[Springer Proceedings in Physics] Advances in Nanoscale Magnetism Volume 122 || Structural and Magnetic Properties and Preparation Techniques of Nanosized M-type Hexaferrite Powders

10

Structural and Magnetic Propertiesand Preparation Techniquesof Nanosized M-type Hexaferrite Powders

T. Koutzarova, S. Kolev, C. Ghelev, K. Grigorov, and I. Nedkov

Institute of Electronics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chausseebuld., 1784 Sofia, Bulgaria, [email protected], [email protected],[email protected], [email protected], [email protected]

Summary. In recent years, the scientific efforts of a large number of research teamshave been concentrating on developing, exploring and applying nanosized magneticferroxides. In this review, we consider the fundamental structural and magneticcharacteristics of nanosized particles of barium hexaferrite. We discuss in some detailthe most common techniques for preparation of nanosized ferroxide powders. Finally,we present original results on applying a promising chemical technique, namely, thesingle microemulsion technique, for the synthesis of barium hexaferrite powdersconsisting of homogeneous in shape and size particles.

10.1 Introduction

In the past decades, magnetic nanoparticles have been the focus of intenseresearch activities not only because of their unusual behaviour compared tothe bulk materials but also for their wide applications in the practical world.The scientific and technological importance of magnetic nanostructures hasthree main reasons [1]:

• There is an overwhelming variety of structures with interesting physi-cal properties, ranging from naturally occurring nanomagnets and com-paratively easy-to-produce bulk nanocomposites to demanding artificialnanostructures,

• The involvement of nanoscale effects in the explanation and improvementof the properties of advanced magnetic materials, and

• Nanomagnetism has opened the door for completely new technologies.

Hard magnetic hexagonal ferrites have been extensively used as permanentmagnets [2–4], magnetic recording media [5], magnetic tapes and floppydisks [6], magneto-optic materials, microelectromechanical systems [7] and

184 T. Koutzarova et al.

microwave filters and devices [8–11] and in recent years, as materials withpotential bio-medical applications [12]. Their great attraction is mainly dueto the abundance of the raw materials and low production costs. The develop-ment of radar electronics and wireless technologies requires planar and low lossmagnetic microwave devices (isolators, filters, phase shifters, and circulators,etc.) [13–17], which can be realized by the integration of a ferrite material withsemiconductor platforms. Barium hexaferrite with M-type hexagonal crys-talline structure (BaFe12O19 or BaM) has attracted a great deal of attentionfor microwave device applications because of its bulk properties, namely, highpermeability, low conductive losses, and, particularly, large uniaxial anisotropy(HA = 17kOe) with the easy magnetization direction along the c-axis [18].

These materials exhibit high magneto-crystalline anisotropy, high Curietemperature, high coercivity and relatively high saturation magnetization, aswell as excellent chemical stability and corrosion resistivity required for manyapplications [19,20]. The interest in these nanosized particles lies in our abilityto affect their physical properties through manipulation of size, compositionand aspect ratio to produce changes in the overall physical properties [20].The properties of the nanoparticles are of interest for the investigation ofnanowires, dot arrays, thin films and bulk composites [1]. It is also well knownthat the magnetic properties strongly depend on the particles’ microstructure[21, 22].

10.2 Crystalline Structure

The hexaferrites form a group of complex oxides in the system AO–Fe2O3–MeO, where A is a large divalent cation, i.e. Ba, Sr, Ca, and Me are a smalldivalent cations, i.e. Mn, Fe, Co, Ni, Cu, Zn. They can be classified on thebasis of chemical composition by varying the A–Me combination and, respec-tively, the crystal structure. Thus, they are subdivided into five fundamental,simplest structural types: M, W, Y, X, U and Z [23–25] . Figure 10.1 showsthe known hexaferrite types, while the most common types are summarizedin Table 10.1 [26].

