STUDIES ON RARE EARTH DOPED NICKEL FERRITE Chapter V

Bnl L The nickel A samples were prepared as described in chapter II.

The nickel ferrite samples of composition NixFe3.x04 of concentrations x = 0.3,

0.5, 0.7 and 0.9 were taken. The samples were doped with neodymium and

gadolinium 5% (weight I weight).

5.1 Structural Studies By X Ray Diffraction

The X-ray diffraction of Nio3Fe2,04 doped with ~ d ~ ' and Gd3' is shown

in figures [5.1 . I &5.1.2]. It exhibits cubic spinel structure for all the samples of

the series Ni,Fe3x04 of Fd3m space group. It is clear from the table [5.1] that

the lattice parameter goes on increasing w~th the Increase in nickel content in

the series Ni,Fe3.,04. The observed increase in the 'a' with the increase in

concentration can be explained on the basis of ionic radi~ of the rare earth ions

substitution and distribution in the structure. Both ~ d ~ ' and Gd3' ions prefer

octahedral site (B-site) and replace ~ e ~ ' Ion in the splnel structure. Since ~ e ~ '

ions are being replaced by the bigger ions ~ d ~ ' and Gd3', the increase in 'a' is

obvious. The occupancy of ~ e ~ ' ions by rare earth ions in the octahedral site is

explained based on IR spectral studies. Broadening of high and low frequency

bands indicates the rare earth ions in octahedral site.

It was reported [Rezlescu.N et al. (1994), Rez1escu.N et al. (1998), Sattar

A.A et al. (2002) and Hua Yang (1996)l that when some Fe ions in ferrite

lattices are substituted by Gd and Nd ions, the lattice parameters will change.

5.2 Spectral Studies Using FTlR

The IR spectra were recorded using a FTlR Shimadzu 8700

model. IR spectrophotometer in the range of 1000-400 cm" at RT. In

recording IR spectra, each of the samples was mixed with potassium bromide

powder and pressed into pellets. The procedure for making KBr pellets was as

Chapter V

discussed in the previous chapter. The infrared spectra of all the rare earth

mixed Ni ferrites are shown in Figs. (5.2.1 8 5.2.2). The absorption band v,,

observed at around 590cm-' is due to metal oxygen stretching vibrational

modes and it is attributed to F$-0' vibrations at the tetrahedral slte, whereas

v 2 observed at 470cm-' is assigned to octahedral F ~ ~ ' - o ~ - group complexes

[Josyulu 0.S et.al (1981)l. Spectral bands of the and Gd3' doped nickel

ferrite samples are glven in the tables (5.2.1, 5.2.21. For gadolinium doped

nickel ferrlte, bands are observed in the range 550,470cm-'. It is found that the

positions of bands are composition dependent. The bands observed in this

range are weak in intensity for both the dopants. The bands are broadened due

to the addition ~ d ~ ' and Gd3' in the nickel ferrite samples. This fact IS also

supported by [C M Sr~vastav et al. (1982)]. On substitution of ~ d ~ ' and Gd3',

the position of v2 band shifts towards lower side, as compared to undoped

samples. Which suggests the occupancy of ~ d ~ ' & Gd3' at Octahedral (B) site.

However, on substitution of impurities as said before, broadening of v2 band

takes place which may be due to occupancy of cations of different character on

the same site. The vibrational frequency v for the diatomic molecule IS given

by the relation

11 = Vibrational frequency of the ion

K = force constant of the ion

M = reduced mass of the ion

The decrease in vibrational frequencies indicates that the bond length

decreases. It is found to decrease the octahedral force constant, which

supports the occupancy of neodymium and gadolinium on B site. The

compositional dependent behaviour of force constant is attributed to the cation

oxygen bond distances [C.B Kolekar et al. (1994 )] Though the d~atomic

approximation is quite good, one can get only qualitative information using this.

The vibrational modes of the unit cell will be the more accurate one.

