directed assembly of bafe 12 o 19 particles...

Published: October 24, 2011

r 2011 American Chemical Society 14014 dx.doi.org/10.1021/la2032863 | Langmuir 2011, 27, 14014–14024

ARTICLE

pubs.acs.org/Langmuir

Directed Assembly of BaFe12O19 Particles and the Formationof Magnetically Oriented FilmsDarja Lisjak* and Simona Ovtar

Department for Materials Synthesis, Jo�zef Stefan Institute, Ljubljana, Slovenia

bS Supporting Information

ABSTRACT:

We have studied the preparation of oriented BaFe12O19 films produced using electrophoretic deposition (EPD). Highly anisotropic,platelike BaFe12O19 particles were synthesized under hydrothermal conditions, and from these particles, stable suspensions wereprepared in 1-butanol by the addition of dodecylbenzene sulfonic acid as a surfactant. The interplay of the interaction forces betweenthe suspended particles and the forces acting on the particles during the EPD directed the particles’ assembly in the plane of thesubstrate. The most significant effect on the orientation of the films was the diameter-to-thickness ratio of the particles, which wasexperimentally confirmed with X-ray analyses, electron microscopy, and magnetic measurements. The abnormal grain growth thataccompanied the sintering at 1150 �C further improved the overall orientation of the films, which showed highly anisotropicmagnetic behavior with a remanent-to-saturation magnetization ratio exceeding 0.8.

’ INTRODUCTION

Barium ferrite (BaF) with the chemical formula BaFe12O19 is awell-known ferrimagnetic material. Its structure is of the magne-toplumbite type,1 where two structural blocks, R (BaFe6O11

2�)and S (Fe6O8

2+), are combined in an alternating fashion, RSR*S*(where * denotes a 180� rotation) in the direction of the hexa-gonal c axis, forming a highly anisotropic crystal lattice. BaFcrystals preferentially grow in the ab plane with a high diameter-to-thickness aspect ratio, forming thin hexagonal plates withtheir easy magnetic axis coinciding with the crystallographicc axis (i.e., perpendicular to the ab plane).1,2 Consequently, BaFalso possesses a high magnetic anisotropy and a high ferromag-netic resonance and is therefore suitable for applications at milli-meter wave frequencies in nonreciprocal devices or in electro-magnetic absorbers.3�5 It was reported5 that the most suitableform of material for such millimeter wave frequency applicationswould be magnetically oriented (i.e., self-biased BaF films with athicknesses of 10�100 μm). Such films could also be used inminiaturized electronic components wheremagnets are required.This is because BaF is also a known permanent-magnet material.Most of the widely used techniques for the preparation offilms are suitable either for thin films only (i.e., pulsed-laser

deposition, sputtering)4,6,7 or for thick films with a limitedorientation (i.e., sputtering, electron-beam evaporation).8,9

However, thick, oriented BaF films were previously preparedwith liquid-phase epitaxy10 and with screen printing followedby annealing in a magnetic field.4,6,11 Another promising methodfor the preparation of thick films is electrophoretic deposition(EPD).

EPD is a well-known method that is suitable for the prepara-tion of films and coatings with various thicknesses and even bulkceramics.12 Putting it simply, EPD can be regarded as anelectrically driven assembly of charged particles. It is dividedinto two steps: (i) the flow of charged particles under an electricfield toward an oppositely charged electrode (also called electro-phoresis) and (ii) the deposition step, during which the chargedparticles deposit on the oppositely charged electrode substrate. Itis important that the particles are dispersed in a solvent and notagglomerated. The attractive van der Waals forces, responsiblefor the agglomeration, can be overcome by electrostatic, steric, or

Received: April 29, 2011Revised: October 21, 2011

14015 dx.doi.org/10.1021/la2032863 |Langmuir 2011, 27, 14014–14024

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electrosteric repulsion with the selection of a suitable surfactant.However, magnetic particles are additionally attracted by themagnetic dipole�dipole forces that are stronger at larger separa-tion distances than the van der Waals forces. Therefore, muchstronger repulsive forces are required to stabilize the suspensionsof magnetic particles than in the case of nonmagnetic particles ofthe same size. For this reason, stable magnetic suspensions, orferrofluids, are most often prepared from superparamagneticparticles.13�15 Such particles showmagnetic behavior only underan applied magnetic field. A much more challenging task is toprepare stable suspensions from hard magnetic particles thathave a permanent magnetic moment. It was shown previouslythat stable suspensions of hard magnetic BaF particles can beprepared providing there is a strong electrosteric repulsion.16

The deposition step of the EPD involves electrochemical andagglomeration phenomena.12 The initial stage of the depositionfrom very dilute suspensions can be explained by the neutrali-zation of the particles upon contact with the electrode.17

However, other mechanisms explain the formation of thickdeposits: (i) the reduction of the repulsive forces between theparticles due to the increased ionic strength next to theelectrode,18 (ii) the double-layer distortion as a particle movestoward the electrode with a thinner double layer ahead and awider double layer behind the particle,19 or (iii) the electro-osmosis around the particles near the electrode.20 Providedthere is an additional external force, an oriented deposition ofthe anisotropic particles can be achieved. For example, orientedAl2O3 and TiO2 films were obtained with EPD in a magneticfield (g10 T) strong enough to orient the diamagnetic particleswith an anisotropic magnetic susceptibility.21,22 Oriented filmsof BaNd2Ti5O14 (BNT) were prepared with the EPD ofhighly anisotropic, elongated particles.23 However, previousattempts24 to prepare BaF films with EPD combined with anexternal magnetic field resulted in only a limited film orienta-tion and thickness. The application of the magnetic field aloneprovided good orientation but poor film densities. Surprisingly,BaF films of comparable or even superior quality were obtainedusing EPD without any external magnetic field with an opti-mized size distribution of the original particles.25 The aim ofthis work was to study the orientation in BaF films preparedwith classical EPD.

