Influence of heat treatment temperatures on structural and magneticproperties of Sr0.50Ca0.20La0.30Fe11.15Co0.25O19 hexagonal ferrites

Yujie Yang, Xiansong Liu n, Dali JinEngineering Technology Research Center of Magnetic Materials, School of Physics & Materials Science, Anhui University,Anhui Province, Hefei 230601, PR China

a r t i c l e i n f o

Article history:Received 15 January 2014Received in revised form17 March 2014Available online 15 April 2014

Keywords:M-type ferriteCalcination temperatureSintering temperatureMagnetic propertyRadical shrinkageX-ray diffraction

a b s t r a c t

M-type ferrite Sr0.50Ca0.20La0.30Fe11.15Co0.25O19 magnetic powders and magnets were prepared by aceramic process. The phase identification of magnetic powders was performed by X-ray diffraction.At calcination temperatures ranging from 1170 to 1270 1C, the phase compositions of the magneticpowders consist of M-type hexaferrites together with small amount of impurity phases such as α-Fe2O3,LaFeO3 and CoFe2O4. At calcination temperatures above 1270 1C, single-phase M-type hexaferrites can beobtained. The microstructures of the magnets were investigated by field emission scanning electronmicroscopy. The particles appear in hexagonal plate-like shape and the particles are distributedhomogeneously. The radial shrinkage of the magnets increases with the increase of calcination orsintering temperature. The magnetic properties of the magnets and magnetic powders were measuredby a permanent magnetic measure equipment and a vibrating sample magnetometer, respectively. Forhigh remanence, intrinsic coercivity, magnetic induction coercivity and maximum energy product, theoptimized calcination and sintering temperatures are 1250 1C and 1190 1C, respectively.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Hexagonal ferrites, such as Sr(Ba)Fe12O19, have been widely usedas permanent magnets due to cheap raw materials, outstandingchemical stability and high uniaxial magnetocrystalline anisotropy[1]. In addition, they show promising properties for their applicationsin microwave devices, magneto-optics andmagnetic recording media[2,3]. Ba- and Sr-hexaferrites have been synthesized by variousmethods, including the ceramic process [4,5], chemical coprecipita-tion method [6–11], hydrothermal method [12], sol–gel method [13],organic acid precursor method [14] and glass crystallization method[15].

In order to fulfill various applications, many attempts have beenmade to improve magnetic properties of M-type hexaferritesconcerning cationic substitution. La3þ substitution for Sr2þ andsubstitution of transition metals such as Zn2þ , Co2þ , Mn2þ , Ni2þ ,Ti2þ and Ti4þ on Fe3þ have been investigated [16–19]. On the otherhand, hexagonal ferrites with combined substitution such as La–Co,La–Zn, Co–Ti, etc. were synthesized by sol–gel and ceramic methods[20–22]. Partial substitution of Sr2þ–Fe3þ by La3þ–Co2þ in M-typehexaferrites is beneficial in achieving high magnetic properties [20].

At the same time, reducing material cost without compromis-ing the magnetic properties is still an important aspect. Using

cheap Ca2þ to replace Ba2þ or Sr2þ is a new way to reduce costand expand material sources [23,24]. A Japanese patent hasreported oxide magnetic materials whose main phase was theM-type hexaferrite phase by applying Ca2þ and La3þ to replacemost of Sr2þ and Co2þ to replace one part of Fe3þ [25]. It is wellknown that the heat treatment temperatures can affect thestructural and magnetic properties of M-type hexaferrites [26–29].

In the present study, M-type hexaferrite Sr0.50Ca0.20La0.30Fe11.15Co0.25O19 magnetic powders and magnets were synthesizedby the ceramic process. The effects of calcination temperature andsintering temperature on the structural and magnetic propertieshave been studied systematically.