The IR absorption bands of solids in the range 100- 1000 cm-' are

usually assigned to vibrations of ions in the crystal lattice [Brabers V.A.M

(1969)l. According to [Waldron(1955)], the ferrites can be considered

continuously bonded crystals, meaning that the atoms are bonded to all nearest

neighbours by equivalent forces (ionic, covalent or Van der Waals). The

frequency distribution of vibrations is given by a Debye treatment of the

classical mechanical problem. In ferrites the metal ions are situated in two

different sublattices designated tetrahedral (A site) and octahedral (Bsite)

according to the geometrical configuration of the oxygen nearest neighbours. [

Waldron1955 ] and [ Hafner 1961 ] have attributed the band around 600 cm-' to

stretching vibrations of the tetrahedral groups (v l ) and around 400 cm-' to the

octahedral groups (v2 ).

Barbers and Vandengerh (1973) have shown that the fine structure In the

IR spectrum can be considered indicative of the presence of crystallographic

ordering. In the IR spectra of the Li05+~ rxTix Fez 5.1 5 x 0 4 composition, the fine

structure is much less pronounced with Increasing titanium concentration. It can

be inferred that the weak fine structure in the IR spectra IS correlated with this

short-range order. The gradual weakening of the fine structure in the IR spectra

with increasing Ti content indicates that long-range order transforms more and

more into short-range order. It can be considered that the symmetry of the

crystal is lowered as a result of the loss of long-range order. This result will be

reflected in the magnetic properties such as the magnetization and the Curie

point. The results concerning this have been published recently [Mazen S A et

al. (1996)l. In the composition of Zn-Mg ferrite doped with Nd ions, the position

of v2 band shifts towards lower side, which suggests the occupancy of ~ d ~ ' ion

on octahedral (B) site. The results regarding DC electrical resistivity study

support the B-Site occupancy of Nd and it shows increase in the DC resistivity

due to its impediment to the electrical conduction [Ladgaonkar et al. 20011 The

magnetization study also supports the occupancy of neodym~um ion on 0- s~te

showing dilution in the magnetization [Ladgaonkar et al. 2000al. However, on

Nd3+ ion substitution, the broadening of v2 band takes place, which may be due

to occupancy of cations of different characters on the same site [Kolekar et al.

19941.

5.3 Magnetic Studies Using Vibrating Sample Magnetometer

The magnetization recording technique using VSM is same as discussed

in the previous chapter at RT.

5.3.1 Saturation Magnetization

The saturation magnetization was found to be lncreaslng with the

concentration as shown in flg(5.3.1). From the table [5.3] it is evident that for

~ d ~ ' d o p e d nickel ferrite M, value increases from 11 emulg to 24emu/g, where

as for Nd3' doped sample it increases from 12 emulg to 26 emulg. The lower

value of magnetization for Gd doped samples may be explained as follows: The

magnetic moment of ~ d ~ ' is 7.94~0, whereas for Nd3' ~t is 3 . 5 ~ 0 . As the net

magnetization is due to antiferromagnetic alignment, the Gd3' doped samples

will have smaller magnetization compared to Nd3' doped samples. Thus, the

sample doped with neodymium has less magnetization compared to gadol~nium

doped sample. It was reported earlier by Lijun Zhao et a1.(2006) that the

saturation magnetization for Nio-lMno3GdxFe2.x04 (Ni-Mn ferr~te doped with Gd),

the values are close to the undoped sample at 800' and 850°c. When the

crystallite sizes are about 30- 40 nm, the samples have similar M, values. This

may be due to the following facts. The magnetic moment of Gd ions is 7.94118,

and Gd is the only rare earth element that has a curie temperature T, (293.3K)

close to room temperature. When the Fe ions are substituted by Gd ions at

Chapter V

lattice sites, the rare earth ion interactions are stronger than Fe-Fe interactions,

so the M, may be increased siginificantly. But as the Gd doped content is less

than 10% of Fe content, the microsubstitution is not enough to increase the

saturation magnetization siginificantly. When the contents of the Gd ions

increase, the doped Gd ions cannot enter into the ferrite lattice totally, as the

ionic radii of the ~ d ~ ' ions are larger than those of ~ e ~ ' ions. Hence the Fe-Fe

~nteractions decrease due to the reduction in the concentration of Fe ions on

the B- sites. All the explanations are based on the assumption that the rare

earth ions occupy the B-sites [HuaYang et al. (2004)l. This fact is in support of

the present observation.

