Indian Journal of Pure &. Applied Physics Vol. 37, January 1999,pp. 51-56

Influence of Sn/Nb on the resistivity of Mn-Zn ferrites

A D P Rao, B Ramesh I, P R M Rao2 & S B Raju I

Department of Nuclear Physics, Department of Physics I , Department of Inorganic and Analytical Chernistrl

Andhra University, Visakhapatnam 530003

Received 9 September 1997; revised 10 July 1998; accepted 3 September 1998

Effect of Sn4~ INb5+ on dc resistivity as a function of dopant concentration, temperature and duration of applied field for

Mn- Zn ferrites has been studied. Frequency response of ac resistivity for all the specimens has also been evaluated. Results are discussed considering vacancies, influence of vacancies, valence fluctuations along with hopping mechanism. Two transition temperatures Ts and Tc are observed relating to change in the conduction mechanism at low temperature range and in high temperature range due to transformation from ferromagnetic to paramagnetic state. Activation energies obtained in low temperature range are lower than high temperature range activation energy values and are explained. The decrease of ac resistivity with frequency is attributed to the dielectric relaxation caused by bound charge carriers.

1 Introduction

Mn-Zn ferrites are widely useful for high frequency applications. During the last decade these were used

1 in

the frequency range 16-25 kHz. With the emergence of switched - mode power supply, rapid development of high frequency ferrites has been increased in electronics ind~stry. Now power supplies are available to operate at 1 MHz and above. Demand for compact power supply increased due to the computers, microprocessors and VCR systems. This increased the demand for soft fer­rites. Mn-Zn ferrites can be used in the place ofNi-Zn mixed ferrites, for devices in which low resistivity is required.

Substitution of a cation in the matrix of ferrite alters its physical properties. According to available litera­ture2

-4, the preferred sites for high valency cation namely Ti, Sn and Nb are octahedral (B). Tin has strong ten­denc/ to occupy B sites,though it occupies tetrahedral (A)sites under certain conditions. Das et al.6 concluded that these cations occupied non-preferential (A) sites at lower concentrations and preferential(B) sites at higher concentrations in Ni-Zn mixed ferrites. Substitution7 of Sn in Ni-Zn ferrites resulted in a property of limited solubility and additional phase formation due to unre­acted material. Contrary to this, in Mn-Zn ferrites it was soluble and did not influence microstructure up to a content 5-7 mol%. Resistivity is one of the important parameters to assess usefulness of ferrite in any device,

which depends on the site occupancy of substituent cation and microstructure. Hence, keeping in view the above predictions, the present authors have studied the effect of tin andl6tlniobium on the resistivity of Mn-Zn ferrites.

2 Preparation of Samples

Ferrite with the composition Zno.37 MIlo.58 Fe2.o5 0 4 is widely useful9 in recording head applications and trans­formers. Series of ferrites with the composition MIlo.58-xl2Zn o.37-x12MxFe2.o504 are prepared using AR grade oxides, where M = Sn4

+ 1Nb5+ andx = 0.05 to 0.50 in steps

of 0.05.Conventional ceramic procedure was followed by double sintering method. Final sintering was done at 1380°C for one hour in air and lowered to 1280°C and kept for one hour. For Nb substituted ferrites, from x =

0.35 to 0.50 samples were not retained properly, since it is known \0 to have diffusion of niobium leading to the formation of micro voids and vacancies. Hence, these were sintered at 1050°C for one hO\1r in air and later lowered to 950°C and kept for an hour. X-ray patterns of all the samples confirm the single-phase formation of ferrite compound except at higher concentrations of tin substitution.

The samples were polished to obtain smooth surfaces, on which silver paste was coated to act as contact electrodes. Using two- probe method, electrical resistiv­ity measurements were performed. In the present paper, the variation of dc resistivity as a function of dopant

52 INDIAN J PURE APPL PHYS. VOL 37, JANUARY 1999

concentration (x),temperature (1) and duration of ap­plied field are presented. AC resistivity for few speci­mens were also evaluated.

