Influence of the presence of Fe2+ ion in nickel-zinc ferrite

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Influence o f the presence o f Fe 2 ÷ ion in nickel-zinc ferrite C M S R I V A S T A V A , S N S H R I N G I , M J P A T N I * and S M J O G L E K A R

Department of Physics, *School of Materials Science, Indian Institute of Technology, Powai, Bombay 400076, India

Abstract. The presence of small amounts of Fe 2 + ion in nickel zinc ferrite significantly influences some of its magnetic properties. The lattice parameter increases slightly and the increase Aa is independent of x. The variation of magnetization with zinc.concentration is explained on the basis of the Yafet-Kittel model. Increase in the N6el temperature on Fe 2 + substitution in ZnxNil -~Fe204 is remarkable. This has been explained on the basis of a four sublattice model. Our analysis shows that dab (d 5 - d 5) interaction is most affected. The influence of Fe 2 + ions on the relaxation processes in the M6ssbauer line shapes of Ni-Zn ferrite is also investigated and is compared with Cu 2 + doped Ni-Zn ferrite.

Igeywords. Electron exchange; exchange constants; Jahn-Teller ion spinels; sublattices;

nickel-zinc ferrite.

1. Introduction

I n t r o d u c t i o n o f small a m o u n t s ( ~ 10 7o) o f J a h n - T e l l e r ion like C u 2 ÷ in N i - Z n and F e - Z n ferrites produces local distortion which significantly affects their various magnetic properties (Srivastava et al 1981a) a n d influences their M 6 s s b a u e r spectra (Srivastava et al 1981b). T o further investigate the effect o f such a crystal distortion o n the properties o f exchange coupled magnetic ions in magnetically ordered systems we have i n t r o d u c e d small q u a n t i t y o f Fe 2÷ ions in N i - Z n ferrites. In the present investigation we have studied the c o m p o s i t i o n s N t l - x - r Z n x F e r ' 2 + ( 0 < x < 0 " 7 ; y = 0 a n d 0-1) Fe~+O4. T h e results o b t a i n e d f r o m N6el temperature, m a g n e t i z a t i o n a n d M d s s b a u e r m e a s u r e m e n t s are discussed here.

2. Experimental

Samples with c o m p o s i t i o n s Ni I _ x _ y Z n x F e r F e 2 0 , t (0 < x < 0-7; y = 0 a n d 0.1) were p r e p a r e d using the ceramic technique. In o r d e r to m a i n t a i n the ferrous content, the samples were sintered in purified nitrogen a t m o s p h e r e . T h e presintered samples were sintered between 1250 a n d 1350°C. N i t r o g e n a t m o s p h e r e was also maintained during the cooling cycle.

T h e x-ray analysis carried on the a b o v e samples identified t h e m as single phase spinels. T h e lattice p a r a m e t e r a is plotted in figure 1. T h e lattice constant increases slightly for all values o f x and this increase, Aa, is i n d e p e n d e n t o f x. T h e ferrous content o f the samples was determined using the s t a n d a r d chemical analysis (Srivastava et al 1976).

8.3.5

8.30

J

o'.~ o'.~ o13 o?~ o~.s ,,16 0'.7 ~8 d.9 ~'.o

Z n ( x )

(a)

aatg)

0.02 0 0"02 O-Or,

(b)

I b

o-z 0.,: o'.s o~.a

×

Figure 1. a. Variation of lattice parameter with Zn concentration in Ni-Zn and Ni-Zn-Fe systems, b. Change in lattice parameter as a function of Zn concentration for Ni-Zn-Cu and Ni-Zn-Fe systems.

3. Results and discussion

3.1 The N~el temperature measurements

The N~el temperature (TN) for Nio.9_~ZnxFeo2.~Fe204 series is found by the initial susceptibility (~i) method. A typical Zi vs T curve is shown in figure 2. The sharp fall in the Zi curve as seen in figure 2 is characteristic o f a single phase spinel ferrite. Variation of T N with Zn concentration is given in figure 3. The T N is found to increase on Fe 2+ (,-,10~o) substitution in Ni-Zn ferrites. The change in T N for 10% Fe z+

substitution in Zn~Nil _ xFe204 is about 100°C for values o f x from 0 to 0.7. So far such a large increase in T N has not been reported. The observed variation of T N can be explained on the basis of the four sublattice model. The tetrahedral (A) site in the spinnel ferrite is occupied by Fe 3 + ions while the octahedral (B) site is split into three sublattices, corresponding to Fe 3 +, Fe 2 + and Ni 2 + ions. On fitting the experimental data on the above model one observes that the exchange constants show a very minor dependence on Zn concentration. The JAa(d 5 - d s) interaction is most affected, and seems to have largest influence on T N. In Ni-Zn ferrite this is 30°K while in the N i - Z n F e 2+ ferrite it is 35°K. The increase in JAB(Fe a +-Fe 3÷) interaction on Fe 2+

introduction may be attributed to the propagating local distortion resulting from electron hopping Fe 3 ÷ ,~ Fe 2 + on the B-sublattice. The experimental and theoretically obtained T N is plotted in figure 3.

