New nanoparticulate Gd1−xZrxFe1−yMnyO3 multiferroics: Synthesis, characterization and evaluation of electrical, dielectric and magnetic parameters

Journal of Alloys and Compounds 585 (2014) 790–794

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New nanoparticulate Gd1xZrxFe1yMnyO3 multiferroics: Synthesis, characterization and evaluation of electrical, dielectric and magnetic parameters Aneela Sultan a, Azhar Mahmood a, Nazia K. Goraya b, Ashfaq M. Qureshi b, Iqbal Ahmad c, Muhammad N. Ashiq b,⇑, Imran Shakir d, Muhammad F. Warsi a,⇑ a

Department of Chemistry, The Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan Institute of Chemical Sciences, Bahauddin Zakariya University, Multan 60880, Pakistan c Department of Chemistry, University of Gujrat, Gujrat, Pakistan d Deanship of Scientific Research, College of Engineering, PO Box 800, King Saud University, Riyadh 11421, Saudi Arabia b

a r t i c l e

i n f o

Article history: Received 19 September 2013 Received in revised form 1 October 2013 Accepted 3 October 2013 Available online 12 October 2013 Keywords: Nanostructures Chemical synthesis Dielectric properties Electrical properties Magnetic materials

a b s t r a c t Nanoparticles of Gd1xZrxFe1yMnyO3 (x, y = 0–1) were synthesized by micro-emulsion method. The structural elucidation was accomplished by X-ray diffraction (XRD), Fourier transformed infrared spectroscopy, scanning electron microscopy (SEM) and thermogravimetric analysis that confirmed that all the nanoparticles are crystalline in orthorhombic phase. The particles size was found in the range 12–48 nm (determined by XRD) and 33–48 nm (estimated by SEM). The nanoparticles were then evaluated for electrical, dielectric and magnetic properties. The electrical resistivity studies exhibited the transition between metal to semiconductor in the range 340–380 K, besides overall electrical resistivity was decreased with the increased Zr–Mn contents. The maximum electrical resistivity (80.95  108 X cm) was exhibited by Gd0.75Zr0.25Fe0.75Mn0.25O3 nanoparticles. The dielectric behavior was found to increase with increased Zr–Mn contents. The magnetic behavior confirmed the transition of magnetic order from antiferromagnetic to the ferromagnetic as the Zr and Mn contents were increased. The new structurally stable nanostructured multiferroics can be utilized for fabricating high frequency and magnetic recording devices. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Nanomaterials are a new class of advanced multifunctional materials that show very interesting and fascinating properties considerably different from the respective bulk materials [1–3]. For example the surface plasmon band, a characteristic property of metal nanoparticles, is not exhibited by the bulk metals [4]. Therefore nano-science and nanotechnology have attracted a significant number of scientists of all disciplines since last two decades [5–7]. Among various nanomaterials, the transition metal oxides have a wide range of applications due to inherent rich chemistry that is attributed to their variable oxidation state. Further, ferrites, the metal oxides that contain more than 50% iron, have very important technological applications like in telecommunication, microwave devices etc [8–12]. The perovskites are another class of metal oxides with potential applications as electrode materials in fabrication of energy storage related devices [13,14]. Researchers are always searching a single material that possesses as much different functionalities as possible. To enhance

⇑ Corresponding authors. Tel.: +92 (0) 3455411391; fax: +92 (0) 62 92 55474. E-mail address: [email protected] (M.F. Warsi). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.10.018

the applications spectrum and meet the new generation needs, nanostructured metal oxides are under extensive research. Metal oxides are important because they have versatile applications. For example LiMn2O4 and its derivatives have been evaluated for applications as electrode materials in Li-ion batteries [15]. On the other hand, several metal oxides are reported as dielectric materials that have applications in high frequency devices [16,17]. Here in this article, we report the GdFeO3 system and its derivatives i.e. Gd1xZrxFe1yMnyO3 in the nanoscale range for evaluation of their dielectric and magnetic behavior. The main purpose of the study is to observe the effect of Zr and Mn substitution on the crystalline structure as well as on various physical, electrical, dielectric and magnetic behaviors of nanostructured perovskites.

