Journal of Magnetism and Magnetic Materials 323 (2011) 945–953

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Journal of Magnetism and Magnetic Materials

0304-88

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/jmmm

Preparation and magnetic properties of barium ferrites substitutedwith manganese, cobalt, and tin

Yue Liu a, Michael G.B. Drew b, Ying Liu a,n

a College of Chemistry and Life Science, Shenyang Normal University, Shenyang 110034, PR Chinab School of Chemistry, The University of Reading, Whiteknights, Reading RG6 6AD, UK

a r t i c l e i n f o

Article history:

Received 20 September 2010

Received in revised form

16 November 2010Available online 21 November 2010

Keywords:

Co-precipitation/molten salt method

Barium ferrite

Doping

Saturation magnetization

Coercivity

53/$ - see front matter & 2010 Elsevier B.V. A

016/j.jmmm.2010.11.075

esponding author. Tel.: +86 024 86578790.

ail address: yingliusd@163.com (Y. Liu).

a b s t r a c t

Barium ferrites substituted by Mn–Sn, Co–Sn, and Mn–Co–Sn with general formulae BaFe12�2xMnxSnxO19

(x¼0.2–1.0), BaFe12�2xCoxSnxO19 (x¼0.2–0.8), and BaFe12�2xCox/2Mnx/2SnxO19 (x¼0.1–0.6), respec-

tively, have been prepared by a previously reported co-precipitation method. The efficiency of the

method was refined by lowering the reaction temperature and shortening the required reaction time, due

to which crystallinity improved and the value of saturated magnetization increased as well. Low

coercivity temperature coefficients, which are adjustable by doping, were achieved by Mn–Sn and

Mn–Co–Sn doping. Synthesis efficiency and the effect of doping are discussed taking into account

accumulated data concerning the synthesis and crystal structure of ferrites.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Extensive investigations have been carried out on the synthesis andproperties of barium ferrite. The usual methods of synthesis include co-precipitation [1–3], molten salt [1,2,4], and sol–gel [3,5], as well ashydrothermal methods [6]. All these synthesis methods, including co-precipitation and molten salt methods, involve the process of convert-ing rhombohedral a-Fe2O3 to barium ferrite. Since a-Fe2O3 is stable athigh temperature, long reaction time is necessary to convert it tobarium ferrite using BaO. Thus, the appearance of a-Fe2O3 as a by-product cannot be avoided as some elemental Fe in the reactant is stillconverted to the form of a-Fe2O3 in the product. By contrast, theinvention of the sol–gel method led to the improvement of synthesisefficiency through formation of a cubic intermediate g-Fe2O3 [7] andthus suppression of the formation of a-Fe2O3. We have followed asimilar route to raise the efficiency of the hydrothermal method, i.e.introducing Fe2+ ions as a reactant to conduct the synthesis via a cubicintermediate Fe3O4 [6]. However, it is difficult to improve the co-precipitation and molten salt approaches via the cubic iron oxideintermediate since high temperatures are involved. At high tempera-ture a-Fe2O3 is likely to be formed, and cubic g-Fe2O3 and Fe3O4 arelikely to be converted to a-Fe2O3.

In fact barium ferrite is composed of cubic S and hexagonal Rblocks. In order to form barium ferrite, Fe2 +, Ba2 +, and O2� in thereactant mixture must be self-assembled to S and R structures at

ll rights reserved.

the reacting temperature in a suitable time. Any conditions thatwould encourage the formation of g-Fe2O3 and Fe3O4 in thesynthesis process would also encourage the formation of bariumferrite since g-Fe2O3 and Fe3O4 would be more easy to convert tothe S block in barium ferrite than a-Fe2O3. This does not mean thathigher synthesis efficiency can be achieved simply by employingg-Fe2O3 or Fe3O4 as a reactant, as high temperature and longreaction time are still needed, and the cubic oxides are likely to beconverted toa-Fe2O3 easily under the experimental conditions. Wenoted that BaFe2O4 may or may not appear as an impurity amongthe products [8,9], although a-Fe2O3 is usually found to be a by-product of the synthesis. Hence studying the synthesis efficiencycould possibly reveal the role played by BaFe2O4.

We have overcome the difficulty of raising the efficiency of theco-precipitation method by coupling the co-precipitation andmolten salt methods and by using a low temperature pretreatment[1,10]. This novel approach has made the formation of S block inbarium ferrite easier in the self-assembly reaction using conditionsthat are also favorable for the formation of cubic iron oxides, stuffedtridymite orthorhombic monoferrite BaFe2O4, or spinel BaFe2O4

[11], since the coupling reduced the reaction temperature byconverting the solid diffusion reaction in the co-precipitationreaction to a molten solvent reaction. The reduced temperaturesuppresses the formation of a-Fe2O3 and favors the formation ofcubic ferric oxides, and hence should also encourage the formationof the S block in barium ferrite in the self-assembly reaction. Pre-heating favors the increase in preparative time for cubic iron oxideformation, thus prolonging the preparative time for S blockformation.

Y. Liu et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 945–953946

Methods with greater efficiency are better for doped bariumferrites since they are more difficult to synthesis. We have appliedthe above method to the synthesis of Zn–Co–Sn [1], La–Mn, andLa–Co [10] doped barium ferrites and found that efficiencies ofsynthesis were greatly increased in that not only were the reactiontemperature and time reduced, but also crystallinity of the productwas improved, the formation of impurities was reduced, and themagnetic properties of the samples synthesized were improvedcompared to those synthesized through high temperature pro-cesses with longer reaction times. This synthesis method isrelatively new and more experiments should be conducted toverify its validity and provide more facts for more extensivediscussion.

