Barium Hexaferite

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*Corresponding author. Tel.:#49-3641-302-711; fax:#49-

3641-302 799.

E-mail address: [email protected] (R. MuK ller)

Journal of Magnetism and Magnetic Materials 201 (1999) 34}37

Barium hexaferrite ferro#uids } preparation andphysical properties

R. MuK ller*, R. Hiergeist, H. Steinmetz, N. Ayoub, M. Fujisaki, W. SchuKppel

Institut f u(r Physikalische Hochtechnologie e.V. Jena, Helmholtzweg 4, D-07743 Jena, Germany

Yarmouk University, Department of Physics, Irbid, Jordan

Abstract

Barium hexaferrite BaFe\V

TiV

CoV

O

ferro#uids have been prepared for the "rst time using oleic acid as surfactant

and Isopar M as carrier liquid. The initial susceptibility versus temperature for zero-"eld cooling of the ferro#uid was

obtained by a vibrating sample magnetometer. TEM pictures of the #uid show isolated particles and only small

agglomerates and a mean particle diameter of approx. 8 nm. Numerical calculations of the magneto-viscous e!ect, based

on the local-equilibrium magnetic state model, clearly show the bene"t for Ba-ferrite ferro#uids resulting from the high

uniaxial anisotropy compared to magnetite ferro#uids. Rheological measurements were performed with a rotational-type

viscometer with magnetic "eld perpendicular to the hydrodynamic vortex axis. 1999 Elsevier Science B.V. All rights

reserved.

Keywords: Ferro#uids; Rheological properties

1. Introduction

Ba}ferrite (Ba}F) magnetic liquids are notknown in the literature up to now. One reason

could be that usual powder preparation methods

supply ferrite particles with mean diameters above50 nm, which results in a fast sedimentation of the

particles in the liquid. Using a modi"ed glass crys-

tallization method [1] we were able to prepare

uniaxially anisotropic single-crystalline Ba}F par-

ticles with mean sizes below 15 nm and with a high

anisotropy constant. Numerical calculations of the

magneto-viscous e!ect for individual spherical par-

ticles in the framework of the local-equilibriummagnetic state (LEMS) model, presented in this

paper, show that a much higher speci"c viscosity

gain should be obtained for barium hexaferriteferro#uids (FF) in comparison to FFs based on

magnetite for particles of the same volume.

2. Preparation

For preparation of such "ne BaFe\V

TiV

CoV

O

powders (x"0.8 and 1.2) a glass melt

4 0 B a O!33 B

O!2 7 ( F e

O

#CoO#TiO

)

(mol%) was quenched with approx. 10 K/s to

0304-8853/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 3 0 4 - 8 8 5 3 ( 9 9 ) 00 0 1 6 - 5


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Fig. 1. TEM picture of a Ba}ferrite ferro#uid.

Fig. 2. Initial susceptibility (magnetic moment) vs. temperature

after ZFC measured at H"4 kA/m. The irregularity (see arrow)

is caused by melting of the carrier liquid.

obtain amorphous #akes. Crystallization of ferrite

particles within a barium borate matrix was

achieved by heat treatment of the #akes at 5803C

for 10 days. The ferrite particles were isolated by

dissolving the matrix in acetic acid. The slurry was

rinsed and freeze dried. The powder shows almost

pure superparamagnetic behaviour. Typical valuesof the magnetic parameters (for x"0.8) are: specif-

ic saturation magnetization: 23.2 Am/kg; coerciv-

ity: 0.95 kA/m; relative saturation remanence:

0.006. Its speci"c surface (measured by BET) is

126 m/g which means an average particle diameter

of approx. 9 nm. The crystalline lattice shows a dis-

torted hexaferrite structure and must be subject tofurther investigation. Ba}F FFs from these nano-

crystalline powders were prepared by use of oleic

acid as surfactant and Isopar M

(EXXON) ascarrier liquid: NaOH and oleic acid were added to

the powder to encourage the surfactant in the form

of oleate to adhere to the surface of the particles.

The amount of oleic acid has to "t to the speci"c

surface of the powder. The oleate was converted

again to oleic acid by adding nitric acid in a next

step. All the mixing steps were done in a ball mill in

order to destroy large agglomerates. Then the sub-

stance was washed by acetone and hot water alter-

nately. After sedimentation in acetone the

supernatant liquid was removed and the carrierliquid was added. The number of aggregates was

reduced by centrifuging the sample at 5000 g. The

supernatant liquid now is the FF. A TEM picture

of a concentrated suspension (Fig. 1) shows isolated

particles and only small agglomerates. From this

picture a particle size distribution with a mean

particle size in the FF of about 8 nm could be

determined which is in good accordance with the

value (6.9$0.5) nm calculated from the initial

magnetic susceptibility.

