Study of cation distribution in the MnZn ferrites using Mössbauer effect

~ )

Solid State C o m m u n i c a t i o n s , Vol. 84, N o . 7, pp. 717-719, 1992. Printed in Great Britain.

STUDY

OF CATION DISTRIBUTION

0038-1098/9255.00+.00 P e r g a m o n Press Ltd

IN T H E M n - Z n

FERRITES

U S I N G M(~SSBAUER EFFECT

M. Arshed, N.M. Butt, M. Siddique, M. A n w a r - u l - l s l a m Pakistan I n s t i t u t e of Nuclear Science & Technology P.O. Nilore, Islamabad, Pakistan T. Abbas, M. Ahmed D e p a r t m e n t of Physics, Baha-ud-Din Z a k a r i y a University, Multan, Pakistan.

(Receivedfor publication 18 August 1992, by M.

Cardona)

'

MSssbauer s t u d i e s w e r e made on Mn1_xZnxFe204 (x=O.O0, 0.25, 0.50). All the t h r e e p a r a m e t e r s , i.e. intensities, magnetic fields and line w i d t h s c o r r e s p o n d i n g to the t e t r a h e d r a l (A) and octahedral (B) sites, 2+ indicate t h a t Zn ions p r e f e r A s i t e s in the spinel s t r u c t u r e of t h e s e f e r r i t e s . Only B s i t e s show the sign of spin f l u c t u a t i o n f o r x=O.50.

Introduction

c a r r i e d out in a i r a t m o s p h e r e and alumina crucibles w e r e used f o r the p r e p a r a t i o n of the samples. The s a m p l e s w e r e b r o u g h t out of the f u r n a c e a t elevated t e m p e r a t u r e s and exposed to a i r f o r cooling. Each time an X - r a y d i f f r a c t i o n p a t t e r n w a s taken to make s u r e that the reaction had been completed, o t h e r w i s e the sample w a s again put in the f u r n a c e . When no t r a c e s of p a r e n t oxides w e r e found in the d i f f r a c t i o n p a t t e r n s , it w a s a s s u m e d t h a t the r e a c t i o n had been completed. Mossbauer e f f e c t m e a s u r e m e n t s w e r e made using a Co-57(Rh-matrix) source in the t r a n s m i s s i o n geometry. The f i t t i n g to the s p e c t r a w e r e made using a c o m p u t e r p r o g r a m m e , a s s u m i n g t h a t all peaks a r e Lorentzian in shape.

MSssbauer e f f e c t has been extensively used to study the f e r r i t e s since t h i s e f f e c t can provide a r a n g e of i n f o r m a t i o n such as their magnetic ordering, hyperfine field distribution, cation distribution, dynamical p r o p e r t i e s of c a t i o n s (e.g. spin f l u c t u a t i o n ) etc [I]. F e r r i t e s aaopt spinel s t r u c t u r e in which the c a t i o n s occupy the t e t r a h e d r a l (A) and o c t a h e d r a l (B) s i t e s in a fcc oxide sublattice. Spinels w i t h only divalent ions in A s i t e s a r e called normal and in B s i t e s a r e called inverse. In mixed spinels divalent and t r i v a l e n t cations occupy both A and B sites. In spinels the m a j o r i n t e r a c t i o n s a r e between A site and B site cations, o t h e r i n t e r a c t i o n s a r e much w e a k e r [I] Dilution of magnetic spinels with diamagnetic ions gives r i s e to an interesting phenomenon in which the f l u c t u a t i o n of spin can be w i t n e s s e d [2]. Here the results of the MSssbauer effect measurements on diluted spinels Mn Zn Fe 0 (x=O.O0, 0.25, 0.SO), which a r e l-x

x

2

Results and Discussions The room t e m p e r a t u r e M~ssbauev spectra o f Mn Zn Fe O w i t h x=O.O0, 0.25, 0.50 are I-X

3

inverse spinels, are presented. Such m e a s u r e m e n t s were made to study the e f f e c t of the dilution of MnFe O with zinc and locate 2 4 the positions p r e f e r r e d by the Zn z* ions in these ferrites.