Excess of barium was used in the experiments. From earlystudies, it was found that using barium in excess favors theformation of barium ferrite [12–14]. Enough work has beenpublished on the synthesis to reach a clear consensus [6] concern-ing the reasons for the effect observed with the use of excessamounts of barium. Besides the investigations on intrinsic bariumferrite, considerable work has been devoted to cation doping inorder to optimize its properties for different uses. One of thepurposes of doping is to prepare barium hexaferrites with a lowtemperature coefficient of coercivity (TCC) since the stability of thecoefficient of coercivity Hc with respect to temperature is impor-tant for some applications. Fang et al. [15] obtained a low positiveTCC of 1.9 Oe/K (0.02 kA/m K) in BaFe12�2xZnxSnxO19, wherex¼1.1, prepared using the co-precipitation method. The lowestpositive TCC obtained by Slama et al. [16] is reported to range from0.01 to 0.08 kA/m K for BaFe12�2xMxZrxO19, where x¼0.4–0.6 andM¼Ni2 +, Co2 +, or Zn2 +, prepared using mechanical alloying andcitrate precursor methods. Yang et al. [17] reported a negative TCCobtained after two component doping with Ni/Ti, while for threecomponent doping with Ni–Zn–Ti, the coefficient falls in the rangeof 0.2–0.9 Oe/K (0.02–0.07 kA/m K). Bai et al. [18] reported anegative TCC in La/Zn substituted Sr-magnetoplumbite ferrite.We reported previously a negative TCC in barium hexaferritesfor three component doping with Co, Zn, and Sn synthesized by theco-precipitation/molten salt method [1]. Here the same method isused to prepare BaFe12�4xCoxMnxSn2xO19 (x¼0.1–0.6) and thesecompounds also exhibit a low TCC. Thus we demonstrate that threecomponent doping of Co and Sn along with Mn or Zn can be used tolower the TCC.

There are several strategies for changing the relative propor-tions of the substituents in three component doping. Yang et al. [17]adopted a strategy for Ni–Zn–Ti substitution to prepare BaFe11.2-

NixZny�xTiyO19 in which y was fixed at 0.4 while x was varied fromx¼0.0 to 0.4, thus making the total substitution amount ofNi+Zn equal to 0.4. In our syntheses of BaFe12�2xMnxSnxO19 (x¼

0.2–1.0), BaFe12�2xCoxSnxO19 (x¼0.2–0.8), BaFe12�2xCox/2Mnx/2

SnxO19 (x¼0.1–0.6), and BaFe12�2xCox/2Znx/2SnxO19 (x¼0.0–2.0)the amount of doping was changed systematically while alwaysensuring that charge balance was retained. Similar formulae havealso been adopted by Ghasemi et al. [19], Ghasemi and Morisako[20], and Gairola et al. [21] to study magnetic and microwaveabsorption properties of BaFe12�x(Mn0.5Cu0.5Zr)x/2O19 andBaFe12�x(Mn0.5Co0.5Zr)x/2O19 (x¼1, 2, 3) prepared by sol–gelprocessing and BaCoxMnxTi2xFe12�4xO19 (0rxr0.5) prepared bya solid-state technique.

1 We note that Ghasemi and Morisako [20] have assumed that m(Fe3+)¼5mB

and m(Mn2 +)¼0mB, which may be an improper use of representation and what the

0mB really means should be that replacing Fe with Mn will result in no magnetization

change.

2. Experimental methods and results

2.1. Synthesis and characterization methods

Ultrafine particles of BaFe12�2xMnxSnxO19 (x¼0.2–1.0),BaFe12�2xCoxSnxO19 (x¼0.2–0.8), and BaFe12�2xCox/2Mnx/2SnxO19

(x¼0.1–0.6) were prepared via the chemical co-precipitation/molten salt route. All the starting materials (FeCl3, BaCl2, CoCl2,SnCl2, MnCl2, and anhydrous Na2CO3) were of analytical grade.Aqueous solutions of the metallic chlorides Ba2 + , Fe3 + , Co2 +, Mn2 + ,and Sn4 + were prepared in the required ratio for each specificferrite, except that the initial mole ratio of the sum of Fe anddopants Co–Sn, Mn–Sn, and Co–Mn–Sn to Ba for reactants waschosen to be 11:1, where the amount of Ba was in excess. Thesewere then added to an aqueous solution of 60% in excess ofanhydrous Na2CO3. The resulted solutions were stirred for 1 h at70 1C, and then cooled, and filtered off. The intermediate precipi-tates were washed thoroughly with deionized water until no Cl�

could be detected and then dried at a temperature of 80 1C. KCl fluxand appropriate amounts of the intermediate were then mixed in1:1 weight ratio. The co-precipitate/molten salt mixtures werethen heated at 450 1C for 2 h. The temperature was then raised from450 to 950 1C at a rate of 15 1C/min, maintained at 950 1C for 4 h,and then cooled. The products were washed with hot deionizedwater several times until the water became Cl� free. The sub-stituted barium ferrite hexagonal particles were then dried at atemperature of 80 1C.

The crystalline phases of the samples were identified throughpowder X-ray diffractograms with Cu Ka radiation using an X-raypowder diffractometer (Y-500, Dandong/China). Magnetic proper-ties were measured at room temperature and at 303 and 85 Kwith a vibrating sample magnetometer (VSM-7300, Lakeshore).The maximum applied field of 10 kOe (0.8 kA/m) was used toevaluate the magnetic parameters. Environmental scanning elec-tron microscopy/electron dispersive X-ray analysis (ESEM/EDX,FEI/Philips XL-30) was used for the estimation of crystallite size andaggregation.

2.2. Results

Theoretically it can be predicted that doping by the non-magnetic Zn2 + ion, which occupies the 4f1 sites, will increasethe value of specific saturated magnetization Ms [22], but experi-mentally the opposite has usually been found, wherein Zn2 + dopingreduces Ms, especially with larger amounts of dopant [10]. Wetherefore used Mn2 + instead of Zn2 + in the three componentZn–Co–Sn doping [1] since Mn2 + and Fe3 + have the same electronconfiguration of 3d5 both with m(Mn2 +)¼5mB.1

2.2.1. Structure and particle morphology of the samples

Fig. 1 shows the XRD patterns of the prepared ferrite powdersfor BaFe12�2xCox/2Mnx/2SnxO19 with x ranging from 0.1 to 0.6. Allsamples were confirmed by XRD to consist of a single phase of themagnetoplumbite structure (JCPDS 43-0002) thus showing that thedopants replace Fe without significant change of structure. Thesmall peak shifts showed that the cell parameters changed alongwith the doping amount (Table 1). The dopants entered the latticeand altered the relative peak intensity in XRD but not the type oflattice.