We will now consider in detail the structure of the M-type hexaferrites.Barium hexaferrite (BaFe12O19) is the M-type hexaferrite family’s best knowncompound. It has the crystal structure of the mineral magneto-plumbite.The crystallographic unit cell corresponds to the space group P63/mmc andcontains two molecules of the chemical composition BaFe12O19 [27]. Thedimensions of the unit cell are a = b = 5.88 A and c = 23.20 A [26]. The basicstructure of the unit cell is built up by ten layers of oxygen ions that are formedby a close packing of cubic or hexagonal stacked layers alternately along the[001] direction. One O2− ion is replaced by barium, which has a similar ionicradius in every fifth layer. The crystal structure can be divided into severalblocks. The S-block (Fe6O2+

8 ) contains two oxygen layers forming a spinelstructure, where the R-block (MFe6O2+

11 ) is a three layer-block containing the

10 Structural and Magnetic Properties 185

MeO

W

BaO

Y

U

Z

X

M

Fe2O3

Ba2Fe8O14

BaFe 2O4

Fe2O3

S = Me2Fe4O8

Fig. 10.1. Phase diagram of AO–Fe2O3–MeO system

Table 10.1. The most well-known hexaferrite types with their compositions and adescription of their crystal structures. Me stands for Mn, Fe, Co, Ni, Cu, Zn and∗ denotes a rotation of 180◦ around the c-hexagonal axis

Type Nominal composition Nominal composition

M BaFe12O19 RSR*S*W BaMe2Fe16O27 RS2R*S*2

X Ba2Me2Fe28O46 (RSR*S*2)3Y Ba2Me2Fe12O22 (TS)3Z Ba3Me2Fe24O41 RSTSR*S*T*S*U Ba4Me2Fe36O60 RSR*S*T*S*

layer with the barium ion. The whole structure can be symbolically describedas RSR*S*, where the R*- and S*-blocks are built up by rotation of 180◦

around the hexagonal c-axis. Within the basic structure the Fe3+ ions occupyfive different interstitial sites. Three sites, named 12k, 2a and 4f2, have octahe-dral coordination, one site (4f1) has tetrahedral coordination and the 2b sitehas a fivefold coordination [28, 29]. The iron ions in the trigonal bipyramidare not in a symmetry plane but are displaced along the threefold/L3 axisand occupy randomly one of two equivalent position separated by 0.156 Afrom the symmetry plane of the bipyramid (Fig. 10.2) [30]. The 4f1 positionsand the 2a octahedral positions are occupied by Fe3+ in the S block. Fe3+

in the R block occupies octahedral sites in the octahedra shared by commonfaces (4f2), in octahedra at the interface of adjacent blocks (12k), and trigonalbipyramidal sites (2b). The presence of magnetic Fe3+ cations in these posi-tions is responsible for the barium hexaferrite’s magnetic properties and forits magneto-crystalline anisotropy (Table 10.2) (K1 = 3.3 × 105 J m−3) [31].


Ba

O

Fe (12k)

Fe (4f )

Fe (2a)

Fe (2b)

1

Fe (4f )2

2-

2+

Fig. 10.2. M-type barium hexaferrite structure [32]

Table 10.2. Crystallographic and magnetic properties for the various cationsublattices of M-type hexaferrite [28]

Sublattice Coordination Block Ions per Spinformula unit direction

12k Octahedral R–S 6 ↑4f1 Tetrahedral S 2 ↓4f2 Octahedral R 2 ↓2a Octahedral S 1 ↑2b Fivefold coordination

(trigonal bipyramidal)R 1 ↑

10.3 Magnetic Properties

The fundamental properties of magnetic materials are the saturation mag-netization, the coercivity, the magneto-crystalline anisotropy constant andthe Curie temperature. Intrinsic properties, such as the spontaneous mag-netization Ms, the first uniaxial anisotropy constant K1 and the exchangestiffness A, refer to the atomic origin of magnetism. As a rule, the intrinsicproperties are realized on length scales of at most a few inter atomic dis-tances and tend to approach their bulk values on a length scale of less than1 nm [1]. Extrinsic properties, such as the remanence Mr and the coercivityHc, are non-equilibrium properties-related to magnetic hysteresis- and exhibita pronounced real-structure dependence [1, 33].