2. Experimental procedure

All samples of Sr0.50Ca0.20La0.30Fe11.15Co0.25O19 hexaferriteswere prepared by the ceramic process. The starting materials usedin this study were SrCO3 (97% purity), CaCO3 (99% purity), La2O3

(99% purity), Fe2O3 (98% purity) and Co2O3 (98% purity), which aremixed together in the chemical composition of Sr0.50Ca0.20La0.30Fe11.15Co0.25O19. Mixtures of starting materials were milled inwater for 6 h with an angular velocity of 80 rpm and a ball-to-power weight ratio of 12:1. Milling processes were performed in aball mill using hardened steel balls (diameter 8 mm). The milledpowders were dried, crushed, and sifted. The sifted powders were

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Journal of Magnetism and Magnetic Materials

http://dx.doi.org/10.1016/j.jmmm.2014.04.0120304-8853/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel./fax: þ86 551 65107674.E-mail address: xiansongliu@ahu.edu.cn (X. Liu).

Journal of Magnetism and Magnetic Materials 364 (2014) 11–17

made into balls (diameter about 8 mm), and the balls were thencalcined in a muffle furnace at various temperatures from 1170 to1310 1C for 2 h in air. The calcined balls were pulverized by avibration mill, and then wet-milled with additives (CaCO3, SrCO3,SiO2, Cr2O3, Al2O3 and H3BO3) for 13 h using the ball mill. Thefinely milled slurry with a diameter of about 0.75 μm was pressedinto disk-shaped compacts (diameter 30 mm, thickness 15 mm )by wet compacting under 310 MPa in a pulsed magnetic field of900 kA/m, which was parallel to the pressing direction. The greencompacts were sintered in the muffle furnace at various tempera-tures from 1180 to 1200 1C for 1.5 h in air. The radial shrinkage ofthe sintered magnets is obtained from the following equation:

Ψ ¼D0�DD0

�% ð1Þ

where Ψ denotes the radial shrinkage, D0 is the diameter of thegreen compact, and D is the diameter of the sintered compact.

X-ray diffraction patterns of the magnetic powders wereobtained by a PANalytical X'Pert Pro diffractometer in continuousmode with Cu Kα radiation source (λ¼1.5406 Å). The morpholo-gies of the green compacts, sintered compacts and polishedsintered compacts were observed by a Cannon IXUS130 digitalcamera. The microstructures of the magnets were investigated bya HITACHI S-4800 field emission scanning electron microscope(FESEM). The magnetic properties of the magnets were measuredat room temperature by a permanent magnetic measure equip-ment (MATS-2000, manufactured by the National Institute ofMetrology of China) in a maximum applied field of 1.2 T. The

magnetic properties of the magnetic powders were measured atroom temperature by a Riken Denshi (BH-55) vibrating samplemagnetometer (VSM) with a maximum applied field of 1.0 T.

3. Results and discussion

3.1. Crystal structure and morphology

Fig. 1 shows the X-ray diffraction patterns of the magneticpowders calcined at various temperatures for 2 h. In comparisonwith the standard strontium ferrite JCPDS Card no. 80-1198, it canbe seen that at calcination temperatures ranging from 1170 to1270 1C, the phase compositions of the magnetic powders consist ofM-type hexaferrites together with small amount of impurity phasessuch as α-Fe2O3, LaFeO3 and CoFe2O4, whereas at calcinationtemperatures above 1270 1C, single-phase M-type hexaferrites canbe obtained. It is apparent that the relative intensities of the peakscorresponding to M-type hexaferrite increase with the calcinationtemperature from 1170 to 1310 1C, while relative intensities of thepeaks corresponding to α-Fe2O3, LaFeO3 and CoFe2O4 decease withthe calcination temperature from 1170 to 1270 1C. This implicatesthat the La–Co substituted M-type Sr–Ca hexaferrites do not changethe magnetoplumbite-type structure of SrFe12O19, and La3þ , Co2þ

and Ca2þ are substituted into the crystal lattice [20,24,30].The lattice constants a and c are calculated from the values of

dhkl corresponding to (107) peaks and (114) peaks according to thefollowing formula:

dhkl ¼43

�Uh2þhkþk2

a2þ l2

c2

!�1=2

ð2Þ

where dhkl is the inter-planer spacing, and h, k and l are theMiller indices.