3.2 Magnetization measurements

The observed variation of the magnetization at room temperature for Ni-Zn and N i - Z n - F e 2+ systems is shown in figure 4. It is observed that the magnetization

I 1

z

05

/

T~'C)

Figure 2. Initial susceptibility v s temperature.

increases with Zn-concentration upto x = 0.3 and then falls off with further increase in Zn-concentration. This variation o f magnetization with Zn concentration can be explained on the basis of the Yafet-Kittel model o f spin-canting on the B-sublattice

(Satya Murthy et al 1969).

3.3 M 6 s s b a u e r effect measurements

Introduction of 10 % of Cu 2 + in Ni-Zn ferrite creates local distortion inhibiting the localised domain wall oscillations responsible for the occurrence of central doublet superimposed on the magnetic spectra (Srivastava et al 1981b). However, when Fe 2 + ion which is also a Jahn-Teller ion is introduced in Ni-Zn system, it is observed that the central doublet continues to be present in the system and does not disappear as in the case of Cu substitution.

A possible explanation for this observation is as follows.

From previous studies (Srivastava et al 1976)it was shown that the atoms which lie within the range of domain wall displacements, give rise to the doublet. A local distortion produced due to Cu 2+ seems to inhibit the localised domain wall oscillations. However, if the distortion propagates at the rate higher than the rate of the domain wall oscillations then it is possible to see again the effect o f localised domain wall oscillation in the M6ssbauer spectrum. Introduction of Fe 2+ leads to a propagating local distortion due to electron hopping Fe 2 + ~ - F e 3 + on the B-sublattice.

IN[K)

t

500 400

3000

X

o',. o'o o'8 ;!o x Zn

(-)

~ooo 9OO

TN(K ) 800 l 700 600 500 4O0 30O 0

o

o"

c • o •

Z n x NJ9-x Fe.21 + Fe2

04

• EXPERIMENTAL o THEORETICAL

i J ~ 1 '/J0

02

04

(b)

Figure 3. a. Dependence of the N6el temperature T N on Zn concentration of the N&I temperature T N on Zn concentration.

b. Dependence

N i - Z n . N i - Z n - Fe.1 o

100

80

t 0 - \

20 b

t L I i

0-2 04. 0-6 0-@

Zn (x)

F i p r e 4. Variation of magnetization with Zn concentration (x), in N i - Z n and Ni-Zn-Fe systems at 300 K.

u~

~loc

l , Z n 0 . 6 Nio. 3 Cuo. ! Fe 2 O,,

21L.

210

~ 2 0 6 i.J

202 I I I I A I I I

- 8 - 6 - - ' - 2 0 2 ' 8 1O

V e l o c i t y mm / s e c

(a)

• . . °o °% • o.."

'" "'.' T : 300K • • ° • °

Figure 5.

at 300 K,

%

x

VELOCITY {mm 15e¢)

(b)

M6ssbauerspectraofa. Zno.6Nio.3Cuo.lFe204and b. Zno.6Ni0.3Fe0.1Fe204

This electron exchange frequency is higher than the frequency of domain wall oscillations. As a result the M6ssbauer spectrum remains unaffected after Fe 2÷

substitution. For comparison we give 300 K M6ssbauer spectra of Ni-Zn-Cu2o.~ and Ni-Zn-Fe2.] for x = 0"6 in figure 5.

4. C o n c l u s i o n s

The effect of crystal distortion due to Jahn-Teller effect on thermally-excited localised domain wall oscillations in Ni-Zn spinel ferrites has been studied by substituting small amounts of ferrous ions in these ferrites. The changes in lattice constant, magnetization,

References

Satya Murthy N S, Natera M G, Youssef S I, Begam R J and Srivastava C M 1969 Phys. Rev. I$1 969 Srivastava C M, Shringi S N, Srivastava R G and Nandikar N G 1976 Phys. Rev. BI4 2032

Srivastava C M, Patni M J, Srinivasan G and Srinivasan T T 1981a Butt. Mater. Sci. 3 225

Srivastava C M, Shringi S N and Joglekar S M 1981 Proceedings International Conference on the Applications of M6ssbauer Effect (ICAME), Jaipur, Published by INSA, New Delhi p. 813