cubic spinel phase of Fe3O4 (Fd-3m). Raman spectroscopy measurements have been carried out to understand the structure. Among various iron oxides, magnetite (Fe3O4) is poor Raman scatterer [32]. Hence, laser wavelength, gratings and exposure time are few key parameters for recording Raman spectrum. Effect of laser power on sample has been explored with variation of laser intensity. Different groups have reported different characteristic Raman modes for various iron oxides. Raman modes according to literature are summarized in Table S2. Symmetric stretching modes are Raman active and antisymmetric stretching modes are infrared active. For hematite (a-Fe2O3) vibration modes are given by Ref. [33].

Table 1 Calculated and fitted parameters from Rietveld refinement of XRD pattern. Sample

t (nm)

a(Ǻ)

RA (Ǻ)

RB (Ǻ)

dAO (Ǻ)

dBO (Ǻ)

dAB (Ǻ)

Bulk magnetite Sample A Sample B Sample C Sample D Sample E Sample F

e 20.40 18.62 13.90 13.0 12.8 6.41

8.39 8.3549 ± 0.0003 8.3718 ± 0.0004 8.3542 ± 0.0007 8.3663 ± 0.0008 8.3519 ± 0.0018 8.3550 ± 0.0038

0.496 0.596 0.539 0.645 0.513 0.495 0.461

0.777 0.706 0.650 0.678 0.759 0.764 0.784

1.8165 1.8093 1.8126 1.8106 1.8152 1.8091 1.81

2.0975 2.0892 2.0929 2.0907 2.0961 2.089 2.09

3.4783 3.4645 3.4077 3.4671 3.4759 3.4642 3.4658

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Fig. 2. TEM images of sample A (top panel), sample E (middle panel), sample F (bottom panel).

Gvib ¼ 2A1gþ3A1uþ3A2gþ2A2uþ5Egþ4Eu A1g and A2u are optically silent and Raman modes at 660 cm1(Eu) has been attributed to Raman forbidden and IR active longitudinal optical Eu mode which is activated by disorder. For maghemite vibration modes are given as [33].

Gvib ¼ A1g þ E1g þ T1g þ3T2gþ2A2uþ2Euþ4T1uþ2T2u T1g, A2u, Eu, T2u are optically silent and 4T1u is infrared active. Magnetite vibration modes are given by Ref. [33]. Gvib ¼ A1gþ2T1g, among this A1g modes are more pronounced. In Fig. 3, Raman modes (217 -223 cm1), (270-290 cm1), (400488 cm1), (610-653 cm1) correspond to hematite phase, (490500 cm1), (680-700 cm1) correspond to maghemite phase and (660-670 cm1) correspond to magnetite phase. Some of the Raman spectra show direct conversion of Fe3O4 particles into aFe2O3, while others show gradual transition from Fe3O4 to g-Fe2O3 to a-Fe2O3. Smaller sized particles can complete oxidation at low temperature to give metastable maghemite phase, while larger particles convert directly to a-Fe2O3. Although the size limit for the gradual and sharp transition of phase with laser intensity is given in previous literature is 300 Å [34], but we have observed the transition phenomena within the particle size 3e15 nm itself. However, maghemite also converts to hematite at higher laser intensity due to its metastability. Model proposed by Gallagher et. al. [34].states that oxidation of magnetite leads to cationic holes that develops lattice constant gradient and prompts lattice strain. Developed lattice strain is crucial driving force for nucleation of a-Fe2O3.

Therefore, larger particle cannot accommodate the developed stress and transforms to stable hematite phase which is again confirmed by our study where it is observed that sample A transformed to hematite phase directly with laser oxidation, while sample F has shown gradual transformation via metastable maghemite phase. Thermal analysis (TGA-DTA) experiments have been carried out in the temperature range 30  Ce1000  C in air atmosphere to investigate thermal stability and phase transformation from magnetite to hematite is given by Ref. [33].

