effect of mn substitution on the electric and magnetic...

SUST Journal of Science and Technology, Vol. 20, No. 6, 2012; P:8-14

Effect of Mn Substitution on the Electric and Magnetic Properties of

Nanocrystalline NiMnCu Ferrites

(Submitted: July 18, 2012; Accepted for Publication: November 29, 2012)

S. Nasrin1*

, Mohammad Hafizuddin Haji Jumali

2, and A. K. M. Akther Hossain

1

1Department of Physics, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh, 2Faculty of Science and Technology, Universiti Kebangsaan Malaysia , 43600 UKM Bangi, Selangor, Malaysia.

*E-mail: [email protected]

Abstract

Nanosize powders of NiMnCu ferrites were prepared by combustion technique. The X-ray diffraction (XRD) patterns exhibited well defined single crystalline phase and formation of spinel structure. From the XRD data lattice constant was calculated. It is found that lattice constant increases with increase of Mn content, which indicates that the present compositions obey the Vegard’s law. It is also found that due to substitution of small amount of Mn average grain size, initial permeability, saturation magnetization and dielectric constant enhanced. On the other hand, resistivity showed opposite trend. The DC magnetization showed that at room temperature all samples are in ferrimagnetic state. Keywords: X-ray diffraction, powder diffraction, nanocrystal, ferrites, permeability, dielectric constant, lattice constant, DC magnetization, grain size, combustion technique, resistivity.

1. Introduction

Nowadays there are new interests in the field of nanostructured materials. The nanostructured magnetic particles have different properties from the corresponding bulk material due to their reduced size and effect of magnetic interaction between particles. Spinel type M2+M3+

2O4 is interesting because of their diverse applications [1]. In the case when M3+ is Fe, the resulting spinel ferrites having a general chemical composition of MFe2O4 (M= Mn, Mg, Zn, Ni, Co) are among the most widely used magnetic materials [2]. The spinel structure is essentially cubic, with the O2- forming fcc lattice. The cations (usually metals) occupy 1/8 of the tetrahedral sites and 1/2 of the octahedral sites and there are 32 O2-

in the unit cell. They are preferred because of their high permeability and saturation magnetization in the radio- frequency (RF) region, high electrical resistivity, mechanical hardness and chemical stability [3]. The Ni-Zn-ferrites are one of the most popular soft ferrites that are used commercially due to their important magnetic and electrical properties. The dependence of the magnetic and electrical properties of Ni-Zn ferrites on composition and temperature has been extensively studied [4]. Several investigators have focused their attention on Ni-Cu mixed spinel ferrite because Cu containing ferrites have interesting electrical and magnetic properties [5-6]. The electrical and magnetic properties of ferrites are dependent on several factors such as sintering process, preparation method and additives. Then it was reported that combustion technique and substitution of Mn had an influence on the electrical and magnetic properties of these ferrites. In the present research, electrical and magnetic properties of NiMnCu ferrites prepared by combustion technique were carried out.

2. Materials And Methods

Nanocrystalline Ni0.50-xMnxCu0.50Fe2O4 (with x=0.00, 0.05) were prepared by combustion technique. The analytical grade of Ni(NO3)2.6H2O, MnCl2.4H2O, Cu(NO3)2.3H2O and Fe(NO3)3.9H2O were used as raw materials. Stoichometric amount of materials have been mixed thoroughly in ethanol in a glass beaker. This mixture was placed in a constant temperature bath at 70-80˚C, followed by an ignition, fine powders have been precipitated, then calcined at 700°C for 5 hours. The combustion was completed within a few seconds and fine nano-sized powders

Effect of Mn Substitution on the Electric and Magnetic Properties of Nanocrystalline NiMnCu Ferrites 9

were precipitated. The calcined powder thus obtained was pressed to form pellet and toroid-shaped samples. These are then sintered at various temperatures (1150-1300°C) in air for 6 h. During sintering samples have been heated/cooled in various heating and cooling rates. The structural characterization of each composition was carried out with an X-ray diffractometer. The microstructure was studied by optical microscope. Average grain sizes of the samples were determined from optical micrographs by linear intercept technique. The frequency dependent initial permeability spectra, the ac resistivity, dielectric constant (κ) were investigated at room temperature using a Wayne Kerr precision impedance analyzer (Model no. 65120B) in the frequency range 1 kHz-120 MHz. The DC magnetization measurements were made using the SQUID magnetometer (MPMS-5S; Quantum design co. Ltd.).