In this study, we report on three mechanisms that have acrucial effect on the magnetic orientation of BaFe12O19 thickfilms prepared with EPD: (i) the interaction forces between theBaF nanoparticles in a polar solvent, (ii) the electrophoresis andthe deposition of particles, and (iii) the anisotropic and abnormalgrowth of BaF grains during the sintering. The effect of thosemechanisms on the orientation of the BaF film strongly dependson the particles’ shape anisotropy and the particle size distribu-tion of the feedstock particles.

’MATERIALS AND METHODS

Powders. The BaFe12O19 (BaF) particles were synthesized hydro-thermally as described in detail in ref 15. For the preparation of powderA, nitrates of Fe and Ba were dissolved in a 5:1molar ratio in water. Thenhydroxides were precipitated using excess Na hydroxide. This suspen-sion of hydroxides was then decanted into an autoclave. The autoclavewas heated to 160 �Cwith a heating rate of 3 �C/min and then cooled toroom temperature. The resulting particles were washed with water toremove the Na ions and with nitric acid to dissolve the Ba carbo-nate. Powder B was prepared with a slightly modified procedure. The

synthesized yield was around 70% for both powders. Surfactant dode-cylbenzene sulfonic acid (DBSa) was added to the suspension ofhydroxides before the hydrothermal synthesis. The suspension was thendecanted into the autoclave and heated to 240 �C. The resulting powderwas washed with water. The adsorption of DBSa onto the particlesurfaces occurred with the addition of nitric acid, which was otherwisealso used to dissolve the Ba carbonate.

The dried particles were deposited on a Cu grid and observed with atransmission electron microscope (TEM, Jeol 2100). The particles wereplatelets, and their equivalent diameter was determined from the surfacewith Gatan digital micrograph software. Although the statistics of thedetermined thicknesses were poorer than those of the diameters(because of the preferential alignment of the anisotropic particles inthe plane), we managed to get reliable values by inspecting a sufficientnumber of TEM images. Powder A was homogeneous with the plateletshaving a diameter of 11 ( 3 nm and a thicknesses of around 3 nm(Table 1 and Figure 1). Powder B consisted of platelets having differentsizes (Table 1, Figure 1, and Figure S1 in the Supporting Information).Almost half of the particles had a diameter of 20 nm; however, around5% of the particles were smaller, with a diameter of 10 nm, and another5% were very large particles, with diameters between 200 and 350 nm.The rest of the particles ranged between 30 and 190 nm in similar

Table 1. Basic Data Used in the Calculationsa

property suspension A suspension B

particles size (nm) 2r = 11 ( 3 2r = 10�350

h = 3 h = 5.6 ( 2.6

Ms (particles) (emu/g) 1015 35 (all particles)

6.4 (small fraction, 2r e 30 nm)

41.3 (large fraction)

γ (particles) (g/L) 7 7

|ζ| (mV) 86 120

σ (μS/cm) 1 11

c (free DBSa) (mmol/L) 0.27 1.12

1/k (nm) 12.4 6.07

ϕi 0.092 0.166a γ denotes the concentration of particles in the suspension, σ denotesthe conductivity, c denotes the molar concentration, ϕi denotes thevolume fraction of adsorbed DBSa in an overlapping layer, and the otherabbreviations are the same as used in the text.

Figure 1. TEM images of the dried suspensions. Enlargements areshown on the right-hand side.


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fractions. The thicknesses of all of the particles ranged between 3 and10 nm (mean thickness = 5.6 ( 2.6 nm) regardless of the particlediameters, with the exception of the smallest particles (with a diameter ofup to 20 nm) that were all around 3 nm thick.

The magnetic properties of the powders were measured with avibrating-sample magnetometer (VSM, Lake Shore 7312) to up to 10kOe and with a Quantum Design SQUID magnetometer to up to 50kOe. The high magnetic field of 50 kOe was necessary to saturate thesamples fully, and the saturation magnetization (Ms) values are listed inTable 1. In our case, the estimated error in the magnetization values permass was a maximum of 10% and the estimated error in the determina-tion of coercivity (Hc) was (50 Oe. Powder B exhibited ferrimagneticbehavior, with anMs of 35 emu/g, anHc of 1620 Oe, which is typical forhard magnets, and a remanent magnetization (Mr) of 16 emu/g. PowderA showed inferior magnetic properties with respect to powder B, with anMs of 10 emu/g, an Hc of 400 Oe, and anMr of 2.5 emu/g.15 Such lowmagnetization values are typical for very small nanoparticles and are aconsequence of the small particle size and the significant influence of themagnetically disordered particles’ surfaces.26 However, theHc value wasnot as low as expected for such 10 nm BaF particles. A closer TEMinspection revealed that powder A contained a few larger particles withdiameters of up to 100 nm (Figure 1), which was the reason for therelatively high Hc value.Suspensions. Powder A was dispersed in 0.032 M nitric acid, and

10% DBSa per powder mass was added. The adsorption of DBSa ontothe particle surfaces was obtained when the suspension was held at100 �C for 2.5 h. The powder was removed from the suspension bycentrifugation and was then washed with water and acetone. To increasethe adsorption of the surfactant onto the particles in powder B, itssuspension in nitric acid was held at 100 �C for 2.5 h. After this, powdersA and B were dispersed in 1-butanol under ultrasound. A pulsed ultra-sound of 300 W (VCX500 Ultrasonic Processor, Sonics & Materials)was used for 5 min with a pulse of 2 s on and 1 s off. (See Figure S2 in theSupporting Information for photographs of both suspensions.)

