Structural, magnetic and Mössbauer spectral studies of Sm3+ ions doped Mg ferrites synthesized by solid state reaction technique

Journal of Alloys and Compounds 552 (2013) 264–268

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Structural, magnetic and Mössbauer spectral studies of Sm3+ ions doped Mg ferrites synthesized by solid state reaction technique Jagdish Chand a,b,⇑, Satish Verma a,b, M. Singh a a b

Department of Physics, Himachal Pradesh University, Shimla 171005, India Department of Physics, Govt. P.G. College, Solan, India

a r t i c l e

i n f o

Article history: Received 3 August 2012 Received in revised form 5 October 2012 Accepted 10 October 2012 Available online 23 October 2012 Keywords: Initial permeability Saturation magnetization Mössbauer spectroscopy Retentivity

a b s t r a c t Sm3+ ions doped MgSmxFe2xO4 (x = 0.0, 0.05 and 0.1) ferrites have been synthesized by solid state reaction technique. Micro-structural and magnetic properties have been investigated at room temperature by means of X-ray diffraction, scanning electron microscope, Vibrating sample magnetometer and Mössbauer parameter measurements. The addition of samarium in Mg ferrite has been shown to play a crucial role in improving the dc resistivity and magnetic properties. Lattice constants have been found to be increasing with an increase in samarium ions concentration in Mg ferrite. The values of saturation magnetization, residual magnetization and hyperfine magnetic field have been decreased due to the doping of Sm3+ ions in Mg ferrite. Mössbauer spectra of all the samples exhibit normal Zeeman split sextets show the ferromagnetic behavior of the ferrites. Variations of Mössbauer parameters such as isomer shift, quadrupole splitting and hyperfine magnetic field with Sm3+ ions concentration have been studied and discussed in details. Mössbauer study is also supported by cation distribution, magnetization verses applied field curve and initial permeability measurements. The mechanisms responsible to these results have been discussed in this paper. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Spinel ferrites have been extensively investigated in recent years for their useful electrical, dielectric, magnetic properties, applications in information storage systems, magnetic bulk cores, magnetic fluids, microwave absorbers and high frequency devices [1]. The intrinsic parameters of ferrites depend on the chemical composition, microstructure, sintering parameters and type of additive or substituted ions [2,3]. The excellent combination of high dc resistivity and low loss factor of rare earths doped Mg ferrites can be used to fulfill the future demand for high-frequency applications [4]. The practical applications of nanoferrites are growing in leaps and bounds with the advancement in nanotechnology [5]. High resistivity (107 X-cm) and low dielectric loss factor (103) makes this samarium doped magnesium ferrite suitable for high frequency applications. The dc resistivity of MgSm0.1Fe1.9O4 ferrite has been increased by two times as compared to Mg ferrite [6]. The dielectric loss factor has been decreased due to the doping of Sm3+ ions in Mg ferrite. The important magnetic property of ferromagnetic spinel ferrites mainly depends on the magnetic interactions between cations with magnetic moments that are situated in the tetrahedral (A) and the octahedral (B) sites [7,8]. In the present study, we have ⇑ Corresponding author at: Department of Physics, Himachal Pradesh University, Shimla 171005, India. Tel.: +91 9418461190. E-mail address: [email protected] (J. Chand). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.10.041

investigated the variation of Mössbauer parameters such as isomer shift, quadrupole splitting and hyperfine magnetic field with Sm3+ ions concentration measured at room temperature. In addition to this, the effects of Sm3+ ions doping on the cation distribution and magnetic properties were investigated and reported in the present work.