No impurities such as a-Fe2O3 and BaFe2O4 were detected byXRD, which was attributed to the high efficiency of the newsynthesis approach. An atomic level blending of the constituentsis achieved by the co-precipitation approach. As mentioned in theintroduction, the problems of the diffusion-controlled solid statereaction in the co-precipitation and ceramic approaches wereavoided by the coupled molten salt approach with KCl flux, which

Y. Liu et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 945–953 947

enabled the synthesis temperature to be greatly reduced. The pre-heating adopted was found to be a useful novel technique inensuring good crystallinity and fewer impurities. In the usualstand-alone co-precipitation or the molten salt method, highersynthetic temperature and longer reaction time are needed, and theappearance of a-Fe2O3 is an inevitable consequence.

However, the XRD diffraction peaks became weaker and broaderalong with the shifts in peak positions and alterations of relativepeak intensities as x was increased (Fig. 1), indicating that crystal-linity decreased and particle grains became smaller as Fe3 +

was substituted by large amounts of Mn2 +, Co2 +, and Sn4 +. Theresults are consistent with those found in three Zn–Co–Sn

Fig. 1. XRD patterns of the samples BaFe12�2xCox/2Mnx/2SnxO19 at different values

of x calcined at 450 1C for 2 h and then at 950 1C for 4 h: (a) x=0.1, (b) x=0.2, (c) x=0.3,

(d) x=0.4, (e) x=0.5, and (f) x=0.6.

Table 1Lattice parameters of the substituted BaFe12�2xCox/2Mnx/2SnxO19 hexaferrites.

x a (nm) c (nm)

0.00a 0.589 2.318

0.30 0.589 2.321

0.40 0.590 2.321

0.60 0.590 2.323

0.80 0.590 2.331

a Data at x¼0.0 are taken from JCPDS 43-0002.

Fig. 2. SEM imagines of BaFe12�2xCoxSnxO

component doping [1]. The other two series, for Mn–Sn and Co–Sn, give similar results.

The lattice parameter a (¼b) almost remains constant but c

increases with x for BaFe12�2xCoxSnxO19 as shown in Table 1. Theradii of Mn2 +, Co2 +, and Sn4 + are 0.080, 0.074, and 0.071 nm,respectively, a little larger than the 0.064 nm of Fe3 +. By substitut-ing the Fe3 + ions with the dopants the parameter c increases byapproximately 0.06% from 2.318 to 2.331 nm although the P63/mmc space group is maintained.

Representative SEM images of the series are shown in Figs. 2 and 3.The phases were perfect crystalline hexaferrite when x¼0.2–0.5 forthe samples doped with Mn/Sn and for x¼0.2–0.6 with Co/Sn (Fig. 2),and also with Mn–Co–Sn as shown in SEM representative micro-graphs (Fig. 3). As shown in the figures the plane dimensions of mostof the grains were about 500 nm. The grains became smaller andthinner as x increased, consistent with the results of XRD, wherediffraction peaks became weaker and broader at large values of x. Butno apparent deterioration of crystallinity was observed at these smallvalues of x. The effect of doping amount x on thinning the plates wasalso observed in Zn–Co–Sn doping and became more evident when x

became larger [1]. It is common for grain size to decrease withincreased doping concentration at larger values of x. But contradictingresults were also reported in literature. Although the diffraction peaksbecame weaker and broader as x increased, which can also been seenfrom the XRD provided in Refs. [19–21] for other three-componentdopings, the uniformity of grain size increased with dopant concen-tration of BaCoxMnxTi2xFe12�4xO19 [21] but the grain size wasnot noticeably affected by doping in BaFe12�x(Mn0.5M0.5Zr)x/2O19

(M¼Cu2+ [19] or Co2+ [20]).Concomitant with an increase in the length of the c-axis,

agglomeration along the c-axis was also observed at small valuesof x, but as x increased further it reduced, and it became difficult toobtain perfectly crystalline hexaferrite plates as thinning becamemore apparent and smaller grains were obtained. The agglomera-tion along the plates in a and b directions also increased as thedoping amount increased.

2.2.2. Magnetic properties

Small amounts of doping do not significantly affect particlemorphology but have a significant influence on magnetic proper-ties. As x increased, the particles became smaller. The effect ofdoping on grain size is significant for anisotropy as indicated by Hc

(coercivity). Hc of intrinsic barium ferrite is high due to stronguniaxial anisotropy along the c-axis. Magnetocrystalline aniso-tropy (c-plane anisotropy) became important when the platesbecame thinner. Therefore, the Zn–Co–Sn [1] and Mn–Co–Sn

19 samples: (a) x¼0.2 and (b) x¼0.6.

Fig. 4. Hysteresis loops of the BaFe12�2xCox/2Mnx/2SnxO19 samples.

Fig. 5. Relationship of saturation magnetization Ms and coercivity Hc with

substituted content x in samples: (a) BaFe12�2xCoxSnxO19, (b) BaFe12�2xMnxSnxO19,

and (c) BaFe12�2xCox/2Mnx/2SnxO19.

Fig. 3. SEM images of BaFe12�2x Mnx/2Cox/2SnxO19 samples: (a) x¼0.2 and (b) x¼0.6.

Y. Liu et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 945–953948

substitutions can lead to a decrease in c-axis anisotropy ormodification in anisotropy from the c-axis to the c-plane.