3 Results and Discussion

Due to simultaneous presence of ferrous and ferric ions at octahedral sites of ferrite, low values of resistiv­ity occurs. Resistivity is sensitive to the chemical com­position and condition under which the sample has been prepared, like pressure, heating and cooling rate during sintering, porosity and grain size.

3.1 Variation of resistivity with dopant concentntion

Fig. 1 shows the variation of dc resistivity with dopant concentration for both tin as well as niobium substituted ferrites . The ;esistivity(p) decreases for Nb doped fer­rites up to x = 0.30 and for x = 0.35 enormous increase of resistivity is observed. For Sn doped ferrites also p decreases initially. There is no continuous increase or decrease of p for these materials. From x = 0.05 to 0.15, p value increases and then decreases for x = 0.20. Again from x = 0.20, the value of p increases and at higher concentration it decreases.

In the present studies x/2 (Mn2+) and xl2 (Zn2+) ions were replaced by x (Sn4) or x (Nb5+). High valence ions are able to form certain vacancies in the ferrite I I. For the present ferrites also, there is a possibility for the forma­tion of vacancies. Some of these may be due to thermal vibrations of atoms at elevated temperature. This is because of the probability of jumping of an atom, posi­tion of lowest energy increases with increasing thermal energy. In addition to this, the fluctuation of niobium

7·S

70

60

5·5

Fio 1 - Variation of res istivi tv with substituent concentra-e ' 4~. 5+

tion (Sn INb )

valence state as Nb3+ and Nb5+, the resistivity decreases. The enormous increase of p is due to decrease of sinter­ing temperature. The increase of resistivity with the decrease of sintering temperature was already reported by Naik and Powerl2 for Ni-Zn ferrites; and Ravinder13

for Li ferrites. The predominance of vacancies on the resistivity of tin doped ferrites is lesser when compared with the same on the resistivity of Nb doped ferrites, since tin does not exhibit bulk diffusion property leading to the formation of more vacancies in the ferrite-like niobium. The initial decrease of p for tin substituted ferrites is attributed to the valence fluctuations of tin as Sn2+ and Sn 4+. This can be understood with the proposed cation distribution for these ferrites given elsewherel4, for which theoretically evaluated lattice parameter (a) was in good agreement with the experimentally obtained values. A tetravalent ion is able to generate Fe2+ ions at A sites, which do not contribute to conduction processl5 , but will form stable covalent bonds with the tetravalent ions II. In the present investigations also, the increase of resistivity (p) is attributed to the formation of stable

4+ d 2+ A' Th f bonds between Sn an Fe at SItes. e presence 0

dopants in A sites at lower concentrations (that is for Sn doped ferrites from x = 0.00 to x = 0.15 and Nb doped

. ferrites from x = 0.00 to 0.20) had also resulted in the magnetic properties like saturation magnetization and initial permeability reported elsewherel6. Dopant starts to enter into octahedral sites also, resulting in the de­crease of p, for Sn doped ferrites at x = 0.20 generation of Fe2+ may take place and eventually electron hopping occurs between Fe3+ and Fe2+ on the octahedral sites. Dopant was introduced in these ferrites at the expense of A sites divalent cations, some of Fe3+ ions B sites would diffuse to A sites for higher concentrations of dopant, when it enters into B sites displacing Fe3+ ions. This reduces the generation of Fe2+ ions at B sites, resulting in a decre~se in electron hopping and finally it increases resistivity. For further concentrations of dopant,the prominence of vacancies comes into play causing decrease in p. At x = 0.50 for tin doped ferrite, the increase of p is ascribed to the formation of Sn02 significantly, at grain boundaries of ferrite. Earlier Varshney and Puri7 observed the formation ofSn02 at grain boundaries in the ferrite Nil+x-yZnySnxFe2-2x04 .