The zeta potentials of the saturated suspensions in 1-butanol weremeasured with single-point measurements (ZetaProbe Analyzer, NorthAttleboro and Zeta PALS Zeta Potential Analyzer, Brookhaven Instru-ments Corporation), taking into account the solvent’s dielectric con-stant (17.84), viscosity (2.99 mPa s), and refractive index (1.3993). Theconcentrations of the suspensions were analyzed after the preparationand after the centrifugation at 5000 rpm by weighing the suspensionbefore and after a heat treatment at 460 �C. In this way, the totalconcentration of DBSa was also determined. The concentration ofDBSa, dissolved in 1-butanol, was determined from a measurement ofthe conductivity (Conductometer Knick � Portamess, cell constant0.475) using a standard addition method. To the BaF suspension in1-butanol, a known amount of DBSa solution with a concentration of 10mmol/L was added. During the addition of the DBSa solution, theconductivity of the suspension was measured. The concentration of freeDBSa in the suspension (Table 1) was determined from a linearextrapolation of the conductivity versus the added DBSa concentrationto 0 mS/cm. The volume fraction of adsorbed DBSa on the particlesurfaces (Table 1) was calculated from the difference between the totaland the free concentration of DBSa considering the volume of thedouble layer. Dried particles from the suspensions were observed withthe TEM, as described above in the Powders section.

In an additional experiment, the small particles from suspension Bwere magnetically separated from the larger particles with a FrantzIsodynamic Magnetic Separator (S. G. Frantz Co. Inc.). The appliedvoltage was 40 V, which gives amagnetic field of 3.7 kOe. Steel wool filler(6.0 g) was inserted into the separation column. A stable suspension(120 mL) with a concentration of 7 g/L was poured into the columnwhen the magnetic field was applied. The smaller particles do notmagnetically interact with the filler, so they flow out of the column. The

column was then washed with 1-butanol to remove all of the smallparticles. After this, the magnetic field was turned off and the largerparticles were washed out of the column with the 1-butanol. The particlesize distribution of both fractions is shown in Figure S3 in the SupportingInformation. The small particles were no larger than 30 nm (Figure S4,Supporting Information). Suspension C was prepared from the smallparticles in a similar way to the other two suspensions. The particles weredispersed in water, and the pH was lowered to 1.5. The suspension wasstirred for 2.5 h at 100 �C. Then the particles were separated from thesuspension, washed, and dried. The dried particles were redispersed in1-butanol.

The magnetic properties of the two separated fractions were mea-sured as described previously in the Powders section (Table 1 and FigureS5 in the Supporting Information). The small-particle fraction, with anaverage diameter of 10( 5 nm (used in suspension C), exhibited weakmagnetic properties (Ms = 6.4 emu/g and Hc = 180 Oe) that were evenpoorer than those of powder A. As mentioned previously, the very lowMs values are a consequence of the small particle size and the significantinfluence of the magnetically disordered particle surfaces.26 The large-particle fraction, however, exhibited typical ferrimagnetic behavior (Ms =41.3 emu/g and Hc = 1500 Oe) similar to that of powder B. The lattershows a lowerMs because it is a mixture of the two separated fractions.Films. Stable suspensions A�C were used for the preparation of

deposits A�C, respectively, using electrophoretic deposition (EPD).The electrophoretic cell was composed of the Al anode and the cathodesubstrate, which was coated with alumina with a 50-nm-thick Pt layer.The deposits were obtained at a constant voltage of 50 V and aseparation distance between the electrodes of 7 mm during the deposi-tion time of 15 min. The deposits were held at 460 �C for 2 h to removethe organic phase. After that, the deposits were sintered at 1150 �C for5 h to obtain films.

The grain sizes, the microstructures, and the thicknesses of the filmswere investigated with a scanning electron microscope (SEM, Jeol7600F). The orientations of the grains in the films were calculatedfrom the X-ray patterns obtained with an X’Pert PRO diffracto-meter (PANAnalytical) using Cu Kα1 radiation and the equation(P � P0)/(1 � P0), where Por P0 = ∑I00L/∑ Ihkl is the peak area ofthe (00L) peaks and Ihkl is the area of all of the peaks. P

0 corresponds tothe randomly oriented BaHF powder, and P corresponds to the orientedfilm. The orientations of the grains in the films were also estimated fromthe magnetic measurements with the VSM, where the magnetic proper-ties were measured out of plane (with the magnetic field applied

Scheme 1. Presentation of the Interaction Energies betweenTwo Approaching BaF Particles, Which Move/Rotate FreelyBecause of Brownian Motion (Large Separation Distance, l),Orient Because of Magnetic (Em) Attraction, and Agglomer-ate Because of Em and van der Waals (Evdw) Attractions

a

a Possible stabilization mechanisms are electrostatic (ER) or stericrepulsion (Es). Precise E and l values vary with the particle size.


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perpendicular to the film plane) and in plane (with the magnetic fieldapplied parallel to the film plane).

’THEORY AND CALCULATION

Interaction Forces between Particles in a Polar Solvent.The interactions between the two approaching BaF particles arepresented in Scheme 1. The studied BaF particles are so smallthat the Brownian motion prevents their sedimentation becauseof gravity (Table S1, Supporting Information), and at the sametime they move and rotate randomly in the absence of anyexternal or interparticle forces. In this case, an effective particlecan be regarded as a sphere with a diameter of 2r. When two BaFparticles in a solvent approach each other, they first becomeattracted by the magnetic dipole�dipole force. Single-domainBaF particles align with their large planes together and formcolumnar agglomerates. Namely, a single-domain magneticparticle is fully saturated, so it aligns in the direction of an appliedmagnetic field, which is in this case the magnetic easy axis of BaFthat is perpendicular to the large plane of the particle. Single-domain BaF particles have sizes of between 10 and severalhundred nanometers,27 like the sizes of the studied particles.Consequently, the BaF plates cannot be approximated withspheres anymore but rather with thin discs. At even smallerseparation distances of up to a few nanometers, they areadditionally attracted by the van der Waals force. The respectiveinteraction energies, Em and Evdw, for the geometry shown inScheme 1 can be calculated with eqs 1 and 2, respectively.28�31

Equation 2a is valid for two flat surfaces with the interacting areaSint, and eq 2b is valid for two spherical particles.

