PARTICLE SIZE AND MAGNETIC PROPERTIES OF BaFe12O19 PREPARED BY THE ORGANOMETALLIC PRECURSOR METHOD

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PARTICLE SIZE AND MAGNETIC PROPERTIES OF BaFe12O19 PREPARED BY THE

ORGANOMETALLIC PRECURSOR METHOD

Maxime Vallet, P. Rodriguez, X. Obradors, A. Isalgué, J. Rodriguez, M.

Pernet

To cite this version:

Maxime Vallet, P. Rodriguez, X. Obradors, A. Isalgué, J. Rodriguez, et al.. PARTICLE SIZE AND MAGNETIC PROPERTIES OF BaFe12O19 PREPARED BY THE ORGANOMETAL- LIC PRECURSOR METHOD. Journal de Physique Colloques, 1985, 46 (C6), pp.C6-335-C6-338.

�10.1051/jphyscol:1985661�. �jpa-00224916�

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J O U R N A L DE PHYSIQUE

Colloque C6, suppl6ment au n09, Tome 46, septembre 1985 page C6-335

PARTICLE SIZE AND MAGNETIC PROPERTIES OF BaFe12019 PREPARED BY THE ORGANOMETALLIC PRECURSOR METHOD

M. Vallet, P. Rodriguez, X. ~bradors+, A. 1salgu6+, J. ~odriguez' and M. pernet++

Quimica Inorgbnica, Universidad Conrplutense Madrid, 28040 Madrid, Spain ' ~ a c u z t a t de F i s i c a , U n i v e r s i t a t de Barcelona, Diagonal 645, 08028 Barcelona, Scain

+ Laboratoire de C r i s t a Llographie, C. N.R. S . , a s s o c i h d l ' U . S . M . G., 166 X , 38042 Grenob Ze Ceder, France

RCsum6 - Des particules monodomaine de BaFe1201g ont 6t6 prepar6es par la methode des precur-seurs organometdlliques & basses tempera-tures de frittage.

On rnontre que la taille des particules peut Btre facilement contr816e & par- tir de la temperature et dutemps de &action. L'aimantation $ saturation des

rains

fins diminue avec la diminution de la caille. La variation du champ coercitif avec la taille des particules peut Stre expliquie avec la theorie du superparamagngtis~ne et la nuclgation des domGnes. On montre qu'il n'y a pas de comportement superparamagngtique dans l'echelle des temps de la spectroscopie Wijssbauer.

Abstract - Single domain BaFe1201g particles have been prepared by using the organometallic precursor method with low reacting temperatures. It is found that an easy control of particle size may be obtained by controlling the firing temperature and reaction time. Saturation magnetization of the fine particles decreases with decreasirlg par'ticle size. The dependence of the coercive field on particle size may be understood from superparamagnetic theory and mltidomain nucleation formation. It is found that no superparamagnetic behaviour does exist in the W6ssbauer characteristic time.

I

-

INTRODUCTION

In recent years a large effort has been devoted to the preparation methods of poly- crystalline M-type,BaFe12019 hexagonal ferrite. This is so because a strict control

- ~

of homogeneity, particle size and shape arid magnetic characteristics are required for technological applications such as perpendicular magnetic recording /I/.

Besides the classical ceramic method /2/, other preparative methods have been analy- zed: chemical coprecipitation /3/, hydrothermal synthesis/4/ and glass-crystallization /I/. However, very recently a new rnethod with very promisinq features has been proposed by Licci and Besagni /5/. The method consists in the formation of a solidi- fied orgarlometallic solution with the desired stoechiometry of metallic ions which is then reacted to get the desired oxide. As it has been pointed out by these authors the advantage of the method is an improved reactivity together with a good particle homogeneity and size control.

In this work we present a preliminar study of the dependence of magnetic properties of BaFe12019 particles prepared by this method on the particle size and thermal treatments.

I1 -SYNTHESIS PROCEDURE AND STRUCTURAL CHARACTERIZATION

Several BaFeL201g samples have been prepared by the organometallic precursor method proposed by iccl and Besagni / 5 / . The Fe ions were introduced as nitrates which were Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985661

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C6-336 JOURNAL DE _PHYSIQUE

dissolved i n distilled water, precipitated with an amnonium solution and washed with water u n t i l neutrality. A stoechiometric mixture of t h i s precipitate and Ba carbo- nate were dissolved i n c i t r i c acid with ethylene-glycol added in order to increase

the viscosity. Finally, t h i s amorphous organic s o l i d w a s decomposed at 723 K, and d i f f e r e n t thermal treatements were performed. I n table I we report the d i f f e r e n t treatments t o whlcn the samples were subjected. fil t h e samples between B and G had cumulative thermal treatments, while those named H and I were directly prepared from stage A.

Table I

Size (A) (+)

Sample Firing temperature Firing time T.E.M. X. R. S (m2/g)

A 723 K - - - -

B 823 K 17 h

-

- -

C 923 K 1 h 350 500 8

D 1073 K 1 h 650 790 7

E 1173 K 0.5 h 1000 -

-

F 1273 K 0.08 h 1250 950

-

G 1273 K 72 h 2300

H

923 K 42 h 300 12

I 973 K 10 500

(+) The s i z e measured by T.E.M. correspond t o the mean diameter of p l a t e l e t - l i k e par- t i c l e s , while t h a t reported from X-ray d i f f r a c t i o n is the diameter of spheres used i n the Scherrer formula.