2. Experimental Ferrites powder of compositions MgGdxFe2xO4 (x = 0.0, 0.05 and 0.1) were prepared by solid state reaction technique. Analytical grade reagents MgO (>97%, Merck, India), Sm2O3 (>99%, Merck, Germany) and Fe2O3 (>98.5%, Loba Chemie, India) were weighted in appropriate proportions and mixed thoroughly by wet blending with de-ionized water in an agate mortar and pestle. The powder is crushed and mixed for 5–10 h in de-ionized water to break it into small crystallites of uniform size. The mixed powders were dried and calcinated at 1073 K for 3 h to improve the homogeneity of the constituents. The rate of heating is maintained at 5.83 °C/ min during samples preparation. The reacted material was well milled by adding a small quantity of polyvinyl alcohol as a binder. The powders were compressed into toroids uniaxially under a pressure of 3–5 ton/in2 in a stainless steel die. The powders and toroids were finally sintered at 1273 K for 3 h at a heating rate of 5.83 °C/min and slowly cool down to room temperature. The phase structure of ferrites was analyzed by XRD (XPERT PRO X-ray diffractometer) using Cu-Ka source. The values of initial permeability and relative loss factor were determined by Agilent Precision LCR meter (Agilent Technologies, Model HP4285A, Japan) in the frequency range 0.1 MHz–30 MHz. The magnetic hysteresis measurements have been performed using a commercial available Vibrating Sample Magnetometer (LakeShore, Model 7140, USA). For the measurements of initial permeability and relative

J. Chand et al. / Journal of Alloys and Compounds 552 (2013) 264–268

265

loss factor (RLF) the toroids were wounds with 55 turns of 32 SWG (Standard Wire Gauge) enameled copper wire. The initial permeability was calculated by using the following relation [9]

li ¼ L=Lo

ð1Þ

where L is the measured inductance of the sample and Lo is the inductance with air core and Lo = 4.6N2d log (OD/ID)  109 H, N being the number of turns, d is the thickness of the toroid in meter and OD and ID are the outer and inner diameters of the toroid respectively. 57Fe Mössbauer measurements were carried out in transmission mode with 57Co/Rh radioactive source in constant acceleration mode using standard PC-based Mössbauer spectrometer equipped with Weissel velocity drive. Velocity calibration was done with natural iron absorber. The spectra were analyzed using least square fitting program NORMOS (SITE/DIST).

3. Results and discussion 3.1. Structural study Fig. 1 shows the X-ray diffraction patterns of the prepared ferrites MgSmxFe2xO4 (x = 0.00, 0.05 and 0.1). All the samples can be indexed as the single-phase cubic spinel structure. The lattice parameters of the prepared ferrites are found to be increasing with the increasing concentration of Sm3+ ions. The lattice parameter a, was calculated by using the following relation [4] 2

2

2

a ¼ dhkl ðh þ k þ l Þ1=2 ;

ð2Þ

where the values of lattice parameter a, for all the samples are calculated. The lattice parameter increases from 8.3798 Å to 8.3845 Å with an increase in Sm3+ ions concentration (x). Such a change in the lattice parameter is expected because Fe3+ ions, which are of smaller radius (0.067 nm), are being progressively replaced with Sm3+ ions, which are of larger size (0.0958 nm). The morphology and size of the grains was studied using a scanning electron microscope. Fig. 2 display SEM images grain size distribution of MgSmxFe2xO4 (x = 0.00 and 0.1) ferrite samples. The average grain size is about 0.1–2 lm. Fig. 2. SEM of MgSmxFe2xO4 (x = 0.0 and 0.1) ferrites power sintered at 1273 K.

3.2. Magnetic study

Fig. 1. XRD patterns of MgSmxFe2xO4 (x = 0.0, 0.05 and 0.1) ferrites samples sintered at 1273 K.

Magnetic characterization of the ferrite samples were done by using vibrating sample magnetometer at room temperature with maximum applied field up to 20 kOe as shown in Fig. 3. The magnetization of pure Mg ferrite sample and Mg–Sm samples attain saturation magnetization in the applied field. For MgSmxFe2xO4 (x = 0.0, 0.05 and 0.1) samples the saturation magnetization, Ms is determined by extrapolating the M vs. I/H curve to I/H = 0 [10]. The values of saturation magnetization and remnant magnetization have been decreased significantly due to the doping of Sm3+ ions in Mg ferrite. The saturation magnetization (Ms) obtained at room temperature has been decreased from 30 emu/g to 8.0611 emu/g and the value of remnant magnetization (Mr) has been decreased from 3.6573 emu/g to 2.30465 emu/g due to the doping of Sm3+ ions in Mg ferrite. This decrease in saturation magnetization and remnant magnetization is attributed to the weak magnetic interactions in Mg–Sm ferrites. The magnetic order in the cubic ferromagnetic spinels is due to super exchange interaction mechanism occurring between the metals ions in the tetrahedral (A) and octahedral (B) sub-lattices. The A–A interactions as well as the B–B interactions exist but they are very weak. Since the A–B interactions are the strongest, it will align all the magnetic moments (spin) at the (A) site in one direction and those at (B) site in the opposite direction, thus constituting two saturated and