The conductivity of a ferrite is sensitive to two layered capacitors, namely well conducting grains, which are separated by grains boundary layers. In the Nb5+ doped ferrites, good and homogeneous grains were formed, while for lower concentrations of tin doped ferrites the

RAO et al.: RESISTIVITY OF Mn-Zn FERRITES 53

fonnation of smaller grains, relatively with the same of the Nb doped ferrites was observed and reported else­where l6

• The resistivity values of tin doped ferrites are compared with resistivity values of Nb doped ferrites and found to be higher. This can be understood in terms of above mentioned smaller grains, which in general will have larger grain boundaries contributing to lower con­centration insulating property. Here grain-to-grain boundary ratio decreases, resulting in an increase in the resistivity. But, at high concentrations of tin, . larger grains were observed due to disappearance of small neighbouring grains. The root mean square of the dc conductivity (adc)is directly proportional the static di­electric constant E' and this was reported by earlier investigators for Ni-Zn, Li-Ni and Mn-Mg ferrites I7

.19

•

For the present ferrites also, a similar behaviour was reported elsewhere20 at lower concentrations of dopants. But for higher concentrations of dopants, effect of va­cancies became prominent and such variations were not observed.

3.2 Variation of resistivity with temperature

The variation of resistivity with temperature was studied for all the materials. Fig.2 shows the plot oflog resistivity with the reciprocal of temperature (n for one of dopant cations, i.e. niobium substituted Mn-Zn fer­rites. The variation of p with T for ferrites has a general character, like for semiconductors. This can be depicted by the Wilson's equation9 as

Pdc = po exp(Elkn ... (1)

where po is a constant, E is the activation energy for electrical conduction and K is Boltzmann's constant. Fig. 2 shows the change in the slope of p with inverse of temperature, at Ts and Tc reflecting three regions of conduction. The.values of Ts, Tc and activation energies obtained are given in Table l.Ts corresponds to change in the conduction mechanism of the ferrites and Tc corresponds to Curie temperature. The experimental Tc values of these ferrites were also obtained by the method reported by Laroia and Sinha21 and are given in Table 1. Recently Bhise et al.I I had observed two breaks and three regions in the log p versus liT for Mn2+ Ti4+ substituted Ni-Zn ferrites.

The three different regions of conduction are due to change in the conduction processes. From the values of activation energy in the region-I (which vary from 0.10 to 0.20 eV; in Table I) it is evident that the conduction process in this region takes place due to cumulative effect of donors and acceptors which fonned during sintering from the loss of oxygen (ionization energy of

-e u

E .c ~ .. ~ 0 -'

3·2

2·a

2·0

1·6·

1·2

o·a

- . . - .. 0·00

- ' - ' - ' 0 ·05 "'- " - '" 0 ·15

---- 0·25 'f--->f--Ji 0 .] S

............... D·4S

./

Fig. 2 - Variation ofresistivity with temperature for Nb substituted ferrites

donors or acceptors is 0.10 eV), electron hopping b F 2+ d F 3+ . (F 2+-F 3+ . . etween e an e Ions e e tranSItIon energy is 0.20 e V) and fluctuation of valence states of substi­tuted cations viz. SnINh. The first temperature Ts is attributed to the change in the conduction mechanism, as observed earlier for tin doped copper ferrites22

. For lower (x = 0.05 to 0.30 ) concentrations of niobium doped ferrites, the .same value of Ts (Ts = 345°C) may be due to some structural aspects by the influence of Nb and more studies around this temperature (Ts) are needed for clearer u,nderstanding. The activation energies ob­tained in the region-II (maximum value is 0.27 eV) are higher for the same obtained in the region-I. In this region, conductivity takes place via Mn2