Em ¼ � μ0πF2Ms1Ms2V1

2V22

4D3ð1Þ

Evdw ¼ � ASint12πl2

ð2aÞ

Evdw ¼ � A12

yx2 þ xy þ x

þ yx2 þ xy þ x þ y

�

þ 2 lnx2 þ xy þ x

x2 þ xy þ x þ y

�ð2bÞ

where x = l/2r1 and y = r2/r1.Here we considered crude BaF particles with no surfactant

layer having a thickness h and a radius r, where the suffix 1 standsfor the larger of the two particles, 2 represents the smaller of thetwo particles, F = 5300 kg/m3 is the particle density, D is theseparation distance between particle centers, and μ0 = 4π� 10�7

J/A2 m is the permeability of a vacuum. The Hamaker constant(A) for the BaF/1-butanol system, which was estimated as in refs32 and 33, was approximately A ≈ 5.3 � 10�20 J.Stable suspensions can be prepared when a large enough

electrostatic and/or steric repulsion is provided between theparticles by the application of surfactants or polymers.14,34�38

The Derjaguin�Landau�Verwey�Overbeek (DLVO) theorydescribes the electrostatic interaction between charged par-ticles in polar solvents based on the formation of an electricaldouble layer (Scheme 1). The electrostatic interaction energy(ER) between two flat BaF particles can be calculated usingeq 3a, and for two spherical particles it can be calculated using

eq 3b.34,35

ER ¼ εrε0ψ2k2π

Sint1� e�2kl

e2kl � e�2kl ð3aÞ

ER ¼ εrε0r1r24ðr1 þ r2Þ 2ψ1ψ2 ln

1 þ e�kl

1� e�kl

"

þ ðψ12 þ ψ2

2Þ lnð1� e�2klÞ� ð3bÞwhere

1=k ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiε0εrkT

e20∑iniZ2

i

vuutHere, ψ is the surface potential and can be approximated for

organic media with the measured zeta potential (ζ),19,28 k is thereciprocal Debye�H€uckel parameter, ε0 = 8.854� 10�12 As/Vmis the permittivity constant of a vacuum, εr is the relativepermittivity of a solvent, ni is the number density of ion i inthe medium, Zi is the charge of ion i, e0= 1.6022� 10�19,As is theelementary electron charge, k = 1.38 � 10�23 J/K is theBoltzmann constant, and T is the absolute temperature.Macromolecules or polymers that are adsorbed or grafted onto

the particle surfaces are responsible for the steric interaction. Thissteric interaction acts over only relatively short separations (i.e., upto double the thickness of the surfactant layer, 2t in Scheme 1) andconsists of two contributions:36�38 (i) the mixing or osmoticinteractions due to the mixing of adsorbed surfactant molecules inthe overlapping layer (Scheme 1) and (ii) the volume restriction orelastic contribution due to the decrease in the configurationalentropy when two surfactant layers approach each other. Themixing steric contribution is repulsive only when the surfactant issoluble in a solvent, meaning that the Flory�Huggins parameter isχe 0.5.34 In contrast to this, the elastic steric contribution is alwaysrepulsive. An exact calculation of the steric energy is very difficultbecause of the complicated experimental determination of some ofthe parameters. Regardless of this problem, a strong steric repulsionoccurs as soon as the separation distance between the two particlesbecomes smaller than double the thickness of the surfactant layer(le 2t). Therefore, some assumptions that simplify the calculationscan be applied without any significant influence on the final sign ofthe total interaction energy, repulsive or attractive. It was shown thatin real systems the mixing mechanism alone based on osmoticrepulsion can be applied as a good approximation to the estimationof steric repulsion energy.36,37 Using the hard-sphere approximationand considering a good solvent for the surfactant with steric repulsion(χ = 0), the steric repulsion energy (Es) can be calculated witheq 4.29,38 Equation 4a is valid for two discs that are attractedwith theirlarge planes (Scheme 1), and eq 4b is valid for two spheres.

Es ≈kT

12πa3ln

11� ϕ

!� ϕ

" #Vint ð4aÞ

Es ≈kTa3

ln1

1� ϕ

!� ϕ

" #πδ2

126

r1 þ r22

þ l

� �� δ

� �

ð4bÞHere, a is a monomer length (in this study, the length of the largestfragment, 0.24 nm, was considered), Vint is the interacting volume,


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and ϕ is the volume fraction of the surfactant in the overlapping layerwith a thickness t (2� 2 nm in our study). The thickness of the over-lapping layer isδ=0 for lg 2t andδ= 2t� l for l< 2t. Obviously, thesteric repulsion,Es > 0 J, occurs only at separation distances l<2t. Thetotal interaction energywas calculated to beET=Evdw +Em+ER+Es.The Em increases significantly with the increasing particle volume

and the consequent increase in their magnetic moment. In contrast,Evdw becomes significant with respect to Em but only for the smallestparticles with small magneticmoments. At the same time, the surface-to-volume ratio increases with the decreasing particle size and enablesthe adsorption of a larger fraction of macromolecules per particlevolume. Therefore, it is to be expected that the stability of thesuspensions will increase with a decreasing particle size. For thefurther orientation of the BaF particles during the EPD, it is crucialthat the particles are dispersed in a solvent. Otherwise, agglomeratesof randomly oriented particles would be deposited.Forces Acting on the Particles during the EPD.Three main

forces act on theBaFparticles during theEPDwith the experimentalsetup used in this study (Scheme 2): gravity, electric, and hydro-dynamic. Gravity, in general, orients the particles in parallel with thehorizontally positioned electrode�substrate. However, its effect onthe orientation of the studied particles is negligible because the rateof Brownian motion exceeds, by several orders of magnitude, thegravity-settling velocity (Table S1, Supporting Information).No significant effect on the particle orientation is expected

for the electric (or electrophoretic) force, which is directlyrelated to the electrophoretic mobility. Equation 512,39 showsthat the electrophoretic mobility (μEP) is directly proportionalto the zeta potential (ζ) and as such to the surface potential ofthe particles, whereas the permittivity and the viscosity (η) areproperties of the solvent. X = 1 for particles with 2r , 1/k andX = 2/3 for particles with 2r > 1/k.