266

J. Chand et al. / Journal of Alloys and Compounds 552 (2013) 264–268 40

x = 0.0 x = 0.05 x = 0.1

380

x = 0.0 x = 0.05 x = 0.1

30

360

Initial permeability

M (emu/g)

20 10 0 -10

340 320 300 280

-20 260

-30 -40

240

-20000

-10000

0

10000

0

20000

5

10

15

20

25

30

Frequency (MHz)

Applied Field (Oe)

Ms ¼ MB  MA

ð3Þ

In case of MgSmxFe2xO4 samples the Fe3+ ions are replaced by the Sm3+ ions at the (B) site. The octahedral site preference energy is large for Sm3+ ions as compared to tetrahedral site preference energy. So due to the large octahedral site preference energy Sm3+ ions occupy (B) sites only. Since the magnetic moment of Sm3+ ions is less than Fe3+ ions, a replacement of Fe3+ ions by Sm3+ ions results a decrease in the magnetic moment of the magnetic ions. This results a decrease in the magnetic moment at the (B) sub-lattice, so that the overall total magnetic moment decreases. Fig. 3 shows that saturation magnetization decreases with an increase in Sm3+ ions concentrations. 3.3. Initial permeability The variation of initial permeability (li) of MgSmxFe2xO4 ferrites with frequency measured at room temperature is shown in Fig. 4. The variation of li with frequency can be understood on the basis of Globus model [12,13]. The increase in li above 15 MHz may indicate the beginning of a resonance with peaks occurring at higher frequencies. The resonance occurs due to the matching of applied field frequency with the precession frequency of magnetic spins in ferrites. This matching leads to energy transfer from the field to the ferrite system in orienting the dipoles. Fig. 4 shows that the values of initial permeability decreases with an increase in Sm3+ ions concentration. These variations can be explained from the following dependence of li [13]

3.4. Relative loss factor The variations of relative loss factor (RLF) of MgSmxFe2xO4 ferrites with frequency measured at room temperature are shown in Fig. 5. Relative loss factor (RLF) is expressed as the ratio of magnetic loss to the initial permeability i.e.

Relative loss factor ¼ Magnetic loss=Initial permeability

ð5Þ

The loss is due to the lag of domain walls with respect to the applied alternating field and is attributed to imperfections in the lattice. The values of RLF are observed to decrease initially with frequency, reaching a minimum value, and then become almost constant up to 20 MHz. The decrease in relative loss factor with increasing frequency is due to the fact that beyond certain frequency of the electric field, the domain wall motion cannot follow the applied electric field. The increase in relative loss factor above 20 MHz may indicate the beginning of a possible presence of resonance with peaks occurring at higher frequencies. At resonance, maximum energy is transferred from the applied field to the lattice resulting in the rapid increase in relative loss factor. It is clear from the Fig. 5 that the relative loss factor is increasing with an increase in Sm3+ ions content. As the RLF is inversely proportional to initial

x = 0.0 x = 0.05 x = 0.1

70

60

50 -5

oppositely magnetized sub-lattices at 0 K. The resultant magnetization is, therefore, the difference between the magnetization of (B) and (A) sub-lattices, the former generally having larger value. The dominant A–B interactions lead to complete or partial (noncompensated) ferrimagnetisms. Thus the net saturation magnetization is given by equation [11]

Fig. 4. Variation of initial permeability of MgSmxFe2xO4 (x = 0.0, 0.05 and 0.1) ferrites with frequency measured at room temperature.

RLF X 10

Fig. 3. Variation of magnetization of MgSmxFe2xO4 (x = 0.0, 0.05 and 0.1) ferrites with applied field at 300 K.