+ or Fe3+ neigh­

bours as Mn2+ + Fe3+ <=> Fe2

+ + Mn3+. The second temperature (Tc) is the Curie temperature, where the material undergoes transfonnation from ferromagnetic to paramagnetic state. Theoretically it was shown23 that the slope of log p versus liT changes while passing through the Curie point. The Curie temperature of this depends on the strong A-B interactions. According to Elwell and Dixon24

, conduction in this region takes place due to thermally activated hopping process. Experimen­tally the transition near Curie temperature has been observed for the present Mn-Zn ferrites, in accordance

54 INDIAN J PURE APPL PHYS. VOL 37, JANUARY 1999

Table I - Calculated activation energy values at two regions along with transition (Ts) and Curie (Tc) temperatures for tin or

niobium substituted ferrites

S.No. Dopant Sn Cone.

EI £2 Ts Tc (Exp.) (eV) (eV) (K) (K)

0.00 0.31 364.9 489.6

2 0.05 0.20 0.25 370.6 482.1

3 0. \0 0.18 0.26 350.9 492.1

4 0.15 0.13 0.27 322.6 509.4

5 0.20 0.20 0.25 377.4 511.9

6 0.25 0.17 0.20 384.6 546.2

7 0.30 0.20 0.18 370.0 524.2

8 0.35 0.18 0.24 381.7 538.8

9 0.40 0.12 0.20 377.4 567.9

10 0.45 0.15 0.19 384.6 526.7

II 0.50 0.19 0.08 384.3 519.3

with the the'Ory devel'Oped by Irkhin and Turov25

. Acusp­like minimum in the neighbourh'Ood 'Of Tc was 'Observed due t'O filling up 'Of 'Oxygen vacancies and migrati'On 'Of i'Ons fr'Om 'One site t'O the 'Other, causing a reducti'On in the c'Oncentrati'On 'Of m'Obile electr'Ons as 'Observed earlier

22

in tin d'Oped c'Opper ferrites. In Table 1 the Tc values are c'Ompared with 'the experimental values rep'Orted ear­lier l 6. They are in good agreement. Ab'Ove the Curie temperature, c'Onducti'On is influenced by the magnetic 'Ordering changes.

3.3 Effed of applied field on the resistivity

The variati'On 'Of resistivi ty (p) with applied field 'On SnINb d'Oped Mn-Zn ferrites was studied. Fig. 3 gives the variati'On 'Of p with the applied field f'Or 'One 'Of the substituents, i.e . Sn4

+ d'Oped ferrites . The value 'Of p decreases with the applied field . One 'Of the fact'Ors which influence the bulk resistivity 'Of a specimen is sum 'Of the grains and grain(s) b'Oundaries c'Ontributi'On t'O the resistivity. The c'Ontributi'On 'Of a grain t'O resistivity is less than its b'Oundary. At grain b'Oundary it was kn'Own

26

that l'Ocalized states existed in the f'Orbidden gap, which c'Orresp'Ond t'O the surface states in additi'On t'O the impu­rity states c'Orresponding t'O the surface defects and im­purities. In the absence 'Of applied external field, s'Ome 'Of the states might be 'Occupied by electr'Ons and p'Oten­tial barriers are created 'On either side 'Of the b'Oundary. The variati'On 'Of resistivity with the increase 'Of applied

Nb

Tc (from EI E2 Ts Tc (Exp.) Tc (from p) (eV) (eV) (K) (K) p) (K) (K)

476.2 0.31 364.9 489.6 476.2

476.1 0.17 0.27 344.8 487.0 487.1

487.8 0.13 0.25 344.8 516.8 500.3

540.5 0.11 0.23 344.8 551 .0 500.0

512.8 0.12 0.24 344.8 563.8 500.0

500.0 0.11 0.23 344.8 572.8 550.2

526.3 0.13 0.25 344.8 646.8 618.1

540.5 0.15 0.20 381.7 694.4 645.1

526.3 0.08 0.24 377.4 680.1 680.3

538.5 0.16 0.20 357.3 685.0 689.6

0.20 0.19 430.1 630.3 625.0

).)