μEP ¼ Xε0εrζ

ηð5Þ

No correlation between the geometry of the particles and theelectrophoretic mobility can be deduced from eq 5. Similar tothis, no effect of the electric field on the orientation of BNT filmsconsisting from anisotropic grains was observed.23 The theore-tical and experimental study of Jimenez and Bellini41 on non-spherical (including disklike) colloid particles showed only aminor shape effect on the electrophoretic mobility for weaklycharged particles. However, the shape effect increased with theparticle charge and could not be neglected for highly chargedparticles. Mittal and Furst42 showed that a disklike particle canorient along an applied ac field’s direction or perpendicular to it,depending on the frequency. At low frequencies, the particlespreferentially aligned perpendicular to the applied field by

connecting the large disk planes. In contrast to this, Oshima40

showed theoretically that cylindrical particles in an incompres-sible liquid possess a different electrophoretic mobility (up to20%) in two directions, perpendicular and parallel to their longaxis. Verde et al.45 showed that ZnO plates orient in plane withthe substrate during the EPD. They explained such an orientationby the effect of hydrodynamic forces, which change their direc-tion in the vicinity of a substrate/electrode, as is presented inScheme 2a. In a regular flow, the hydrodynamic friction opposessuch an orientation because the larger surface induces morefriction than the side surfaces and favors the direction of theparticle flow with their large plane perpendicular to thesubstrate.43 The hydrodynamic friction force (FF) depends onthe orientation of arbitrarily shaped particles and is given byeq 6.44

FF ¼ fijvj, fij ¼ 13kTðln p þ CtÞ

πηhð6Þ

Here, fij is the translational friction tensor or friction coefficient.The friction coefficient strongly depends on the shape of theparticles, p = h/2r for disklike particles. Ct = 0.312 + 0.565/p +0.100/p2 is a numerical constant and is valid for particles with p =0.1�20.The hydrodynamic force changes its direction in the vicinity of

the electrode, where particles tend to aggregate because of theconvection caused by the electro-osmosis around the particles(Scheme 2a).20,45 Unfortunately, a high particle concentrationdiminishes or even prevents the ordering because the electro-osmotic effect preferentially induces agglomeration. On the basisof the above, we can expect that the key to an oriented depositionis highly anisotropic and fully dispersed BaF particles in dilutedsuspensions.Anisotropic and Abnormal Grain Growth during Sinter-

ing. During normal grain growth, finer grains possess a muchlarger driving force for their growth than do large grains.46 Fromthis point of view, one would prefer to use suspensions madefrom fine particles. However, fine particles with a lower shapeanisotropy are expected to show a poorer in-plane orientationthan are the larger particles with a larger shape anisotropy. (Seethe Forces Acting on the Particles during the EPD section.)Therefore, we expected that the optimum solution would be acombination of large particles, which are likely to orient prefer-entially in plane with the substrate, with fine particles that fill theempty space between the large particles (Scheme 2b). In thiscase, exaggerated or abnormal growth of the large particles at theexpense of the small ones can be expected. Because the differencein the chemical potential across a curved grain boundary providesthe driving force for the boundary to move to the center of thecurvature, large grains with convex boundaries grow at theexpense of small grains with concave boundaries.46 This abnor-mal grain growth can be additionally promoted by impurities, thepresence of a liquid phase, and anisotropic grain growth as aresult of the anisotropic crystal structure or the constrainedsintering in films.2,23,47�50 Hexaferrites tend to grow anisotropi-cally with extensive growth in the ab plane and limited growth inthe c direction, thus forming hexagonal platelike grains.2 Assum-ing the preferential orientation of the as-deposited particleswithin the plane of the film, a higher degree of orientation canbe expected for the larger grains than for the smaller ones. (Seethe Forces Acting on the Particles during the EPD section.) It hasbeen shown that the orientation of grains tends to increase in the

Scheme 2. Schematic Presentation of the Forces Acting onthe BaF Particles during the EPDa

a (a) Gravity, g; electrophoretic, El; electro-osmotic, Eo; and hydro-dynamic, Hy. (b) In-plane view of an oriented deposit.


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direction of the preferred orientation of the large grains.47�49

This can be explained by the driving force for the growthof abnormal grains into normal, which is proportional to(1/rnormal)� (1/rabnormal).

51 The abnormal grains can be severalorders of magnitude larger than the matrix grains, and they willgrow fast until all of the matrix grains are consumed. At this stage,the abnormal grains become normal. In contrast to this, thedriving force for the growth of the grains with a homogeneoussize is much smaller than for the former. In our study, the growthout of the plane of the film is additionally limited by theconstraints of the substrate.23 On the basis of this, we wouldpredict that any grain growth will increase the degree of orienta-tion of the BaF film.Therefore, in an ideal case, the suspension should be composed

of large and, at the same time, very thin plates that would assembleduring the EPD in parallel with the substrate, together with verysmall particles that would fill the empty space between the largeplates, as is schematically presented in Scheme 2b. However, thisis possible only if the electrostatic attraction between the smallparticles and the electrode is stronger than the magnetic attractionbetween the deposited particles and the incoming small particles.In the opposite case, the small particles would deposit on top of thelarge particles, thus forming columns, as shown in Scheme 1. (Seealso the inset in Figure S2 in the Supporting Information.)