2. Experimental 2.1. Materials/chemicals The chemicals used for the synthesis of GdFeO3 and its Zr–Mn substituted derivatives were GdCl36H2O (Sigma–Aldrich, 99%), Fe(NO3)39H2O (Sigma–Aldrich, 98%), MnCl24H2O (Sigma–Aldrich, 99%), ZrOCl24H2O (BDH, 96%), Cetlytrimethylammonium bromide (CTAB) (Fluka, 98%) and aqueous ammonia (BDH, 35%). These chemicals were used as such without any further treatment.

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A. Sultan et al. / Journal of Alloys and Compounds 585 (2014) 790–794 2.2. Experimental procedure The Gd1xZrxFe1yMnyO3 nanoparticles were synthesized by the normal microemulsion method [8,18]. The stoichiometric amounts of metal salts were dissolved in the deionized water to prepare the solutions of required molarities. All these aqueous solutions were mixed in a beaker and stirred on a hot plate. Then cetyltrimethylammoniumbromide (CTAB) solution was added by maintaining a ratio of 1:1.5 between metals and CTAB. Aqueous ammonia solution (2.0 M) was added drop wise to get the precipitates while keeping the pH between 10 and 11. After mixing the mixture was stirred four hours. After that the precipitates were washed several times with deionized water till the pH reached to neutral level i.e. 7. These precipitates were finally washed with methanol. The precipitates were then dried in an oven at 100 °C and finally annealed at 700 °C for 8 h in muffle furnace model Vulcan A-550.

2.3. Characterization The thermogravimetric analysis (TGA) for the un-annealed GdFeO3 sample was carried out using thermal analyzer (SDT Q600 V8.2 Build 100). The sample was heated with the heating rate of 10 °C min-1. The FTIR analysis was carried out to investigate the formation of Gd1xZrxFe1yMnyO3 using Nexus 470 spectrometer. The crystallinty and the phase purity of the synthesized materials were confirmed by Philips X’ Pert PRO 3040/60 X-Ray diffractometer that uses the Cu Ka as radiation source. The SEM/EDX analysis of nanoparticles was done to estimate the surface morphology and composition using Jeol JSM-6490A electron microscope. The dc electrical resistivity was measured by the two point probe method in the temperature range of 300–700 K by using kiethly-2400 source meter. The dielectric measurements were performed at room temperature using Wayn ker WK6500B Precision instrument. The pellets of 13 mm diameter and of 2.5 mm thickness were used for the resistivity and dielectric measurements. The vibrating sample magnetometer (VSM) Lakeshore-74071 was used to measure the hysteresis loops and various magnetic properties such as magnetization, retentivity and coercivity were determined [19].

3. Results and discussion 3.1. Structural analysis The thermogravimetric (TG) curve for the unannealed sample Gd0.75Zr0.25Fe0.75Mn0.25O3) is shown in Fig. 1. The curve shows the weight loss in three stages. The weight loss at 208 °C is due to the loss of hydrated water which was trapped in the pores of the synthesized materials. The weight loss at 276 °C may be attributed to the decomposition of organic contents such of cetyltrimethylammoniumbromide (CTAB) which was used as template during the synthesis process. The last weight loss at 417 °C is due to the formation of oxides from the hydroxides of various metals and the beginning of the formation of orthorhombic phase [19–21]. All other samples are annealed at 700 °C to obtained pure orthorhombic phase.

The XRD patterns for Gd1xZrxFe1yMnyO3 (where x, y = 0.0, 0.25, 0.5, 0.75 and 1.0) are presented in Fig. 2. The XRD peaks with h k l 0 2 0, 1 1 2, 2 0 2, 0 0 4 and 2 0 4 appeared at 2h values 31.82, 32.84, 40.90, 47.40 and 59.26, respectively perfectly matched with the standard pattern (ICSD 00-015-0196). This indicates that the synthesized materials are formed in single orthorhombic phase without any other impurity. The lattice constant (a), cell volume (Vcell), X-ray density (qX-ray) and crystallite size have also been calculated from the XRD data using their respective relations [8] and their values are given in Table 1. The lattice constant and cell volume decrease with the increase in Zr–Mn concentration (Table 1). The reduction in cell parameters is due to the smaller ionic radius of substituted Zr4+ (0.80 Å) as compared to that of Gd3+ (0.94 Å). Although the doped Mn2+ (0.72 Å) has greater ionic radius than Fe3+ (0.64 Å) but the collective effect of Zr–Mn results in the reduction of cell parameters. The other reason for the decrease in the cell parameters may be due to the higher interaction of Zr4+ with oxygen as compared to Gd3+ which results in the contraction of lattice constants [22]. The X-ray density is calculated using following relation [23].