The identification of the M-structure was made from X-ray diffractograms while the correct chemical composition of samples were ascertained by means of X-ray

fluorescence and atomic absorption analysis. The particle shape and size were

determined from transmission electron micrographs. The mean particle size was studied both from the 205 diffraction line broadenings, using an Ag powder as instrumental standard, and the classical Scherrer formula. Finally, the B.E.T. specific surface area of the particles was measured for several samples. The minimum temperature at which we got a complete transformation to M phase was 923K. Nevertheless, if the reaction temperature is augmented to 973K the time necessary to complete the reaction is strongly reduced.

In figure 1 we show some characteristj.~ electron microscopy photographs from which the mean particle size reported in table I were determined. We note that a fairly good agreement is obtained for the sizes determined from T.E.M. and X-ray diffraction.

Our results show that a full correlation among particle size, heating temperature and firing time does exist.

Figure 1 - Electron microscopy photographs of samples C, E , and G.

Thus, it i s c l e a r t h a t the present preparative method allows t o control the p a r t i c l e diameter above about 300

A.

Dloreover, theelectron micrographs show that a good homogeneity is obtained and t h a t the sintering process among the p a r t i c l e s begins

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only at about 1273 K. Finally, the measured B.E.T. specific surface areas ranged from 7 t o 12 m2/g, which a r e much lower than those reported by Kubo e t al. f o r p a r t i c l e s with s i m i l a r s i z e s prepared by the glass-crystallization procedure /6/. Thus, it seems t h a t the present method provides samples with lower porosity.

111 - MAGNETIC PROPERTIES

The saturation magnetization and coercive f i e l d s were deduced from room temperature hysteresis loops measured by means of a vibrating sample magnetometer i n magnetic f i e l d s up t o H= 60 KOe. Several room temperature 5 7 ~ e K6ssbauer spectra were also r e c o r d e d i n a convention^ spectrometer using a 10 di 5 7 ~ o : ~ h single l i n e source.

I n figure 2 we represent the dependence of saturation magnetization and coercive f i e l d on the mean p a r t i c l e diameter, as obtained from T.E.M. photographs. It may be observed t h a t a decrease of saturation magnetization does appear when the p a r t i c l e diameter is reduced below about 1250

A.

This is a c o m n phenomenon i n f i n e p a r t i - c l e s and may be understood as a r i s i n g from spin non-collinearity at the surface of the c r y s t a l s /7/. The experimental dependence of the coercive f i e l d on the p a r t i c l e s i z e is very similar t o those previously observed by Kubo e t al. /6/ i n BaFe,.O,,

L L -u

p a r t i c l e s prepared by the g l a s s c r y s t a l l i z a t i o n method. The coercive f i e l d i s maxi- rmun at a diameter of about 1000

A

while it decreases at smaller and larger diameter values. The decrease of Hc below 1000

A

may be interpreted i n the scope of the superparamagnetic theory of uniaxial f i n e p a r t i c l e s /8/, while the Hc decrease of p a r t i c l e s with higher diameter is associated t o domain w a l l nucleation /6/.

Ms

(emu lg )

Figure 2

-

Dependence of saturation magnetization and coercive f i e l d on mean p a r t i c l e diameter.

75

70

65

Finally, we recorded several room temperature Mijssbauer spectra i n order t o g e t a microscopic characterization of the samples. For instance, Mdssbauer spectroscopy would enable us t o know i f the BaFe1201g f i n e p a r t i c l e s obtained by the present method show superparamagnetic behaviour. I n figure 3 the room temperature M6ssbauer spectrum of sample C is shown. In s p i t e of being the sample with smallest s i z e the f i t t e d hyperfine parameters a r e very similar t o those found i n BaFe12019 bulk samples and sharp absorption l i n e s a r e obtained. T ~ U S , we may conclude t h a t no spin relaxa- t i o n appears within the MBssSauer characteristic time (about s). The thermal activation of spins responsible for the decrease of coercive field will have a higher relaxation time.

0

1000 2000

mean diameter ( A )

-I

-

- -

-

5000

H' ( O e )

4000

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961 , , , , , , i , , , , , ,

1 -

^9.5 ⁰ ^9.5

v e l o c i t y (mm/s)

Figure 3

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Room temperature Mijssbauer spectrum of sample C

As a final conclusion we may state that the organometallic precursor method provides homogeneous single domain BaFe1201g particles at low reacting temperatures with good magnetic properties and with an easy control of particle size.

Acknowledgements - We are grateful to J. Gonzalez-Calbet for help in the electron microscopy measurements.

REFERENCES

Kubo 0.

,

^{Ido T.}

,

Yokoyama H.

,

^IEEETrans. on Mag. MAG18 ( 1982) 1122 Kojima H., "Ferromagnetic Materials", vol. 3 , E.P. Wohlfarth ed., North- Holland (1982) p. 305

Kaneda

K.,

Miyakawa C., Kojima H., J. Am. Ceram. Soc.

57

(1974) 354 Kiyama M., Takada T., Nagai N., Horriish N., 4th Int. Conf. Ferrites, San Francisco (1984), Adv. in Ceram. ("in ^thepress")

Licci F., Besagni T., IEEE Trans. on Magn. MAG20 (1984) 1639

Kubo O . , Ido T., Yokoyama H., Koike Y., Magn. Magn. Mater. Conference, San Diego (USA) (19841, J. Appl. Phys. ("in the press")

Morrish A. H., Kaneda K., J. Magn. Magn. Mater. 35, (1983) 105

Herpin A.

,

"Theorie du Magnetisme", Presses ~niversitaires de France ( 1968)