Structural, Dielectric and Mossbauer Properties of Mg_0.2Mn_0.5Ni_0.3Al_yFe_2-yO₄ Nanoferrites Prepared by Citrate Precursor Method

Structural, Dielectric and Mossbauer Properties of Mg_0.2Mn_0.5Ni_0.3Al_yFe_2-yO₄ Nanoferrites Prepared by Citrate Precursor Method

Satish Verma, Jagdish Chand, Pooja Dhiman, Sarveena and M. Singh

Department of Physics, Himachal Pradesh University, Summer-Hill, Shimla-171005, INDIA. Email: satishapurva@gmail.com

Abstract

Polycrystalline Mg

Mn Ni

Al Fe O

(y=0.0-0.10)

5000

(311)

0.2 0.5

0.3 y 2-y 4

Relative Intensity(arb.units)

nanoferrites were prepared by citrate precursor method. Particle size decreases from 102.25 nm to 41.65 nm with increasing Al3+ ions concentration. The unit cell parameter

3+

4000

3000

(440)

a is found to decrease linearly with Al ions due to its

(220)

(511)

smaller ionic radius. Theoretical lattice parameter ath, volume V of the unit cell, oxygen positional parameter u, ionic radii of A-site and B-site were determined. 57Fe Mössbauer measurements performed at room temperature shows a six line pattern for sample with larger particle size

2000

1000

(400)

(422)

(222)

30 40 50 60

2(deg.)

and a doublet pattern for sample with smaller particle size, owing to weakening of magnetic exchange interaction, which is indicative of their super paramagnetic nature. Dielectric constant and dielectric loss were determined in the frequency range 0.075 MHz 30 MHz. Dielectric behaviours have been attributed to the MaxwellWagner type interfacial polarization.

1. Introduction

   The iron oxide and substituted iron oxide nanomaterials known as spinel ferrites are used for magnetic memory and magneto optical devices, as contrasting agents in magnetic resonance imaging, for refrigeration and as sensors. Spinel ferrites are still one of the basic materials of modern electronics and computers technology. When Fe3+ ions are replaced by Al3+ ions, the crystal structure becomes a cubic spinel structure. The addition of Al3+ ions gives interesting Mössbauer spectra and drastically changes magnetic hyperfine fields and other Mössbauer and X-ray parameters. The aim of present work is to study effect of Al3+ ions substitution on physical, electric and Mössbauer properties of Mg0.2Mn0.5Ni0.3AlyFe2-yO4 nanoferrites.

2. Results and Discussion

XRD pattern of Mg0.2Mn0.5Ni0.3Fe2O4 ferrite, Figure 1 indicate that material has a well-defined crystalline cubic single phase. Crystalline size was estimated using Scherrers formula and it decreases from 102.25 nm to 41.65 nm with

increasing Al3+ ions concentration.

FIGURE 1. XRD pattern of Mg0.2Mn0.5Ni0.3Fe2 O4 ferrite.

The radii of tetrahedral and octahedral sites were calculated, Table 1, using lattice parameter a and equations given by Smit and Wijn [1]:

Rtetra = 3 (u ¼) a R0 (1)

Rocta = (5/8 u) a – R0 (2)

Here R0 = 1.32 Ã… is the radius of oxygen atom. Tetrahedral and octahedral site radii decrease continuously with increasing Al3+ ions concentration. The oxygen positional parameter and theoretical value of lattice parameter were calculated using relations [2]:

u = [(Rtetra + R0)1/a3 + 0.25] (3) ath = 8/33 [(Rtetra + R0) + 3 (Rocta + R0)] (4)

Figure 2 shows Mössbauer spectra of y=0.0 and 0.10 at room temperature. For y=0.0, Mössbauer spectrum exhibits a superposition of two Zeeman sextets, one sextet correspond to higher magnetic field attributed to Fe3+ ions on B-site, and other sextet correspond to lower magnetic field attributed to Fe3+ ions on A-site. Mössbauer spectra of y=0.10 is characterized by simultaneous presence of central paramagnetic doublet.

