Magnesium ferrite nanoparticles inserted in a glass matrix—Microstructure and magnetic properties

Materials Chemistry and Physics 132 (2012) 264–272

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Magnesium ferrite nanoparticles inserted in a glass matrix—Microstructure and magnetic properties M.G. Ferreira da Silva a , M.A. Valente b,∗ a b

Glass and Ceramic Engineering Department (CICECO), Aveiro University, 3800-193 Aveiro, Portugal Physics Department FSCOSD (I3N), Aveiro University, 3800-193 Aveiro, Portugal

a r t i c l e

i n f o

Article history: Received 16 March 2011 Received in revised form 29 September 2011 Accepted 24 October 2011 Keywords: Sol–gel Magnesium ferrite Nanoparticles Ferrimagnetism Superparamagnetism

a b s t r a c t Glasses with composition xFe2 O3 –5MgO–(95 − x)SiO2 , where x is equal to 1.25, 2.5, 5 and 10 mol%, were prepared by the sol–gel method. The gel samples were heat-treated at temperature between 500 and 1000 ◦ C. The samples were studied using X-ray diffraction (XRD), scanning electron microscopy combined with X-ray microanalysis by energy dispersive spectrometer (SEM-EDS), visible–near infrared spectrometry (VIS-NIR) and dc magnetic measurements. In most samples the presence of magnesium ferrite was detected by XRD. However, in the brown and transparent 1.25Fe2 O3 –5MgO–93.75SiO2 sample heat-treated at 500 ◦ C, there was no evidence, by XRD, of the presence of magnesium ferrite. In 10Fe2 O3 –5MgO–85SiO2 composition, heat-treated at 500 and 1000 ◦ C, hematite was also present. By the Scherrer method it was determined that the average size of magnesium ferrite crystals, in all the samples heat treated at 1000 ◦ C, varied from 8 to 10 nm. All the samples heat treated at 1000 ◦ C exhibited ferro or ferrimagnetic interactions combined with superparamagnetism with a blocking temperature. The sample with composition 1.25Fe2 O3 –5MgO–93.75SiO2 treated at 500 ◦ C showed a paramagnetic behaviour down to 5 K. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Magnesium ferrite, MgFe2 O4 , is a soft magnetic n-type semiconducting material which finds a great number of applications in heterogeneous catalysis, adsorption, sensors, magnetic technologies and components for biomedical applications [1,2]. MgFe2 O4 crystallizes in the cubic spinel structure which contains two different cation sites: eight tetrahedral A and 16 octahedral B sites per f.u. The structural formula of Mg-ferrite is usually written as (Mg1−y Fey )[Mgy Fe2−y ]O4 , where the round and square brackets denote sites of tetrahedral (A-site) and octahedral (B-site) coordination, respectively, and y represents the degree of inversion (defined as the fraction of (A) sites occupied by Fe3+ cations) [3]. The magnetic properties of a spinel ferrite are strongly dependent on the distribution of the different cations among (A) and (B) sites [4], which depend on the shape and size of the nanoparticles and the thermal history [3,5]. Nanocomposites represent a new class of materials with different physical and chemical properties compared to the bulk material [3,5–10]. Due to the small size of the nanocrystals an important part of the atoms are located at the surface. Therefore, nanocrystals have special properties compared to the polycrystalline material, obtained by conventional methods. Nanocrystallites have a

strong aggregation tendency which, usually, makes more difficult to exploit their unique properties. The dispersion of nanocrystals in amorphous matrixes represents a route to obtain very fine nanocrystals by hindering their growth in the pores of the matrix and their aggregation [6,11,12]. MgFe2 O4 nanoparticles possess typical superparamagnetic properties. Superparamagnetism is a unique feature of magnetic nanoparticles and has great relevance to modern technologies including magnetic resonance imaging contrast agents, data lifetime in high density information storage, ferrofuid technology, and magnetocaloric refrigeration [2,3,5,6,13]. There are numerous papers about ferrite magnesium nanoparticles but, from our knowledge, no study related with the behaviour of magnesium ferrite nanocrystalline particles embedded in a silica matrix. The objective of this work is the preparation and characterization of silica sol–gel derived glass–ceramics containing nanoparticles of Mg-ferrite. The samples were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) with X-ray microanalysis (SEM-EDS), visible and near-infrared spectrophotometry (VIS–NIR) and dc magnetic measurements. 2. Experimental procedure 2.1. Sample preparation