The position of the magnetic ions and orientation of the spins in the crys-tal lattice were determined by Gorter by considering exchange interactions inbarium hexaferrite [34]. The magnetic moments of the iron ions are arranged


parallel to the hexagonal c-axis, but with opposite spin directions of the sub-lattices. The iron ions in the 12k, 2a and 2b sites have their spins alignedparallel to each other and the crystallographic c-axis, whereas those of 4f2and 4f1 point in the opposite direction [35]. The resulting magnetization Mat a temperature T of BaFe12O19 per formula unit can be approximated bysimple summation according to the formula

M(T ) = 6σ12k(T ) − 2σ4f1(T ) − 2σ4f12(T ) + σ2a(T ) + σ2b(T ) (10.1)

where σi stands for the magnetic moment of the i-Fe3+ ion. Assuming a mag-netic moment of 5µB per Fe3+ ion at 0 K (µB is the Bohr magneton) the netmagnetization is of 20µB per formula unit of barium hexaferrite [28].

Mossbauer spectroscopy is a basic technique for exploring the fine mag-netic structure of magnetic materials. The Mossbauer spectrum of bariumhexaferrite below the Curie point contains a superposition of five magneticallysplit subspectra associated with the five different iron sites [36]. Figure 10.3presents a typical spectrum of nanosize barium hexaferrite; the data thusobtained is summarized in Table 10.3 [37]. The spectrum was fitted with fivesix-line sub-patterns. The five six-line sub-patterns were assigned to the 12k,4f2, 4f1, 2a and 2b sites of the hexagonal crystal structure.

Fig. 10.3. Mossbauer spectrum of BaFe12O19 powder at room temperature [37]

Table 10.3. Hyperfine parameters of BaFe12O19 with average particle size 80 nmHhf , hyperfine magnetic field; δF e, isomer shift; 2ε, quadrupole splitting; RA, relativearea) [37]

Hhf 107 (Am−1) δFe (mm s−1) 2ε (mm s−1) RA (%)

12k 3.28 0.35 0.42 504f2 4.10 0.38 0.20 164f1 3.89 0.26 0.24 192a 4.03 0.34 0.06 102b 3.19 0.27 2.23 5


The energy of a magnetic material depends on the orientation of themagnetization with respect to the crystal axes, which is known as magneticanisotropy. The magnetic anisotropy affects strongly the hysteresis loop shapeand the values of the coercivity and the remanence. It is, therefore, of con-siderable importance for the practical applications of magnetic materials in,e.g., magnetic recording media. For example, permanent magnets need highmagnetic anisotropy to keep the magnetization in a desired direction. Themagneto-crystalline anisotropy is an intrinsic property of the ferrimagneticmaterials which does not depend on the particles’ shape and size. For a sin-gle crystal, it is the energy necessary to re-orient the magnetic moment ofthe crystal from the easy magnetization axis of to the hard magnetizationaxis. The existence of these two axes of magnetization arises from the interac-tion between the spin magnetic moment and the crystal lattice (spin–orbitalcoupling).

Generally, ferrites with hexagonal structure have two types of anisotropy,namely c-axis anisotropy and c-plane anisotropy, which are associated withthe easy magnetization along the c-axis and in the c plane, respectively. In thebarium ferrite family, only the Y-type barium ferrite has c-plane anisotropy,while the others have c-axis anisotropy [38, 39]. The BaFe12O19 exhibits oneof the highest values of the magneto-crystalline anisotropy – K1 = 3.3 ×105 Jm−3 [31]. The energy EK per unit volume of the magneto-crystallineanisotropy for uniaxial anisotropy can be written as follows [40]:

EK = K1 sin2 θ +K2 sin4 θ + · · · , (10.2)

where θ is the angle between the magnetization and the c-axis. K1 and K2

are the first and the second anisotropy constant. The direction along whichEK has an absolute minimum is called the easy magnetization axis. The easyaxis is determined by the sign and relative value of K1, and when K1 > 0 itcoincides with the hexagonal axis of symmetry (001), while for K1 < 0 it liesin the basic plane [41]. It is often convenient to express anisotropies in termsof anisotropy fields Ha.

The law of approach to saturation is often used to estimate the anisotropyfield Ha and the magneto-crystalline anisotropy K1 [42].

