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Journal of Magnetism and Magnetic Materials 310 (2007) 2399–2401

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Hysteresis losses in iron oxide nanoparticles prepared by glasscrystallization or wet chemical precipitation

Robert Mullera,�, Silvio Dutza, Rudolf Hergta, Christopher Schmidta,b, Hanna Steinmetza,Matthias Zeisbergera, Wolfgang Gawaleka

aInstitute for Physical High Technology, Albert-Einstein-Street 9, 07745 Jena, GermanybDepartment Materials Engineering, University of Applied Science, 07745 Jena, Germany

Available online 17 November 2006

Abstract

Ferrofluids were prepared from glass crystallized as well as wet precipitated iron oxide particles. Comparing hysteresis losses versus

applied field amplitude from particles in immobilized state (powder) and in fluid state (ferrofluid) shows in some cases anomalous large

losses at low magnetic fields. The influence of texture on the losses was investigated.

r 2006 Elsevier B.V. All rights reserved.

PACS: 75.50Mm; 75.50Tt; 75.60

Keywords: Magnetic particle; Ferrofluid; Hysteresis loss; Texture

1. Introduction

Magnetic iron oxide particles are promising materials formedical applications, e.g., hyperthermia for tumor treat-ment. The heat generation of these materials in ACmagnetic fields is based on Neel or Brown relaxation orhysteresis losses. Using Brown losses in medical hyperther-mia is problematic as the mobility of the particles in thehuman tissue is hard to control, and the particles can evenbe completely immobilized. For such particles, the Neelrelaxation is the dominant loss mechanism at smalldiameters (o20 nm for magnetite) [1]. For larger particlesand sufficient field amplitude (4Hc), the heat is generatedby hysteresis losses. As the usual preparation methodsresult in a more or less broad distribution of particle sizes,even samples with a mean diameter o20 nm contain acertain fraction of hysteresis-dominated particles.In this paper, we report about two methods of

preparation with the aim to obtain particles beyond thesuperparamagnetic range. Therefore, the magnetic char-acterization is focused on the measurement of the hyster-

- see front matter r 2006 Elsevier B.V. All rights reserved.

/j.jmmm.2006.10.772

onding author. Tel.: +493641 206109; fax: +49 3641 206199.

ddress: robert.mueller@ipht-jena.de (R. Muller).

esis losses. To influence the size distribution we work ontechniques where nucleation and growth of particles can beinfluenced independently.

2. Particle and ferrofluid preparation

The preparation of magnetic iron oxide powder bycrystallization from CaO–Fe2O3–B2O3–glass during atemperature treatment and subsequent dissolving of thematrix was shown in Ref. [2]. Two separated heating stepsfor nucleation and particle growth at temperatures of500–700 1C were carried out.The separated powders show magnetization values up to

70Am2/kg close to literature values of g-Fe2O3. Investiga-tions by X-ray diffraction (XRD) (Xpert, Philips) showthat there is a mixture of Fe3O4 and g-Fe2O3 (JCPDS Nos.19–0692 and 39–1346). There was no hematite found byXRD. The mean particle size of the further investigatedpowder (powder 1) was about 16 nm.A water-based charge stabilized ferrofluid (fluid 1) was

prepared from powder 1 in connection with the dissolvingof the borate by acetic acid [2]. The number of aggregatesin the fluid was reduced by centrifugation which may resultin a change of the size distribution between particles in

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Fig. 1. Hysteresis losses of powder and fluid samples in dependence on the

applied field amplitude.

Fig. 2. Ratio of hysteresis losses at different directions with respect to the

texture axis (01) versus applied field strength.

