HIGH FIELD MAGNETIZATION STUDY OF SODIUM-ZINC SPINEL FERRITES

HAL Id: jpa-00224948

https://hal.archives-ouvertes.fr/jpa-00224948

Submitted on 1 Jan 1985

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

HIGH FIELD MAGNETIZATION STUDY OF SODIUM-ZINC SPINEL FERRITES

X. Obradors, M. Pernet, Maxime Vallet, A. Isalgué, J. Rodriguez, A. Labarta

To cite this version:

X. Obradors, M. Pernet, Maxime Vallet, A. Isalgué, J. Rodriguez, et al.. HIGH FIELD MAGNETI-

ZATION STUDY OF SODIUM-ZINC SPINEL FERRITES. Journal de Physique Colloques, 1985, 46

(C6), pp.C6-445-C6-448. �10.1051/jphyscol:1985686�. �jpa-00224948�

JOURNAL

DE

PHYSIQUE

Colloque C6, suppldment au n09, Tome 46, septembre 1985 page (36-445

HIGH FIELD MAGNETIZATION STUDY OF SODIUM-ZINC SPINEL FERRITES

X. Obradors, M. pernet+, M. Vallet ++ , A. Isalgu6, J. Rodriguez and A. Labarta

FacuZtat de Fisica, U n i v e r s i t a t de BarceZona, DiagonaZ

645,

08028 BarceZona, Spain

+ ~ a b o r a t o i r e de CristaZZographie,

C.N.R.S.,

associg

a'

Z 'U.S.M.G.,

166 X ,

38042 Grenob Ze Cedex, France

+ + ~ e ~ a r t a m e n t o ~ u i r n i c a Inorg&ica, Universidad ConrpZutense de Madrid, Ciudad Universi ta r i a , 28040 Madrid, Spain

R6sum6 - ^A partir des mesures d'aimantation et de diffraction des rayons X nous montrons que la structure magnktique des spinelles Na Znl-2xFe2+x04

X

est non - collinkaire avec un angle moyen de canting supgrieur celui des spi nelles isomorphes Li - Zn. Les dsultats exp6rimentaux sont analys6s dans le cadre d'un modsle modifii: de spin canting. Le comportenent de l'aimantation saturation est bien repmduit avec ce modGle contrairement la susceptibili- tk diffgrentielle A fort champ. On en conclut que le modsle de spin canting est trop grbssier pour expliquer certains resultats exp6rimentawc.

Abstract - From isothennal magnetization and X - ray diffraction measurements it is shown that NaxZnl-2xFe2+x04 spinels own a n o n

-

collinear magnetic st- ture with a mean canting angle greater than the isomorphous Li - ^Zn spinels.

The experimental results are analyzed in terms of a modified random spin can- ting model. The behaviour of saturation magnetization is well reproduced with this model but not that of the high field differential susceptibility.

It

is argued that the crudeness of the random spin canting model prevents a full ex- planation of the experimental results.

I - INTRODUCTION

Mixed spinel oxides are very interesting materials because the existence of compe- ting antiferromqnetic interactions give rise to new magnetic phenomena such as spin glass behaviour /I/. For instance, from the early times of ferrimagnetism the exis- tence of non collinear spin structure in disordered spinel ferrites was known and different theoretical models were proposed to account for it /2/. However, the mag- netic behaviour of these compounds is complex and a correlation among experimental data arising from magnetization

/2/,

neutrondiffraction

/3/

and Missbauer spectros- copy /4,5/ measurements is still lacking.

It was reported recently

/6/

that new disordered spinels with conposition

NaxZnl-2xFe2+x04 can be prepared with the content x=0.25. From a magnetic point of view these compounds could be considered equivalent to Li

-

Zn spinel ferrites,ho- wever, because of the greater size of Na ions the intrinsic magnetic properties may be strongly modified

/ 7 / .

In this work we report a study of the low temperature saturation magnetization and differential susceptibility of Na Zn

0

spinels together with a X

-

ray dif- fraction investigation of cation 8is%~%%$~.~~ina~y, a theoretical analysis of the experimental results is carried out in the scope of the spin canting model /2/.