The variations of lattice constants a and c of the magneticpowders calcined at various temperatures for 2 h are shown in

Fig. 1. X-ray diffraction patterns of the magnetic powders calcined at varioustemperatures for 2 h.

Fig. 2. Lattice constants a and c of the magnetic powders calcined at varioustemperatures for 2 h.

Table 1ρx of calcined magnetic powders, ρm and P of the magnets calcined at varioustemperatures for 2 h.

Calcination temperature (1C) ρx (g/cm3) ρm (g/cm3) P (%)

1170 5.162 5.051 2.151190 5.169 5.063 2.051210 5.166 5.068 1.901230 5.164 5.077 1.681250 5.162 5.082 1.551270 5.186 5.086 1.931290 5.188 5.091 1.871310 5.191 5.099 1.77

Y. Yang et al. / Journal of Magnetism and Magnetic Materials 364 (2014) 11–1712

Fig. 2. It can be seen that the lattice constant c decreases from22.95 Å (at 1170 1C) to 22.92 Å (at 1190 1C). At calcination tem-peratures ranging from 1190 to 1250 1C, the lattice constant c

increases to 22.95 Å. With a further increase in calcinationtemperature from 1250 to 1310 1C, the lattice constant c decreasesto 22.90 Å. However, compared with the lattice constant c, thevariation of the lattice constant a is not substantial in thecalcination temperature range. In La–Co substituted M-type Sr–Ca ferrites, La3þ and Ca2þ ions are expected to enter Sr2þ sites,and Co2þ ions are located in Fe3þ sites [20,23]. X-ray diffractiondata are used to estimate the X-ray density (ρx) of magneticpowders. The bulk density (ρm) and porosity (P) of the magnetsare also calculated. The following equations are used to obtain thevalues of the parameters listed in Tables 1 and 2:

V ¼ 0:8666a2c ð3Þ

ρx ¼2MNV

ð4Þ

ρm ¼ 2mπr2h

ð5Þ

P ¼ 1�ρmρx

ð6Þ

where m is the mass; M is the molar mass of the magneticpowder; N is Avogadro's number; r is the radius of the magnet;and h is the height of the magnet. The values of ρx increase withthe calcination temperature from 1170 to 1190 1C, and decreasewith the calcination temperature from 1190 to 1250 1C. With afurther increase in the calcination temperature from 1250 to1310 1C, the values of ρx increase to 5.191 g/cm3. Tables 1 and 2show that the values of ρm are smaller than those of ρx. This can bedue to the existence of pores in the magnets. From Table 1, it isclear that the values of ρm increase with the calcination tempera-ture from 1170 to 1310 1C, while the values of P fluctuate in thecalcination temperature range. From Table 2, it can be seen thatwhen the sintering temperature increases from 1180 to 1200 1C,

Table 2ρx of the magnetic powder calcined at 1250 1C for 2 h, and ρm and P of the magnetssintered at various temperatures for 1.5 h.

Sintering temperature (1C) ρx (g/cm3) ρm (g/cm3) P (%)

1180 5.162 5.067 1.841185 5.162 5.075 1.691190 5.162 5.082 1.551195 5.162 5.087 1.451200 5.162 5.094 1.32

Fig. 3. Morphologies of (a) green compacts (diameter 30 mm, thickness 15 mm),(b) sintered compacts (diameter about 27 mm, thickness 12 mm) and (c) polishedsintered compacts (diameter about 27 mm, thickness 10 mm).

Fig. 4. FESEM micrographs of the magnets made from the magnetic powders calcined at (a) 1170 1C, (b) 1210 1C, (c) 1250 1C, and (d) 1310 1C for 2 h.

Y. Yang et al. / Journal of Magnetism and Magnetic Materials 364 (2014) 11–17 13

the values of ρm increase from 5.051 to 5.099 g/cm3, while thevalues of P decrease from 2.15% to 1.77%.