M = Ms(1 − A

H− B

H2· · · ) + χpH, (10.3)

where A is the inhomogeneity parameter, B is the anisotropy parameter andχp, the high-field differential susceptibility. The factor B is proportional toK2, where K denotes the effective anisotropy constant. In the spatial case ofBaFe12O19, which possesses uniaxial crystalline anisotropy along the c-axisand K2 � K1, the factor B may be expressed as [43]:

B =H2

a

15=

4K21

15M2s

. (10.4)


Coercivity is one of the most important characteristics of the hexaferritesin what concerns their potential applications. It describes the stability ofthe remanent state and gives rise to the classification of magnets into hardmagnetic materials. A widely used phenomenological coercivity expressionis [44]

Hc = αK2K1

µ0Ms−DeffMs −�H(T, η), (10.5)

where αK is the real-structure-dependent Kronmuller parameter [45, 46],Deff is a magneto-static interaction parameter and ∆H is a fluctuation-field correction due to thermal activation and η = dH/dt is a sweep rate[1, 33, 44, 47].

A fundamental characteristic of the coercivity is its dependence on theparticles’ size, which explains the unceasing development of techniques forpreparation of hexaferrite powders with high homogeneity and ever smallerparticles’ size. Below a certain critical size (Dcrit) the particle become mon-odomain; due to the hexaferrites’ magneto-crystalline anisotropy, this size issignificantly higher than that of ferrites with a spinel structures.

Figure 10.4 presents schematically Hc as a function of the size D of super-paramagnetic (SPM), monodomain (MD) and polydomain (PD) particles[48].

The critical size for monodomain BaFe12O19 particles can be calculatedby the following expression [26]:

Dcrit =9σw

2πM2s

(10.6)

where σw = (2kBTc|K1|/a)1/2 is the energy density of the domain wall, |K1|is the magneto-crystalline anisotropy constant, Tc is the Curie temperature,Ms is the saturation magnetization, kB is Boltzmann constant and a is thecrystal lattice constant. In particles with size D > Dcrit one observes a poly-domain state. Below this critical size, the particles exhibit only one zone ofspontaneous magnetization and absence of domain wall, i.e., they become

Fig. 10.4. Schematic presentation of the coercivity Hc dependence on the particles’domain structure at room temperature


monodomain. For barium hexaferrite, using the values of the single crystalparameters [49], one calculates the value Dcrit 460nm.

When a monodomain particle is very small, the anisotropy energy becomescomparable to or less than the thermal energy kBT ≥ KeffV ; the magneticstate of the particles is then defined as superparamagnetic [50]. Keff is the con-stant of effective anisotropy, which includes the magneto-crystalline anisotropyand the anisotropy of shape [51]. Thus, there exists a specific limiting size, Ds,for a particle to be monodomain under which the coercivity of a particle iszero. The initial rise in Hc as the particle’s size rises (aboveDs) (Fig. 10.4) can,therefore, be explained by the rise in the number of monodomain particles.As the particles’ size increases further, the coercivity reaches a maximum andthen drops down again. This coercivity reduction for sizes exceeding Dcrit isrelated to the appearance of domain walls. The transition from a monodomainto a polydomain state results in a decrease of Hc, since the magnetizationmechanism changes, namely, shifting the domain walls becomes energeticallymore advantageous than rotating the individual atomic spins.

Another important parameter used to describe the properties of hexafer-rites is the saturation magnetization Ms. The relation between the domainstate and the saturation magnetization can be divided into four regions [52]:

• For very small superparamagnetic particles (D < Ds), the variation in Ms

is due to thermal processes• For particles with sizes (Ds < D < Dtrans) the variation in Ms is

independent of the particles’ size and is related to rotational processes;• In larger particles (Ds < D < Dcrit; processes of inhomogeneous magneti-

zation arise and the coercivity decreases• As the particles’ size is increased further (D > Dcrit) the monodomain

particles become polydomain, where the variation of the saturation mag-netization has to do with domain wall motion.

Table 10.4 presents data on the magnetic characteristics of single crystalBaFe12O19 [2, 26, 53].