200o C

400o C

Fe3 O4 !gFe2 O3 !aFe2 O3 Figure S3 shows TGA curve for as synthesized samples that show three step change in weight (%): first weight loss can be due to decomposition of adsorbed water molecules due to presence of moistures, followed by weight gain due to conversion of Fe3O4 to gFe2O3.This transition is designated by small intermediate hump and has been observed within the temperature region (192e276  C) for sample A, (186e275  C) for sample B, (156e274  C) for sample C, (162e274  C for sample D and (155e192  C) for sample E. The intense intermediate hump is found to reduce as particle size decreases and completely disappears for sample F. Hence, the sample was heated at 250  C and intermediate maghemite phase was further confirmed by Raman spectroscopy (Fig. 4). Weight change within temperature range (590e640  C) for different sample corresponds to conversion into hematite, which is associated with strong DTA peak. Strong DTA peak clearly remarks the complete oxidation of g-Fe2O3 to a-Fe2O3. All synthesized samples are

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Fig. 3. Raman spectroscopy of sample A (a), sample B (b), sample C (c), sample D (d), sample E (e), sample F (f).

reactivity in nanoregime. In DTA curve (Fig. S4)), exothermic peaks are observed for all samples within 540e640  C. Exothermic peak which defines phase transition of magnetic magnetite to nonmagnetic hematite phase is found to be shifted to lower temperature range with decreasing particle size. This is mainly due to excessive surface energy of the smallest sized particles due to presence of higher number of surface atoms and excess dangling bonds which enhance surface reactivity and needs smaller energy to complete phase transformation, therefore leads to shift of transition temperature towards lower temperature region [35]. However, systematic peak shift is observed in DTA except sample F. Hence, distinct nature in TGA and DTA curve confirms the dependence of thermal behaviour and stability on particle size and synthesis parameters.

Fig. 4. Raman spectrum of heat-treated sample F.

observed to lose magnetic property followed by heat treatment and XRD of resulting sample (after DTA run) confirms the presence of nonmagnetic a-Fe2O3. Weight change percentage enhances with decreasing particle size which is expected due to enhanced surface

4.2. Magnetic property analysis Further high temperature magnetization measurement was carried out for two extreme sized samples (Sample A and Sample F) which show completely distinct behaviour with respect to the temperature. Fig. 5 shows high temperature M-T curve of sample A and F. High temperature M-T curve of Sample A simply depicts

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Fig. 5. High temperature M-T curve for sample A and sample F.

ferrimagnetic nature with magnetization dropping to zero at 817 K. This is curie temperature Tc for sample A that is mainly attributed to phase transition from magnetic magnetite phase to nonmagnetic hematite phase. High temperature M-T curve for sample F shows two step dip in magnetic moment. First and second dip was observed at 469 K and 779 K respectively. First dip may be ascribed to transition to maghemite (g-Fe2O3) phase while second dip probably corresponds to transition to non-magnetic hematite phase. The observed fact corroborates well with our previous Raman spectroscopy and thermal measurement observation which predicts that smallest sized particle undergoes gradual phase transition in comparison with the largest sized particle. Fig. 6 a shows room temperature magnetization (M) as a function of applied field (H) and the measurements were carried out up to 2 T magnetic field. All the samples exhibit saturation magnetization (MS) in the range of 53e81 emu/g. Large sized Fe3O4 particles synthesized in hydrothermal show soft ferrimagnetic behaviour with coercivity 40 Oe and saturation magnetization (Ms) of 81.5 emu/g. With decreasing size, saturation magnetization also decreases which may be attributed to randomly oriented surface spins. Magnetic isotherms for samples B, C, D, E, reveal coercivity value within 20 Oe. However for bare iron oxide nanoparticles, presence of dipolar interaction is always observed due to agglomerating tendency of nanoparticles. Hence, it is possible that the presence of coercivity of the order of few oersteds can be due to interaction effects. However the existence of small amount of