qXray ¼

ZM NA V cell

ð1Þ

where Z is the number of formula units in a unit cell, M is the molar mass of the sample, NA is the Avogadro’s number and Vcell is the unit cell volume. The value of X-ray density (Table 1) increases with the Zr–Mn contents which is due to the reduction in cell volume as the X-ray density and cell volume are inversely related with each other (Eq. (1)). The bulk density and porosity are also calculated and their values are given in Table 1. The bulk density decreases with the substituents indicating that the un-doped material is denser than that of doped one. The crystallite size is also calculated using Scherer’s formula [24]. The crystallite size is found in the range of 12–48 nm which is much smaller as compared to other reports regarding substituted ferrites nanoparticles i.e. 70 nm [25], 40–65 nm [26], 30–47 [23], 36–58 nm [27]. This particles size was found in agreement with the particles size (33–48 nm) as estimated by SEM images (electronic Supplementary data). The crystallite size plays an important role in the electronic devices to obtain suitable signal-to-noise ratio. The crystallite size below 50 nm is required in obtaining the suitable signal-to-noise ratio [28]. In the present investigation, the crystallite size is below 50 nm indicating that the synthesized materials can have potential applications in recording media devices fabrication in obtaining the suitable signal-to-noise ratio.

x, y = 1.0

Intensity

x, y = 0.75

x, y = 0.50

x, y = 0.25 x, y = 0

10

20

30

40

50

60

70

80

2 theta / degree Fig. 1. TGA graph of ‘‘Gd1xZrxFe1yMnyO3’’ nanoparticles.

Fig. 2. XRD Patterns of ‘‘Gd1xZrxFe1yMnyO3’’ nanoparticles annealed at 700 °C.

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Table 1 Lattice constants (a, b and c), cell volume, bulk density, X-ray density, porosity and crystallite size of Gd1xZrxFe1yMnyO3 nanoparticles. Parameters

Lattice Constant/a (Å) Lattice Constant/b (Å) Lattice Constant/c (Å) Cell Volume/Vcell (Å3) Bulk Density/qbulk (g cm3) X-ray Density/qx-ray (g cm3) Porosity Crystallite size/D (nm)

x=0

x = 0.25

x = 0.50

x = 0.75

x = 1.0

y=0

y = 0.25

y = 0.50

y = 0.75

y = 1.0

5.3678 5.6345 7.6780 232.2201 2.85 3.93 0.2748 48.38

5.3667 5.6340 7.6776 232.1398 2.77 3.96 0.2729 26.45

5.3652 5.6333 7.6770 232.028 2.57 4.02 0.2900 16.231

5.3634 5.6325 7.6762 231.893 2.48 4.15 0.2552 12.17

5.3629 5.6318 7.6758 231.8305 2.35 4.22 0.2539 16.22

Table 2 The elemental composition of Gd1xZrxFe1yMnyO3 nanoparticles as determined by EDX analysis. Elements (mass %)

Gd Zr Fe Mn

Theoretical Observed Theoretical Observed Theoretical Observed Theoretical Observed

x=0

x = 0.25

x = 0.50

x = 0.75

x = 1.0

y=0

y = 0.25

y = 0.50

y = 0.75

y = 1.0

47.15 51.95 – – 33.63 31.61 – –

37.26 35.17 7.20 7.80 26.58 25.90 8.70 8.25

26.25 26.90 15.22 14.63 18.79 19.05 18.39 18.71

13.92 14.21 24.20 25.60 9.92 10.15 29.25 28.37

– – 34.34 28.50 – – 41.50 38.50

Fig. 4. Effect of Mn and Zr content on resistivity of ‘‘Gd1xZrxFe1yMnyO3’’ nanoparticles.

Fig. 3. Effect of temperature on resistivity of ‘‘Gd1xZrxFe1yMnyO3’’ nanoparticles.