3. Magnetic investigations

The initial susceptibility versus temperature

curve in the range from 4.2 K up to room temper-

ature for zero-"eld cooling (ZFC) was obtained by

VSM. Fig. 2 shows such a curve for our Ba}F FF,

with the typical maximum in the curve character-istic of"ne particle systems in the transition region

between superparamagnetic and stable ferrimag-

netic behaviour [4]. The temperature (approx.175 K) at the maximum suggests an average block-

ing temperature of approx. 120 K and from this an

average anisotropy constant of approx. 240 kJ/m

at this temperature. This value of the anisotropy

constant agrees with a value obtained in Ref. [5] by

a di!erent method. Further magnetic studies on

Ba}F FF were carried out in Ref. [5].

4. Rheological investigations

Numerical calculations of the reduced viscosity

gain due to the magneto-viscous e!ect for the case

of individual spherical particles were done in the

frame of the LEMS model [3]. For these we used

for Ba}F HI"375 kA/m at 203C which gives

a uniaxial crystalline anisotropy energy of

R. Mu(ller et al. /Journal of Magnetism and Magnetic Materials 201 (1999) 34}37 35


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Fig. 3. Reduced viscosity gain /(c

) (c: volume fraction of

particles) vs. applied "eld and volume for spherical particles of

Ba}F (a) and magnetite (b).

Fig. 4. Zero-"eld viscosity vs. volume fraction c of Ba}F. Note

the transition from linear increase for dilute FF (a) towards

a linear behaviour at concentrated FF (b).

Fig. 5. / vs. "eld strength of a concentrated #uid. Note thetransition from linear increase at low "elds (a) towards a linear

behaviour at medium "eld strength (b).

70.5 kJ/m

. This is identical with the energy barrierwhich has to be surmounted in order to rotate the

magnetisation out of the preferred [0 0 1] direction.

In Fe

O

the barrier depth due to crystalline an-

isotropy is much lower: According to Ref. [2] we

have to consider for spherical Fe

O

particles

(at 203C) K"!13.35 kJ/m and K

"

!3.35 kJ/m. For the preferred [1 1 1] direction

we obtain an energy of!4.28 kJ/m. The absolute

value of this energy is an upper limit for the barrier

and is used in these calculations.

The results (Fig. 3) show a much higher speci"cviscosity gain for Ba}F FFs compared with

Fe

O

FFs for the same particle volumes. It should

be mentioned that for nonspherical particles the

anisotropy energy barrier can be higher. Therefore,

the di!erence in the speci"c viscosity gain

can be reduced due to shape e!ects. We can con-

clude that for Ba}F FFs it is easier to obtain highmagneto-viscous e!ects even for low particle vol-

ume and therefore low weight. This may result in an

improved stability of the FF in the gravitational"eld.

Measurements of the rheological properties were

performed with a rheometer of combined searle-

and couette-type concentric cylinder double gap

geometry. Results for di!erent Ba}F volume frac-

tions in zero "eld are shown in Fig. 4. A compari-

son of the data of the linear "t (a) with Einstein's

law [6] "

(1#c) (c is the particle concentra-

tion) reveals that we have to consider a 1.57;larger

particle size than expected from the data of the

Ba}F volume fraction, which were obtained by

measurements of the speci"c weight of the FF. This

is a result of the additional hydrodynamic particle

diameter due to the surfactant, which has for oleicacid a size of about 2;2 nm. Therefore, we obtain

a mean Ba}F particle diameter of approx. 7.0 nm,

which corresponds with the previous results. Fromthe good "t of the linear slope (a) even for volume

fractions up to 0.05, i.e. the missing of terms of

higher order in c, we may conclude that these terms,

which are due to particle}particle interactions, are

suppressed. Results of "eld-dependent measure-

ments of a concentrated #uid with magnetic "eld

perpendicular to the hydrodynamic vortex-axis are

shown in Fig. 5. Measurements of dilute #uids

(c"0.013) reveal even higher values up to/

)7 at 520 kA/m. /

*

cannot be

36 R. Mu(ller et al. /Journal of Magnetism and Magnetic Materials 201 (1999) 34}37


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explained by the LEMS model. Collective phe-

nomena as e.g. the formation of particle chains with

increasing "eld have to be considered in order to

explain these results.

Acknowledgements

This work was supported by DFG under Schu

1023/2 and GTZ 93.2157.1-06.100.

References

[1] R. MuK ller et al., Proceedings of Electroceramics IV 1994,

Aachen, vol. II, p. 1183.

[2] L.R. Bickford et al., Proc. Inst. Electr. Eng. B 104 (1957)

238.

[3] A. Cebers, Magnetohydrodynamics 11 (4) (1975) 439.

[4] H. Pfei!er, R.W. Chantrell, J. Magn. Magn. Mater. 120

(1993) 203.

[5] N.Y. Ayoub et al., J. Magn. Magn. Mater. 201 (1999) 119.

[6] J.W. Goodwin, Colloid Sci. 2 (1975) 246.

R. Mu(ller et al. /Journal of Magnetism and Magnetic Materials 201 (1999) 34}37 37

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