Journal of Magnetism and Magnetic Materials 446 (2018) 118–124

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Journal of Magnetism and Magnetic Materials

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Research articles

Lowering the synthesis temperature of Y3Fe5O12 by surfactant assistedsolid state reaction

http://dx.doi.org/10.1016/j.jmmm.2017.08.0760304-8853/� 2017 Elsevier B.V. All rights reserved.

⇑ Corresponding authors at: School of Physics, Southeast University, Nanjing211189, China (Q. Xu); Department of Physics, Nanjing University, Nanjing 210093,China (J. Du).

E-mail addresses: xuqingyu@seu.edu.cn (Q. Xu), jdu@nju.edu.cn (J. Du).

Fenghua Xue a, Ju Huang a, Tianrui Li a, Zifan Wang a, Xiaochao Zhou a, Lujun Wei b, Baizhi Gao a, Ya Zhai a,Qi Li a, Qingyu Xu a,c,⇑, Jun Du b,c,⇑a School of Physics, Southeast University, Nanjing 211189, ChinabDepartment of Physics, Nanjing University, Nanjing 210093, ChinacNational Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China

a r t i c l e i n f o

Article history:Received 6 June 2017Received in revised form 21 July 2017Accepted 26 August 2017Available online 1 September 2017

Keywords:YIGSurfactantSolid state reactionFerromagnetic resonance

a b s t r a c t

There is an urgent technical requirement of lowering the sintering temperature of Y3Fe5O12 (YIG) for itspractical applications. In this paper, a modified solid state reaction method is reported by adding the sur-factant of cetyltrimethylammonium bromide (CTAB). A high sintering temperature of 1200 �C is requiredfor the formation of YIG phase without adding CTAB, which is effectively decreased to 1050 �C by addingCTAB. The morphology studies show that the sintering temperature plays the main role in the crystalgrowth and excludes the possible contribution of CTAB. The prepared YIG ceramic samples show soft fer-romagnetic properties, with coercivity of only 21.2 Oe for the sample prepared with CTAB at 1050 �C,which decreases with increasing sintering temperature. The main role of adding CTAB is preventingthe agglomeration of ball milled ultrafine source particles, which may facilitate the interdiffusion amongthem and promote the reaction at lower temperatures. Furthermore, the Gilbert damping constant is sig-nificantly reduced for YIG prepared by adding CTAB, which is one order smaller than that without CTAB.

� 2017 Elsevier B.V. All rights reserved.

1. Introduction

Y3Fe5O12 (YIG) is a versatile garnet ferrite and has been widelystudied for its unique electromagnetic and magneto-optical prop-erties, with high melting point, large resistivity, high electromag-netic properties, high thermal stability, low thermal expansion,high chemical stability, and high thermal conductivity [1]. YIG isa ferrimagnetic material with a cubic structure (space groupIa3d), in which one cell embodies eight molecules. There are threecrystallographic lattice sites marked as a, c, and d, which are sur-rounded by O octahedron, dodecahedron and tetrahedron, respec-tively. Y3+ ions occupy the c sites and the magnetic Fe3+ ionsoccupy both a sites and d sites, whereas oxygen ions are dis-tributed at the interstitial sites [1]. The ferrimagnetic behavior ofYIG mainly comes from the opposite Fe3+ ions’ spin orientation atthe a and d sites [2]. YIG has large Faraday rotation, high initial per-meability, high saturation magnetization and small coercivity [3].YIG is also a well-known microwave ferrite, has been widely

applied in microwave, magnetic, and magneto-optical devicesdue to its low dielectric loss in microwave region, and small line-width in ferromagnetic resonance [4].

Industrially, YIG ceramics are fabricated using conventionalceramic production techniques where Y2O3 and Fe2O3 are mixed,sintered at high temperature. However, in order to obtain YIGceramics with high density, high sintering temperatures over1450 �C and high soaking times over 10 h are required [5]. Mucheffort has been devoted to lower the sintering temperature [5–8].Chemical methods, such as co-precipitation, sol-gel, citrate-nitrate gel combustion, etc. have been employed to produce YIGceramic powders at low sintering temperature, which is not suit-able for the large scale industry production [3,5]. Ion substitutionis also an efficient technique to lower the sintering temperature,such Bi, Ce, Gd, etc. [6,9–12]. However, ion substitution may inevi-tably introduce unwanted negative effects on the structural andmagnetic properties of YIG, such as the decrease of saturate mag-netization, increase of coercivity, etc. [6,10–12].

In this paper, we introduce a surfactant, Cetyltrimethyl Ammo-nium Bromide (CTAB), into the conventional solid state reactionprocess for the fabrication of YIG ceramics. The sintering tempera-ture has been effectively lowered from 1200 �C to 1050 �C. Further-more, the Gilbert damping constant is significantly reduced for YIG

F. Xue et al. / Journal of Magnetism and Magnetic Materials 446 (2018) 118–124 119

prepared by adding CTAB, which is one order smaller than thatwithout CTAB.