0·00

).1

29 0-IS 0 ·10

2-7

2·5

"" '" 9 2

2·4

Fig. 3 - Variation ofresistivity with applied de field for Sn substituted ferrites

field can be interpreted 'On the basis 'Of Heywang's

m'Odel27• Because 'Of applied field, the Fenni level shifts

and p'Otential barrier is als'O m'Odified, resulting in the

decrease 'Of the grain b'Oundary resistance.

3.4 AC Resistivity

The ac resistivity 'Of the ferrite samples were evalu­

ated using the f'Ormula given by P'Older28,as ... (2)

RAO et 01.: RESISTIVITY OF Mn-Zn FERRITES 55

E " E .II:.

o

7

6

y = 0·00 Q = 0·15

X = 0.25

(:, • 0 ·35

• = 0"5

7

.6

5

, .....:.. E

3

.II:. .... ... o -

0~---f----i===~1;:~~~==~S~==~6----~7-----180 f , )06 kHz ( 5 n '~)

Fig. 4 - ac resistivity variation with frequency for Sn substituted ferrites

where EO is the free space pennittivity, E" = E' tan e the imaginary part of the dielectric constant (E'), tan e the dielectric loss, E' the dielectric constant and ro = 2IVthe angular frequency. Values of E' and tan e were obtained as a function of frequency (j) from the measurements made with a HP 4192A impedance analyser. The vari­ation of pac {for few specimens of Sn doped ferrites) with freq~ency is shown in Fig. 4. The dispersion of Pac

value was observed. After attaining a particular fre­quency ( 3 x 106 kHz), the Pac value remained practically constant. The results can be explained by considering the real part of ac resistivity. The real part of ac conduc-. , ( ) be ' 29 tiVlty a ac can wntten as

a=at(1)+a2(ro) ... (3)

The temperature dependent first term at (n is related to drift of electric charge carriers. It is independent of frequency and is given by

al (n = ao exp(-Elkn .. . (4)

where the symbols have their usual meaning. The second term, a2( ro) is related to the dielectric relaxation caused by bound charge carriers and is dependent on frequency. The terms can be written as30

a2(ro) = B'ro" ... (5)

where ao, B and n are constants and other symbols have their usual meaning.

The obtained ac resistivity of all the present ferrites showed strong dependence on frequency (at low values). Since in ferrimagnetic region, electron hopping mecha­nism between nearest neighbours ofFe2

+ and Fe3+ domi­

nates, so the resistivity decreases with increasing frequency. The pac reaches a constant value means that the natural frequency of the electron which jumps between Fe2

+ and Fe3+ is associated with the applied

frequency. The dispersion of pac can be explained by the Maxwell-Wagner two layer or the heterogeneities model for soft ferrites31

•32

• The heterogeneities of the material (dielectric structure) consist of two layered capacitors based on the idea that the well conducting grains are separated by grain boundaries of lower con­ductivity. The ac resistivity with frequency (at low values) is related to the resistive grain boundaries and at high frequency ac resistivity is due to the conductive grains33

. Recently, increase in aac with frequency had been reported by some researchers34 for SbNi ferrites and explained on the basis of Maxwell-Wagner two­layer model.

4. Conclusions

1. The variation of Pdc with dopant concentration of SnINb substituted ferrites is influenced by the formation

56 INDIAN J PURE APPL PHYS. VOL 37, JANUARY 1999

of vacancies, valence fluctuations of dopant ions and by electron hopping mechanism.

2. The variation of Pdc due to formation vacancies for Nb substituted ferrites is more than Sn substituted fer­rites. The diffusion property of Nb is leading to the formation of voids and vacancies in the host lattice.

3. Two transition temperatures Tsand Teare observed in support of change in the conduction mechanism and ferrimagnetic-to- paramagnetic state of the ferrite mate­rial. These Te values are in good agreement with the Te values reported earlier.