∗ Corresponding author. E-mail address: mav@fis.ua.pt (M.A. Valente). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.10.030

Glasses with the compositions xFe2 O3 –5MgO–(95 − x)SiO2 with x equal to 1.25, 2.5, 5 and 10 (mol%), were prepared using tetraethylorthosilicate (TEOS),

M.G. Ferreira da Silva, M.A. Valente / Materials Chemistry and Physics 132 (2012) 264–272

1400

500 oC

265

MF - magnesium ferrite H-hematite

MF + H

1200

MF + H

1000

Intensity (a.u.)

10Fe2O3-5MgO-85SiO2

800 5Fe2O3-5MgO-90SiO2

600 400

2.5Fe2O3-5MgO-92.5SiO2

200 0

1.25Fe2O3-5MgO-93.75SiO2

4

14

24

34

44

54

64

74

2 Fig. 1. XRD patterns of the 1.25Fe2 O3 –5MgO–93.75SiO2 , 2.5Fe2 O3 –5MgO–92.5SiO2 , 5Fe2 O3 –5MgO–90SiO2 and 10Fe2 O3 –5MgO–85SiO2 samples heat-treated at 500 ◦ C.

magnesium and iron nitrates. A mixture of TEOS, ethanol, magnesium and iron nitrates, dissolved in HCl acidified water (acidified water/TEOS molar ratio of 10), was stirred during 1 h at room temperature. Solutions were allowed to gel and dry at 60 ◦ C in Petri dishes under ambient conditions. The obtained gels were heat-treated at 120 ◦ C during two days. These 120 ◦ C treated samples were further heat-treated at 500 and 1000 ◦ C during 1 h, under ambient air conditions.

The temperature dependence of the dc magnetic susceptibility,  = M B−1 where M is the magnetization (emu g−1 ) and B the applied magnetic field (B = 100 G), of the 1.25Fe2 O3 –5MgO–93.75SiO2 samples heat-treated at 500 and 1000 ◦ C, measured in the zero field

2.2. Samples characterization The samples were characterized by X-ray powder diffraction (XRD), scanning electron microscopy combined with X-ray microanalysis by energy dispersive spectrometer (SEM-EDS), visible and near-infrared spectrophotometry (VIS–NIR), dc magnetic susceptibility and magnetization measurements. The scanning electron microscope was a Hitachi S4100-1 equipped with an EDS Brucker AXS-Quantax 800. The X-ray diffraction spectra were obtained, at room temperature, on a Rigaku XDMAX diffractometer using a monochro˚ at 40 kV and mated (monocrhromator graphite) CuK␣ radiation ( = 1.54056 A) 30 mA, by step scanning and powdered samples. Visible and near-infrared spectrophotometry (UV–VIS–NIR UV-2101/3101PC spectrophotometer) was used to obtain the spectra of the samples 1.25Fe2 O3 –5MgO–(93.75)SiO2 (brown) and 2.5Fe2O3 –5MgO–92.5SiO2 (ruby) heat-treated at 500 ◦ C. The average size of the nanocrystalline particles was calculated, based on the XRD powder patterns, using the Scherrer equation D = 0.9/ˇ cos , where  is the radiation wavelength, ˇ the size contribution to the integral breadth of diffraction peak,  the angle of diffraction and D the apparent size of particles (in Å) along the corresponding reciprocal lattice vector. The dc magnetic measurements were performed on powder samples (100–150 mg) using a vibrating sample magnetometer – VSM (Cryogenic – Cryofree). The dc magnetic susceptibility was recorded under zero-field cooled (ZFC) and field-cooled (FC) sequences under 0.1 T, between 5 and 300 K. Typical hysteresis curves were obtained at several temperatures (5–300 K), below and above the blocking temperature, in magnetic field from −10 to 10 T. The magnetic parameters such as magnetization of saturation (Ms ), coercivity (Bc ) and retentivity (Mr ) are obtained from the VSM results.