remanence suggests the presence of ferrimagnetic phase. On the other hand the presence of broad distribution of particles sizes can also lead to features that resemble two phases namely a) superparamagnetic and b) ferrimagnetic phases. Sample prepared in reverse coprecipitation doesn’t show any coercivity and remanence which may be attributed superparamagnetic nature. Considering synthesized sample as perfect ferrimagnet, (moment)/(formula unit) is calculated [36]. Ideal ferrimagnet has 4mB moment/formula unit. Sample A has shown largest magnetic moment/formula unit while reducing particle size value of ferrimagnetic moment reduces which indirectly confirms possibility of SPM phase. Table S3 shows calculated moment/formula unit for all samples. All room temperature magnetic isotherms were initially fitted with usual FM function to fit hysteresis curves, but it is observed that sample A data follows this function well. However magnetic isotherms for other samples fail to follow the ferrimagnetic behaviour (Fig. 6b). Therefore, SPM part (Langevin function) is added to account for the non-saturated behaviour at higher fields and it is perceived that FM þ SPM model perfectly fit with the M vs. H of other samples. All M vs. H were fitted according to following equation [37].

MðH; TÞ ¼

     H±Hc pS mH tan þ Msp L tanh Hc 2 KT p

2Ms

(6)

Fig. 7 shows SPM þ FM fit of room temperature magnetic isotherm. First term in equation (6) denotes ferrimagnetic contribution, Ms is ferrimagnetic saturation magnetization, H is applied external field, Hc is coercivity, S ¼ Mr/Ms is squareness ratio, Mr is remanent magnetization. Second term of the equation accounts for superparamagnetic component and Msp is corresponding magnetization. From these fits SPM, FM contributions could be estimated, and it is found that with decreasing size, FM contribution reduces. However, equation (6) didn’t yield good fit to magnetization data of smallest sized particles (sample F). Parameters obtained from all the fitting was summarized in Table 2. In order to understand magnetic state of the particles further we have carried out low temperature magnetic measurements on three samples (sample A, E and F) synthesized in hydrothermal, microwave and chemical precipitation route were carried out at 5 K (Fig. 8). The increase in coercivity at low temperature could be due to increase in anisotropy. At 5 K, coercivity of sample F is highest

Fig. 6. Room temperature magnetic isotherms for different samples (a), FM fit (b).

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Fig. 7. FM þ SPM fit for sample A (a), sample B(b), sample C (c), sample D(d), sample E (e), and sample F (f). Red curve denotes theoretical fit while blue and dark brown graph depicts the simulated part. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Table 2 Parameters obtained from FM þ SPM fit. Method

Saturation SPM magnetization (Msp) (emu/ Saturation FM Magnetization (Ms) (emu/ Coercivity g) g) (Hc)(Oe)

Remanent magnetization Mr (emu/g)

FM moment (mB)

SPM (mB)

Sample A Sample B Sample C Sample D Sample E Sample F

e 18.35 20.91 17.04

61.09 65.24 61.09 61.09

40 15.41 11.05 17.09

8.91 3.56 2.98 4.20

3.39 2.70 2.53 2.45

e 2854 2310 2450

15.96 e

47.13 e

6.19 e

1.61 0.25

1.95 e

3380 e

compared to the other two samples. This distinct nature can be accredited to blocked state of smallest sized particle. It is well known that SPM (unstable single domain) particles go to a stable single domain state below the blocking temperature of the particles. It is well known that in this state high field is needed to reverse the magnetization direction due to high anisotropy and can lead to larger coercivity.

The ZFC and FC magnetization curves for A, E, and F samples are shown in Fig. 9. It is seen that for sample F bifurcation of ZFC, FC curves starts at 209 K due to blocking of SPM particles below this temperature (TB), followed by ZFC maximum (Tp) at 136 K. For, T < TB, particle moments are aligned along easy direction and due to lower thermal energy, it becomes impossible for the moments to cross anisotropy energy barrier and moment remains blocked in

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Davg through the standard relation

Fig. 8. Magnetic isotherm for sample A, sample E, sample F at 5K. Inset shows magnified version of low field region.