Fig. 1. XRD patterns of YIG ceramic samples sintered at various temperatures withor without CTAB. The insets are the photos to show the color.

2. Experimental details

YIG ceramics were synthesized by solid state reaction. Y2O3 andFe2O3 (purity of AR) were weighted in a mole ratio of 5:3 andmixed together, which was then put into a ball milling tank and1 wt% of CTAB was added into the pot as the surfactant. BecauseCTAB is soluble in organic solvent, we chose ethyl alcohol asball-milling dispersing medium. The mixture was ball milled for12 h in the rotation speed of 450 round per minute. After that,the mixture was dried at the temperature of 50 �C for 2 h. The driedmixture was pre-sintered in a furnace at 900 �C for 3 h, since CTABcan burn in air and be totally removed at this temperature [13].After that, ball milling with the same parameters was applied onthe mixture again with same amount of CTAB and ethyl alcoholadded. After drying, the mixture was put into the oven, and heatedto the set sintering temperature for 3 h, and then cooled down toroom temperature naturally. For the ferromagnetic resonance(FMR) measurement, the obtained ceramic powders were pressedinto discs, which were sintered at the same temperature as thepowders for 3 h.

The structures of samples were examined using X-ray diffrac-tion (XRD, Rigaku Smartlab3). The morphologies were studied bya scanning electron microscope (SEM, FEI Inspect F50). The fielddependence of magnetization was measured using a vibratingsample magnetometer (VSM, Microsense EV7) at room tempera-ture. FMR experiments were carried out by using a Brucker Elec-tron Spin Resonance (ESR) equipment (ER-200D-SRC) with aTE102 rectangular resonant cavity at various microwave frequen-cies (4, 6, 8, 10, 12 GHz). FMR is performed by putting the discson a coplanar waveguide whose magnetic radio frequency (RF)field is used for excitation. The setup was placed in a homogenousexternal magnetic field. In the following, we name the sample withCTAB to indicate the addition of CTAB during the ball milling, andthe number denotes the sintering temperature.

3. Results and discussion

Fig. 1 shows the XRD patterns of YIG ceramics powders sinteredat different temperature with or without adding CTAB during theball milling. It can be seen that the sintering temperature has sig-nificant influence on the formation of YIG phase. Without the addi-tion of CTAB, YIG can only be formed when the sinteringtemperature is above 1200 �C. When the sintering temperature is1100 �C, the most diffraction peaks in the XRD pattern cannot beindexed by the standard data of YIG (PDG#43-0507). The insetshows the photo of the corresponding sample. The dark red coloris mainly from the source of Fe2O3, confirming the incompletereaction at such low temperature. With increasing sintering tem-perature to 1200 �C, all the diffraction peaks can be indexed toYIG. The color shown in the inset changes to yellow green, confirm-ing the complete reaction of source materials and formation of YIG.With the addition of CTAB, the sintering temperature was signifi-cantly reduced. As can be seen, the XRD pattern for YIG sinteredat 1050 �C can be mostly indexed to YIG, with only several weakpeaks from impurities remaining. The color shown in the insethas been changed to yellow green already with very faint red color.With increasing the sintering temperature to 1100 �C, the reactionhas been completed, and pure phase of YIG has been formed.

Similar yellow green color as that of YIG-1200 can be observedin the inset.

The morphologies of YIG ceramic samples were investigated bySEM. As can be seen from Fig. 2, the grain size increases withincreasing the sintering temperature. However, for the YIG ceramicsamples sintered at the same temperature with or without addi-tion of CTAB, the particles have the similar size. This indicates thatthe addition of CTAB has little influence on the growth of grains,and the sintering temperature plays the main role. Based on theXRD and SEM results, the role of CTAB on the fabrication processof YIG can be understood in the following way. The reaction ofthe materials in solid state reaction process mainly depends onthe contacting areas of each ingredient. During the ball milling pro-cess, the particle size of each ingredient can be effectively reducedinto sub-micrometer. However, the agglomeration of source parti-cles is inevitable, leading to the large particles and uneven mixture,which will impede the chemical reaction process. To precede thechemical reaction process further, higher sintering temperature isneeded for the prolonged diffusion length of ions from each ingre-dient. This is the condition for the general solid state reaction pro-cess. CTAB is a widely used surfactant with both hydrophilic andhydrophobic groups. The surface wettability can be modified bythe chemical or physical adsorption of CTAB, thus the particle sizecan be effectively reduced and the mixture of ingredients can bemore uniform [13]. This will effectively shorten the diffusionlength of the ingredient ions and promote the chemical reactionprocess. Thus the sintering temperature can be significantly low-ered. However, the grain growth is a typical crystal growth process,and the growth kinetics mainly depends on the temperature, nomatter the uniformity of the distribution of the ingredients. Thus,the YIG ceramics powders show similar size when the sampleswere sintered at the same temperature, no matter CTAB was addedor not.