3. Results and discussion In the samples with x ≤ 5, heat-treated at 500 ◦ C, it was not possible, by XRD, to know if magnesium ferrite (MF) or hematite (H) is inserted in the glass matrix (Fig. 1). However, the SEM micrographs denote the presence of small particles (Fig. 2) and the SEM-EDS spectra reveals that the elements Si, Mg, O and Fe are also present in the samples heat-treated at 500 ◦ C (Figs. 3 and 4). Moreover, the VIS–NIR spectra of the samples with x = 1.25 and 2.5 (Fig. 5) show a large band, centered at approximately 880 nm, indicative to the presence of colloidal iron oxide aggregates in the samples [14]. Furthermore, in the XRD patterns of the samples with the same composition, heat-treated at 800 ◦ C, it is evident the presence of MF in the 2.5Fe2 O3 –5MgO–92.5SiO2 and 5Fe2 O3 –5MgO–90SiO2 samples (Fig. 6) and MF and H in the 10Fe2 O3 –5MgO–85SiO2 sample.

Fig. 2. SEM micrographs of the 1.25Fe2 O3 –5MgO–93.75SiO2 2.5Fe2 O3 –5MgO–92.5SiO2 samples heat treated at 500 ◦ C.

and

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Fig. 3. XRD microanalysis (EDS) of the 1.25Fe2O3 –5MgO–93.75SiO2 sample heat-treated at 500 ◦ C.

cooled (ZFC) and field cooled (FC) conditions, between 4 and 300 K, are registered in Fig. 7. For the samples with x ≤ 5 heat-treated at 500 ◦ C it is observed an overlap of the ZFC and FC curves for all the temperatures indicating no magnetic transition in this temperature range. For these samples it is, also, observed a small shoulder at approximately 20 K (Figs. 7–9). In the sample with x = 10, heat-treated at 500 ◦ C, the overlap of the ZFC and FC curves are observed for temperatures above 60 K and it was observed a small peak at 25 K in the ZFC curve (Fig. 8). In order to compare the values of the dc magnetic susceptibility of the samples heat-treated at 500 ◦ C the results are normalized in function of the amount of iron oxide. Thus, we consider the value of the magnetic susceptibility of the sample with x = 1.25 as reference and divide the magnetic susceptibility of the samples with x = 2.5, 5 and 10 by two, four and height, respectively. The normalized magnetic susceptibility, decreases with the increasing of the iron oxide concentration (Fig. 9). The normalized magnetic susceptibility are greater for the sample with x = 1.25 and are almost the same for the samples with x = 2.5 and 5 (Fig. 9). The temperature dependence of the reciprocal susceptibility (−1 ) is plotted in Fig. 10 for the samples heat-treated at 500 ◦ C. For the samples with x ≤ 5 and for temperatures higher than 40 K, the −1 data are, approximately, linear functions of the temperature indicating that the magnetic susceptibility obeys the Curie–Weiss law, dc = C/(T − p ), where p is the paramagnetic

Table 1 Paramagnetic curie temperature, p , and effective magnetic moment, eff , of the samples heat-treated at 500 ◦ C. Sample

Heat-treated at 500 ◦ C C (Curie constant)

1.25Fe2 O3 –5MgO–93.75SiO2 2.5Fe2 O3 –5MgO–92.5SiO2 5Fe2 O3 –5MgO–90SiO2 10Fe2 O3 –5MgO–85SiO2

p (K)

eff (␮B )

8.24 × 10−4 0±2 4.00 ± 0.08 1.17 × 10−3 0±2 3.39 ± 0.06 2.34 × 10−3 0±1 3.46 ± 0.02 Without paramagnetic behaviour

Curie temperature, C the Curie constant and T the temperature. p is indicative of the strength of the average interaction between the magnetic ions (Fe). The observed p , equal to zero, for the samples with x ≤ 5, within the range of the error of the results, can be ascribed to a paramagnetic interaction between the magnetic ions or particles (Fig. 10 and Table 1). From the straight line the magnetic moment, eff , per ion and p are calculated (Table 1). The values of the effective magnetic moment per ion (Table 1), eff , are lower than the values of 5.92 and 4.9 ␮B of the Fe3+ and Fe2+ free ions, respectively [15,16]. These low values of the ␮B suggest that the magnetic ions are antiferromagnetically coupled in these samples or as a consequence of some type of canting of the magnetic moment of the ions located on the surface of the particles due to the small size of the particles (Fig. 2). The sample with x = 10, heat-treated at 500 ◦ C, shows a different behaviour. In the studied temperature range it is not possible to fit

Fig. 4. XRD microanalysis (EDS) of the 2.5Fe2 O3 –5MgO–92.5SiO2 sample heat-treated at 500 ◦ C.