The composition of the synthesized materials is confirmed by the EDX analysis and the theoretical and experimental mass % of each element is given in Table 2. It is clear from the Table that theoretical and observed values of mass % for the substituted and host elements are in good agreement with each other. The contents of Fe and Gd are found to decrease as Zr–Mn contents were increased. This confirmed the successful substitution of Gd and Fe with Zr and Mn respectively in the lattice. The reduction in the values of lattice parameters also indicate that the substituents are doped in the perovskites lattices. The FTIR spectrum of Gd1xZrxFe1yMnyO3 (x, y = 0.5) (electronic Supplementary information, Figure S1) exhibited the metal oxygen bands at 414 cm1 (Gd–O), 465 cm1 (Zr–O), 506 cm1 (Mn–O), 480 cm1 (Fe–O) [8,19]. The typical SEM image of the prepared nanoparticles is shown in the electronic Supplementary information (Figure S2). The SEM image shows that the nanoparticles have narrow size distribution and are spherical in morphology. Some aggregated nanoparticles could also be seen in SEM image.

Fig. 5. Effect of frequency on the dielectric constant of ‘‘Gd1xZrxFe1yMnyO3’’ nanoparticles.

3.2. Electrical and dielectric properties The DC electrical resistivity of all the synthesized nanomaterials has been determined by the two-point probe method in the temperature range of 300–675 K. The variation of dc resistivity with temperature is shown in Fig. 3. It is clear from the Figure that the resistivity increases with the temperature showing the metallic behavior and reached to a maximum value at specific temperature known as transition temperature. After the transition temperature the resistivity starts to decrease with temperature showing the

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Fig. 6. Effect of frequency on the dielectric loss of ‘‘Gd1xZrxFe1yMnyO3’’ nanoparticles.

Fig. 7. Effect of frequency on the ‘‘tan d’’ of ‘‘Gd1xZrxFe1yMnyO3’’ nanoparticles.

semiconducting nature of the ferrites materials. Such type of resistivity behavior with temperature also observed in spinel as well as hexagonal ferrites and our results are in agreement with the already reported in the literature [28–30]. Such metal to semiconductor transition takes place due to phase transition in the materials but at such low temperature there is no possibility of phase transition. This type of behavior occurs in ferrites due to the spin canting i.e. the angle of electron spin changed with the increase in temperature and substituents concentration. The room temperature resistivity as a function of Zr–Mn concentration is shown in Fig. 4. The dc resistivity of the material decreases with the increase in Zr–Mn content. The conduction

GdFeO3 is due to the hopping of electrons between ferrous and ferric ions. When Mn2+ replaces the iron ions, although the numbers of iron ions decrease but Mn2+ also oxidize to Mn3+ producing holes. Now the conduction is due to hopping of electrons as well as transfer of holes, so collectively it results in the reduction of dc resistivity. The significant decrease in electrical resistivity from 28.81  108 X cm to 2.81  108 X cm suggested that the new nanostructured perovskites can be utilized for switching applications. The variation of dielectric constant, dielectric loss and tangent loss with frequency is shown in Figs. 5–7 respectively. All these parameters are decreased with the increase in frequency. The reduction of these parameters is rapid at low frequency which become slower and then almost become constant at higher frequencies. This is the normal behavior of ferrites which is already observed by many researchers and is explained on the basis of Maxwell-Wagner model as reported earlier [8,31]. With increasing the frequency of externally applied electric field, the electronic exchange between Fe2+ and Fe3+ as well as Mn2+ and Mn3+ ions cannot follow the alternating field, resulting in deterioration in dielectric constant. Space charge carriers in a dielectric materials such as multiferroics requires finite time to line up their axes parallel to an alternating electric field, if the frequency of the field reversal increases, a point will be reached when the space charge carriers cannot keep up with the applied external field and the alternation of the field, thus resulting in a reduction in the dielectric constant of the material. The compositional dependence of dielectric constant, dielectric loss and tangent loss at three different frequencies (0.25, 0.5 and 5 MHz) are given in Table 3. All these parameters are found to increase with the increase in Zr–Mn contents. It has been reported that the dielectric polarization mechanism is similar to that of electrical conduction process [32]. The increase in dielectric constant and dielectric loss is due to the decrease in resistivity of the synthesized materials as discussed above. The dielectric and dc resistivity data are in good agreement with each other.