The most important micromagnetic phenomenon is magnetic hysteresis,which refers to the dependence of the magnetization as a function of theexternal magnetic field. Hysteresis is a complex non-linear, non-equilibriumand non-local phenomenon, reflecting the existence of anisotropy-relatedmetastable energy minima separated by field-dependent energy barriers. Onan atomic scale, the barriers are easily overcome by thermal fluctuations,but on nanoscale or macroscopic length scales the excitations are usually

Table 10.4. Magnetic characteristics of single crystal BaFe12O19

Tc (◦C) Hc (Am−1) Ms (emug−1) K1 (Jm−3) Ha (Am−1)

BaFe12O19 450 5.3 × 105 72 3.3 × 105 1.35 × 108


Fig. 10.5. Hysteresis loop of nanosized barium hexaferrite

Fig. 10.6. Determination of the saturation magnetization value

too weak to overcome the barriers. The determination of the local magne-tization M(r), from which the hysteresis loop is obtained by averaging, iscomplicated by the influence of the magnet’s real structure (defect structure,morphology, metallurgical ‘microstructure’) [1]. Figure 10.5 presents a typ-ical hysteresis loop of nanosized barium hexaferrite in high magnetic fieldsup to 2.5 × 106 Am−1. In this case the magnetization curve does not reachsaturation, so that data on the remanent magnetization (Mr) and coerciv-ity field (Hc) can only be obtained. The saturation magnetization value canbe estimated by extrapolating the curve for H → ∝. Barium hexaferritebeing a hard magnetic material, it reaches saturation at very high magneticfields, where one can determine the saturation magnetization value (Fig. 10.6).Figure 10.7 illustrates the magnetization variation of barium hexaferrite withparticles’ size of 80 nm with ellipsoidal shape as the magnetic field is raised


Fig. 10.7. Magnetization variation of barium hexaferrite with particles’ size of80 nm [37]

to 2.4 × 107 Am−1 [37]. As is seen, no saturation is reached; this behavior isrelated to the relative increase of the surface as the particle size is decreasedand, respectively, to the increased role of the disordered magnetic structureof the surface layer. This effect should be the object of further studies, sinceone might thus be able to clarify the contribution of the various types ofanisotropies on the magnetic properties of this type of particles.

10.4 Methods for Preparation

It is well known that the electrical, optical and magnetic properties of mate-rials vary widely with the particle sizes and shape and with the degreeof crystallinity. At present, tremendous efforts have been made in improv-ing their magnetic capabilities by using different synthesis methods [35]. Atthe same time, the research on their structural and physical properties hascontinued [4, 35, 54, 55].

Recent studies have shown that physical properties of nanoparticles areinfluenced significantly by the processing techniques [56]. Since crystallite size,particle size distribution and inter particle spacing have the greatest impacton magnetic properties, the ideal synthesis technique must provide superiorcontrol over these parameters [57]. A variety of techniques have been employedfor the synthesis of nanoparticles with definite shapes and sizes [20,58–60] . Atypical method of obtaining ferrimagnetic hexagonal oxide particles in generalis the solid-state reaction. The conventional solid-state method for prepar-ing BaFe12O19 is to fire an appropriate mixture of α-Fe2O3 and BaCO3 atvery high temperatures (1,150–1,250◦C). The resulting powder is then groundto reduce the particles’ size. Although high-temperature firing assures theformation of the required ferrite phase, larger particles (>1 µm) are oftenobtained in this firing process. It has been shown that the theoretical intrin-sic coercivities of ferrites can be approached only when the particle sizes are


below 1 µm [61]. On the other hand, grinding may introduce impurities intothe powder and cause strains in the crystal lattices, which has unfavorableeffect on the magnetic properties [62]. To overcome these problems, variuossoft chemical methods have been developed in order to reduce the particlesize and obtain highly homogeneous ultra fine single-domain particles of bar-ium hexaferrite. Among the most popular techniques we should mention: theglass-ceramic method [63,64], chemical co-precipitation [65–68], hydrothermalprocesses [69–71], the ammonium nitrate melt method [72], sol–gel [73–77],pyrolisis of aerosol [78,79], the mechanochemical method [80,81], auto combus-tion [20,82,83]. In all these processes, precursors are used that have ultra-finesize and high surface area; thus conventional restrictions of phase equilib-ria and kinetics can be easily overcome, which leads to lowering of sinteringand solid-state reaction temperatures and increased sintering rate [57]. Thesemethods are widely known and commonly used in the synthesis of magneticoxides. We will now consider some of them in more detail.

