MÖSSBAUER STUDIES OF SUBSTITUTED LITHIUM FERRITES

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MÖSSBAUER STUDIES OF SUBSTITUTED LITHIUM FERRITES

J. Dormann

To cite this version:

J. Dormann. MÖSSBAUER STUDIES OF SUBSTITUTED LITHIUM FERRITES. Journal de

Physique Colloques, 1980, 41 (C1), pp.C1-175-C1-176. �10.1051/jphyscol:1980149�. �jpa-00219718�

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JOURNAL DE PHYSIQUE Colloque Cl, suppl&ment au n O 1, Tome 41, janvier 1980, page C1-175

~&SSBAUER STUDIES OF SUBSTITUTED LITHIUM FERRIES

J. L. Domann

Laboratoire de Magne'tisme, C. N . R.S., I , PZace A. Briand

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⁹²¹⁹⁰BeZZevue, France.

INTRODUCTION

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The magnetic properties of Li-Zn and Li-Ti f e r r i t e s have been already reported /1-3/. I t has been shown t h a t f o r L i - T i f e r r i t e s , Ms values are. consistent with a Nee1 c o l l inear ferrimagnet f o r Ti amounts

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0.7 atoms per formula u n i t , while f o r Li-Zn f e r r i t e s the addition of more than about 0.4 atom of Zn r e s u l t s in a decrease in magnetiza- t i o n . Recently, neutron data /4/ and high f i e l d magnetization data /5/ have c l e a r l y shown t h a t t h i s property r e s u l t s from a localized canting of B spins, which i s q u a l i t a t i v e l y explained by the mo- del introduced by Rosencwaig /6/. This was furthe- more verified by high f i e l d Mtssbauer measurements on Ni-Zn f e r r i t e s /7/. The i n t e r p r e t a t i o n of Wss- bauer data i s complicated by the presence, a t high temperatures, of a relaxation behaviour when the Zn s u b s t i t u t i o n i s > 0.3. This i s a general feature of Zn-substi tuted f e r r i t e s

.

Bhargava e t a1 /8-9/ unam- biguously showed t h a t i n Ni-Zn and Co-Zn f e r r i t e s the shape of the Mdssbauer spectra i s due t o spin- spin relaxation. The relaxation time slowly changes with T and with applied f i e l d ( t h i s i s not the case

'

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f o r superparamagnetism). The Mtssbauer spectra could be interpreted by considering a superposition of magnetic and paramagnetic s i t e s ; however the f i t so obtained i s poor. The f i r s t data on Li-Zn f e r r i t e /lo/ were interpreted by using such an hypothesis.

On the other hand, when relaxation behaviours are ngt too important, Midssbauer spectra should present several s i t e s

.

The supertransferred hyperf i ne f i e 1 d

!-ISTHF depends on the number of non magnetic nearest neighbours /11/. An analysis taking i n t o account of relaxation behaviour should include the variations

Of H ~ ~ ~ ~ '

EXPERIMENT

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The general composition of the poly- c r i s t a l l i n e f e r r i t e was

Zn Ti Yn 0 Fe2.5-~.5z-1.5t-mLi~.5+~.5t-~.5z z t' m 4 For O<z<0.7, O<t<0.8, m i s e i t h e r 0 o r 0.035

The sample preparation has been described e l - sewhere /12/. The cation d i s t r i b u t i o n was checked by neutron and X-rays d i f f r a c t i o n ; no s i g n i f i c a n t

deviations have been observed /13/. MBssbauer expe- riments were carried out using a conventional spec- trometer operated i n the constant acceleration mo- de. The source used was ^~0~~i n Pd.

RESULTS AND ^'DISCUSSION

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Spectra of pure LiFe508 with low s t a t i s t i c a l noise were f i r s t analysed ; a t Teast three s i t e s a r e necessary t o obtain a good f i t . Even four s i t e s should be introduced t o ob- t a i n the c l a s s i c a l values of the isomer s h i f t s and of the hyperfine f i e l d H. The superstructure of L i atoms has no influence on the hyperfine parame- t e r s /14/.