’RESULTS AND DISCUSSION

Interactions between the BaF Particles in a Polar Solvent.Figure 1 shows TEM images of the dried suspensions. Suspen-sion A contains particles of a relatively homogeneous size:platelets with a diameter of 11 ( 3 nm. No agglomeration wasobserved between the small particles. The high-magnificationimage shows that the particles are not in contact with each other.This can be a consequence of a steric barrier, formed by theDBSa, adsorbed at the particle surfaces. A few particles withdiameters of around 100 nm were also observed in suspension A,together with a limited agglomeration around them. Thisagglomeration is most likely to occur during drying becausesuspension A had a high stability with a high zeta potential of86 mV (Table 1) and no sedimentation at all. (Figure S2, Sup-porting Information.) Suspension B (Figure 1) contains smallparticles as well as larger ones. (See also Figure S1 in the

Supporting Information for the particle size distribution.) Mostof the particles have diameters of around 20 nm, but a consider-ably smaller fraction of particles, with diameters larger than200 nm, was observed. The measured thicknesses were in therange of 3�10 nm for all of the inspected particles. Someagglomeration around the large particles was observed, and italso affected the suspension’s stability. Suspension B remainedstable for hours (Figure S2, Supporting Information), and theparticles started to settle after 1 day. Consequently, the concen-tration of dispersed particles decreased from 7 g/L in freshsuspension B to 5.5 g/L in 1-week-old suspension B. Therefore,only fresh suspensions were used for the EPD.Figure 2 shows the calculated total interaction energy (ET =

Evdw + Em + ER + Es from eqs 1�4) between two BaF particles ofdifferent sizes. The basic material parameters used in the calcula-tions are listed in Table 1. As discussed in the previous section (seealso Scheme 1), the BaF particles were considered to be spheres atlarge separations, where the magnetic attraction energy was negli-gible with respect to the thermal energy (|Em| < kT), whereas atsmaller separation distances the BaF particles were considered to bethin discs. These calculations provide an estimation of the respectiveenergies. (See details on the related assumptions and simplificationsin the Theory and Calculation section.) Nevertheless, they explainthe experimental results well and are therefore interpreted as such.In suspension A, the repulsion energy prevails over the

attraction and the total interaction energy is repulsive (ET > 0kT), regardless of the separation distance (l). Em in suspension Ais negligible in comparison to Evdw. (See also Figure S6 and arelated detailed explanation in the Supporting Information.) Ahigh primary maximum (.5kT) confirms the experimentally

Figure 2. Calculated interaction energy between BaF particles of different sizes with respect to the separation distance with an enlargement on the right-hand side. (Legend) Two values show diameters of the interacting particles from suspension B.

Figure 3. SEM images of the surfaces of the deposits fired at 460 �Cwith encircled misoriented particles.


Langmuir ARTICLE

observed high stability for suspension A. The stability of suspen-sion A results mostly from the steric repulsion originating from alarge DBSa molecule (benzene ring + C12 chain).Particles of various sizes constitute suspension B (Figures 1

and S1 in the Supporting Information). The stability ofsuspension B was estimated from the interaction energiesbetween the particles of the main fractions (Figure 2; see alsoFigure S6 and the corresponding discussion in the SupportingInformation). Apart from a primary maximum due to stericrepulsion, a secondary minimum is also observed for the pair ofparticles, where at least one of them is larger than 30 nm (2r1 >30 nm). The secondary minimum increases with the particlesize because of the increasing Em. However, it is partiallyreduced by the electrostatic repulsion, which cannot beneglected for such large particles. The significant effect ofthe electrostatic repulsion was also observed experimentallybecause the rate of sedimentation increased with the increasingconductivity (i.e., with the increasing ionic strength). When thesecondary minimum (absolute value) exceeds the thermal en-ergy, the particles agglomerate before they approach closeenough to be repelled by the steric barrier. Such an example isshown for a pair of particles with 2r1 = 80 nm and 2r2 = 20 nm(Figure S6, Supporting Information). Therefore, we can con-clude that BaF particles that are larger than 50 nm cannot bedispersed in 1-butanol and will agglomerate. However, thisagglomeration is not fast: suspension B remained stable forhours. This can be explained by the small concentration of thelargest particles (Figures 1 and S1 together with the discussionrelated to Figure S6 in the Supporting Information).Assembly of the BaF Particles during the EPD.As discussed

in the Theory and Calculation section, the hydrodynamic andelectro-osmotic forces acting on anisotropic BaF particles in thevicinity of the electrode could induce a preferential orientation ofthe particles parallel to the substrate. Our results show that thiswas true and that the BaF particles indeed preferentially orientedin the plane of the substrate electrode. This is presented inFigure 3. We can see that most of the particles on the depositsurfaces are lying in the plane of the film. Some misorientedparticles, highlighting the deposits’ surfaces, are also seen. Only afew particles with diameters of around 100 nm can be observed inthe matrix of nanosized particles (hardly seen with the SEMbecause of their small sizes) in deposit A, as in dried suspension A(Figure 1). In contrast to this, a much larger fraction of particles

with diameters of 100�500 nm can be observed in deposit B thanin dried suspension B (Figures 1 and 3), suggesting particlegrowth during the firing at 460 �C. The reason for the limitedparticle growth in deposit A when compared to that in deposit Bis in the higher particle-size homogeneity of the former. Theexplanation is similar to that in the Anisotropic and AbnormalGrain Growth during Sintering section.The magnetic hysteresis loops (Figure 4) of deposit B

measured in two directions clearly show anisotropic behavior,suggesting the preferential orientation of the deposited BaFparticles. The larger Hc, Mr, and Mr/Ms ratio for the out-of-plane measurement when compared to those values measured inthe plane (Table 2) further suggest that the BaF particles arepreferentially oriented in the plane of the film. As mentionedbefore, the magnetic easy axis of BaF is perpendicular to theparticle plane, and when it is parallel to the applied field (i.e., anout-of-planemeasurement), largerMr andHc values are expected(see the Interactions between BaF Particles in a Polar Solventin Theory and Calculation section for a more complete ex-planation) than when the applied field is perpendicular to theeasy magnetic axis (i.e., an in-plane measurement). Some mis-oriented particles, which can be seen in Figure 3, contributed tothe Mr and Hc values measured in the plane (Table 2).If we compare deposit A to deposit B, we can see that the

former shows no distinct magnetic anisotropy and hence nosignificant particle orientation. As previously shown,45 the hydro-dynamic force in the vicinity of the electrode orients anisotropicparticles with their large dimensions in plane with the electrode.Solomentsev et al.20 showed that electro-osmosis favors agglom-eration (i.e., the deposition of an incoming particle on an already-deposited particle rather than the deposition of a new incomingparticle on a free space on the electrode). Because of the mag-netic anisotropy of BaF platelets, columnar agglomerates areformed (Scheme 1). Therefore, once the first layer is preferen-tially deposited in a plane with a substrate the agglomerationshould not affect the orientation significantly. The effect of thehydrodynamic forces during the EPD must have been muchstronger on the larger particles with the higher shape anisotropy(i.e., a higher diameter-to-thickness aspect ratio) that were veryrare in suspension A. The absence of any distinguishablemagnetic orientation of deposit A suggests that the hydrody-namic forces had no significant effect on the small particlesduring the EPD because the shape anisotropy was too low.