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Table 2 The maximum magnetization (Mmax ) at 5 K and 10 T for the samples heat-treated at 500 ◦ C, Mmax (emu g−1 ), and the magnetic saturation for the samples treated at 1000 ◦ C, Ms (emu g−1 ). The magnetic saturation, MsMF (emu g−1 MgFe2 O4 ).

100

T(%)

Sample

Heat-treated at 500 ◦ C Mmax (emu g−1 )

80

1.25Fe2 O3 –5MgO–93.75SiO2 2.5Fe2 O3 –5MgO–92.5SiO2 5Fe2 O3 –5MgO–90SiO2 10Fe2 O3 –5MgO–85SiO2

60

20

0 700

800

900

1000

1100

1200

wavelength (nm) Fig. 5. vis–NIR spectra of the 1.25Fe2 O3 –5MgO–93.75SiO2 2.5Fe2 O3 –5MgO–92.5SiO2 samples heat treated at 500 ◦ C.

Ms (emu g−1 )

MsMF (emu g−1 of MgFe2 O4 )

2.09 3.48 6.58 3.98 (Mmax )

49.7 42.7 42.7 21.3 (Mmax )

the formation of MgFe2 O4 (MF). Thus, even assuming that all the possible ferrite MgFe2 O4 is grown it remains a significant amount of iron oxide that can be segregated as nanoparticles. In fact, only a small part of iron oxide is structurally inserted as Fe3+ in the gel–glass matrixes [14]. The value of the normalized magnetic susceptibility of this sample is the lowest of the samples heat-treated at 500 ◦ C (Fig. 9). Despite the positive contribution of iron oxide that is structurally inserted as Fe3+ or as iron oxide nanoparticles in the glass matrix the increase of the ferrimagnetic interaction, between the iron ions inside the magnesium ferrite, can justify the decrease of the normalized susceptibility and the eff , of the samples heat-treated at 500 ◦ C with the increase of the iron oxide concentration. The isothermal magnetization measurements were taken at different temperatures (5, 10, 100 and 300 K) from −10 up to 10 T. Fig. 11 represents the magnetic moment (emu g−1 ) vs. B (T), at 5 K, of the samples heat-treated at 500 ◦ C. It was verified that the magnetization of saturation is not reached and small hysteresis loops were observed at temperatures below 10 K (Fig. 11 and Table 3 ) for the samples with x ≥ 2.5, due to the slow response of the magnetic moments to the applied magnetic field, suggesting a ferrimagnetic behaviour. These results indicate the existence of two kinds of magnetic interactions: a ferrimagnetic interaction inside the nanoparticles and a paramagnetic interaction between the nanoparticles. The paramagnetic behaviour can be due to the large distance between the nanoparticles (Fig. 2) or the low value of the magnetization. The no detection, by XRD, of MF can be ascribed

40

600

4.54 3.46 6.02 5.14

Heat-treated at 1000 ◦ C

and

the results −1 vs. T with a straight line (Fig. 10) and the overlap between the ZFC and FC curves is observed, only, at temperatures above 100 K (Fig. 8). Moreover, the XRD (Fig. 1) and SEM results show the presence of MF and/or H particles. On the other hand for x = 10 the iron oxide concentration cannot be completely used in

800ºC

1200

MF - magnesium ferrite 1000

MF

MF

Intensity (a.u.)