particle region which leads to higher coercivity. The broader ZFC maxima also indicate wider size distribution of particles in the system which has been confirmed from TEM analysis. Since in our study, we have not used any dilution media and agglomerating nature of prepared sample was confirmed from microscopic images, it can be expected that interaction effects of the moments can lead to observed ZFC/FC nature. Conversely, no ZFC maxima or blocking temperature (TB) was observed below room temperature for sample A, while for Sample E, bifurcation is observed at 280 K without any prominent ZFC maximum. The curvature near 300 K indicates the possible existence of blocking temperature just above 300 K. These observations suggest a shift in blocking temperature with particle size which can also be validated by Stoner-Wohlfarth model (SW) [19]. The position of the maximum of the ZFC curves of a nanoparticles system TP gives an estimation of the average NP size

!1 3 =

6 25 Davg ¼ T p keff p

(7)

Where, Keff is an effective magnetic anisotropy constant. By taking bulk magnetite anisotropy Keff ¼ 2.5  105 erg/cm3 at low temperatures, one gets Davg ¼ 15 nm for sample F. Blocking temperature shifts towards higher temperature value as volume of particle increases. Shift of blocking temperature to higher region can also be influenced by enhanced dipolar interaction below TB. Dipolar interaction energy is given by: Edipole ¼ -m0M2/4pd3, where m0 is free space permeability, M is magnetic moment, and d is interparticle distance [19]. Further, difference of ZFC and FC magnetization (DM) with respect to temperature was explored to get an idea about anisotropy as well as blocking temperature distribution. As TEM particle distribution suggests distribution of particles size, so it can be assumed that there will be distribution of blocking temperature as well. DM basically represents the magnetization of blocked particles. DM is expressed as [38,39].

29M 2s H DM ¼ MFC ðH; TÞ  MZFC ðH; TÞz 3Ka < TB >

∞ ð

TB f ðTB ÞdTB

(8)

T

Here, Ms represents saturation magnetization, H is applied field, Ka is anisotropy constant, and average blocking temperature < TB > ¼ TB0 expðl2 =2Þ; TB0 is most probable blocking temperature and l is distribution width. f(TB) represents fraction of volume of particles with blocking temperature with in TB to TB þ dTB which can be expressed as following [39]:

Fig. 9. FC/ZFC magnetization curve at 100 Oe for sample A, sample E, sample F.

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Table 3 Parameters obtained from fitting of equations (9) and (10).

8   2 9 > > > > > > ln TTB0B < = 1 f ðTB Þ ¼ pffiffiffiffiffiffiffi exp  2 > > 2p TB lB > > 2lB > > : ;

(9)

Sample

TP (K)

TB0 (K)

Ka (erg/cm3)

lB



Sample E Sample F

>300 138

92.48 57.14

8.23106 1.17107

0.5687 0.4183

108.70 62.37

The above integral can be solved in terms of error function by   T

ln T pffiffiffi B0 considering TB ¼ TB0 expð 2 lB xÞ, and x ¼ pffiffiffi

2lB

and can be

expressed as [40].

8 39 2   > > T > > ln = < TB0 6 29Ms2 H lB 7 7 6 DM ¼  pffiffiffi 5 1  erf 4 pffiffiffi 6Ka > 2 lB > 2 lB > > ; :

(10)

Experimentally observed difference of ZFC and FC magnetization as a function of temperature was further fitted for samples E and F (Fig. 10). It is observed that equation fits perfectly well for smaller sized sample F but deviates for sample E. This is consistent with our observations for the room temperature isotherms discussed earlier, where we suggested that the sample E contains both SPM and FM components, while sample F has SPM phase alone. Anisotropy constant, Ka value for sample F is found to be 1.17х107 erg/cm3 which is higher compare to the bulk magnetite. This is because when the particles size reduces, along with magnetocrystalline and shape anisotropy, surface anisotropy also contributes due to enhanced surface to volume ratio and also due to randomly oriented spins. Apart from that, strong interaction among nanoparticles also contributes towards the enhancement of anisotropy. Anisotropy for sample E is found to be slightly higher than the bulk magnetite. As the TEM images of sample E and F confirmed the distribution of particle sizes which can be directly correlated to distribution of blocking temperatures. Hence, we have further analysed distribution of blocking temperatures obtained from fitted parameters of DM vs. T and it is noticed that value of TB0 and increases with sample F to sample E which is in agreement with experimental data as well as with SW model. The fitted parameters obtained were tabulated in Table 3. Further average particle diameter is calculated using the following formula:

¼

2 30kB < TB > p ¼ < DTB > 3 el =3 Ka 6

(11)

Using above formula average particle diameter of sample F is calculated out to be 3.32 nm which is in great agreement with TEM particle size. However average particle diameter of sample E is found to be 4.2 nm which is closer to the TEM mean particle diameter 6.9 ± 1.9 nm. The deviation from TEM particle size is probably due to underestimation Ka and because of occurrence of bifurcation point at highest measuring temperature as well as range of particle distributions for sample E lies within 2e50 nm. Hence obtained value probably the average over all particle sizes which may lead to discrepancy with experimental result. Further to investigate superparamagnetic nature or presence of interaction effect, temperature dependent M-H measurements were carried out for sample F at different temperature above its blocking temperature. An ideal superparamagnet system satisfies the criteria, such as, zero coercivity and remanence, follows Langevin’s function. In addition to this, superposition of magnetization curves when magnetization is plotted against H/T above blocking temperature. However, in cases finite coercivity is observed and also the scaling behaviour of magnetization is not observed. These features are observed even in dilute systems and attributed to dipolar interaction effects. By taking interaction effects into consideration, an interacting superparamagnet (ISP) model has been suggested to account for the deviations. It is observed that initial magnetization above blocking temperature deviates from scaling behaviour. This deviation is smaller for the temperatures well above blocking temperature (220e280 K) compared to the temperature closer to the blocking temperature (140e180 K). In an interacting superparamagnet, Langevin function is modified by taking temperature contribution from dipolar interaction effect among moments. ISP model for isothermal magnetization of a nanoparticle system is given by following equation [41].

 M ISP ¼ N mL

mH

kB ðT þ T * Þ

"

 ¼ MS L

MS H

NkB þ aM 2S

# (12)

Here L indicates langevin function, Ms is saturation magnetization, and correction term in the denominator of langevin function

Fig. 10. Experimentally observed and fitted DM (MFC e MZFC) as a function of temperatures for sample E (a) and sample F (b). Inset shows corresponding blocking temperature distribution.

Please cite this article as: A. Ghosh et al., Comprehensive structural and magnetic properties of iron oxide nanoparticles synthesized through chemical routes, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152931

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A. Ghosh et al. / Journal of Alloys and Compounds xxx (xxxx) xxx

implies dipolar energy explained by equation kBT* ¼ am2/ d3 ¼ aNm2 ¼ aM2s /N, where N is number of nanoparticles per unit volume and the above expression can be extended to a distribution of nanoparticles. Taking account of temperature effect in Langevin function, it is perceived that M-H curve at different temperature superimpose on each other which clearly remarks towards the interaction effect present in sample F (Fig. 11).

highest coercivity at 5 K compared to other samples further confirms the existence of superparamagnetic phase. 4. Dipolar interaction effect above blocking temperature has been investigated by incorporating ISP model and it is observed that scaling law corroborates well for smallest sized particles. Thus, tuning synthesis routes and parameters and controlling particle size as well as distribution can help towards potential applications of Fe3O4 NPs in various fields depending on its structural and physical properties.

5. Conclusions The detailed analysis of structural and magnetization data of Fe3O4 nanoparticles synthesized in six different chemical routes (keeping ratio of trivalent and divalent cation constant) result in the following conclusions. 1. Fe3O4 nanoparticles show single and pure magnetite phase, but with change in oxygen positional parameter (u), lattice parameter and bond lengths with varying synthesis parameters and particle size suggesting the structural distortion among synthesized samples. Movement of oxygen ions with respect to octahedral and tetrahedral sites tunes the cationic distribution and additionally change in lattice constant indicating the compression or expansion of lattice. Thus, different physical and magnetic properties are mainly attributed to the structural deviations among particles. 2. Size dependent vibration spectra with laser intensity variation are mainly attributed to uncompensated lattice strain in larger sized particle compared to the smallest size particles. Transition of comparatively larger sized Fe3O4 particles directly to nonmagnetic hematite phase (a-Fe2O3) with lower laser intensity while gradual transition to hematite phase via metastable magnetite phase for smallest sized particles is observed. Although the earlier model claims the size limit to observe the particle size dependent Raman spectra for 300 Å, but in present study it has been observed within 30e200 Å. 3. Room temperature isotherms of different sized samples exhibit distribution of magnetic moments within range of 56e81 emu/g which is ascribed to presence of dual phase (FM and SPM) in different proportions as confirmed from fitted magnetic isotherms to FM and Langevin functions respectively. This also claims the evolution of exchange interaction term with change in particle size and its size distribution. Presence of ZFC maxima at 138 K, higher anisotropy constant of 1.17х107 erg/cm3 and