Fig. 3(a)–(c) shows the morphologies of green compacts (dia-meter 30 mm, thickness 15 mm), the sintered compacts (diameterabout 27 mm, thickness 12 mm), and the polished sintered com-pacts (diameter about 27 mm, thickness 10 mm), respectively.Fig. 4 shows the FESEM micrographs of the magnets made fromthe magnetic powders calcined at 1170, 1210, 1250 and 1310 1C for2 h. It is clearly seen that the particles appear in hexagonal plate-like shape and the particles are distributed homogeneously. Themean particle sizes of the magnets slightly increase with theincrease of calcination temperature.

Fig. 5(a) shows the influence of calcination temperature on theradial shrinkage of the magnets. As seen from Fig. 5(a), the radialshrinkage continuously increases from 8.5% (at 1170 1C) to 10.37%(at 1310 1C). This is not in agreement with that reported bySharma et al. [26]. The difference can be attributed to tworeasons. One is that the strontium ferrites in the study of Sharmaet al. are different from La–Co substituted M-type Sr–Ca ferritesin this study. The other reason is that during the experimentalprocedure in the study of Sharma et al. the aqueous solution of 5%polyvinyl alcohol (PVA) was added into the magnetic powderbefore mixing. Fig. 5(b) shows the influence of sintering tem-perature on the radial shrinkage of the magnets. The radialshrinkage increases with the sintering temperature from 1180to 1200 1C. This is in agreement with that reported by Sharmaet al. [26].

3.2. Magnetic properties

Fig. 6 shows the influence of calcination temperature on theremanence (Br), intrinsic coercivity (Hcj), magnetic inductioncoercivity (Hcb) and maximum energy product [(BH)max] of themagnets. From Fig. 6(a), it is clear that the variation in theremanence (Br) of the magnets is not substantial in the calcina-tion temperature range. Br increases from 66.38 emu/g (at1170 1C) to 67.27 emu/g (at 1250 1C). With a further increase incalcination temperature from 1250 to 1310 1C, Br decreases to66.69 emu/g. In M-type hexaferrite crystals, Fe3þ ions are dis-tributed in five interstitial crystallographic sites, namely threeoctahedral (2a, 2b and 12k), one octahedral (4f1), and onetrigonal bipyramid (2b). Three parallel (2a, 2b and 12 k) andtwo antiparallel (4f1 and 4f2) sub-lattices, which are coupled bysuperexchange interactions through O2� ions, form the ferro-magnetic structure. Mos̈sbauer investigations of La–Co substi-tuted M-type strontium ferrites have demonstrated that most ofCo2þ ions are substituted for Fe3þ ions in the octahedral 2a and4f2 sites, and a valence change of some Fe3þ ions to Fe2þ ionsoccurs at 2a sites [20]. Li et al. have reported that Ca2þ ions arelocated in Sr2þ ions in SrLaCo ferrites [23]. Therefore, if theamount of Fe3þ ions which changes to Fe2þ ions is x, thechemical composition will be changed to Sr0.50Ca0.20La0.30(Fe3þ)11.15�x(Fe2þ)xCo0.25O19. There are two possible reasons.On the one hand, when the calcination temperature is increasedfrom 1170 to 1250 1C, the amount of α-Fe2O3 decreases, which isconfirmed by X-ray diffraction analysis shown in Fig. 1. This willincrease the amount of Fe3þ ions, and the Fe3þ–O–Fe3þ super-exchange interaction at 2b and 12k sites is enhanced, which willresult in an increase of Br. At calcination temperatures above1250 1C, the decrease of the lattice constant c can be one of thereasons for lower Br as magnetization is bound to the hexagonalaxis [31]. On the other hand, magnetization reversals in thesintered magnets take place not only by coherent rotation, butalso by reverse-domain nucleation rotation [32].