5Fe2O3-5MgO-90SiO2 800

MF 800ºC

600

400

2.5Fe2O3-5MgO-92.5SiO2

200 1.25Fe2O3-5MgO-93.75SiO2 0 5

15

25

35

45

55

65

75

θ Fig. 6. XRD patterns of the 1.25Fe2 O3 –5MgO–93.75SiO2 , 2.5Fe2 O3 –5MgO–92.5SiO2 , 5Fe2 O3 –5MgO–90SiO2 and 5Fe2 O3 –5MgO–90SiO2 samples heat-treated at 800 ◦ C.

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FC- 1000

1.E-03

Susc (emu/(g.Gauss))

1.E-03

ZFC- 1000

1.E-03 8.E-04 6.E-04 4.E-04

FC- 500 ZFC-500

2.E-04 0.E+00 0

50

100

150

200

250

300

T(K) Fig. 7. Temperature dependence of the magnetic susceptibility,  = M B−1 (M – emu, B – 100 G) per g, of the 1.25Fe2 O3 –5MgO–93.75SiO2 sample heat-treated at 500 and 1000 ◦ C, for zero field cooled (ZFC) and field cooled (FC).

2.0E-03

Susc (emu/(g.Gauss))

1.8E-03

FC 1000

1.6E-03 1.4E-03

ZFC 1000

1.2E-03 1.0E-03 8.0E-04 6.0E-04 4.0E-04

FC 500

2.0E-04

ZFC 500

0.0E+00 0

100

50

150

200

250

300

T(K) Fig. 8. Temperature dependence of the magnetic susceptibility,  = M B−1 (M – emu, B – 100 G) per mole, of the 10Fe2 O3 –5MgO–85at-treated at 500 and 1000 ◦ C, for zero field cooled (ZFC) and field cooled (FC).

to an incipient crystallization which may explain the low ferrimagnetic behaviour observed at high magnetic fields (Fig. 11) and the paramagnetic behaviour observed by the susceptibility measurements (Fig. 10). For the sample with x = 1.25 the coercivity (Bc ) and retentivity (Mr ) are equal to zero, and the samples with x = 2.5 and 5 have low values for Bc and Mr at temperatures below or equal to 10 K. At these temperatures the Bc and Mr values are greater for the sample with x = 10 and are zero at 300 K for all samples heat-treated at 500 ◦ C.

The maximum values of the magnetization (Mmax ) of the samples heat-treated at 500 ◦ C, measured at 5 K and 10 T, are 4.74, 3.46, 6.02 and 5.14 emu g−1 for the samples with x = 1.25, 2.5, 5 and 10, respectively (Fig. 11 and Table 2). These values agree with the measured values for the dc magnetic susceptibility. Although the sample with x = 1.25 has the lowest concentration of iron oxide its magnetization is larger than the magnetization of the sample with x = 2.5. The largest value of the magnetization is observed for the sample with x = 5 but it is not the double of the Mmax of the sample

1.4E-04

Susc N (emu/(g.gauss))

1.2E-04 1.0E-04 8.0E-05

1.25 Fe 2.5 Fe

6.0E-05

5Fe 10Fe

4.0E-05 2.0E-05 0.0E+00

0

50

100

150

200

T(K) Fig. 9. Temperature dependence of the zero field cooled (ZFC) normalized magnetic susceptibility,  (T), of the xFe2 O3 –5MgO–(95 − x)SiO2 samples heat-treated at 500 ◦ C.

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269

2.5E+05

Susc-1 (emu/(g.Gauss))-1

1.25Fe 2.0E+05

2.5Fe

1.5E+05 5Fe

1.0E+05

5.0E+04

0.0E+00

10Fe

0

30

60

90

120

150

180

210

240

T(K) Fig. 10. Temperature dependence of the inverse of the magnetic susceptibility curve, susc−1 vs. T of the samples heat-treated at 500 ◦ C.