3. Results and Discussion

A. X-Ray Diffraction Analysis, Lattice Parameter and Crystal Size: The X-ray diffraction patterns of mixed spinel ferrites sintered at 1200°C in air for 6h are shown in FIG. 1. Diffraction peaks have been indexed to (111), (220), (311), (222), (400), (422), (511) and (440). The results of XRD data indicated that the samples have cubic spinel structure. From the XRD patterns it is observed that the positions of the peaks comply with the reported value [7] and no traces of raw materials were found, there by confirming that the chemical reaction was completed.

20 30 40 50 60

0

4000

8000

Ts=1200oC

Ni0.5-x

MnxCu

0.5Fe

2O

4

(44

0)

(22

2)

2θ θ θ θ (degree)

Inte

nsity

(ar

b.u)

(220

)

(11

1)

(311)

(400)

(422

)

(511

)

x=0.00x=0.05

FIG.1. The X-ray diffraction patterns for Ni0.50-xMnxCu0.50Fe2O4.

From the XRD data lattice constant was calculated by Nelson-Riley function. The lattice constant, a0, increases with increase of Mn2+ content in Ni0.50-xMnxCu0.50Fe2O4. The increase in lattice constant with Mn content indicates that the present compositions obey the Vegard’s law. This increase can be attributed to the ionic size differences since the unit cell has expanded when substituted ionic size is larger. The ionic radius of Mn2+ (0.89Å) is greater than that of Ni2+ (0.77Å) [8]. When the larger Mn ions enter the lattice, although the unit cell expands it preserves the overall cubic symmetry. In the XRD pattern of the calcined powder it was observed broader peak which indicating nano-sized particle nature. The average particle size of the sample was calculated using the (311) diffraction peak as shown in FIG. 2 by using the Debye Scherrer formula D = 0.9λ/βcosθ, where λ is the wavelength of X-ray, θ is the angle of the incident beam in degree and β is the full width at half maximum (FWHM) of the fundamental reflection (311) in radian. The average particle size of the sample Ni0.50Cu0.50Fe2O4 was found 27 nm.

10 S. Nasrin, Mohammad Hafizuddin Haji Jumali and A. K. M. Akther Hossain

34.8 35.2 35.6 36.00

400

800 Ni0.5

Cu0.5

Fe2O

4

TS=1200oC

x=0.00

2θ (degree)

FIG.2. The (311) powder diffraction peak of Ni0.50Cu0.50Fe2O4.

B. Microstructure:

FIG.3. The optical micrographs for the Ni0.50-xMnxCu0.50Fe2O4 sintered at 1200˚C. The optical micrographs of Ni0.50-xMnxCu0.50Fe2O4 sintered at 1200˚C are shown in FIG. 3. Average grain diameters of the samples are determined from optical micrographs by linear intercept technique [9]. The average grain sizes and the types of grain growth of the samples, which influence the magnetic and electrical properties of the materials. The microstructural study shows that at increases of small amount of Mn when x=0.05 average grain size is very large. This is probably due to the lower melting temperature of Mn (1245˚C) compared to Ni (1453˚C) or due to modification of chemical properties. C. Magnetization as a function of fields: FIG. 4 shows magnetization as a function of field (MH) at room temperature. It was observed that magnetization of the samples increases with increasing the applied magnetic field up to 0.7×105 (A/m). Beyond this applied field we find magnetization increases slowly and then saturation occurs. Therefore, it is clear that at room temperature the samples are in ferrimagnetic state. The saturation magnetization, Ms, as a function of Mn content are shown in FIG. 4 and find that the Ms increases with increase of Mn content.


0.0 4.0x105 8.0x10550000

100000

150000

200000

TS=1200oC

Ni0.5-x

MnxCu

0.5Fe

2O

4

x=0.00 x=0.05

Ma

gn

etiz

ati

on

(A

/m)

Magnetic field H (A/m)

FIG.4. Variation of magnetization as a function of magnetic field for Ni0.50-xMnxCu0.50Fe2O4 sintered at 12000C.