Hysteresis loops of BaFe12�2xCox/2Mnx/2SnxO19 are shown inFig. 4. Note that Ms increases with x at small values of x up to 0.4(Fig. 5). The hard ferrite became softer as more Fe3 + was sub-stituted by the dopants. Thus for BaFe12�2xCox/2Mnx/2SnxO19,Hc¼270.6 kA m�1 at x¼0.1, Hc¼201.0 kA m�1 at x¼0.3, andHc¼88.6 kA m�1 at x¼0.5.

The value of Ms was determined by the law of approach tosaturation (LAS) as Eqn. (1) following ref. [1]:

MðHÞ ¼Msð1�a1=H�a2=H2�a3=H3ÞþwpH ð1Þ

where M(H) is the specific magnetization at magnetic field strengthH; a1 the inhomogeneity parameter; a2 the anisotropy parameter;and wp the high field differential susceptibility.

To obtain Ms, wp in Eq. (1) can be neglected since the curve inhysteresis loops levels out at high magnetic field and a3 can beneglected when the magnetic field is high. So a simplified equationcan be used to calculate Ms as Eqn. (2):

MðHÞ ¼Msð1�a1=H�a2=H2Þ ð2Þ

As x increases, Ms first increases and then decreases for all threeseries. For a low doping amount, Ms of the Co/Sn doping series ishigher than that of the Mn/Sn doping series, but the decrease in Ms

in the Co/Sn doping series occurs much earlier than in the other twoseries at x¼0.3 (Fig. 5). The Ms from samples of Mn–Co–Sn dopinghas intermediate values at most values of x and reaches a maximum

of 79.19 A m2/kg at x¼0.4. Ms for the three component doping ofBaFe12�2xCox/2Znx/2SnxO19 was also observed to have values inter-mediate between those for the two component doping ofBaFe12�2xZnxSnxO19 and BaFe12�2xCoxSnxO19 at the same level ofdoping[1]. Since Ms increases for a larger range of x in the Mn/Snseries, adding Mn2 + to the Co–Sn series in three component dopingwill lessen the point at which Ms begins to decrease with increase inx (Fig. 5). The effect of Mn2 + on Ms was also observed by comparingthe La–Mn and La–Co doped series [10].

As x increases, the grain size decreases. Hc decreases for all threeseries of ferrites since the mobility of the domain wall is limited bythe small size. For BaFe12�2xMnxSnxO19, Ms reaches a maximumat x¼0.8, where Ms¼86.46 A m2/kg while Hc¼48.0 kA m�1.For BaFe12�2xCoxSnxO19, Ms reaches a maximum at x¼0.3 with Ms¼

80.65 A m2 kg�1 and Hc¼103.3 kA m�1. At x¼0.3 Ms of BaFe11.4Co0.3

Sn0.3O19 was higher than that of BaFe11.4Mn0.3Sn0.3O19, but starts todecrease at a lower value of x than do the Mn/Sn dopant compounds.

The anisotropy constant K1 and the anisotropy field HA arerelated to a2 and defined in Eqn. (3) following ref. [17]

a2 ¼ 4K21=15Ms

2¼H2

A=15 ð3Þ

Values of HA and K1 versus x of the three series are shown inFig. 6. As x increases, K1 decreases. But K1 is greater than zero, whichmeans the facile magnetizing direction is along the c-axis. K1 of thethree component Mn–Co–Sn system is higher than that of all othersystems considered, e.g., BaFe12�2xMnxSnxO19, BaFe12�2xCoxSnx

O19, BaFe12�2xZnxSnxO19, and BaFe12�2xCox/2Znx/2SnxO19. The

Fig. 6. Effect of x on HA and K1 of (a) BaFe12�2xCoxSnxO19, (b) BaFe12�2xMnxSnxO19,

and (c) BaFe12�2xCox/2Mnx/2SnxO19.

Fig. 7. Effect of x on TCC in BaFe12�2xMnxSnxO19 and BaFe12�2xCox/2Mnx/2SnxO19

powders.

Y. Liu et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 945–953 949

higher value of K1 obtained by the three component Mn–Co–Sndoping also results in a value of HA that is higher than that of theother doped ferrites, although the values of both HA and Hc decreaseas x increases.

Hc is closely related to both K1 and HA. The decrease in K1 as x

increases is reflected in the values of Hc and HA. In a hexagonalcrystal, the energy EK along different directions can be calculated[18] as Eqn. (4):

EK ¼ K1 sin2yþK2 sin4yþK3usin6yþK3 sin6ycos6Fþ � � � ð4Þ

Where y and F are spherical coordinates; y represents thedeviation from the c-axis; and F defines the direction in theperpendicular plane.

As shown in Eq. (4), EK varies with y. K1 has the greatestinfluence on EK at small values of y. If y represents the deviationfrom the easy facile magnetization direction and when there is noexternal magnetic field, the M(H) vector will lie along this direction.K1, Hc, and HA decrease as x increases.

The effect of varying the amount x of the substituent on theaverage TCC in the Mn/Sn and Mn–Co–Sn series is shown in Fig. 7.The average temperature coefficient of Hc was obtained as follows:

DHc=DT ¼ ½Hcð303KÞ�Hcð85KÞ�=218K ð5Þ

Hc (303 K) and Hc (85 K) are the values of Hc at temperatures 303and 85 K, respectively. With increase in x, TCC and DHc/DT showed

a decrease from positive to negative. DHc/DT approaches zero, thepoint at which coercivity is independent of temperature, at valuesof x between 0.5 and 0.7 in the Mn/Sn series. For Mn/Sn doping inthis region Ms increases, Hc decreases, and the magnetic propertiesbecame more stable with respect to temperature when DHc/DT¼0.At room temperature, for x¼0.5, Ms¼83.07 A m2 kg�1, Hc¼103.3kA m�1 and for x¼0.7, Ms¼84.33 A m2 kg�1, Hc¼53.0 kA m�1. Forthe Mn–Co–Sn series, DHc/DT approaches 0 between 0.5 and 0.6.When x¼0.5, Ms¼77.31 A m2 kg�1, Hc¼88.6 kA m�1 and whenx¼0.6, Ms¼76.78 A m2 kg�1, Hc¼73.3 kA m�1 at room tempera-ture. For x in the range 0.5–0.6, not only is coercivity stable withrespect to temperature, but also Ms and Hc are relatively high. As x isfurther increased, Ms is relatively constant but Hc and DHc/DT

decrease.