Table 3 The magnetic saturation (Ms ), the maximum magnetization (Mmax ), the coercivity (Bc ) and retentivity (Mr ) at different temperatures. x 1.25 2.5

5

500 ◦ C

10

1.25

2.5 1000 ◦ C

5

10

Measurement T (K)

Ms (emu g−1 )

Bc (T)

Mr (emu g−1 )

5/10/100/300 5 10 100/300 5 10 100–300 5 10 300

– – – – – – – – – –

0 0.03 0.01 0 0.06 0.04 0 0.13/−0.18 0.15/0.12 0

0 0.05 0.02 0 0.09 0.03 0 0.22 0.16 0

5 100 300 5 100 300 5 100 300 5 100 300

2.09 1.69 (Mmax ) 1.30 (Mmax ) 3.48 3.05 (Mmax ) 2.39 (Mmax ) 6.58 6.16 (Mmax ) 4.88 (Mmax ) – – –

0.01 0.01 0.004 0.01 0.01 0.004 0.017 0.01 0.01 0.03 0.02 0.01

0.45 0.23 0.15 0.65 0.60 0.16 0.97 0.78 0.53 0.93 0.51 0.18

with x = 2.5. On the other hand the value of Mmax of the sample with x = 10 is lower than the Mmax of the sample with x = 5. The size of the hysteresis loops increases with the iron oxide concentration. This behaviour can be related with the different interactions between the iron ions as discussed before, for the magnetic susceptibility. It is possible to represent the isothermal magnetization as a function of a more convenient variable B T−1 (magnetic 5

6

10

Moment (emu/g)

-5

0

5

10

-3 -5

10K

4

2.5

2 -10

5

1.25

4

-1

field/temperature) [15,17]. Fig. 12 shows the collapse on a single curve of the magnetic moment vs. B T−1 of the sample with x = 1.25 heat treated at 500 ◦ C. This behaviour is characteristic of a paramagnetic interaction between the magnetic moments [17]. The absence of collapse of M vs. (B T−1 ) curves for the others samples heattreated at 500 ◦ C, suggests antiferromagnetic interactions between the magnetic ions. Fig. 13 is an example for the sample with x = 2.5.

5K

3

Moment (emu/g)

Heat-treated at

2

50K

1 -2.0E+04

-1.5E+04

-1.0E+04

-5.0E+03

100K

200K 0 0.0E +00 5.0E+03 -1

1.0E+04

1.5E+04

2.0E+04

-2 -3 -4

-7

H(Tesla) Fig. 11. Magnetization vs. B at 5 K, of the samples xFe2 O3 –5MgO–(95 − x)SiO2 heattreated at 500 ◦ C.

-5

B/T Fig. 12. Magnetization vs. B T−1 (magnetic field/temperature), of the sample 1.25Fe2 O3 –5MgO–93.75SiO2 heat-treated at 500 ◦ C.

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10K

3

5K

Momet (emu/g)

2 50K 1

-2.0E+04

-1.5E+04

-1.0E+04

-5.0E+03

100K 0 0.0E+00 5.0E+03 -1

1.0E+04

1.5E+04

2.0E+04

-2 -3 -4

B/T

Fig. 13. Magnetization vs. B T−1 (magnetic field/temperature), of the sample 2.5Fe2 O3 –5MgO–92.5SiO2 heat-treated at 500 ◦ C.

In the samples, heat-treated at 1000 ◦ C, magnesium ferrite (MF) was detected by XRD and SEM (Figs. 14 and 15). Nevertheless, in the sample with composition of 10Fe2 O3 –5MgO–85SiO2 was found the presence of magnesium ferrite and hematite (Fig. 14). The average size of MF particles, determined by the Scherrer method, are 8 and 10 nm for samples with x ≤ 5 and x = 10, respectively. However, the H particles presents in the 10Fe2 O3 –5MgO–85SiO2 sample have an average size of 55 nm. The magnetic susceptibility, of the samples heat-treated at 500 ◦ C, is very much lower than that observed in the samples, with the same composition, heat-treated at 1000 ◦ C (Figs. 7 and 8). The ZFC susceptibility vs. temperature of the samples heat-treated at 1000 ◦ C shows a well-defined broad peak at 20 K (Figs. 7 and 8). For these samples with x ≤ 5 the ZFC and FC curves diverge at temperatures below 60 K (Fig. 7) and for the sample with x = 10, this divergence starts at 160 K. This temperature is normally named blocking temperature, TB . The samples heat-treated at 1000 ◦ C, with x = 1.25, 2.5 and 5, have the magnetization of saturation of 2.09, 3.48 and 6.58 emu g−1 , respectively, at 5 K. These results show that the magnetization of saturation increases almost linearly with the increasing of the iron oxide concentration. The sample with x = 10 does not reach the magnetization of saturation (Fig. 16) and the maximum magnetization observed, 3.98 emu g−1 (Fig. 16). This behaviour can be due to the magnetic interaction inside the particles of magnesium ferrite

Fig. 15. SEM micrographs of 1.25Fe2 O3 –5MgO–93.75SiO2 10Fe2 O3 –5MgO–85SiO2 samples heat-treated at 1000 ◦ C.