The synthesis procedure and elementary structural characterization of the Na

-

Zn spi nels were reported earlier

/ 6 / .

Two series of compounds differing in their thermal treatments were prepared: i) A series with

a

slow cooling from T

=

1273 K a t p T

=

50 K / h, ii) B series where the samples were quenched to liquid N2 from T

=

1273 K.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985686

C6-446 JOURNAL D E PHYSIQUE

The i m t k m d magnetization curves of both s e r i e s were measured a t T = 4.2 K i n mag- netics f i e l d s up t o H = 150 KOe by using the axial extraction technique i n connection with a B i t t e r magnet of the Service National des Champs Intenses, Grenoble. The ca- tion distribution of A-Na0.25%.5Fe2.2504 compound was investigated from an X-ray diffraction pattern collected i n an automatic Siemens D500 powder diffractometer using pyrolithic graphite monochromatised CuK,radiation. The explored angular ran- ge w a s 12O ,( 2 8 6 120° and the angular step was he = 0.05O

.

The analysis of the dif- fraction pattern was carried out by the Rietveld method by using the GBW3.2 version of the Wiles and Young program /8/. The crystal structure refinement w a s made with pseudo

-

Voigt profile f o r the diffraction ,peaks and an overall isotropic tempera- ture factor.

I11 - RESULTS

In Figure 1 the measured isothermal magnetization curves of A s e r i e s compounds are shown. A similar s e t of measurements were carried out f o r the B s e r i e s compouhds.

F m these curves it is seen that magnetic saturation was reached, then i n the high f i e l d region the magnetization curves may be represented by M = M,+JH

,

where M, is the saturation magnetization and is the d i f f e r e n t i a susceptibility.

1-2xFe2+x04 spinels a t T=4.2 K.

Fig.1

-

Isothermal magnetization curves of A-Na

Zn

I n Figure 2 the composition dependence of

M e

is indicated together with the values reported f o r Li

-

Zn spinel f e r r i t e s /3/. The experimental values o f X are reported i n Figure 3. The differences of M, and

X

values among A and B s e r i e s were lower than about 2%. Then we may conclude that thermal treatment has a minor influence on the cation distribution and magnetic properties. In Table 1 we s m i z e the ;e- fined structural parameters of A-Na0.25Zn0.5Fe2.2504. Our results show that Na ions only occupy the octahedral B s i t e s , similarly t o L i i n L i

-

Zn spinel f e r r i t e s /9/.

The occupation of Zn(2+) ions, on the other hand, could not be conclusively stabli- shed because of the d l difference among Fe(3+) and Zn(2+) atomic form factors.

I f we assume that Zn ions only occupy the tetrahedral A s i t e s , as i n L i - Zn spinel f e r r i t e s /9/, the calculated saturation magnetization i n the scope of a Ngel type collinear spin structure i s much higher than the experimental values. Hence, both the low values of saturation magnetization add the experimental high f i e l d diffe- rential susceptibility allow us t o conclude that the spin structure of a l l the stu- died compounds is non collinear.

Fig.

2 -

Saturation magnetization versus composition.

A

Na -

Zn

ferrites,. Li

- Zn

ferrites. The lines are the results of the calculations with the indicated values o f & = J bb / Jab, where

Jbb

and

Jab

stand for octahedral-octahedra1 and tetrahedral- octahedral exchange interactions, respectively.

Fig.

3

- High field differential susceptibility

as

a function of the composition.

The continous lines are the results of the calculations with s

⁼

^Jbb / Jab= 0.40 and Jab values indicated.

IV - RANDOM SPIN CANTING MODEL AND DISCUSSION

In order to explain the non collinear magnetic stmcture of Na - Zn spinels we have

developped a theoretical analysis of the saturation magnetization and high field di-

fferential susceptibility in the scope of a modified random spin canting model.