3. Discussion

Experimental data on the effects of doping are being accumu-lated with contributions from many workers, reflecting the impor-tance of such research. Evidence has been obtained indicating, e.g.,which ions prefer which site and how magnetic properties areaffected by different ions in different sites. Unlike the results fromintrinsic barium ferrite for which a consensus is now generallyaccepted, the effects of doping on barium ferrite are much morecomplicated and there are conflicting reports on magnetic proper-ties [23]. For a basic understanding of the effects of doping, a clearpicture on the structure of intrinsic barium ferrite needs to beconsidered first.

3.1. Structure and efficient synthesis

Some of the packing possibilities for barium ferrites are shownin Fig. 8. Only the combinations of S, RS, and TS blocks are proven tobe possible in various types of barium ferrites. T is defined as theAC0ACA block. There are different types of barium ferrites made upof combinations of the basic building blocks M, R, and S shownin Fig. 8. These include M¼R+S in the M-type; W¼M+S in theW-type; X¼M2S in the X-type, which also includes M4S and M6S;Y¼T+S in the Y-type; U¼M2Y in the U-type, and Z¼M2Y2 in theZ-type, which also includes M4Yn, M6Y14, and M8Y27. Only pureproducts of the M-, Z-, Y-, and W types can be obtained. Selectivelysubstituting Fe3 + at different sites by different cations is importantin altering the magnetic properties of barium ferrites. It has beenshown that excessive cation doping can convert the M-type bariumferrite to the W-type [23].

M-type barium ferrite belongs to the space group P63/mmc witha complex hexagonal magnetoplumbite structure. The latticeparameters are a¼b¼0.5892 nm and c¼2.3183 nm. The coordi-nates of the atoms in the unit cell have been obtained from thesymmetry of the space group and powder diffraction studies[23–25]. Fe3O4 is one of the intermediates in the synthesis ofM-type barium ferrite obtained using Fe2 + as one of the reactantsby the hydrothermal method [6]. The structure of barium ferriteBaFe12O19 (see Fig. 8) can be considered to be constructedfrom building blocks in two different ways, both of which involvethe Fe3O4 moiety. First, unit barium ferrite cell can be described interms of four blocks S–R–Sn–Rn, where the cubic S block (ABCpacking) consists of [Fe6O8]2 +

¼2[Fe3O4]+ with some similarityto the structure of Fe3O4 and the hexagonal R block (AB0AB orAC0AC packing) of [BaFe6O11]2�

¼[(BaFe2O4)Fe4O7]2�¼[(BaFe2O4)2

(a-Fe2O3)O]2� with some similarity to a-Fe2O3. All Fe in bariumferrite is in the trivalent state. Second, it can be described as the fourblocks S4–B1–S4

n–B1n where the cubic S4 block consists of [Fe9O12]3+

¼3[Fe3O4]+ and the hexagonal B1 block of [BaFe3O7]3�¼[BaO

(Fe3O4)+O2]3�¼[BaO (a-Fe2O3)FeO3]3� . From Fig. 8 it can be seen

Fig. 8. Structural representation of BaFe12O19. The c-axis is vertical; the horizontal

direction is the cell diagonal. Large open and shaded circles represent O2� and Ba2 +

ions, respectively. Small open, shaded, black, open enclosing +, and open enclosing

� represent Fe3 + ions in 4f2, 4f1, 12k, 2b, and 2a sites, respectively. nIndicates

symmetry related blocks. Layers A, B, and C contain only oxygen ions. Layers B0 and

C0 are the same as layers B and C but with one oxygen ion replaced by Ba2+ in the cell.

The small arrows indicate the directions of z component of magnetic moment of the

Fe3 + ions.

Y. Liu et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 945–953950

that the S4 or S cubic block of the ABC packing is set between twohexagonal B1 and B1

n blocks of AB0A and AC0A or between R and Rn

blocks. This feature is usually described as two spinel blocks in a unitcell each being in the middle of the hexagonal layers that contain Ba2+.The presence of BaFe2O4 and BaFeVO4 (V: vacancy) moieties in thehexagonal R or B1 blocks may involve only two layers and theirstructures have some similarity with those found in the spinelcompound BaFe2O4[11].

There is evidence for the appearance of Fe3O4 from othermethods for preparing barium ferrite. In a combustion method,Castro et al. [26] detected the intermediate Fe3O4 though this issurprising as only Fe3 + was used in their starting material and thelikelihood of reducing Fe3 + to Fe2 + in a combustion environmentseems remote even though organic reducing reagents wereincluded. Lisjak et al. [27] also detected Fe3O4 in their preparationof BaCoTiFe10O19 coatings on glass using atmospheric plasmaspraying and with no Fe2 + provided from outside. Seifert et al.[28] found Fe3O4 in their preparation of La-substituted M-type Srhexaferrites, but in this case it seems possible that trivalent La3 +

may force Fe3 + to be reduced to Fe2 + when it substitutes fordivalent Sr2 +. It has been proposed [27] that the formation ofbarium ferrite from BaO and a-Fe2O3 is irreversible but from thereported XRD patterns it seems likely that barium ferrite hasdecomposed partially to BaO and Fe3O4[27].

The synthesis of barium ferrite by the sol–gel route should be adiffusion-controlled solid state reaction, much the same as theco-precipitation route. a-Fe2O3 has been found as an impurity inmany reports through the sol–gel approach. But the synthesistemperature of the sol–gel approach might be lower than that ofthe stand-alone co-precipitation approach. It should be noted thatcubic g-Fe2O3 is usually found to be present when using the sol–gelapproach. This is an important clue as to why the sol–gel approachcan be made more efficient.