4. The activation energies of low temperature range are lower than the activation energies obtained in the high temperature range, due to change in conduction mechanism.

5. The effect of applied field on the Pdc is interpreted by Heywang's model.

6. The decrease of resistivity (Pac) with the increase of frequency is attributed to the dielectric relaxation caused by bound charge carriers, which is dependent on frequency.

References

Deshpande C E & Date S K,/ndian J Chem, 35 (1996) 353.

2 Gorter E W, Philips Res Reps, 9 (1954) 403.

3 Puri R K & Varshney U, J Phys Chem Solids, 44 (1983) 655.

4 Blasse G, Philips Res Reps, 18 (1963) 400.

5 Klnowles J E, Philips Res Rep, 29 (1973).93.

6 Das A R, Anatham V S & Khan D C, J Appl Phys, 57 (1985) 4189.

7 Varshney U & Puri R K,lEEE Trans on Mag, 25 (1989) 3109.

8 Jain G C, Das B K & Kumari S, IEEE Trans on Mag, 16 (1980) 1428.

9 Rao K H, Electrical, Magnetic and Mossbauer studies in Cr, In and Al doped Zinc-Manganese Ferrites, Ph.D. Thesis, Andhra University, India, (1981).

10 Kim H T & 1m H B, IEEE Trans on Mag, 18 (1982) 1541.

II Bhise B V, Mahajan V C, Patil M G, Lotke S D & Patel S A, Indian J Pure & Appl Phys, 33 (1995) 459.

12 Naik A B & Power J I, Indian J Pure & Appl Phys, 23 (1985) 436.

13 Ravinder D, Cryst Res Technol, 26 (1994) 457.

14 Rao A D P, Ramesh B, Rao P R M & Raju S B, II Nuovo Cimento, 20D (1998) 199.

15 Rezlescu N, Rezlescu E, Pasnicu C & Craus M C, J Mag & Mag Mater, 136(1994)319.

16 Rao A D P, Ramesh B, Rao P r. M & Raju S B, J Appl Phys, (In Press) (1997).

17 Elliti M A,J Mag Mater, 136 (1994) 138.

18 Reddy P V & Rao T S,J Less-Common Metals, 80 (1982) 255.

19 Reddy P V & Rao T S,J Less-Common Metals, 105 (1985) 63.

20 Rao A D P, Ramesh B, Rao P R M & Raju S B, Presented in the 7th Int. Conf. on Ferrites, Bordeaux, France, Sept. 3-6, 1996.

21 Laroia K K & Sinha A P B, Indian J Pure & Appl Phys, I (1963) 215.

22 Patil B I, Sawant S R, Vanshi S S S & Patil S A, Bull Mater &i, 16 (1993) 267.

23 Smit J & Wijn HPJ, Ferrites, Philips Tech Library Series

24

25

26

27

28

29

30

31

32

33

34

(Cleaver-Home, London), (1959) 234.

Elwell D & Dixon A, Solid State Commun (USA), 6 (1968) 585.

Irkhin Y U P & TurovE A, Sov Phys JETP, 33 (1957) 673.

Lenzinger M & Now E H, J Appl Phys, 40 (1969) 278.

Heywang W,J Am Ceram Soc, 47 (1964) 484.

Polder D, Proc lEE, 97 (1950) 246.

Jonscher A K, Dielectric relaxation in s'olids (Chelsea Dielec­tric Press, London) 1983.

Mostafa M F, Abd Elkadar M M, Atallah A S & Elnimr M K, Phys St Solidi A, 135 (19?3) 549.

Larsen P K & Metselaar R, Phys Rev B, 8 (1973) 2016.

Jankowski S,J Am Ceram Soc, 71 (1988) C~176. Haberey F & Wijn H P, Phys St Solidi (a), 26 (1968) 231.

Ahmed M A, EI Hiti M A, EI Nimr M K & Amer M A, J Mag & Mag Mater, 152 (19%) 391.

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