In the sol–gel synthesis the term sol refers to a suspension or dispersion ofdiscrete colloidal particles, while gel represents a colloidal or polymeric solidcontaining a fluid component, which has an internal network structure whereinboth the solid and fluid components are highly dispersed. The cations firstform a sol of either hydroxides or citrates or acetates. The discrete colloidalparticles slowly coalesce together to form a rigid gel. Since the particle sizesare very fine, these gels can be calcined at much lower temperatures than theconventionally derived powders to obtain a homogeneous product. Atomiclevel mixing of constituents in the sol–gel process leads to the formation ofsingle-phase products much more easily than by other process. The purity,microstructure and properties of the product can be controlled by the properselection of starting precursors, solvent, pH, of sol, calcinations temperatureand processing environment. The main problems in the hexaferrite preparationby the sol–gel technique are the gel formation and the deviation of measuredand expected values of the specific saturation magnetization [84].

The citric acid precursor method originated from the Pechini method.Pechini developed this method in 1967 and applied for patent in the UnitedStates (Patent No. 3 330 697). In the precursor method, the metallic salts aredissolved in water to have the required metallic ions well mixed. The metallicions are then chelated by a poly-acid (e.g., citric acid), and esterification ofchelated cations is carried out by adding poly-alcohol (e.g., ethylene glycol)at appropriate temperatures. After dehydration, a solid ester precursor withwell-mixed metallic ions can be obtained. The solid precursor is subjected toproper heat treatment to form the final ceramic particles. Lucchini et al. [85]showed that using pectic acid to chelate barium and iron ions in an aqueoussolution of nitrates and heating in air at 700◦C can produce crystalline bariumferrite with particle sizes less than 1 µm in diameter [62].

The hydrothermal process is used to synthesize pure, ultra-fine, stress-free barium hexaferrite powder with a narrow size distribution at relativelylow temperature (200–300◦C). This synthesis uses different precupsors as


Ba(NO3)2 and Fe(NO3)3.9H2O mixtures in the presence of NaOH/KOH/NH4OH, (C2H5)4NOH [69]; FeOOH and Ba(OH)2 mixture; αFe2O3 andBa(OH)2 mixtures; FeCl3 and Ba(OH)2 mixtures.

The low temperature combustion route is based on the gelling and sub-sequent combustion of an aqueous solution containing salts of the desiredmetals and some organic fuel, giving a voluminous and fluffy product withlarge surface are. This method has been proved to be a novel, extremely facile,time-saving and energy-efficient route for synthesis of ultra-fine powders [86].Using this method, Huang et al. [86] synthesized barium hexaferrite powdersbased on the combustion of nitrate-citrate gels due to an exothermic redoxreaction between nitrate and citrate ions. The particles have sizes between 80and 120 nm and Ms = 59.36 emug−1 and Hc = 4.4 × 105 Am−1.

In the aerosol process, a solution of the cations is passed trough an aerosolgenerator in the form of fine droplets, which are subsequently dried to form finepowders on passage through vacuum. The particles are than carried througha heated reactor tube in which the precursor compounds react to yield fineparticulates, which are then collected on a filter. Monosized spherical particlescan also be obtained by controlling the droplet size and contamination canbe avoided to a large extent by this method; powders having various sizedistributions can also be synthesized.

The chemical co-precipitation method is a cheap and easy choice for massproduction [43]. In this process, the cations are generally precipitated fromsolutions, such as hydroxides or carbonates. Co-precipitation of multivalentcations in a multicomponent system is difficult because the precipitatingagent (OH−, CO3

2−) form insoluble species with cations, which can haveapproximately the same solubility product only under very narrow boundarycondition of pH, temperature, dielectric constant of solvent. In the hydroxideprocess, the cations are precipitated from the solutions by using NaOH/KOHor NH4OH as precipitating agent. The carbonates are precipitated from themetal salts solution by adding Na/K-carbonate or (NH4)2CO3. Jacobao et al.[87] and Roos [88] used the coprecipitation method to prepare barium ferriteand showed that by heating the coprecipitates at relatively low temperatures(≤800◦C), submicron BaFe12O19 particles can be obtained. W. Ng et al. [67]studied in detail the influence of the heat treatment temperature on bar-ium hexaferrite’s magnetic properties. In general, this method does not allowone to control the size and size distribution of the particles [89]. In order toovercome these difficulties, the microemulsion method was proposed [90–93],which will be discussed in more detail later.