The number of Bohr magneton, n (µB) is calculated using the relation,BA

sB N

MMn

µ= , where M is the

molecular weight, Ms is the saturation magnetization, NA is the Avogadro’s number and µB is the Bohr magneton. The observed variation in saturation magnetization can be explained on the basis of cations distribution and exchange interaction between A and B sites. It is known at Ni2+ and Cu2+ occupy B sites, although Fe3+ and Mn2+ exist at both A and B sites [10]. When Mn2+ is introduced at the cost of Ni2+, some of the iron ions migrate from A site to B sites. This increases the iron ion concentration at B sites. As a result, the magnetic moment of B sub-lattice increases for small manganese concentration. However as manganese concentration increases, the magnetic moment of A sub-lattice remains constant. D. Complex initial permeability

FIG.5. shows the complex initial permeability spectra for Ni0.50-xMnxCu0.50Fe2O4 sintered at 1200°C. It is observed that the real and imaginary part of initial permeability increases with increase of Mn content.

102 103 104 105 106 107 108

20

40

60

x=0.00 x=0.05

TS=1300

0C

Ni0.5-x

MnxCu

0.5Fe

2O

4

Rea

l p

art

of

init

ial

per

mea

bil

ity,u

i/

Frequency (Hz)

106 107 1080

2

4

6

8

10

Imagin

ary

part

of

init

ial

per

mea

bil

ity,u

i//

x=0.00x=0.05

TS=1300

0C

Ni0.5-x

MnxCu

0.5Fe

2O

4

Frequency(Hz)

FIG.5. (a) The real part (b) the imaginary part of the initial permeability for Ni0.50-xMnx Cu0.50Fe2O4 sintered at 1200˚C.


The /iµ remains constant in the frequency range up to some critical frequency which is called resonance frequency,

fr. For further increase in frequency a sharp decrease in/iµ

. We can say that at the increases of Mn content grain

size increases and the Ms also increases with increasing Mn content. The permeability of nanocrystalline ferrite is related to two different magnetizing mechanisms: spin rotation and domain wall motion [11-13] which can be

described as follows: /iµ =1+χw+χspin where χw is the domain wall susceptibility, χspin is intrinsic rotational

susceptibility. χw and χspin may be written as : γπχ 43 2DMsw = and KMsspin22πχ = where Ms saturation

magnetization, K the total anisotropy, γ the domain wall energy and D the average grain diameter. It is generally believed that larger the grain sizes, the higher the saturation magnetization and initial permeability. Therefore, in the present case variation of the initial permeability strongly influenced by its grain size and DC magnetization property. There is a decreasing trend in permeability with increase in frequency. This is because at higher frequencies nonmagnetic impurities between intragranular pores and grains act as pinning points and increasingly hinder the motion of spin and domain walls thereby decreasing their permeability [14]. E. AC resistivity as a function of frequency:

The ac resistivity is decreased with increase of small amount of Mn (as shown in FIG. 6), may be due to the increase in grain size. As we know, larger grains results in less number of grain boundaries, which act as scattering centre for the flow of electron. Resistivity showed opposite trend of the initial permeability and the saturation magnetization.

102 103 104 105 106 10710-1

101

103

105

x=0.00 x=0.05

TS=1200oC

Ni0.5-x

MnxCu

0.5Fe

2O

4

log

ρ

Frequency (Hz)

FIG.6. Variation of resistivity as a function of frequency at sintering temperature 12000C.

F. Frequency dependence dielectric constant:

The variations of dielectric constant as a function of frequency for samples are shown in FIG. 7(a). It is clear that the dielectric constant increases with the increases of small amount of Mn and decreases rapidly with increasing frequency. The decrease of dielectric constant (the real and imaginary part) with increasing frequency is a normal dielectric behavior of spinel ferrites. The decrease in dielectric constant is rapid at low frequency and becomes slow at higher frequencies approach to frequency independent behavior. The dielectric dispersion curve can be explained on the basis of Koop’s two-layer model and Maxwell-Wagner polarization theory. To interpret the frequency response of dielectric constant in ferrite materials, Koops suggested a theory in which relatively good conducting grains and insulating grain boundary layers of ferrite material can be represented with the behavior of an inhomogeneous dielectric structure [15], as described by Maxwell [16]. The large value of dielectric constant at lower frequency is due to the predominance of species like Fe2+ ions, oxygen vacancies, grain boundary defects, etc. [17], while the decrease in dielectric constant with frequency is natural because of the fact that any species contributing to polarizability is found to show lagging behind the applied field at higher frequencies [18]. Loss


tangent (tan δ) with composition (x) are shown in FIG. 7(b), tan δ are found to increase with increasing the concentration. Loss factor tan δ is represented the energy dissipation in the dielectric system. It is considered to be caused by domain wall resonance. The value of tan δ is large at lower frequencies, while becomes lower at higher frequencies. At higher frequency the losses are found to be low since domain wall motion is inhibited and magnetization is forced to change rotation.