5.3.2 Retentivity

The effect of retentivity with the composition of the nickel ferrite is shown

in fig(5.3.2). The M, values are presented in the table [5.3], which shows the

Increase M, for increase in concentration of nickel in nickel ferrite for both Nd

and Gd doped ferrite. For Nd doped nickel ferrite, the retentivity increases from

3.30 to 5.1 lemulg and for Gd it is 2.99-5.03 emulg. Substitution of rare earth

ions has not significantly changed the retentivity when compared to the

undoped samples as shown in (table 4.3 and 5.3) .

5.4 Electrical Studies

The AC electrical conductivity studies are done using LCZ Zentech 3305

Automatic Component Analyzer at RT. The preparation of the sample is same

as that in the previous chapter. The variation of conductivity with the applied

ac frequency is shown in figures 5.4.l(a-d) and 5.4.2(a-d).

5.4.1 Electrical Conductivity

The AC electrical conductivity is measured at different frequencies for Nd

and Gd doped nickel ferrite. They are shown in tables [5.4.1 & 5.4.21 and the

variation of conductivity with frequency is shown in figures 5.4.l(a-d) & 5.4.2(a-

d). AC conductivity increases with the increase of frequency. At lower

frequencies conductivity is independent up to 100KHz. Doping of these

samples wlth neodymium I gadolinium has a strong influence on the

conductivity. In all the samples the conductivity of neodymium doped samples is

higher than gadolinium doped samples. The higher magnitude in neodymium

doped sample may be expla~ned as follows: Neodymium has four unpaired

electrons in the 4f orbital whlch contributes to the conductivity whereas

gadolinium has one electron in 5d orbital whlch contributes to conductivity to a

smaller extent [ M.A.Ahmed et al. (2003) 1. A small change In the interactions

between the Gd I Nd ions with nickel Ions can cause a change in the

conductivity. In this case the conductivity of the both samples is low with a

magnitude of about 1pSeimenslm at 9OOKHz

The frequency dependence of AC electrical conductivity of the Ni -Mg

ferrite sample is reported by L.John Brechmans et al. (2004). It IS observed that

the AC conductivity Increases with increasing applied frequency. Since the

increase In frequency enhances the hopping frequency of the charge carriers

Fez' and ~ e ~ ' the conduction is increased. The conduction mechanism of

ferrite is explained on the basis of hopping of charge carriers between ~ e ' ' and

Fe3' on the octahedral sites. The present findings of increase in conductivity

with frequency are supported by this report

5.4.2 Dielectric Constant

Doping of these samples with Nd IGd the dielectric constant decreases

with the frequency and composition. Tables [5.5.land 5.5.21 and figures

5.4.3(a-d) 8. 5.4.4(a-d) represent these data. The dielectric properties may be

explained in terms of the assumption that the mechanism of dielectric

polarization is similar to that of conduction (that it depends depends on the

charge carriers). Since the rare earth ions radii are large compared to that of

the Ni. Fe. The occupancy of Nd3' I Gd3+ into the octahedral sites is probable.

The dielectric polarization depends on number ferrous ions which take part in

the electron exchange interaction. Thus the dielectric polarization is decreasing

which tends to decrease dielectric properties.

An similar explanation was proposed by [Reddy and Rao 19821 for Li -TI

ferrites. They reported that the electron exchange interactions [Waldron RD

19551 result in a local displacement of electrons in the directions of the electric

field, which determines the polarization of the ferrites. The ferrous ions which

take part in the electron exchange ~nteraction between Fez' and Fe3' are

formed during the sintering process and their presence is confirmed by IR

absorption spectra. Hence, the presence of Fez+ ions is responsible for the

polarization. The number of ferrous ions decrease and hence, the polarization

decreases. Thus, the number of Fez' ions play a dominant role in the

mechanism of dielectric polar~zation.