From Fig. 6(b), it can be seen that Hcj and Hcb of the magnets firstincrease with the calcination temperature from 1170 to 1250 1C, andthen begins to decrease at calcination temperatures above 1250 1C.When the calcination temperature is increased to1250 1C, Hcj andHcb of the magnets reach the maximum values of 5349 and 3904 Oe,respectively. The variation of the coercivity is in agreement with thatreported by Nga et al. [33], Mendoza-Suárez et al. [34] and Ketovet al. [35]. For magnets, further grain growth does not take placeduring sintering as the coarsening of grain has already occurred athigher calcination temperature [26]. The critical size of a single-domain particle is estimated by the equation given as [36]

Dm ¼ 9sw

2πM2s

ð7Þ

where sw is the wall density energy, Ms is the saturation magnetiza-tion. The coercivity of the M-type hexaferrite increases when thegrain size in a crystalline body or the particles size of a powderapproaches single-domain size [32]. For D4Dm the particles aremulti-domain structures, while for DoDm the particles are mono-domain structures. For M-type strontium ferrites, the value of Dm isabout 0.65 μm. In this study, the mean particle size of the magnets ishigher than 3.0 μm. So the multi-domain structures occur in thegrains of the magnets. Therefore, the variation of the coercivity canbe attributed to the grain growth, on the one hand, and crystallinityimprovement, on the other hand, with the increase of calcinationtemperature.

From Fig. 6(a), it is also clear that (BH)max of the magnetsincreases from 4.17 MG Oe (at 1170 1C) to 4.30 MG Oe (at 1250 1C),and then begins to decrease when the calcination temperature isabove 1250 1C. Since the maximum energy product is the

Fig. 5. Influence of (a) calcination temperature and (b) sintering temperature onthe radial shrinkage of the magnets.

Y. Yang et al. / Journal of Magnetism and Magnetic Materials 364 (2014) 11–1714

maximum area in the second quadrant of the hysteresis loop, thevalues of the remanence (Br) and magnetic induction coercivity(Hcb) have their influence on it. Therefore, the variation of (BH)max

has the same feature as that of Br as shown in Fig. 6(a).Fig. 7 shows the influence of sintering temperature on the

remanence (Br), intrinsic coercivity (Hcj), magnetic inductioncoercivity (Hcb) and maximum energy product [(BH)max] of themagnets. From Fig. 7(a), it is observed that Br of the magnetslinearly increases with the sintering temperature from 1180 to1200 1C. This is in agreement with that reported by Onreabroyet al. [29]. The enhancement of Br can be attributed to the increasein the density of the magnets as shown in Table 2.

From Fig. 7(b), it can be seen that when the sintering tempera-ture is increased from 1180 to 1190 1C, Hcj and Hcb of the magnetsincrease from 5181 and 3759 Oe to 5349 and 3904 Oe, respec-tively. With a further increase in sintering temperature from 1190to 1200 1C, Hcj and Hcb of the magnets linearly decrease to 4957and 3695 Oe, respectively. At sintering temperatures below1190 1C, the solid phase reaction is not complete, and the graincoarsening does not occur [37]. When the sintering temperature isincreased to 1190 1C, the solid phase reaction is complete, and Hcj

and Hcb reach to the maximum values. At sintering temperaturesabove 1190 1C, the coarser grains appear, and this results in thedecrease of Hcj and Hcb [26].

From Fig. 7(a), it is also observed that (BH)max of the magnetscontinuously increases with the increase of sintering temperaturefrom 1180 to 1200 1C. Since the maximum energy product is themaximum area in the second quadrant of the hysteresis loop, thevalues of the remanence (Br) and magnetic induction coercivity(Hcb) will exert their influence on it. Therefore, the variation of

(BH)max of the sintered magnets has the same feature as that of Bras shown in Fig. 7(a).

Fig. 8 shows the hysteresis loops of the magnetic powderscalcined at 1170, 1210, 1250 and 1310 1C for 2 h. The saturationmagnetization (Ms), remanent magnetization (Mr) and coercivity(Hc) of the magnetic powders are determined from the obtainedhysteresis loops and their values are listed in Table 3. As can beseen, Ms, Mr and Hc of the magnetic powders increase with thecalcination temperature from 1170 to 1250 1C, and then begin todecrease when the calcination temperature is above 1250 1C. Ms

and Mr of the magnetic powders are higher than the resultsreported by Hession et al. [9] and Rashad et al. [11], while Hc of

Fig. 6. Influence of calcination temperature on (a) Br and (BH)max, and (b) Hcj andHcb of the magnets.

