*Corresponding author. Tel.: #1-905-525-9140; Fax: #1-905-546-1252.E-mail address: waltond@mcmail.mcmaster.ca (D. Walton).

Journal of Magnetism and Magnetic Materials 226}230 (2001) 1948}1950

Magnetic blocking in ferronematic liquid crystals

Derek Walton��*, Suhaila M. Shibli�

�Department of Physics and Astronomy, McMaster University, 128 Main Street West, Hamilton, Ontario, L8S 4M1, Canada�Instituto de Fisica, Universidade de SaJ o Paulo, SaJ o Paulo, Brazil

Abstract

It is self-evident that ferro#uid particles added to liquid crystals to form ferronematics must be blocked at roomtemperature in order to be able to orient the ferronematics at low "elds (&10��T). Magnetite particles &10 nm arecommonly used, but their blocking temperature is &100K. We will show that they are indeed paramagnetic in theferro#uid, but become blocked when added to the liquid crystal. � 2001 Elsevier Science B.V. All rights reserved.

Keywords: Ferronematics; Superparamagnetism; Ferro#uids; Magnetic relaxation

While both ferro#uids and nematic liquid crystals canbe oriented in a magnetic "eld, the "elds required are&1T. The addition of a few percent of a suitable ferrof-luid to the liquid crystal lowers the "eld required fororientation to &10��T.

The theory of ferronematics is provided in the classicpaper of Brochard and De Gennes [1]. Their approach ispredicated on the presence of single domain magneticparticles whose moment is static, i.e., that the magneticgrains are blocked. The grain size in ferro#uids used tomake ferronematics is on the order of 10 nm. Normallygrains of this size are superparamagnetic at room tem-perature. If that were the case, the moment of the grainwould reverse so often that it would be impossible for itto orient the ferronematic. Indeed it was found that verysmall magnetic particles, &3 nm were incapable oforienting a ferronematic, but the larger grains could.

The lyotropic liquid crystals employed are a mixture ofpotassium laurate/1-decanol/water (KL/DeOH/H

�O),

with concentrations in weight percent of 28.90/7.10/64.0,respectively.

The ferro#uid consisted of magnetite particles, withtypical dimensions of about 10 nm, coated with disper-sive agent (oleic acid) to prevent their aggregation [2,3]

dispersed in water. The concentration was about 10��magnetic particles /cc.

Grains of Fe�O

�of this size can be expected to be

superparamagnetic, and magnetization measurementscon"rm this: The magnetization as a function of time wasmeasured with a SQUID magnetometer. The sampleswere sealed in Pyrex container whose volume was 0.5ml.for the diluted samples. In order to keep the total mo-ment within the range of the magnetometer, a muchsmaller volume was 0.05ml. was used for the concen-trated ferro#uid.

While it was possible to perform the measurements atroom temperature that would result in a contribution tothe magnetization from the physical rotation of thegrains which is uninteresting in the present context. Thusthe freshly prepared samples were cooled to 265K in zeromagnetic "eld, and frozen in a magnetically disorderedstate. A "eld of 0.01T was then applied and the momentmeasured as a function of time.

The results for the ferronematic, a pure ferro#uidsample, and a sample diluted to the same concentrationas the ferronematic are shown in Fig. 1. It is clear that therelaxation times in both ferro#uids are similar, and muchshorter than those in the ferronematic.

Nematic liquid crystals are birefringent, and the relax-ation of the directors can be observed optically. Thusoptical measurements were also performed and the op-tical relaxation, shown in Fig. 2, is very similar to themagnetic relaxation of the ferronematic. The time scalesare di!erent, of course, because the optical relaxation

0304-8853/01/$ - see front matter � 2001 Elsevier Science B.V. All rights reserved.PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 0 1 9 - 1

Fig. 1. Time dependent magnetization of ferronematic and pureand diluted magnetite ferro#uid samples at ¹"265K.

Fig. 2. Optical measurement of the relaxation of the directororientation in a ferronematic sample at room temperature ina 0.01T "eld. The director orientation was "rst established ina 1T "eld, then the "eld was rotated 903, and reduced to 0.01 T.

Fig. 3. Guinier plot of small angle X-ray scattering data at room temperature for a concentrated, and dilute magnetite ferro#uid sample.

involves the rotation of the directors, as well as themagnetic relaxation.

A simple explanation for this behavior would be thatthe particles cluster [4]. This is plausible because it is

known that ferro#uid particles tend to aggregate andcome out of solution upon dilution of the ferro#uid. Inorder to investigate this possibility small angle X-rayscattering (SAXS) measurements were made at the

D. Walton, S. M. Shibli / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1948}1950 1949

synchrotron located at the National Synchrotron LightLaboratory (LNLS)*Campinas, Brazil.

The results yielded a radius of gyration for the grainsin the ferro#uid of 5.6 nm, obtained from the straight linein the Guinier plot [5,6] of the SAXS curves (Fig. 3). Thisagrees closely with an average particle size of 10 nm sizeobserved in electron microscopy [7,8].

When the ferro#uid is diluted with water to the sameconcentration as that in the ferronematic, the radius ofgyration obtained from the other straight line shown inFig. 3 is 4.7 nm; so, if anything, the radius of gyration hasdecreased slightly. Had agregation occurred it wouldhave to increase. Furthermore, the time dependent mag-netization is also very similar (Fig. 1), and electronmicroscopy reveals no evidence of aggregation.

Aggregation is not responsible for the increased relax-ation time, therefore, it must be concluded that the in-crease is the result of some interaction between the liquidcrystal and the individual magnetic grains of the ferro-#uid. Brochard and De Gennes show that a substantialmechanical coupling exists between the micelles of theliquid crystal and the magnetic grains, which is in factresponsible for the mutual alignment of the two sets of

particles. Magnetite has a large magnetostriction, andestimates [9] indicate that the stress in the magneticgrain produced by its displacement of the micelles may besu$cient to raise the blocking temperature of the grain.

We are grateful for support from NSERC of Canada,FAPESP of the state of S. Paulo, and LNLS, Brazil.

References

[1] F. Brochard, P.G. de Gennes, J. Phys. (Paris) 31 (1971) 691.[2] A.R.V. Bertrand, Rev. Inst. Fr. Petr. 25 (1970) 15.[3] R.E. Rosensweig, Ferrohydrodynamics, Cambridge

University Press, Cambridge, MA, 1985.[4] A. M. Figueiredo Neto, private communication, 1999.[5] A. Guinier, G. Fournet, Small Angle Scattering of X-Rays,

Wiley, New York, 1955.[6] A. Guinier, Theorie et Technique de la Radiocristallog-

raphie, Durond, Paris, 1956.[7] L. Neel, Ann. Geophys. 5 (1949) 99.[8] R.S. Carmichael, Physical Properties of Rocks and

Minerals, CRC, Boca Raton, Florida, 1989.[9] D. Walton, S.M. Shibli, to be published.

1950 D. Walton, S. M. Shibli / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1948}1950