Li-Zn f e r r i t e s

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A spin-spin relaxation unambi- guously occurs /8-9/ i n such f e r r i t e s . The re1 axa- t i o n happens even a t 77 K f o r z = 0.4 ; f o r z = 0.6 a t 4.2 K, the spectra are s l i g h t l y perturbed by the relaxation. Spectra a t 4.2 K ( f i g . 1 ) are correct- l y analysed by assuming only one A s i t e , which means t h a t the variation of (HSTHF)A due t o L i i n B s i t e i s weak :

AH^^^^)^

< 2 kOe. This

Fig. 1 MBssbauer spectre Of Li-Zn f e r r i t e s a t 4,.2K

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980149

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c1-176 JOURNAL DE PHYSIQUE

confirms measurements c a r r i e d o u t on L i f e r r i t e ,

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= 45' f o r t = 1.25. ( i i ) i t i s n o t possible t o which show no experimental differences between orde-

r e d and disordered L i f e r r i t e s . HA does n o t d i f f e r much from the value o f L i f e r r i t e , but several 8 s i - t e s are necessary f o r the analysis. I f the i n t e n s i - t i e s o f the s i t e s are equal t o the t h e o r e t i c a l pro- b a b i l i t y t o f i n d 0, I... Zn n.n., the "x2" f a c t o r i s very close t o i t s best value. The as deduced H values lead t o

AH^^,,^)^

= 10 2 kOe ; H f o r no Zn n.n. varies from 533 kOe ( z = 0) t o 545 kOe (z=0.6).

These values are consistent w i t h H values reported for pure ZnFe204. The quadrupole s p l i t t i n g i s a l - ways weak ( < 0.1 mm/s) and bA = 0.35 f 0.02 mm/s,

= 0.46 0.02 mm/s w i t h respect t o m e t a l l i c i r o n .

. . -

e r r i t e

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Unexpected features happen : ( i ) r e l a x a t i o n occurs b u t f o r higher s u b s t i t u t i o n degree and a t higher temperature than f o r Li-Zn f e r r i t e s . For t = 0.735 ( f i g . 2) the 300 K spectra cannot be solved n e i t h e r w i t h the s o l u t i o n used a t 77 o r 4 K, nor w i t h c l a s s i c a l parameters. These r e l a x a t i o n behaviours could be r e l a t e d t o l o c a l i z e d canting which destroys the long range c o r r e l a t i o n between Fe spins.

For L i - T i f e r r i t e , l o c a l i z e d canting i n A s i t e occurs probably, due t o T i atoms i n 8 s i t e s , which ex- p l a i n s the deviation o f Ms values from a Nee1 f e r - rimagnet, the mean angle being

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= 25' f o r t = 1,

f i t the spectra w i t h o n l y one A s i t e ( f i g . 2) : i t i s no more possible t o consider

AH^^^^)^

as n e g l i - g i b l e . The best f i t o f the 4.2 K spectra i s o b t a i - ned by assuming t h a t o n l y T i are s i g n i f i c a n t i n

AH^^^^)^

⁼¹²^f2 kOe. This value d i f f e r s from the usual value

AH^^^^)^

= 3 kOe (however 9 kOe has been reported /11/). Several 8 s i t e s are needed t o f i t the spectra ; t h i s i s due t o the influence o f L i i n A s i t e s through the HSTHF :

AH^^^^)^

⁼

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'

2 kOe. H corresponding t o 0 L i n.n. has the sa- me value than i n pure L i f e r r i t e (H = 533 f 2 kOe).

6A = 0.33 0.02 I I U ~ / S , 6B = 0.48

+ -

^0.02 ^{m / S .}

I n conclusion, we have pointed o u t the e x i s - tence o f r e l a x a t i o n i n L i - T i f e r r i t e s ; such a r e - l a x a t i o n may be r e l a t e d t o l o c a l i z e d canting due t o non magnetic n.n. We have measured AHSTHF i n L i - T i and Li-Zn f e r r i t e s and showed t h a t

AH^^^^)^

f o r L i - T i f e r r i t e i s higher t h a t the values u s u a l l y reported.

The authors wishes t o thank Dr Simonet f o r providing w i t h the samples.

REFERENCES

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/14/ J.L. Dormann, A. Tomas and M. Nogues (unpubl ishdd)

.

Fig. 2 Mtissbauer spectra o f L i - T i f e r r i t e s , t=0.735