Since a-Fe2O3 is stable and easy to form, significant energy isneeded to convert a-Fe2O3 to the B1 block. Fe3O4 is less stable thana-Fe2O3. If the B1 block in barium ferrite can also be formed fromFe3O4, the energy required for the B1 block conversion would belowered. The formation of the S4 block is much easier if it can beformed from cubic ferrite. In the hydrothermal approach, we usedFe2 + to synthesize barium ferrite via the cubic iron oxide Fe3O4 asan intermediate. In this way synthesis temperature is lowered andimpurities are reduced, thus raising efficiency [6].

For the co-precipitation method, the constituent cations in therequired stoichiometric ratio are precipitated simultaneously withhydroxides so that molecular level mixing of the constituenthydroxides is attained. The hydroxides in turn undergo a solidstate reaction basically in the same way as they do in the ceramicmethod to form the final ternary barium ferrite. The efficiency ofthe co-precipitation approach is therefore increased comparedwith the ceramic approach since the reacting species migrate overshorter distances in the diffusion process employed. But the solidstate reaction in the co-precipitation approach still requires a highreaction temperature and a long reaction time so that impuritiessuch as a-Fe2O3 are inevitably formed. Hence it is difficult to raisethe synthesis efficiency in the co-precipitation approach by astrategy that involves an intermediate g-Fe2O3 as in the sol–gelapproach, since the high temperature tends to convert g-Fe2O3 toa-Fe2O3. We [1,10] have therefore applied the molten salt meth-odology and low temperature pre-heating treatment to the co-precipitation precursor. In this method, the reaction temperature islowered by converting the solid state reaction to a molten saltreaction. The pre-heating technique involved heating the co-precipitation precursor and the KCl flux at 450 1C for 2 h beforethe temperature was raised to 950 1C. This proved to be animportant step in raising the efficiency of the synthesis. Thismethod encourages the self-assembly of iron, barium, and oxygenions to barium ferrite for the following reasons. The S4 or S block issimilar to that found in Fe3O4 or g-Fe2O3 and the B1 or R block issimilar to that found in a-Fe2O3. Since a-Fe2O3 is stable and easy toform, a significant amount of activation energy is needed to converta-Fe2O3 to the S4 or S block and to the B1 or R block even thoughtheir structures are similar. Thus high temperature and longreaction time are needed. The S4 or S block, and the B1 or R blockin barium ferrite should be formed more easily from cubic Fe3O4 org-Fe2O3 than from a-Fe2O3. g-Fe2O3 is favored to be formed at alower temperature and is converted to a-Fe2O3 on raising thetemperature. Thus the low temperature and the short reaction timeof the coupled molten salt and pre-heating techniques favorthe formation of Fe3O4 or g-Fe2O3. The conditions that favor theformation of Fe3O4 or g-Fe2O3 would also favor the formation of theS4 or S block in the self-assembly of barium ferrite. Thus the coupledmolten salt reaction causes the iron and oxygen ions to be morereadily assembled to the required spinel structure in barium ferrite.Pre-heating the co-precipitation precursor at 450 1C improves thelikelihood of self-assembly of the iron, barium, and oxygen ions toform barium ferrite by suppressing the formation of a-Fe2O3, andby prolonging the preparation time to form the g-Fe2O3 structure.Though the cubic intermediate Fe3O4 was detected in the hydro-thermal method using Fe2 + as one of the reactants and the spinelblock must be formed for barium ferrite, g-Fe2O3 or Fe3O4 has not

Y. Liu et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 945–953 951

yet been detected as an intermediate in the co-precipitation/molten salt method. Although it is not easy to detect cubic ironoxide intermediates, the synthesis efficiency shows that the effectdoes exist. This favored effect in the formation of the S or S4 blockmight be responsible for the fact that cubic ferric oxides have notbeen detected by XRD.

Spinel BaFe2O4 [11] has structure similar to that of the S4 or Sblock in barium ferrite and is less stable than a-Fe2O3. The stuffedtridymite BaFe2O4 relates to the R block in barium ferrite becausethe size of Ba2 + is responsible for the occurrence of both structures.These facts account for the effect played by intermediate BaFe2O4

formed in the synthesis.

3.2. Site preference and magnetic moment

3.2.1. Magnetic moment for barium ferrite

The z component of magnetic moment of an unpaired electron isgiven as Eqn. (6):

mz ¼ g9e9

2mecm_¼ gmmB ð6Þ

where g is 2 for electron spin, m is 71/2, e and me are the chargeand mass of the electron, respectively, c is the velocity of light, h isPlanck’s constant, and mB is the Bohr magneton. Since the fieldprovided by O2� for Fe3 + is weak, there are 5 uncoupled singleelectrons in Fe3 + and hence 5mB of the z component of magneticmoment for each Fe3 +. Super-exchange interaction makes the spin

of Fe3 + at 4f1 �

and 4f2 �

opposite of those at 2b�!

,2a�!

and 12k�!

as

illustrated in Fig. 8. There are two BaFe12O19 units in an M-type

barium ferrite unit cell, and all the 24 holes of 4f1 �

, 4f2 �

, 2b�!

,2a�!

,and

12k�!

in the unit cell are occupied by the 24 Fe3 + ions from the two

BaFe12O19 units. The total z component of the magnetic moment ofeach BaFe12O19 unit can be calculated from the contribution of oneFe3 + from a 2b site, one from a 2a site, and six from 12k sites, thuseight in one direction; and two Fe3 + from 4f1 sites and two from 4f2

sites, thus giving four in the opposite direction. The total z

component of the magnetic moment of BaFe12O19 can thus becalculated as (8–4)�5mB¼20mB, very close to the experimentalvalue of 19.7mB.

3.2.2. Magnetic moments for spinels

In Fe3O4 the magnetic moments are opposite for the Fe3 + ions intetrahedral and octahedral holes due to the super-exchange effect,

i.e. ½Fe3þ

��t½Fe2þ

�!Fe3þ

�!�oO4. So, the magnetic moment of Fe3O4 can be

calculated as (5+4–5)mB¼4mB, very close to the experimental valueof 4.08mB. There are four units of Fe3O4 in barium ferrite,BaM¼S4B1¼[(Fe3O4)+]3 [BaO (Fe3O4)+O2], but the molecularmagnetic moment of barium ferrite (20mB) is significantly greaterthan that of four units of Fe3O4.