R. Muller et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2399–24012400

flake and in fluid. The particle concentration of theobtained fluid is E1.0mass%.In a second method [3], we prepared our particles by

growing cycle by cycle on initial particles, which were pre-pared by the usual wet precipitation method. A NaHCO3

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solution was slowly added to a FeCl2/FeCl3 solution up topH 7, which lead to the formation of a brownishprecipitate. Then a new Fe2+/Fe3+ mixture was addedand the precipitation was carried out again. This procedurewas repeated three times. After that the solution was boiledfor 10min to form an almost black precipitate. Particles inthe size range from 10 to 30 nm could be prepared.For medical in vivo applications of iron oxide nano-

particles, a biocompatible coating is necessary. We usedcarboxymethyl dextran (CMD) starting from initial mate-rial CMD sodium salt (Fluka). Typically, the nanoparticledispersion was separated magnetically and washed threetimes with water. After adjusting the pH with diluted HClto 3–4, the suspension was warmed to 45 1C and anaqueous solution of CMD was added. The mixture washomogenized by ultrasonic treatment for 1min using aSonopuls GM200 (Bandelin) device. The suspension wasstirred for a further 60min at 45 1C and the coatednanoparticles were separated magnetically and washedonce with water.The ferrofluids with particles oca. 17 nm are stable

concerning sedimentation. Typical dried fluid samplesshow a mass loss (mainly at E220 1C) by thermogravi-metry (Netzsch STA409) of 5–7% what can be interpretedas CMD amount. After about two weeks a considerableagglomeration could be observed in all samples, whichprobably results from an aging effect of the CMD layer.Mean particle sizes by XRD measured on samples

prepared by a three-cycle procedure with bigger iron oxideparticles taken from the sediment as well as from the stablesupernatant after 1 day are 17.4 and 21 nm, respec-tively. The mean size of the original sample (fluid 2) was19.3 nm.The distribution of the hydrodynamic diameter (number

weighted) measured by dynamic light scattering byM. Kettering (University Jena) has a mean value at about80 nm. The coercivity of the immobilized ferrofluid is4.1 kA/m. Powder 2 was prepared by drying of fluid 2.

3. Hysteresis loss investigations

The specific hysteresis loss power at the usual fieldparameters (410 kHz, 11 kA/m) of the dried powder andthe fluid is 21 and 43W/g for sample 1, respectively, and 22and 48W/g for powder 2 and fluid 2. Hysteresis lossinvestigations on ferrofluid samples with different meansizes of 15.8 and 19.3 nm (i.e., different sedimentationstability) revealed interesting behavior. Comparing lossesversus applied field amplitude (Fig. 1) from particles inimmobilized state (dried powder) and in fluid state(ferrofluid) show in some cases (like fluid 2) anomalouslarge losses at low magnetic fields whereas all powder

samples show a similar behavior as already found by Dutzet al. [4].In order to investigate whether there is a field-dependent

influence of an orientation of particles in the sample onspecific hysteresis losses, a Brown relaxation after texturingin a magnetic field of 39.8 and 796 kA/m, respectively, wasreduced using a solid gel sample made from fluid 2.Magnetization loop measurements were performed independence on the direction of the external magnetic field(parallel or perpendicular) with respect to the texture axis.Fig. 2 shows the ratio of the values of the specific losses inboth directions in dependence on the external magneticfield amplitude. The alignment effect of particle momentsmay be clearly seen at lower fields oca. 80 kA/m. Thespecific hysteresis losses vary by a maximum factor of morethan two in case of the high texturing field but only by afactor of 1.16 for the low texturing field. There is only aweak effect at high measuring fields close to saturation.

ARTICLE IN PRESSR. Muller et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2399–2401 2401

The origin of the high losses of the fluid 2 is not clear upto now but may come from oriented particles oragglomerates, connected with interaction effects. Viscouseffects of the fluid cannot be excluded, too.

Acknowledgments

The authors thank M. Kettering (University Jena) andCh. Schmidt. The work was supported by DFG No.Ga662/3-1.

References

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R. Hergt, W. Gawalek, Z. Phys. Chem. 220 (2006) 51.

[4] S. Dutz, R. Hergt, J. Murbe, R. Muller, M. Zeisberger,

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