Fig. 7. Influence of sintering temperature on (a) Br and (BH)max, and (b) Hcj and Hcb

of the magnets.

Fig. 8. Hysteresis loops of the magnetic powders calcined at (a) 1170 1C,(b) 1210 1C, (c) 1250 1C, and (d) 1310 1C for 2 h.

Y. Yang et al. / Journal of Magnetism and Magnetic Materials 364 (2014) 11–17 15

the magnetic powders is lower than that reported by Hession et al.[9] and Rashad et al. [11]. From Figs. 6–8, it can be concluded thatthe optimum calcination temperature is 1250 1C and the optimumsintering temperature is 1190 1C. Fig. 9 shows the typical demag-netizing curve of the magnet which is obtained from the magneticpowder calcined at 1250 1C for 2 h and sintered at 1190 1C for1.5 h. As shown in Fig. 9, the magnet indicates its magneticproperties, including the remanence (Br¼67.27 emu/g), intrinsiccoercivity (Hcj¼5349 Oe), magnetic induction coercivity (Hcb¼3904 Oe) and maximum energy product [(BH)max¼4.30 MG Oe].Br of the magnet is higher than that reported by Liu et al. [38], andis lower than that reported by Tenaud et al. [20]. While Hcj of themagnet is obviously higher than that reported by Liu et al. [38] andTenaud et al. [20].

4. Conclusion

M-type ferrite Sr0.50Ca0.20La0.30Fe11.15Co0.25O19 magnetic pow-ders and magnets were synthesized via the ceramic process. Thephase identification of the magnetic powders was performed byX-ray diffraction. At calcination temperatures ranging from 1170 to1270 1C, the phase compositions of the magnetic powders consistof M-type hexaferrites together with a small amount of impurityphases such as α-Fe2O3, LaFeO3 and CoFe2O4. At calcinationtemperatures above 1270 1C, single-phase M-type hexaferritescan be obtained. The microstructures of the magnets wereinvestigated by FESEM. The particles appear in hexagonal plate-like shape and the particles are distributed homogeneously. Theradial shrinkage of the magnets increases with the increase ofcalcination or sintering temperature.

The magnetic properties of the magnets and magnetic powderswere measured by a permanent magnetic measure equipment anda vibrating sample magnetometer, respectively. The optimizedheat treatment temperatures for high Br, Hcj, Hcb and (BH)max are

as follows: calcination temperature¼1250 1C and sinteringtemperature¼1190 1C. The magnet made from the magneticpowder calcined at 1250 1C for 2 h and sintered at 1190 1C for1.5 h has the highest magnetic properties, including the rema-nence (Br¼67.27 emu/g), intrinsic coercivity (Hcj¼5349 Oe), mag-netic induction coercivity (Hcb¼3904 Oe) and maximum energyproduct [(BH)max¼4.30 MG Oe].

Acknowledgments

The authors acknowledge the financial support from the NationalNatural Science Foundation of China under Grant nos.51072002 and51272003, the Key Program of the Education Department of AnhuiProvince (Grant nos. KJ2010A008, KJ2013B057 and KJ2012A027),Anhui University “211Project” Academic Innovation Team projectsunder Grant no. 02303402, and from the Research Fund for theDoctoral Program of Higher Education of China under project20123401110008.

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Table 3Magnetic properties of the magnetic powders calcined at 1170, 1210, 1250 and1310 1C for 2 h.

Calcination temperature (1C) Ms (emu/g) Mr (emu/g) Hc (Oe)

1170 71.1 49.1 18541210 71.7 49.3 19551250 73.2 51.3 22731310 72.0 50.9 2148

Fig. 9. A typical demagnetizing curve of the magnet which is made from themagnetic powder calcined at 1250 1C for 2 h and sintered at 1190 1C for 1.5 h.

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