5.4.3 Dielectric Loss Tangent

The variation of loss tangent with frequency of the samples is shown in

the tables [5.6.l.and 5.6.21. From the figure 5.4.5(a-d) & 5.4.6(a-d), it is evident

that dielectric loss tangent increases with the frequency. The dielectric loss

tangent shows a hike at the frequency 500 KHz. This may be due to transfer of

electrons between Fez' and Fe3' and holes between ~ i " and Ni" in octahedral

sites. When compared with ~ d ~ ' , Gd3' doped samples, dielectric loss tangent

for Nd is larger than Gd doped samples. Since the ionic radii of Nd is less than

Gd the higher magnitude of loss tangent is seen in Nd doped samples.

The variation of loss tangent with frequency for the Ni- Mg ferrite

samples was reported earlier. An abnormal dielectric behaviour IS observed for

all the samples because the d~electric relaxation peaks are observed at a

frequency of 2KHz. According to Rezlescu model, the relaxation peaks may be

due to the collective contribution of both p and n type of charge carriers

[N.Rezlescu (1974)l. The electronic exchange between between ~ e ' ' and ~ e ~ '

and hole transfer between Ni2' and Ni3' in octahedral sttes are responsible for

such behaviour. Further the jumplng frequencies of localtzed charge carriers

are almost equal to those of the AC electric f~eld at the peak. However, in the

present case there is only an increase in loss tangent with the frequency.

Chapter V

Table 5.1 Lattice constant for nickel ferrite doped with neodymium and

gadolinium

the nickel ferrite

Table 5.2.1 Position of IR vibrational frequency for nickel ferrite doped

with neodymium

- Concentration of T T i k a X n a i Vibrational 1 nickel ferrite Frequency V, frequency v 2

( 1 --led. -

Table 5.2.2 Position of IR vibrational frequency of nickel ferrite doped

with gadolinium

Concentration of nickel ferrite

Vibrational Vibrational Frequency frequency

.l (cm") VI (cm-') -- - - --

Table 5.3 Saturation magnetization and Retentivity of nickel ferrite doped

with neodymium and gadolinium

-- I Concentration [;;r;rt -I ~ a t i r a t i o r 1 of the nickel magnetization magnetization 1 ferrite Neodymium gadolinium

doped emulg doped

t- emulg

1 Nb3Fez704 11 31

Table 5.4.1 Conductivity of nickel ferrite doped with neodymium

Table 5.4.2 Conductivity of nickel ferrite doped with gadolinium

Chaoter V

Table 5.5.1 Dielectric constant of nickel ferrite doped with neodymium

Table 5.5.2 Dielectric constant of nickel ferrite doped with gadolinium

Chapter V

Table 5.6.1 Dielectric Loss tangent of nickel ferrite doped with neodymium

t a n

, i (Nio sFez ~ 0 4 ) (Nio 7Fe2

lo3

Table 5.6.2 Dielectric Loss tangent of nickel ferrite doped with gadolinium

- -

q u e n c y tan 6 1 tan b

(KHz) ' (Nio 3Fe27Od (Nio 5F.2 504) (Nil 7Fe2 304) (Nio .Fez 104) ---p.? 3 o3

Fig (5.1 .I ) XRD pattern for Nio 3Fe2 7 0 4 doped with neodymium

Fig (5.1.2) XRD pattern for Nio 3Fe2704 doped with gadolinium

Fig (5.2.1) IR spectra for nickel ferrite doped with neodymium

Fig (5.2.2) IR spectra for nickel ferr~te doped with gadolin~um

Fig (5.3.1) Saturation magnetization for nickel ferrite doped with neodymium and gadolinium

Chapter V

Fig (5.3.2) Retentlv~ty of nickel ferrite doped with neodymium and gadolinium

I 0

0 8

o 100 100 600 son 1000

Frequency ( K H z )

Fig 5.4.l(a) Conductivity for Nio3Fe2,04, ~ d ~ + d o p e d

Chapter V

I , , , , . , . , 0 200 400 BOD 800 I000

Frequency (KHz)