Figure 4. Magnetic hysteresis loops for the deposits fired at 460 �C.


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Anisotropic and Abnormal Grain Growth during Sinter-ing. Figure 5 shows the XRD patterns of the sintered films. Bothpatterns show the preferential orientation of the films in theplane because the relative intensities of the (00L) peaks are muchhigher than the others. For a comparison, the XRD of a randomlyoriented BaF powder is also shown. Film B shows a higher degreeof orientation, 78%, than film A, 46%. This is to be expectedbecause deposit B showed a higher degree of in-plane orientationthan deposit A (Assembly of the BaF Particles during the EPDsection). The films also had different thicknesses, which could bethe consequence of the different particle sizes. Film A, preparedfrom smaller particles, had a thickness of 6 μm and was thinnerthan film B with a thickness of 17 μm. The small thickness of filmA is also the reason that the substrate peaks (Pt and * in Figure 5)can be observed together with the hexaferrite peaks in thecorresponding XRD. At the same time, two hematite peaks areobserved and are much more intense for film A than for film B.Hematite is antiferromagnetic and has no influence on themagnetic orientation of the films.We can see in Table 2 that all of the sintered films show a

better orientation than the deposits as well as better magneticproperties in general. The latter can be explained by theimproved crystallinity of the BaF grains and the higher densityof the films with respect to the deposits, whereas the abnormalgrain growth can explain the increased orientation after the

thermal treatment. Figure 6 shows the microstructures of thesintered films. It can be seen that the sintering was accompaniedby grain growth (Figures 1 and 3). When compared to theprefired deposits at 460 �C (Figure 3), the grains grow from 10 to100 nm to 0.8( 0.2 μm in film A and from 100 to 500 nm to 1.0( 0.3 μm in film B. Abnormal grain growth, which is typical forgreen compacts with an inhomogeneous grain size, can beobserved in both films, although it was more significant in filmB, which had a larger grain size distribution than did film A. Thiscould be expected because the particle sizes were larger and lesshomogeneous in suspension B and in deposit B than in suspen-sion A and deposit A. The diameter ratio between the abnormaland matrix grains is also larger in film B because of the largerparticle size in the feedstock powder. The abnormal grain growthdominated in the film plane. It was shown previously thatabnormal grain growth occurs in hexaferrites, regardless of theorientation of the large grains in the fine matrix.48 However,when some degree of preferential orientation was induced in agreen compact, the orientation increased with the abnormal graingrowth.2,47 Lee et al.49 showed experimentally that the large orientedgrains formed from smaller ones, which were perfectly oriented in agreen compact before sintering. Fu et al.23 used the elongatedparticles for the preparation of the orientedBNTfilmswith the EPDand observed anisotropic grain growth during sintering that resultedin an increased orientation of the films. The increased orientation ofthe studied films during the thermal treatment can be explained by(i) the abnormal growth of the large grains that preferentially orientin thefilmplane during theEPD, as in refs 2 and 47, (ii) the sinteringof the highly oriented agglomerates formed during the EPD as in ref49, and (iii) the constrained grain growth perpendicular to the filmplane due to the substrate. The last of these also causes constrainedsintering and consequently residual porosity.The preferential orientation of BaF particles within the plane

of the film was also confirmed by the magnetic measurements

Table 2. Magnetic Properties of the Deposits and Films Where OUT and IN Denote the Out-of-Plane and In-PlaneMeasurements, Respectivelya

sample deposit A deposit B deposit C film A film B film C

MsOUT (emu/g) 3.8 11.5 no saturation 13.9 34 45

MrOUT (emu/g) 1.6 4.6 2.3 11.7 28.5 10.1

Mr/MsOUT 0.40 0.40 0.30 0.84 0.83 0.45

HcOUT (Oe) 640 1185 1047 4688 3274 1396

MsIN (emu/g) 3.8 no saturation no saturation no saturation no saturation no saturation

MrIN (emu/g) 1.6 2.1 1.0 5.3 8.2 6.0

Mr/MmaxIN 0.40 0.17 0.11 0.43 0.29 0.15

HcIN (Oe) 640 294 173 3736 2544 1120aMmax denotes the magnetization at 10 kOe when the magnetization did not saturate up to 10 kOe.

Figure 5. XRD patterns of sintered BaF films and randomly oriented BaF,where H denotes hematite, Pt denotes platinum, and * denotes corundum.

Figure 6. SEM images of the sintered film surfaces.


Langmuir ARTICLE

(Figure 7). A higher magnetic anisotropy (i.e., a larger differencebetween themeasurements in the plane and out of the plane) wasmeasured for film B than for film A. In a completely magneticallyoriented material with an easy magnetization axis, the hysteresisloop is squarelike, with high Hc, Mr, and Mr/Ms values whenmeasured out of plane (Scheme 3): the magnetization saturates(Ms) at a low applied field, and the remanent magnetization (Mr)approaches the Ms value. This means that the sample’s easymagnetic axis coincides with that of the applied field (i.e., it isperpendicular to the sample plane). Because the easy magnetic