Experimental Mn

l-x

Zn Fe O x

2

4

samples were prepared in

polycrystalline f o r m by high t e m p e r a t u r e solid s t a t e r e a c t i o n method f o r x=O.O0, 0.25 and 0.50. MnO (99.9Z), ZnO (99.9Z) and Fe 0 2

x

2

4

shown in Fig. l. Each s p e c t r u m c o m p r i s e s of t w o s e x t e t s c o r r e s p o n d i n g to A and B s i t e s of the spinels. The o u t e r one is f r o m A and the inner one is f r o m B s i t e s occupied by Fe sv of the spinels. The information about the cation d i s t r i b u t i o n in the spinels can be obtained f r o m the a r e a s , magnetic fields and the line widths a s s o c i a t e d with the t w o sites. For t h e s e f e r r i t e s A to B a r e a r a t i o is observed to d e c r e a s e with the increase of x. It is 0.64+0.04, 0.$4+0.04 and 0.49+0.02 respectively f o r x=O.O0, 0.25 and O.SO. The d e c r e a s e in the intensity of A s i t e s indicates t h a t as zinc r e p l a c e s manganese, Zn z* ions a p p e a r to e n t e r A positions and the Fea*ions displaced from these A sites enter B positions. For x=O.O0 the t w o e a r l i e r r e p o r t e d values of this a r e a r a t i o a r e 0.12+ 0.02 [3] and 0.30-+0.10 [4]. This d i f f e r e n c e may be due to the d i f f e r e n c e in the p r e p a r a t i o n methods. The change in the magnetic fields a s s o c i a t e d with A and B s i t e s a r e shown in Fig.2. The A s i t e s show a small change in

3

(99.9Z) obtained f r o m M/S Merck, Germany w e r e used as s t a r t i n g m a t e r i a l s . The p a r e n t oxides were weighed in stoichiometric ratios, thoroughly ground in a g a t e m o r t a r and then heated f o r quite long t i m e s at IO00°C and finally for three hours at 1300°C. This facilitated the complete r e a c t i o n producing the f e r r i t e s samples. All heat t r e a t m e n t s w e r e 717

Vol. 84, No. 7

STUDY OF CATION DISTRIBUTION

718

.

I

I I

I

t h e i r m a g n e t i c f i e l d w i t h x a s c o m p a r e d to B s i t e s . Since t he i n t e r a c t i o n b e t w e e n t he t w o s i t e s is of e x c h a n g e n a t u r e , this observed b e h a v i o u r i n d i c a t e s t h a t Zn z÷ ions have m o r e positional preference for A sites. The d e c r e a s e in t h e m a g n e t i c f i e l d on t h e t w o s i t e s w i t h t he i n c r e a s e in x is b e c a u s e of t h e diamagnetic nature of t he Zn z÷ ions. An i n c r e a s e in x c a u s e s a p r o g r e s s i v e d e c r e a s e in t he n u m b e r of A-B i n t e r a c t i o n s . For x=O.O0 t h e m a g n e t i c f i e l d s a s s o c i a t e d w i t h A and B s i t e s , 480+lkOe and 437_+lkOe r e s p e c t i v e l y , a r e in a g r e e m e n t w i t h S a w a t z k y e t al. [31. T h e r e is an i n c r e a s e in line w i d t h s ( b r o a d e n i n g of p e a k s ) o f both A and B s i t e s w i t h t h e i n c r e a s e of x a s shown in Fig.3. The b r o a d e n i n g o c c u r s when t h e r e is a r a n g e of distribution of interacting nuc l e i in the neighbourhood of Mossbauer nuclei. The i n c r e a s e in line w i d t h w i t h t h e i n c r e a s e of x is much more f o r B s i t e s t h a n A sites which i n d i c a t e s t h a t Zn z* ions p r e f e r A p o s i t i o n s in these s pi ne l s . The observed line widths a s s o c i a t e d w i t h B s i t e s a r e 2.12-+0.04 and 2.90-+0.1 l m m / s for x=O. 25 and 0.50 r e s p e c t i v e l y . In t h e s e c a s e s A s i t e s do not have only 6Fe, 5FelMn, 4Fe2Mn and 3FelMn n e a r e s t n e i g h b o u r s t o Fe nuclei a t B s i t e s , a s in t h e c a s e of x=O.O0 [5], but t h e s e n e i g h b o u r s of Fe B s i t e s now a l s o c o n t a i n Zn z÷ ions, w hi c h ma y gi ve rise to additional probable d i s t r i b u t i o n s of t he t ype 5FelZn, 4FelMnlZn, 4Fe2Zn, 3Fe2MnlZn, 3FelMn2Zn, 3Fe3Zn etc. Such p e a k s can be f i t t e d w i t h n u m e r o u s s e x t e t s . Our f i t t i n g i n d i c a t e s t h a t the fields associated with the sextet c o r r e s p o n d i n g t o B s i t e s r a n g e f r o m 350 to 430kOe f o r x=0.25, w i t h a v e r a g e va l ue of 395kOe. For x=O.50 t he MSssbauer s p e c t r u m is .just like a ne c kl a c e , F i g.l c . A similar