Our

model assumes that the magnetic interactions may be represented

w i t h an

isotropic

Heisenberg Hamiltonian. In order to get self consistent local canting angles we have

minimized

:

JOURNAL

DE

PHYSIQUE

6

F' = F

+ 1

( c o s

6 -x ^picas

^{Qi ) where}

^{h is}

a Lagrangian m u l t i p l i e r which a l l o w i=O

t o maintain t h e s e l f c o s i s t e n c y among

Bi

and v a l u e s and F i s t h e f r e e energy at T = 0 K. We n o t e t h a t t h i s formulation minimizes t h e t o t a l magnetic f r e e energy and n o t t h e l o c a l magnetic energy. In Figure 2 t h e c a l c u l a t e d M v a l u e s are shown f o r d i f f e r e n t

d

^{= Jbb}

/

Jab exchange i n t e r a c t i o n s r a t i o s . It may b e , s e e n t h a t a n o v e r a l l agreement among experimental and c a l c u l a t e d values a r e got. We n o t e , however, t h a t t h e b e s t

S

values f i t t i n g t h e experimental d a t a of Na

-

Zn and L i

-

Sn s p i n e l s a r e q u i t e d i f f e r e n t . Thus, it seems t h a t t h e i n c r e a s e i n t h e i n t e r a t o m i c d i s t a n c e s induced by Na i o n s l e a d t o a modification of exchange i n t e g r a l s i n such a way t h a t an i n c r e a s e of t h e mean c a n t i n g a n g l e is produced. This is i n agreement with t h e high s e n s i b i l i t y of t h e non c o l l i n e a r ground s t a t e s t o small v a r i a t i o n s of exchange i n t e g r a l s / 7 / .

F i n a l l y , we have c a l c u l a t e d t h e composition dependence of t h e d i f f e r e n t i a l suscep- t i b i l i t i e s f o r d i f f e r e n t values of Jab (Figure 3 ) . From t h e comparison of t h e o r e t i - c a l and experimental values it can be seen t h a t even t h e q u a l i t a t i v e behaviour is d i f f e r e n t . Thus, it appears t h a t random s p i n c a n t i n g models have a l i m i t e d success i n e x p l a i n i n g t h e magnetic p r o p e r t i e s of t h e disordered s p i n e l f e r r i t e s . Moreover, t h e s p i n g l a s s behaviour previously reported /lo/, and which we have a l s o observed i n a s e t of experiments, now i n p r o g r e s s , s u g g e s t s t h a t a deeper c o n s i d e r a t i o n of t h e exchange f r u s t a t i o n e f f e c t s must be done.

Table 1

-

Cation d i s t r i b u t i o n andcrystallographic parameters o f A-Na0.25%.5Fe2.504

a

(i)

u A site

B

s i t e

B

( i 2 )

REFERENCES

/1/ V i l l a i n J.

,

Z. Phys. B-Condensed Matter B33, 31 (1979).

/2/ P a t t o n C.E., Liu Y.

,

J.Phys. C-Solid S t a r 1 6 ,5995 (1983).

/3/ Maltsev V.I., Vologin V.G., Phys.Stat. Sol

(3

8 5 ,529 (1984).

/4/ Dormann J.L., Rev.Phys. Appl.

2,

1113 (19801.- /5/ Barghava S.C.

,

Z e m N. ,Phys.Rev. B

11,

1726 ( 1980)

/6/ V a l l e t M.

,

P a r r a s M., Obradors X. ,Pernet M. ,Rodriguez J.

,

J o u b e r t J .C. , I .E .E.E Trans. on Pkgn. MAG20, 1515 (1984).

/7/ Globus A. ,Pascard H. ,Cagan V., J.Physique 38, C1-163 (1977).

/8/ Young R.A. , W i l e s D.B., J.Appl.Cryst. 15, 4 z (1982).

/9/ Nogues M. , D o m J.L. ,per& ~ . ; ~ i m o G t W. ,Gibort P., I.E.E.E. Trans. on Magn.

MAG15, 1729 (1979) ..

/lo/

P a t t o n C.E. ,Edmonson C.A.

,

^{Liu Y.H.}

,

J .Appl .Phys. 53,2431 - ( 1982).