axis of BaF is perpendicular to the particle plane, the preferentialorientation of the particles in the film plane can be assumed fromFigure 7. The Mr/Ms ratio is a measure of the magneticorientation, which is 1 for a perfectly oriented sample. In contrastto this, the hysteresis loop measured in plane does not showmagnetic saturation and has low Mr and Hc values. In this case,the magnetic easy axis is perpendicular to the applied field and astrong applied field is required to reverse the particles’ magneticmoments in the field direction (i.e., to saturate the sample). Atthe same time, the magnetic moments can flip easily within theplane, resulting in lowHc andMr values. Therefore, the larger thedifference between the magnetic properties measured in the twodirections, the higher the magnetic anisotropy and the degree oforientation of the sample. TheMr andHc values of the studied filmsmeasured in plane originate from themisaligned samples. AlthoughMr is ameasure of themass/volume fraction ofmisaligned particles,this is not so forHc. The latter depends on the angle between themagnetic easy axis and the applied field (Scheme 3).Hc is largestwhen the magnetic easy axis is parallel to applied field (i.e.,perpendicular to the particle plane), and it decreases with theincreasing angle. Our films show moderate Hc values in plane,which are lower than those out of plane. This indicates that themisaligned particles are aligned at an angle smaller than 90�.Although the Mr/Ms ratio for the out-of-plane measurement

(Table 2) was similar for films A and B (0.83 and 0.84), this valuewas lower for film B when measured in plane (0.29) than for filmA (0.41), confirming the higher magnetic anisotropy of film B.The reason for the comparable orientation of the two films is thatsuspension A also contained some large particles with diametersof 100�300 nm (Figure 1). These large particles could orient inthe plane of the film during the EPD. However, their fractionwas too small for any measurable magnetic anisotropy in depositA (Figure 4). Despite this, these large particles can induceabnormal grain growth during sintering (Figure 6). The highMr/Ms ratio of film A indeed suggests that a large mass/volumefraction of the grains is oriented in the plane of the film.To verify the effect of large particles, suspension C was

prepared from only the small-particle fraction obtained by themagnetic separation of suspension B (Suspension section inMaterials and Methods). No particles larger than 30 nm indiameter were observed in suspension C (Figures S3 and S4 inthe Supporting Information). Film C was prepared from suspen-sion C in the same way as films A and B. The magneticmeasurements (Figure 7, Table 2) showed a significantly lower

Figure 7. Magnetic hysteresis loops of the sintered films.

Scheme 3. Representation of the Effect of the Alignment of aSingle-Domain Particle on the Magnetic Hysteresisa

aThe arrow shows the direction of the applied magnetic field withrespect to the particle.


Langmuir ARTICLE

magnetic anisotropy of film C when compared to those of theother two films: a smaller difference between the in-plane andout-of-planemeasurements, a lowerMr/Ms ratiomeasured out ofplane, and equal Hc values measured in both directions. At thesame time, we can see that the overall magnetization is higher forfilm C than for films A and B, suggesting a higher density for thisfilm. This can be a consequence of (i) the highest stability forsuspension C, resulting in the highest green density of thedeposit and/or (ii) the highest sintering rate for the finestparticles.It was shown previously25 that the density of film B can be

improved by increasing the suspension’s stability during the EPDthat was performed at a lower applied voltage (i.e., at 30 insteadof 50 V). In this case theMs measured out of plane was as high as55 emu/g and the Mr/Ms ratio was 0.87 whereas a 94%orientation was determined from the XRD analysis. We canconclude that dense BaF films with a high magnetic orientationcan be obtained from stable suspensions with a limited fraction oflarge particles together with very small nanoparticles.Finally, we compare the mechanisms of the orientation of the

most promising methods for the preparation of oriented BaFfilms: liquid-phase epitaxy (LPE), screen printing (SP), andEPD. The substrate with a structure similar to that of BaFinduces the crystallization and growth of the BaF film from themelt in the preferred direction during the LPE.10 Prior to this, aBaF seed layer is often deposited on a substrate with pulsed-laser deposition in order to improve the orientation of the film.The orientation of the SP BaF films was not achieved during theSP process but during the subsequent low-temperature anneal-ing under an applied magnetic field.11 Although in both the SPand the EPD presynthesized BaF particles are used as a feed-stock, we showed in this study that magnetically oriented BaFfilms can be prepared without the applied magnetic field. Highlyanisotropic BaF particles assemble during the EPD process inthe plane of the substrate, and the net magnetic orientation canbe further increased by the anisotropic grain growth andabnormal grain growth that accompany the sintering.

’CONCLUSIONS

The magnetic orientation of the Ba ferrite thick films preparedwith EPD was studied. It was shown that these magneticallyoriented films can be prepared without any external magneticfield. The basic condition for the high degree of orientation is astable suspension of highly anisotropic particles. In this case, anindividual particle orients in the plane with a substrate because ofthe hydrodynamic forces acting on a particle during the EPD invicinity of the depositing electrode. The orientation of thedeposits was further improved as a result of the abnormal graingrowth and the anisotropic grain growth that accompany thesintering at 1150 �C. This was especially effective when a stablesuspension of highly anisotropic platelike Ba ferrite particles withan inhomogeneous diameter was used for the EPD. The max-imum orientation of the films determined with XRD was around90%. The remanent magnetization nearly reached the saturationmagnetization (i.e., it was >80%), suggesting that such films aresuitable for applications in the remanent state (i.e., for self-biasedapplications) with no need for any external magnets. We proposethat the same principle can be applied to the preparation of highlyoriented films from thin plates in general.

’ASSOCIATED CONTENT

bS Supporting Information. Particle size distribution inpowder B. Photographs of suspensions A and B. Characterizationof the magnetically separated particles from suspension B. Effectof gravity versus Brownian motion on the particles. Detailedexplanation of the interaction between the BaF particles in apolar solvent. This material is available free of charge via theInternet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected]. Tel: +386-1-4773-872. Fax: +386-1-2519-385.

’ACKNOWLEDGMENT

This work was financially supported by the SlovenianResearch Agency. We acknowledge the CENN Nanocenter forthe use of the TEM. We are grateful to Dr. Marko Jagodi�c for theSQUID measurements and to Mr. Slavko Kralj for the zetapotential measurements.

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directed assembly of bafe 12 o 19 particles...

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