Table 10.5 summarizes the magnetic parameters of barium hexaferrite pro-duced by different soft-chemical techniques. In all cases listed in the table, thevalues of the magnetic parameters are lower than the theoretical ones calcu-lated for single-crystal barium hexaferrite. This is most probably the resultof the presence of magnetic and structural defects on the particles surfaceand, in some cases, due to the worse size homogeneity in the former samples.For particles with size of about 100nm, the lower values of Ms and Hc are


Table 10.5. Summarizes the magnetic parameters of barium hexaferrite producedby different soft-chemical techniques

Synthesis method Temperature Average particle Ms (emug−1) Hc (kAm−1)(◦C) size (nm)

Low temperaturecombustion [86]

850 120 59.36 440.8

Ion-exchangeresin [94]

850 220 71 302.4

Co-precipitation [95] 63.6 381.9Sol–gel [74] 900 130 70 473.4Ultrasonic spraypirolysis [96]

300 51 401.6

Sol–gel [97] 60.6 399Co-precipitation [67] 800 130 57 450Sol–gel [98] 950 85 61.62 442.7Sol–gel [99] 1,000 58.4 405.8Ammonium nitratemelt [72]

850 200 36.7 203.6

Ammonium nitratemelt [72]

900 300 45 243.1

Co-precipitation [100] 800 220 43 358High-energymilling [101]

60.9 381.1

Aerosol route [102] 1,000 108 50.8 290Self-propagation hightemperature [103]

49 190.9

Co-precipitation [104] 800 50–100 67.8 436.7Microemulsion [57] 100 61.2 429.4Microemulsion [105] 925 100 60.48 342.9Co-precipitation [106] 50–100 67 413.8Co-precipitation [105] 925 100 <50 <238.7Aerosol pyrolysis [79] 1,000 50–70 42.6 469.5Mechanicalalloying [107]

900 100 68 477.4

Microemulsion [108] 800 100 58 413.8Spark plasmasintering [109]

800 65.52 111.4

Co-precipitation [110] 830 500 52 2.38(a smallfraction of10 nm)

also due to the fact that the particles have not achieved the perfect hexag-onal shape typical for barium hexaferrite. The low saturation magnetizationvalues can be explained by the fact that the particles are smaller than thecritical diameter for barium hexaferrite and should possess non-compensatedmagnetic moments on the surface.


10.5 Microemulsion Technique

We will now consider the use of the aqueous cores of water-in-oil microemul-sions as reactors for the synthesis of barium hexaferrite nanoparticles. Oneof the reasons to explore this technique more closely is that the precipi-tation reactions in microemulsions offer a novel and versatile technique forsynthesis of a wide variety of magnetic nanoparticles with the ability to con-trol precisely the size and shape of the particles formed, as well as a uniquemethod to control the kinetics of particle formation and growth by varyingthe physicochemical characteristics of the microemulsion [57].

The microemulsion system consists of an oil phase, a surfactant phase andan aqueous phase. The reverse micelles are water-in-oil droplets stabilizedby a surfactant. The high homogeneity of the nanosized precipitate particlesproduced is due to fact that each of the aqueous droplets acts as a nanosizedreactor for nanoparticles formation [111, 112]. One of the advantages of thistechnique is the preparation of very uniform particles (<10% variability) [113].A microemulsion system exhibits a dynamic structure of nanosized aqueousdroplets, which are in constant formation, breakdown, and coalescence. Thisresult in a continuous exchange of solvent. If a nanoparticle is nucleated withinthe water droplet, its growth is limited by the size constraint of the waterdroplet [114]. The size of these aqueous droplets is in the range 5–100nmdepending on the water/surfactant ratio:

Rw =3Vaq[H2O]

σ[S], (10.7)

where Rw is the water droplet radius, Vaq is the volume of the water molecule,σ is the area per polar head of surfactant, [S] is the concentration of surfac-tant [115]. An increase in the ratio increases the size of the water pool insidethe inverse micelle, and therefore allows bigger particles to form [116]. Thus,the surfactants not only reduce the surface energy, but also control the growthand shape of the particles and act against aggregation. The surfactants are ofthree types – non-ionic, anionic and cationic. Various surfactants have beenemployed in the synthesis of hexaferrites, with cetyltrimethylammonium bro-mide CH3–(CH2)15–N(CH3)3Br (CTAB), a cationic surfactant, being mostcommonly used.