3.3. Magnetic properties The hysteresis loops for Gd1xZrxFe1yMnyO3 nanoparticles are shown in Fig. 8. The values of saturation magnetization (Ms), retentivity (Mr) and coercivity (Hc) are calculated from the hysteresis loops and their values are given in Table 4. The antiferromagnetic behavior was exhibited (Fig. 8) by Gd1xZrxFe1yMnyO3 (x, y = 0). Further this sample has very small value of magnetization (0.85 emu g1) and remanence (0.03 emu g1) (Table 4). This weak ferromagnetism may be due spin canting. As the Zr–Mn content was increased the ferromagnetic character was also found to increase i.e. the values of magnetization, retentivity and coercivity were increased with the increase in concentration of substituents. It has been reported that rare earth element magnetic moment is

Table 3 Dielectric constant, dielectric loss factor and dielectric loss of Gd1xZrxFe1yMnyO3 nanoparticles at various frequencies. Parameters

Dielectric Constant (103)

Dielectric Loss (103)

Tangent Loss

Frequency

0.25 MHz 0.5 MHz 5 MHz 0.25 MHz 0.5 MHz 5 MHz 0.25 MHz 0.5 MHz 5 MHz

x=0

x = 0.25

x = 0.50

x = 0.75

x = 1.0

y=0

y = 0.25

y = 0.50

y = 0.75

y = 1.0

0.028 0.014 0.012 0.028 0.002 0.0008 0.658 0.185 0.089

0.031 0.015 0.013 0.036 0.003 0.001 0.663 0.186 0.103

0.413 0.019 0.18 0.093 0.008 0.0015 0.87 0.433 0.124

0.617 0.025 0.021 0.144 0.02 0.0023 1.64 0.781 0.216

1.66 0.034 0.027 1.726 0.028 0.003 6.31 0.866 0.327

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funding of this research through the Research Group Project no RGP-VPP-312. We are also thankful, The Islamia University of Bahawalpur-63100 (Pakistan) and Higher Education Commission of Pakistan ((PM-IPFP/HRD/HEC/2011/2264 (M. F. Warsi) & (20-1515/R & D/09-8049 (M. N. Ashiq)) for financial support, Quaid-e-Azam University Islamabad for XRD, FTIR and electrical measurements, NUST-Islamabad for SEM and dielectric measurements, and institute of solid state physics (Punjab University, Lahore) for magnetic measurements. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jallcom.2013.10. 018. References Fig. 8. Hysteresis loops curves for ‘‘Gd1xZrxFe1yMnyO3’’ nanoparticles at room temperature (298 K).

Table 4 Various magnetic parameters of Gd1xZrxFe1yMnyO3 nanoparticles. Magnetic Parameters

Coercivity (Hc)/G Magnetization (Ms) (emu/g) Retentivity (Mr) (emu/g)

x=0

x = 0.25

x = 0.50

x = 0.75

x = 1.0

y=0

y = 0.25

y = 0.50

y = 0.75

y = 1.0

150.1 0.85 0.03

167.7 4.58 0.66

358.4 4.91 1.97

626.2 7.85 2.14

803.7 13.48 4.36

aligned antiparallel to the magnetic moment of iron ions that affect negatively on the magnetization and retentivity [33] and this is the reason that pure sample with x = 0.0, y = 0.0 has very low value of magnetization. The magnetic moments for Fe3+ and Mn2+ is same i.e. 5 lB, therefore there will be no net change in magnetization with manganese substituent. The zirconium ion (Zr4+) is a nonmagnetic with zero magnetic moment. When a nonmagnetic ion replaces the magnetic Gd3+ whose electrons spin is antiparallel to Fe3+/Mn2+ then the total number of aligned spin at iron site increases and as a result the total magnetization and retentivity increased and the material behave as ferromagnetic material. The increase in magnetic properties suggests that materials in the present investigation can be used for data-storage media. 4. Conclusions Gd1xZrxFe1yMnyO3 nanoparticles are synthesized by a cheap route i.e. micro-emulsion method. The dielectric, dc-resistivity and magnetic parameters are evaluated and found in agreement with each other. The dielectric and dc-resistivity showed that these materials can be utilized in electrode fabrications, switching applications technology etc. The magnetic properties suggest that these nanomaterials have potential applications in magnetic recording devices technology. Acknowledgements The authors would like extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for its

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