Fig. 9. Anisotropy contribution from five different holes occupied by iron ions in barium f

of the site from the perturbing barium ion and this distance results in the relative values

(b) Levels in barium ferrite alongside those in pure O2� packing.

There are other substances [29] such as ½Co2þ

��t½Sn4þ Co2þ

�!�oO4,

½Fe3þ

��t½Co2þ

�!Fe3þ

�!�oO4 (experimental value for its molecular mag-

netic moment is mz¼3.7mB), ½Co2þ

��t½Co3þ

�!Co3þ

�!�oO4, ½Mn2þ

��t½Mn3þ

�!

Mn3þ

�!�oO4, ½Mn2þ

��t½Fe3þ

�!Fe3þ

�!�oO4 (4.6–5mB), and g-Fe2O3 as

½Fe3þ

��t½Fe3þ

5=3�!

V1=3�oO4 (3.15mB), where V indicates vacancy. It can

be inferred that higher valence cations such as Sn4 + or cations withelectron configuration other than d5 or d10 such as Co2 + prefer tooccupy octahedral holes or 4f2 sites. The formula [Fe3 +

8/9,V1/9]t[Fe3 +16/9,

V2/9]oO4 has been proposed for g-Fe2O3. Fe2O3 can be viewed ashaving a structure in which BaO is removed from the Ba layer inBaFe12O19 [30], resulting in Fe12O18.

Doping can also change the super-exchange [6] between sub-lattices where Fe3 + ion sites are filled. For example, when the non-magnetic Zn2 + fully occupies tetrahedral holes, the Fe3 + ions inoctahedral holes couple in an anti-parallel fashion in two sub-lattices by the super-exchange effect; thus the molecular magnetic

moment of ½Zn2þ�t½Fe3þ

�Fe3þ

�!�oO4 is 0.0mB. Thus super-exchange in

ZnFe2O4 when Fe2 + ions in Fe3O4 are fully substituted by Zn2 + isdifferent from that in Fe3O4 and is also different from those inexamples where the doping amount of Zn2 + is small. So, when asmall amount of Fe2 + in Fe3O4 is substituted by Zn2 +, Zn2 + ionsenter tetrahedral holes and force the Fe3 + ions to move tooctahedral holes to increase the ligand field stabilization energy,and the role of Fe2 + is then played instead by Fe3 + in the octahedralholes. The Fe3 + ions remaining in tetrahedral holes have spinsopposite to those in octahedral holes due to super-exchange as isalso found in the inverse spinel Fe3O4. When the amount of Zn2 +

added is small, the amount of Fe3 + in octahedral holes increases,that of Fe3 + in tetrahedral holes decreases, and Ms increases withthe doping amount of Zn2 +. When the amount of substituent islarge, then, super-exchange causes the Fe3 + ions in the two sub-lattices of octahedral holes to have opposite spins and Ms decreaseswith doping amount of Zn2 +. When barium ferrite is doped withZn2 +, a similar effect is expected [1].

3.2.3. Magnetic moments for doped barium ferrites

The above facts are consistent with the fact that Fe3 + ions in 4f2

octahedral holes have a spin opposite to those of the Fe3 + ions in 2aand 12k octahedral holes when barium ferrite is formed. Accordingto Refs. [17,25,31], Fe3 + ions in 4f2 sites have a relatively greatercontribution to K1 as shown in Fig. 9. Since Sn4 + ions preferentiallysubstitute Fe3 + ions in 4f2 holes, K1 will decrease with increase in x.

It can be inferred from cation–anion interaction and ligand fieldstabilization energies [32] that the d5 cation Mn2 + or the non-magnetic d10 cation Zn2 + will occupy the 4f1 site in barium ferrite,leaving the octahedral holes to be occupied by higher valencecations such as Sn4 + or cations with electron configuration otherthan d5 or d10 such as Co2 +, resulting in an increase in ligand fieldstabilization energy. It has been shown that Sn4 + and Co2 + have the

errite. The heights above the horizontal dotted line can be estimated by the distance

of anisotropy according to Refs. [17,25,31] (see text). (a) Levels in pure O2� packing.

Y. Liu et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 945–953952

tendency to substitute Fe3 + at 4f2 sites [15,22] since tin possesses ahigher valence and cobalt an electron configuration other than d5 ord10[32].

The magnetic properties shown in Fig. 5 can now be rationa-lized. Mn2 + is magnetically equivalent to Fe3 + wherein both havethe 3d5 electron configuration. Although Mn2 + will substitute Fe3 +

at the 4f1 sites, this has little effect on the value of Ms. The magneticmoment of Co2 + is 3.7mB. Co2 + prefers 4f1, 2b, and 4f2 sites. Thenon-magnetic Sn4 + prefers 4f2 sites. Sn4 + replaces Fe3 + at 4f2 sites,thus increasing Ms since Fe3 + at 4f1, 4f2 possess a spin opposite tothat of Fe3 + at other holes.