1800 1600

H

1400

H

MF+H

MF+H H

MF

1200

Intensity (a.u.)

MF - magnesium ferrite H - hematite

MF+H

H

H MF

MF

10Fe2O3-5MgO-85SiO2

1000 5Fe2O3-5MgO-90SiO2

800 600

2.5Fe2O3-5MgO-92.5SiO2

400 1.25Fe2O3-5MgO-93.75SiO2

200 0

4

14

24

34

44

54

2θ Fig. 14. XRD patterns of all the samples heat-treated at 1000 ◦ C.

64

74

and

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8

5

Moment (emu/g)

6 4

2.5

2

10 1.25

0 -10

-5

-2

0

5

10

-4 -6 -8

B (Tesla) Fig. 16. Magnetization vs. B at 5 K, of the samples xFe2 O3 –5MgO–(95 − x)SiO2 heattreated at 1000 ◦ C.

and hematite (XRD – Fig. 14) and the distance between the particles (Fig. 15). Table 3 shows the values of the magnetization of saturation (Ms ), the maximum magnetization (Mmax ), the coercivity (Bc ) and retentivity (Mr ) at different temperatures. At room temperature the coercivity (Bc ) and retentivity (Mr ) have low values for all samples. These values increase with the iron oxide concentration and the decrease of the temperature of measurement. If we compare the values of the maximum magnetization (Mmax ) with the magnetization of saturation (Ms ) of the samples heat treated at 500 and 1000 ◦ C, respectively (Figs. 11 and 16 and Table 2), we can see different magnetic behaviours. One difference is that the Ms , observed for the samples heat-treated at 1000 ◦ C, is achieved at low magnetic fields while for the samples heat-treated at 500 ◦ C the magnetization increases even with applied field of 10 T (Figs. 11 and 16). Probably in the samples heat treated at 500 ◦ C the ferrimagnetic interactions between the magnetic ions inside the particles are not very strong and the magnetic moment (Mmax ) can be high. The samples with x = 1.25 has 4.54 emu g−1 for Mmax and 2.09 emu g−1 for Ms , when heat-treated at 500 and 1000 ◦ C, respectively. The low value of this Ms in comparison with the Mmax of sample heat-treated at 500 ◦ C can be due to different ferrimagnetic interactions between the two sub lattices A and B in magnesium ferrite. Bulk MgFe2 O4 is a ferrimagnetic material that crystallizes in the normal spinel structure which contains two different cations sites: eight tetrahedral A and 16 octahedral B sites per f.u. In a fully inverse spinel structure half of the trivalent cations fully occupy the A sites while the B sites are shared by the divalent cations and the remaining half of the trivalent cations. Mixtures between the two configurations occur and are characterized by the degree of inversion which depends strongly on the preparation procedures [1,5]. The magnesium ferrite may have non-zero inversion degrees which leads to different magnetic properties. Namely, they may show ferrimagnetism combined with superparamagnetism. The value of the magnetization of the magnesium ferrite depends strongly on the distribution of the different cations between A and B sites which depends on the shape and size of the nanoparticles and the thermal history of the sample [3,5]. The high values obtained for the magnetization of saturation (Ms ) with low applied fields (Fig. 16) and the low coercive field (Fig. 16 and Table 3) observed for the samples with x ≤ 5 heat-treated at 1000 ◦ C are characteristic of both superparamagnetic and spin–glass systems. The measured values of MsMF (emu (g of MgFe2 O4 )−1 ), Table 3, for the samples with x ≤ 5 are larger relatively to the values of the bulk magnesium ferrite and similar to the values obtained for nanoparticles prepared by others methods [3,5].