The difference between the paramagnetic and the ferromagnetic behaviour can be explained by super paramagnetic relaxation time , using equation:

= 0 exp (KV/kBT) (5)

Magnetic sextets corresponds to ferromagnetic particles of larger size and for these particles ( > ). When is less

obs

than Larmor precession time obs i.e. ( < obs) a super paramagnetic doublet is observed in Mössbauer

TABLE 1. Structural parameters of ferrite samples.

y a(Ã…) ath(Ã…) (Ã…)3 Rtetra Rocta u (Ã…)

0.0 8.399 8.397 592.53 0.5849 0.7294 0.3808

0.05 8.394 8.393 591.58 0.5860 0.7305 0.3807

0.10 8.390 8.388 590.71 0.5914 0.7364 0.3809

800

700

600

0.8

0.7

0.6

Dielectric Loss

0.5

0.4

0.3

0.2

300 K

323 K

348 K

373 K

398 K

423 K

448 K

463 K

(b)

(a)

spectrum. Presence of doublet alone in the Mössbauer spectra can be attributed to super paramagnetic relaxation due to extremely small size of crystallites [3].

y=0.10

500

Dielectric constant

400

300

200

0.1

0.0

0 5 10 15 20 25 30

Temperature (K)

300 K

323 K

348 K

373 K

398 K

423 K

448 K

463 K

0 5 10 15 20 25 30

Frequency (MHz)

Relative Transmission (%)

FIGURE 3. Variation of (a) dielectric constant (b) dielectric loss (inset) with frequency at different temperature of y=0.0.

TABLE 2. Mössbauer and dielectric parameters.

y=0.0

Samples Area ratio

B/A

() 1MHz

()

1 MHz (x10-3)

Tan /

(x10-5)

Q-Factor (x105)

-10 -5 0 5 10

Velocity (mm/s)

FIGUR

y = 0.0 0.710 214 3.9 1.822 0.548

y =0.05 0.556 207 1.9 0.917 1.090

y=0.10 0.492 202 1.3 0.643 1.555

At low frequencies, dipolar and interfacial polarizations are

E 2. Mössbauer spectra of Mg0.2Mn0.5Ni0.3AlyFe2-yO4 ferrite samples.

B

There is no significant change in the values of isomer shift corresponding to Fe3+ ions at A- and B-sites with an increase in Al3+ ions concentration. This indicates that there is negligible influence on the s electron charge density around Fe3+ nuclei at both the sites. Quadrupole interaction has values close to zero for A-site and B-site. The hyperfine magnetic field at A-site and B-site shows a gradual decrease with increasing Al3+ ions concentration. This can be explained on the basis of super transferred hyperfine field at central cation which originates from magnetic moments of nearest-neighbour cations. The introduction of Al3+ ions, which replaces Fe3+ ions from the B-site decreases intra-sub lattice contributions, which in turn decreases hyperfine magnetic field. Net magnetic field is due to dominant Fe3+A – O2- – Fe3+ linkage, because A-B interaction is stronger than A-A and B-B interactions. The distribution of Fe3+ ions amongst the A- and B-sublattices can be understood from the relation between the area ratio of B- to A-sites subspectra and (molar ratio) y, Table 2. The decrease of this ratio against y indicate that the substitution process often reduces the Fe3+ ions number in the lattice at the expense of the number of Fe3+ ions at the B sublattice.

Figure 3 shows the variation of dielectric constant and dielectric loss with frequency at different temperature of y=0.0. There is initial decrease in dielectric constant and dielectric loss with frequency followed by appearance of the resonance which may be due to matching of frequency of charge transfer between Fe2+ Fe3+ ions and that of applied electric field. Dielectric constant of any material in general is due to dipolar, electronic, ionic and interfacial polarizations.

known to play most important role. Both these polarzations are strongly temperature dependent. At high frequencies, electronic and ionic polarizations are main contributors and their temperature dependence is insignificant. Figure 3 [Inset], shows that dielectric loss decreases initially with increasing frequency which can be explained on the basis of Koops phenomenological model [2,4], followed by appearance of a resonance with peak occurring at frequencies higher than 30 MHz.

Conclusions

Mössbauer spectra exhibit broad doublet with increasing Al3+ ions concentration which suggest super paramagnetic nature and collapse of long range ferromagnetic ordering due to smaller particle size. Very low dielectric losses even at high frequencies make these materials suitable for microwave applications.

References

1. Satish Verma, J. Chand, K. M. Batoo and M. Singh, J. Alloys. Compd. 565, 148-153 (2013).

2. Satish Verma, Jagdish Chand and M. Singh, J. Alloys. Compd. 587, 763-770 (2014).

3. Satish Verma, Jagdish Chand, Khalid Mujasum Batoo and M. Singh, J. Alloys. Compd.551,715-721 (2013).