40

30

20

li ¼ M2s Dm =K 1

ð4Þ 10

where Dm is the average grain diameter, K1 is the magnetocrystalline anisotropy constant and Ms is the saturation magnetization. As li is proportional to Ms2, the variation of li with x for Sm3+ should be affected in a manner similar to that of variation of Ms2 with x. Hence the decrease in initial permeability and saturation magnetization with Sm3+ ions concentration can be correlated well with each other.

0

0

5

10

15

20

25

30

Frequency (MHz) Fig. 5. Variation of relative loss factor of MgSmxFe2xO4 (x = 0.0, 0.05 and 0.1) ferrites with frequency measured at room temperature.

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J. Chand et al. / Journal of Alloys and Compounds 552 (2013) 264–268

3.5. Cation distribution analysis The cation distribution in the present system is derived from site preference energies and magnetization method. According to the Neel’s two sub-lattice model of ferrimagnetisms [14] the magnetic moment per formula unit in Bohr magneton (lB); nNB is expressed as

nNB ¼ MB ðxÞ  MA ðxÞ;

nNB ¼ Ms xM=NnB

ðMg0:125 Fe0:877 ÞA ½Fe1:073 Sm0:05 Mg0:875 B

ðx ¼ 0:05Þ

ðMg0:145 Fe0:9215 ÞA ½Fe0:9785 Sm0:1 Mg0:855 B

ðx ¼ 0:1Þ

3.6. Mössbauer study The room temperature Mössbauer spectra for MgSmxFe2xO4 (x = 0.0, 0.05 and 0.1) are given in Fig. 6. From the figures, it is clear that all the samples have six line patterns of Mössbauer spectra. The magnetic hyperfine splitting spectrums are the indicative of magnetic ordering and show the ferromagnetic phase. The spectrum reveals the hyperfine magnetic sextets corresponding to the tetrahedrally (A) and octahedrally (B) coordinated iron cations. In these spectra the dots represent experimental data and solid lines through the data points are least square fit. The solid lines obtained after fitting the spectrum indicates the position of the tetrahedral (A) and octahedral (B) sites. It is observed from the Mössbauer spectra taken at room temperature, that there is no systematic and significant change in the values of isomer shift corresponding to Fe3+ ions at (A) as well as (B) site, with an increase in the content of samarium in the system. The values of isomer shift corresponding to (A) and (B) sites are consistent with high spin Fe3+ charge state [18]. The values of quadrupole splitting obtained for the present ferrites system also confirms the presence of only Fe3+ charge state and not Fe2+ charge state in the system. The values of Q.S. for all the ferrite samples are almost zero within the experimental error. Thus, almost negligible values of the quadrupole splitting obtained system cannot be attributed to the absence of electric field gradient at the nucleus rather it can be explained in terms of the

0.90

0.85

-5

0

5

Velocity (mm/s)

(b)

x=0.05

1.005 1.000

Relative Transmission (%)

0.995 0.990 0.985 0.980 0.975 0.970 0.965 -10

-5

0

5

10

Velocity (mm/s)

(c)

x=0.10

1.005 1.000

Relative Transmission (%)

ðx ¼ 0:0Þ

0.95

0.80

ð7Þ

where M is the molecular weight of ferrite sample, Ms is the saturation magnetization, N is the Avogadro’s number and nB is the Bohr magneton. The nNB (lB) values were calculated by using the free ion magnetic moments of Fe3+(5 lB), Sm3+(1.5 lB) and Mg2+ (0 lB) ions. The ferrite in the present system is mostly inverse. Fe3+ ions show no clear preference to any site [16]. Sm3+ ions have large octahedral site preference energy as compared to tetrahedral site preference energy. So due to the large octahedral site preference energy Sm3+ ions occupy (B) sites only [17]. The octahedral site preference energy for magnesium ions is more as compared to tetrahedral site preference energy, so more ions prefers to go to octahedral site as compared to tetrahedral site [4]. The cation distribution in the present system as derived from site preference energies and magnetization method is represented as Cation distribution

x=0.0

1.00

ð6Þ

where MB(x) and MA(x) are octahedral (B) and tetrahedral (A) sublattice magnetic moments in Bohr magneton. The values of magneton number nBN (saturation magnetization per formula unit in Bohr magneton) at room temperature were calculated, by using the relation [15]

ðMg0:1 Fe0:893 ÞA ½Fe1:107 Mg0:9 B

(a)

Relative Transmission (%)

permeability and from the Fig. 4. the initial permeability is decreasing with an increase in Sm3+ ions content, so the RLF is found to be increasing with increasing Sm3+ ions concentration.