Usually, the synthesis of precursors for oxide particles formation is carriedout by way of mixing two microemulsion systems with identical composi-tions but different aqueous-phase types – the one containing metal ions, theother, a precipitating agent (NH4OH, NaOH, KOH, etc.). The co-precipitationreactions are expected to take place when aqueous droplets containing thedesirable reactants collide with one another, coalesce and break apart. Thecollision process depends upon the diffusion of the aqueous droplets in thecontinuous media, i.e. oil, while the exchange process depends on the attrac-tive interactions between the surfactant tail and the rigidity of the interface,as the aqueous droplets approach closely each other [57, 117]. One of the


NaOH

OH-

OH-

Aqueous phaseBa(NO )3 2

FeCl3

Precipitate

Microemulsion systemone

Microemulsion systemtwo

Single microemulsion Double microemulsion

Fig. 10.8. Schematic diagram of the microemulsion techniques

many microemulsion systems employed to produce magnetic oxides consistsof cetyltrimethylammonium bromide (CTAB) as a cationic surfactant; n-butanol as a co-surfactant; n-hexanol as a continuous oil phase and an aqueousphase [37,118]. An advantage of using CTAB as a surfactant is the possibilityof free passage of OH− ions through the water droplet walls in both direc-tions. This fact allows one to use a single microemulsion system to producenanosized particles when the precipitating agent is NH4OH, NaOH, or KOH.The single microemulsion method is characterized by the presence of only onemicroemulsion system whose aqueous phase contains metal ions only. Oneof the advantages of the single microemulsion technique is that it is muchless expensive than the classical double microemulsion method. Figure 10.8presents schematically the two microemulsion techniques.

The XRD spectrum of the synthesized BaFe12O19 powder is presented inFig. 10.9. It shows the characteristic peaks corresponding to the barium hex-aferrite structure. Scanning electron microscopy is widely used to determinethe grain size and morphology of powders. Figure 10.10 shows the morphol-ogy of the BaFe12O19 powder obtained by single microemulsion. It exhibitsa narrow grain-size distribution, with the average particle size being 130nm.The particles have an irregular shape between spherical and hexagonal. Theprocess of forming the platelet shape typical for BaFe12O19 hexahedral hasnot been completed due to the small particle size. The critical diameter for


Fig. 10.9. X-ray diffraction pattern of barium hexaferrite powder obtained by singlemicroemulsion technique

Fig. 10.10. SEM image of barium hexaferrite powder sample with average particlesize of 130 nm prepared via single microemulsion

Fig. 10.11. Hysteresis loop of barium hexaferrite powder sample with averageparticle size of 130 nm prepared via single microemulsion

single-domain barium hexaferrite particles is about 460 nm [64], so that theparticles are single domain.

The hysteresis loop of the powder sample at room temperature and amaximum applied field of 2.3 × 106 Am−1 is shown in Fig. 10.11. The satu-


Fig. 10.12. Magnetization curve barium hexaferrite powder sample with averageparticle size of 130 nm prepared via single microemulsion

ration magnetization value (Ms) was obtained from the magnetization curvein high magnetic fields up to 1 × 107 Am−1, which is presented in Fig. 10.12.The results of the magnetic measurements, namely, saturation magnetizationMs of 62 emu g−1 and coercivity field (Hc) of 3.9 × 105 Am−1 at room tem-perature are comparable to the best results for powders prepared via thedouble microemulsion method [105, 119]. Such high Ms and Hc values maybe attributed to the high phase purity, the well-defined crystallinity and thehomogeneity with respect to the BaFe12O19 particles size.

It was thus demonstrated that the single microemulsion method, whichis less expensive than the classical double microemulsion method, may beused to prepare powders of monodomain barium hexaferrite nanoparticleswith high size-homogeneity and good magnetic properties in view of possibleapplications.

Acknowledgement

T. Koutzarova was supported by NATO Reintegration Grant(EAP.RIG.981472). The work was supported in part by research agree-ments between the Bulgarian Academy of Sciences and Bulgarian ScientificFund under grant HT-1/01.

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[Springer Proceedings in Physics] Advances in Nanoscale Magnetism Volume 122 || Structural and Magnetic Properties and Preparation Techniques of Nanosized M-type Hexaferrite Powders