102 103 104 105 106 107 108

0

9000

18000

27000

36000 Ni0.50-x

MnxCu

0.50Fe

2O

4

TS=1200oC

x=0.00x=0.05

Rea

l par

t of

diel

ectr

ic c

onst

ant,

ε/

Frequency (Hz) 102 104 106 108

0

500000

x=0.00x=0.05

TS=1200oC

Ni0.50-x

MnxCu

0.50Fe

2O

4

Imag

inar

y pa

rt o

f di

elec

tric

con

stan

t, ε//

Frequency (Hz)

FIG.7(a). Variation of dielectric constant as a function of frequency at sintering temperature 12000C.

102 103 104 105 106

0

10 Ni0.50-x

MnxCu

0.50Fe

2O

4

TS=1200oC x=0.00

x=0.05

tanδ

Frequency (Hz)

FIG.7(b). Tan δ versus frequency at room temperature.

3. Conclusion

Nanocrystalline NiMnCu ferrites are successfully synthesized by combustion technique. The XRD patterns of the samples clearly indicate the formation of spinel structure. From the XRD pattern of the calcined powders, nano-sized particle nature was found. There is an increasing trend in lattice constant with increasing of Mn content. The increase in lattice constant with Mn content indicates that the present system obeys the Vegard’s law. The optical micrographs at sintering temperature 1200°C showed that at the increase of small amount of Mn average grain size is very large. Due to substitution of small amount of Mn for x=0.05 the real part of the initial permeability, saturation magnetization, Ms and dielectric constant (κ) increases. On the other hand the ac resistivity showed opposite trend.

Reference [1] H. Hamdeh, Z. Zia, R. Foehrweiser, J. Appl. Phys.76, 1135 (1994). [2] M. Sugimoto, J. Am. Ceram. Soc. 82, 269 (1999). [3] J. M. Hasting, and L. M. Corliss, Phys. Rev. 102, 460 (1956).


[4] H.Igarashi and K. Okazaki, J. Am. Ceram. Soc. 60, No.1-2, 51 (1976). [5] A. K. M. Akther Hossain, M. Seki, T. Kawai, H. Tabata, J. Appl. Phys. 96, 1273 (2004). [6] A . Verma, T.C .Goel, R. G .Mendiratta, Mater. Sci. Tech. 16, 712 (2000). [7] C. Rath, S. Anand, R. P. Das, K. K. Sahu, S. D. Kulkarni, S. K. Date, N. C. Mishra, J. Appl. Phys. 91, 2211

(2002). [8] R. D. Shannon, Acta crystallogr A 32, 751 (1976). [9] M. Mendelson, I. Ceram, J. Am. Soc. 52(8), 443 (1967). [10] J. Smit and H. P. J. Wijn, Philips Technical Library, Eindhoven, Netherland, 149 (1959). [11] Hu, Jun and Yan, Mi, Journal of Zhejiang University Science B 6 , 580 (2005). [12] S. T. Mahmud, A. K. M. Akther Hossain, A. K. M. Abdul Hakim, M. Seki, T. Kawai and H. Tabata, J. Magn.

Magn. Mater. 305, 269 (2006). [13] T. Tsutaoka, M. Ueshima, T. Tokunaga, T. Nakamura and K. Hatakeyama J. Appl. Phys. 78, 3983 (1995). [14] A. GLOBUS, Thesis, Univ. of Paris, Paris, France (1963). [15] C. G. Koops, Phys. Rev. 83, 121 (1951). [16] J. C. Maxwell, A Treatise on Electricity and Magnetism, Clarendon Press, Oxford, (1982). [17] J.C. Maxwell, Electric and Magnetism, Oxford University Press, NewYork 828 (1973). [18] I. H. Gul, A. Z. Abbasi, F. Amin, M. Anis-ur-Rehman, A. Maqsood, J. Magn. Magn. Mater. 311, 494 (2007).

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