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The difference between the Mmax and Ms are lower for the samples with x = 2.5 and 5 (Table 2). The difference between the values for Bc and Mr (Table 3) of the samples, with the same composition heat-treated at 500 and 1000 ◦ C, confirms the paramagnetic behaviour of the samples with x ≤ 5 heat-treated at 500 ◦ C and the soft ferrimagnetic behaviour of the samples heat-treated at 1000 ◦ C. For samples with x = 10 heat-treated at 500 and 1000 ◦ C the difference between the Mmax is large (5.14 and 3.98 emu g−1 , respectively). This difference can be justified by the different crystallization of these samples. The low Mmax value of the sample with x = 10 relatively to the Ms of the samples with x = 2.5 and 5 heat-treated at 1000 ◦ C, can be associated with a greater inversion degree of the magnesium ferrite due to the larger particles size. We believe that the presence of the hematite favours the antiparallel alignment of the iron ions within the magnesium ferrites which is responsible for the decrease of magnetization. In Figs. 7 and 8 the observed broad peak may be associated with the superparamagnetic blocking temperature, TB , of the nanoparticles. This behaviour is due to the progressive magnetic blocking of these particles whose size determines a magnetic anisotropy comparable to thermal energy. In spin glasses, the FC susceptibility curve, for the temperatures below the bifurcation point is almost flat but in our data (Figs. 7 and 8) the magnetization continues to rise, which is characteristic of a superparamagnetic system. 4. Conclusions 4.1. Samples heat-treated at 500 ◦ C The samples xFe2 O3 –5MgO–(95 − x)SiO2 , with x ≤ 5, have a paramagnetic interaction between the magnetic nanoparticles, p = 0. This paramagnetic behaviour can be due to the great distance between the nanoparticles and the small value of the magnetization. The small hysteresis loops observed at temperatures ≤10 K suggest ferrimagnetic behaviour. These results indicate the existence, of two kinds of magnetic interactions: a ferrimagnetic interaction inside the nanoparticles and a paramagnetic interaction between the nanoparticles. These results are in agreement with the incipient crystallization of magnesium ferrite nanoparticles. The presence of magnesium ferrite phase was not confirmed by XRD. The values of the normalized magnetic susceptibility are greater for the sample with x = 1.25 and are almost the same for the samples with x = 2.5 and 5. The increasing of the ferrimagnetic interactions, with the increase of iron oxide concentration, can justify the decrease of the normalized susceptibility. In the sample with x = 10 the iron concentration is too large to be completely used in the formation of MgFe2 O4 . It remains a significant amount of iron that can be segregated as iron oxide particles and the value of the normalized magnetic susceptibility is the lowest of the samples heat-treated at 500 ◦ C. The low Mmax value of the sample with x = 10 relatively to the Mmax of the samples with x = 2.5 and 5, can be associated with a greater inversion degree of the magnesium ferrite. The presence of the hematite favours the antiparallel alignment of the iron ions within the magnesium ferrites, responsible for the decrease of magnetization. 4.2. Samples heat-treated at 1000 ◦ C The easiness with which the samples heat-treated at 1000 ◦ C reach the saturation magnetization can be ascribed to the existence of magnesium ferrite nanoparticles. The magnesium ferrite may have non-zero inversion degrees which leads to different magnetic

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properties. Namely, they may show ferrimagnetism combined with superparamagnetism. The high values of the saturation magnetization at low applied fields and the low coercive field observed for the samples with x ≤ 5 are characteristic of superparamagnetic systems. The low Mmax value of the sample with x = 10 relatively to the Ms of the samples with x = 2.5 and 5, can be associated with a greater inversion degree of the magnesium ferrite due to the increased size of the particles. The presence of the hematite, favours the antiparallel alignment of the iron ions within the magnesium ferrites and the decrease of magnetization. Acknowledgements The authors thank Julia Manfroi (Brazilian student) for the help in this research work, CICECO and I3N for the financial support. References [1] A. Pradeep, P. Priyadharsini, G. Chandrasekaran, J. Magn. Magn. Mater. 320 (2008) 2774–27799. [2] M. Kubota, Y. Kanazawa, K. Nasu, S. Moritake, H. Kawaji, T. Atake, Y. Ichiyanagi, J. Therm. Anal. Cal. 92 (2008) 461–463.

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