Fig 5.4 . l (b) Conduct~v~ty for Nio5Fe2 5 0 4 , ~d~'doped

1 0 200 400 600 800 7000

Frequency (KHz)

Fig 5.4.l(c) Conductiv~ty for N I O I F ~ ~ 3 0 4 , ~ d ~ ~ d o p e d

a z o o ,370 a o o 800 ,000

Frequency (KHz)

Fig 5.4.l(d) Conductivity for Ni0gFe2 104, ~ d ~ ~ d o p e d

0 200 do0 600 800 1000

Frequency (KHz)

Fig 5.4.2(a) Conductivity for NiosFe2704, ~ d ~ ~ d o p e d

, , , . , . , . , . I o 200 eoc i nc aco ,roo

Frequency ( K H z )

Fig 5.4.2(b) Conductivity for Nio sFe2 5 0 4 , ~ d ~ + d o ~ e d

Frequency (KHz1

Fig 5.4.2(c) Conductivity for Nio 7Fe2.304, ~ d ~ ' d o ~ e d

Chapter V

/ . I . , , / . , . /

0 200 I O U GOO P O P 10011

Frequency (KHz)

Fig 5.4.2(d) Conductivity for NiogFe2 1 0 4 , ~ d ~ + d o p e d

Frequency (KHz)

Fig 5.4.3(a) Dielectric constant for Nio ,Fez 7 0 4 , ~ d ~ ~ d o p e d

, , . , . , , , I ,

0 200 400 600 800 1000

Frequency (KHz)

Fig 5.4.3(c) Dielectric constant for N I ~ ,Fez 3 0 4

Chapter V

I . , . , . , . , . , 0 200 400 800 800 ?OD0

Fig 5.4.3(d) Dielectric constant for NlosFe2 1 0 4 , ~ d ~ * d o ~ e d

Fig 5.4.4(a) Dielectric constant for Nio 3Fen 7 0 4 , ~ d ~ ' doped

5 7 -

5 6 -

5 5 -

; 5 4 -

0 5 3 -

t 5 2 - 0 .

5 , -

5 0 -

1 9 3

, ,

',

. 1 , . , . , . , . I

0 ?OF 100 500 800 1000

Frequency ( K H z )

0 100 400 600 600 1000

Frequency (KHz)

Fig 5.4.4(b) Dielectric constant for NiosFe2504, Gd3+ doped

" " 0 200 400 600 800 1000

Frequency (KHz)

Fig 5.4.4(c) Dielectric constant for Nio ,Fez 3 0 4 , Gd3* doped

3 1 1 200 103 600 800 ,000

Frequency I K H z j

Fig 5.4.4(d) Dielectric constant for NiogFez 1 0 4 , ~ d ~ ' doped

6 l i -

t ' 0 -

6 -

i - i ' , ~ l ~ ~ ~ ~ ~ ~ 0 >oo 40: 600 800 lo00

Frequency [KHz)

Fig 5.4.5(a) Dielectric loss tangent for Nio fez 7 0 4 , ~d~~ doped

Chapter V

Frequency (KHz)

Fig 5.4.5(b) Dielectric loss tangent for Nio ~Fe2 so4, ~ d ~ ' doped

Frequency (KHz)

Fig 5.4.5(c) Dielectric loss tangent for Nio7Fe2.304, ~ d ~ ' doped

Chapter V

Frequency ( K H z )

Fig 5.4.5(d) Dielectric loss tangent for NiogFe2 1 0 4 , ~ d ~ ' doped

Fig 5.4.6(a) Dielectric loss tangent for Nio,Fe~ 704 Gd3+ doped

Chapter V

400 (10" BOO lO"0

Frequency (KHz]

Fig 5.4.6(b) Dielectric loss tangent for NiosFe2 5 0 4 . Gd3' doped

200 a00 6 0 0 qr 1000

Frequency I K H z I

Fig 5.4.6(c) Dielectric loss tangent for Nio ,Fe>3O4, Gd3' doped

Chapter V

Frequency (KHz)

Fig 5.4.6(d) Dielectric loss tangent for NiosFe2 < 0 4 , ~ d ~ ' doped