IA

100

99

98

X= 0 , O 0

|~

! I

I

E

-

I00

O

9g

.,'4

98

o~

(b)

I

X,O 25

.-~

97

E I

tO E

I

IA

10O

O 9g

L

I---

98

X=0.50

! -IO

I -5

Velocity

Fig. l:

MSssbauer

spectra

I

I 5

0

10

Cmm/s)

of

various concentrations x = 0 . 2 5 and (c) x=0.50.

Mn

l-x

(a)

Zn Fe O

for

x=O.O0,

(b)

x

2

3

3.0

500

1

A

400

2.0

E E

v

o

L

J~ v

-i-J;

1.0

300

I

J

200 0

I 0.25 X

Fig.2:

0.0

I 0.50

0.00

I 0.50 X

:-

Change in the internal magnetic field (H1n) of the tetrahedral (o) and octahedral (4) s i t e s as a f u n c t i o n of c o n c e n t r a t i o n x.

I 0.25

Fig.3:

Change in t he line w i d t h s (F) a s s o c i a t e d w i t h t he t e t r a h e d r a l (o) and o c t a h e d r a l (4) s i t e s a s a f u n c t i o n of c o n c e n t r a t i o n X.

VoL 84, No. 7 spectrum

STUDY OF CATION DISTRIBUTION

w a s observed f o r Co Zn Fe O 0.5 O.S 2 4 where it w a s found t h a t spin f l u c t u a t i o n is responsible f o r such a shape of the Spectrum [2]. The s u b s p e c t r u m c o r r e s p o n d i n g to A s i t e s gives r i s e to a six line p a t t e r n with peak intensities 3:2:1. But the sextet c o r r e s p o n d i n g to B s i t e s a p p a r e n t l y s e e m s to collapse to a t w o lines quadrupole splitted pattern which may be due to the spin fluctuation. This indicates that only Fe nuclei at B s i t e s show the sign of spin fluctuation. The isomer s h i f t s of Fe 3÷ ions at A s i t e s show an increase, 0.33 to 0 . 3 5 m m / s , and B s i t e s show a decrease, 0.39 to 0.35ram/s, with the i n c r e a s e of x. Such minute changes in isomer shift indicate that the s-electron d i s t r i b u t i o n at Fe 3. ions a r e not a f f e c t e d by the Zn s u b s t i t u t i o n . For all values of x no quadrupole s p l i t t i n g w a s observed within the e x p e r i m e n t a l e r r o r f o r both A and B s i t e s indicating t h a t the typical cubic s y m m e t r y of ferrites is not disturbed on the Zn substitution. The conclusion is t h a t the change in the t h r e e Mossbauer p a r a m e t e r s , namely A to B area ratios, the magnetic fields a s s o c i a t e d with A and B s i t e s and the line widths

719

corresponding to A and B sites, indicates t h a t Zn z• ions p r e f e r t e t r a h e d r a l (A) sites in Mn-Zn f e r r i t e s , which is in agreement w i t h the study o f Abbas et al. [6]. The Fe nuclei at B sites show a sign o f spin f l u c t u a t i o n f o r x=O.SO.

Acknowledgement We a r e thankful to Mr. M. Shafi f o r his help in conducting MSssbauer m e a s u r e m e n t s .

ReFerences I.

2.

T.C.

Gibb,

Principles

of

Spectroscopy,

Chapman and Hall

1976, p.159. P.K. lyengar

and

S.C.

MSssbauer (London),

Bhargaava,

Phys.

Stat. Sol.(b)46, I17 (1971). 3. 4. S. 6.

G.A. Sawatzky, F. van der Woude and A.H. Morrish, Phys. Left. 25A, 147 (1967). E. Wieser, W. Meisel and K. Kleinsttick, Phys. Stat. Sol. 16, 129 (1966). G.A. Sawatzky, F. van der Woude and A.H. Morrish, Phys. Rev. 187, 747 (1969). T. Abbas, Y. Khan, M. Ahmed and S. Anwar, Sol. Stat. Commun. 82, 701 (1992).