Mn2+ is paramagnetic. Its intrinsic nature, which is responsible forferromagnetism of cobalt, may be responsible for the fact that Ms islarger in the Co/Sn series than in the equivalent Mn/Sn seriescontaining paramagnetic Mn2+ when xo0.3. The Ms decreaseoccurring early in Co/Sn series may be due to the fact that themagnetic moment of Co2+ is less than that of Mn2+. In the inverse

spinel Fe3O4, or½Fe3þ

��t½Fe2þ

�!Fe3þ

�!�oO4, one half of the paramagnetic

Fe3+ ions in tetrahedral holes is responsible for the super-exchangethat causes the Fe3+ ions in tetrahedral and octahedral to possessopposite spins. When the paramagnetic Fe3+ ions in tetrahedral holesare largely or fully replaced by non-magnetic Zn2+ ions as in ZnFe2O4

or ½Zn2þ�t½Fe3þ

�Fe3þ

�!�oO4, the decrease or even disappearance of

magnetic moment in tetrahedral holes changes the super-exchangeand causes the spins of Fe3+ in the two octahedral sub-lattices to beanti-parallel (Section 3.2.2). When Sn4+ preferentially occupiesoctahedral holes, Co2+ and Mn2+ tend to occupy tetrahedral 4f1

holes in barium ferrite. It is reasonable then to assume that the smallermagnetic moment of Co2+ is responsible for the super-exchange thatgives rise to an early decrease in Ms. Since Mn2+ has the same largemagnetic moment as that of Fe3+, doping with Mn2+ does not harmthe super-exchange action of oxygen and does not affect the trend ofincrease in Ms with doping amount of Sn4+ ions in 4f2 sites at low x

and will delay the decrease in Ms to relatively higher values of x.When x40.8, the increased amount of non-magnetic ions Sn4 +

weakens the super-exchange between the sub-lattices of Fe3 +, andtherefore Ms decreases as x increases.

3.3. Anisotropy and doping

The effect of doping on grain size described in Section 2.2.1 canbe explained as follows: the radii of Mn2 +, Sn4 +, and Co2 + are onlyslightly larger than that of Fe3 +, so their substitution in smallamounts will not affect the crystal lattice and morphology of thesamples significantly. But the lattice will be distorted and crystal-linity reduced when the amount is increased.

When these larger ions enter the trigonal bipyramid 2b,tetrahedral 4f1, and octahedral 2a, 12k, 4f2 holes, the regularpolyhedral holes became elongated along the c-direction. In amanner similar to the Jahn–Teller effect, the elongated distortionperturbed by doping is affected by the interaction of degenerateneighbouring holes. This is in agreement with the fact that dopingchanges the c-axis but not the a-axis length [33]. The increase inlattice parameter is shown in Table 1 for Co–Mn–Sn doping but wasalso noted for doping with Mn–Co–Zr [20], Mn–Cu–Zr [19], andMn–Co, Co–Ti. [34]. It is interesting that the elongation of the c-axisresults in agglomeration along the c-axis at lower x and an apparentthinning in the c-direction at larger x. This fact can be rationalizedby considering the fact that growth along the c-direction isenergetically disfavored in comparison with growth in the perfectplane and that growth along the c-direction can be more easilyinterrupted as x increases.

The increase in lattice parameter c with x also alters the distancebetween magnetic ions, resulting in variation in exchange

interaction and thus magnetic properties. The c-axis direction isthe most sensitive to magnetization and the anisotropy of M-typebarium ferrite, which is responsible for Hc, should be dependent onthe Fe3 + ions situated in the five type of holes: 2b, 2a, 12k, 4f1, and4f2. In a spherical ligand field there is no anisotropy. For pureoxygen packing such as in a-Fe2O3 and Fe3O4, the order ofanisotropy of trigonal bipyramid hole 2b, tetrahedral hole 4f1,and the octahedral holes 2a, 4f2, and 12k should be

2b44f144f2 � 2a� 12k

(as shown in Fig. 9a) according to deviations from sphericalsymmetry. In spinel ferrite [Fe3 +]t[Fe2 +Fe3 +]oO4 there are notrigonal bipyramidal 2b holes. But when barium ions are intro-duced, as in barium ferrite, some Fe3 + ions are shifted to these 2bsites of the hexagonal blocks. So, the magnetic properties of thespinel ferrites are very different from those of barium ferrite. In thebarium ferrite unit cell, one of the oxygen atoms in one B or in one Clayer is replaced by barium, resulting in either a B0 or a C0 layer(Fig. 8). This barium replacement has a significant influence on thelattice. The holes that are nearest to the barium atom are 2b, 12k,and 4f2 followed by 2a. The Fe3 + in 4f1 holes can interact only withBa2 + indirectly through O2� . If the variational principle, which hasbeen successfully applied to energy perturbation, can also beapplied to the anisotropy induced by Ba2 +, then the order ofanisotropy in barium ferrite for the Fe3 + occupied five-type holesshould be (Fig. 9b)

2b44f242a44f1412k

where f1 is almost unchanged and 2a is not changed much, but theother sites are split. This order is consistent with the resultsobtained by Xu et al. [31]. The calculated data of Xu can also befound in Refs. [17,25].

4. Conclusions

The novel method of synthesis developed previously was usedfor Mn/Sn, Co/Sn, and Mn–Co–Sn substituted barium ferrites withthe general formulae of BaFe12�2xMnxSnxO19, BaFe12�2xCoxSnxO19,and BaFe12�2xCox/2Mnx/2SnxO19, respectively. The efficiency of thenew method was confirmed i.e. the synthesis temperature waslowered and the reaction time was shortened; crystallinityimproved and fewer impurities were observed. Relatively higherMs values were also obtained by the improved synthesis method.These results combined with the previous work [1,10] show thatthe new method adopted was really effective.

Great advance in understanding is usually achieved fromfundamental thinking [35]. Extended discussions were thereforegiven. The reasons for the synthesis efficiency were explored. Toform the product, iron, barium, and oxygen ions must be assembledto develop the spinel structure in barium ferrite. The coupledmolten salt method lowered the synthesis temperature, increasingthe probability of assembling of the spinel structure; the pre-heating period was used for the preparation time of ion rearrange-ments needed for the S block assembling. Both of the above showntwo techniques facilitate the self-assembly of Fe2 +, Ba2� , and O2�

to form barium ferrite. The effects of doping on magnetic propertieswere also discussed. Products with adjustable low coercivitytemperature coefficient induced by doping were investigated.

Acknowledgements

To Education Ministry of Liaoning Province (L2010518), LiaoningProvincial Federation of Social Sciences (2010lslktjyx-52), andShenyang Normal University.

Y. Liu et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 945–953 953

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