0.995 0.990 0.985 0.980 0.975 0.970 0.965 -10

-5

0

5

10

Velocity (mm/s) Fig. 6. Mössbauer spectra of MgSmxFe2xO4 (x = 0.0, 0.05 and 0.1) ferrites.

presence of the chemical disorder in the compositions which can mask the shifts in the hyperfine field produced by the quadrupole interaction [19]. All the spectra of bulk samples at room temperature have been resolved into sextets belonging to (A) and (B) sites, respectively. The sextets are labeled as (A) and (B) sites on the basis of the values of magnetic hyperfine fields. Since Fe3+ ions present at (B) site experience larger hyperfine fields, the outer sextets have been assigned as (B) site in this Mössbauer spectrum. The internal magnetic field on a nucleus can arise due to various interactions [20,21] and can be written as

Hint ¼ Hcore þ HSTHF þ HTHF þ HD

ð8Þ

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J. Chand et al. / Journal of Alloys and Compounds 552 (2013) 264–268

Table 1 Mössbauer parameters of Samarium doped Mg-ferrite. Sample

Sub-spectrum

WID (mm/s)

I.S. (mm/s)

x = 0.0

(A) (B)

0.46 ± 0.00 0.28 ± 0.00

0.13 ± 0.00 0.28 ± 0.00

x = 0.05

(A) (B)

0.99 ± 0.03 0.45 ± 0.03

x = 0.10

(A) (B)

0.53 ± 0.03 0.77 ± 0.03

Hf (T)

Area (%)

0.03 ± 0.00 0.02 ± 0.01

46.74 ± 0.01 49.99 ± 0.07

70.05 29.95

0.18 ± 0.00 0.24 ± 0.01

0 .06 ± 0.01 0.03 ± 0.01

46.54 ± 0.09 49.88 ± 0.04

62.27 37.73

0.13 ± 0.01 0.24 ± 0.00

0.11 ± 0.00 0.08 ± 0.01

45.45 ± 0.07 49.51 ± 0.04

18.99 81.01

where Hcore is the field due to the polarization of core s-electrons. HTHF and HSTHF are the transferred and supertransferred fields respectively. The term, HD is the dipolar field. From the Table 1 it is observed that the internal magnetic field decreases systematically at (A) as well as at (B) site with an increase in samarium ions concentration. These variations of hyperfine field can be explained using Eq. (8). The variations in the hyperfine field can be attributed to effect on HSTHF and HD whereas the values of Hcore and HTHF do not vary with the concentration (x) of samarium. In case of samarium doped ferrite, the replacement of Fe3+ ions of higher magnetic moment by samarium ions having lower magnetic moment (1.5 lB) effectively decreases the dipolar magnetic field. The variation of hyperfine field with samarium concentration could be understood on the basic of Neel’s model and the supertransferred hyperfine field. It is observed that H(B) decreases with an increase in samarium concentration. We attribute this to the fact that all the samarium ions occupy solely the (B) site. This data is also supported by magnetization vs. applied field curve, initial permeability measurements and cation distribution analysis. 4. Conclusions Single phase Sm3+ substituted Mg ferrites were successfully synthesized by solid state reaction technique. MgSmxFe2xO4 (x = 0.0, 0.05 and 0.1) samples attains saturation magnetization and exhibit ferromagnetic coupling at room temperature. The Mössbauer patterns of all the samples are sextet indicating there is a magnetic coupling. Xrd confirm the spinel structure of the presently studied ferrite. Incorporation of Sm3+ ions results in a decrease in initial permeability, saturation magnetization and

Q.S. (mm/s)

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