Fig. 11. ISP scaling law effect on sample F. Inset shows effect of standard superparamagnetic scaling law effect on sample F.

Associated content Supporting information includes table summarizing oxygen positional parameter, simulated structure, SEM and EDAX of sample B and sample F, literature summary of Raman modes of Fe3O4, TGA and DTA of all samples, table summarizing ferrimagnetic moments of all samples. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Authors acknowledge the Indian Institute of Technology Madras (IITM), India and Ministry of Human Resources and Development, Government of India for financial support. Authors would like to thank Dr. S. Kavita (ARCI Chennai) for room temperature and high temperature magnetic measurements. One of the authors thanks Department of Science and Technology (DST) for the financial support to establish Nano Functional Materials Technology Centre (NFMTC) through SR/NM/NAT/02e2005 project. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.152931. References [1] K.K. Kefeni, T.A.M. Msagati, B.B. Mamba, Ferrite nanoparticles: synthesis, characterisation and applications in electronic device, Mater. Sci. Eng. B SolidState Mater. Adv. Technol. 215 (2017) 37e55, https://doi.org/10.1016/ j.mseb.2016.11.002. [2] D.H. Kumar, Y. Yun, Spinel ferrite magnetic adsorbents : alternative future materials for water purification ? Coord. Chem. Rev. 315 (2016) 90e111, https://doi.org/10.1016/j.ccr.2016.01.012. [3] Q. Zhang, M. Zhu, Q. Zhang, Y. Li, H. Wang, Fabrication and magnetic property analysis of monodisperse manganese-zinc ferrite nanospheres, J. Magn. Magn. Mater. 321 (2009) 3203e3206, https://doi.org/10.1016/j.jmmm.2009.05.049. [4] J. Wan, X. Jiang, H. Li, K. Chen, Facile synthesis of zinc ferrite nanoparticles as non-lanthanide T1MRI contrast agents, J. Mater. Chem. 22 (2012) 13500e13505, https://doi.org/10.1039/c2jm30684k. [5] A. Pandit, S. More, R. Dorik, K. Jadhav, Structural and magnetic properties of Co 1þ y Sn y Fe 2-2y-x Cr x O 4 ferrite system, Bull. Mater. Sci. 26 (2003) 517e521. http://www.springerlink.com/index/p56k85l8q806m2k1.pdf. [6] C. Shao-Wen, Z. Ying-Jie, M. Ming-Yan, L. Liang, Z. Ling, Hierarchically nanostructured magnetic hollow spheres of Fe3O4 and g-Fe2O3: preparation and potential application in drug delivery, J. Phys. Chem. C 112 (2008) 1851e1856, https://doi.org/10.1021/jp077468þ. [7] B. Dutta, N.G. Shetake, S.L. Gawali, B.K. Barick, K.C. Barick, P.D. Babu, B.N. Pandey, K.I. Priyadarsini, P.A. Hassan, PEG mediated shape-selective synthesis of cubic Fe3O4 nanoparticles for cancer therapeutics, J. Alloy. Comp. 737 (2018) 347e355, https://doi.org/10.1016/j.jallcom.2017.12.028. [8] A.H. Lu, E.L. Salabas, F. Schüth, Magnetic nanoparticles: synthesis, protection, functionalization, and application, Angew. Chem. Int. Ed. 46 (2007) 1222e1244, https://doi.org/10.1002/anie.200602866. [9] K. Petcharoen, A. Sirivat, Synthesis and characterization of magnetite

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