Fig. 2. SEM images of (a) YIG-1000, (b) YIG-1100 (c) YIG-1200, (d) YIG-CTAB-1050, (e) YIG-CTAB-1100, (f) YIG-CTAB-1200.

120 F. Xue et al. / Journal of Magnetism and Magnetic Materials 446 (2018) 118–124

The magnetic properties of YIG ceramic powders wereevaluated by a VSM at room temperature, and the field dependentmagnetization (M-H) curves are presented in Fig. 3. As can be seen,the magnetization of YIG-1100 is very small with very large coer-civity of 1000 Oe, which is significantly different from that of YIG[6]. The small magnetization might come from Fe2O3. If the YIGphase has been formed, the soft ferromagnetic properties can beimmediately observed, as shown the clear ferromagnetic hysteresisloops with small coercivity. The saturate magnetization of YIG-

CTAB-1050 (22.0 emu/g) is smaller than that of YIG-1200(31.4 emu/g) and YIG-CTAB-1100 (29.7 emu/g), which might bedue to the small amount of impurities. The coercivity of YIG-CTAB-1050 is 21.2 Oe. This is a typical value for the polycrystallineYIG [6], which is slightly larger than that of YIG-1200 (14.4 Oe) andYIG-CTAB-1100 (16.5 Oe). As can be seen, YIG-1200 shows thehighest saturate magnetization and smallest coercivity, indicatingthe important role of sintering temperature on the magnetic prop-erties of YIG, which is consistent with the previous report that the

Fig. 3. M-H curves of YIG ceramic samples. Top left inset shows the enlarged view of M-H curve of YIG-1100, and the bottom right inset shows the enlarged view of YIG-1200,YIG-CTAB-1050, and YIG-CTAB-1100.

F. Xue et al. / Journal of Magnetism and Magnetic Materials 446 (2018) 118–124 121

saturate magnetization increases and coercivity decreases withincreasing the sintering temperature [4]. The observed saturatemagnetization of YIG-1200 and YIG-CTAB-1100 is larger than thatof single crystalline YIG (27.4 emu/g) [14]. Fe2+ ions have largerradius of 0.77 Å than that of Fe3+ ions (0.63 Å), which will locateat the a sites with larger space [15]. Fe2+ ions have smaller mag-netic moment (4 lB) than that of Fe3+ (5 lB), thus the net magneticmoment between the antiparallel aligned a and d sites becomeslarger [4]. Comparable magnetization has been reported previouslyfor YIG prepared by microwave sintering method [10,16].

Fig. 4 shows the FMR data of YIG ceramic samples. Fig. 4(a)shows the typical FMR spectra measured at frequency of 8 GHz.As expected no signal can be detected for YIG-1100, due to thelacking of YIG phase. For the other three samples, clear FMR curvescan be observed, due to the formation of YIG. With increasing thefrequency, the resonance field (HFMR) shifts to larger value, whichis due to the monotonous increase dependence [17]. The line widthof the FMR spectra (DH) was determined from the peak-to-peakvalue, as shown in Fig. 4(b) [17]. The line width is in the order ofseveral hundred Oe, which is much larger than the reported valueof single crystal YIG (0.30 Oe) and epitaxial YIG film (6.6 Oe)[17,18]. The FMR line width is formed by the relaxation of spinexcitations and by the magnetic inhomogeneity [17]. Comparedwith the single crystal bulks and films, the high concentration ofgrain boundaries will inevitably introduce many defects, whichwill broaden the line width of FMR spectra [19].

Fig. 5 shows the typical curves of HFMR and DH for YIG samplesin dependence on the frequency. The difference in HFMR results

from the different respective saturation magnetizations and differ-ent crystalline anisotropy. The field dependence of HFMR on fre-quency can be fitted to the Kittel equation including theinfluence of anisotropy magnetic field [20]:

f ¼ jcjl0

2pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðHFMR þ HKÞðHFMR þ HK þMSÞ

pð1Þ

where |c|, HFMR, HK, and MS are the gyromagnetic ratio, resonancemagnetic field, anisotropy field, and saturate magnetization, respec-tively. It is clear from Eq. (1) that the Larmor precession is depen-dent on its intrinsic magnetic property and driven by the externalmagnetic fields [20]. The experimental observed saturate magneti-zation was applied in the fitting. From the fitting, |c| of free electroncan be obtained. And the field dependence of line width can be fit-ted to:

DH ¼ DH0 þ 2afffiffiffi3

pjcj

!ð2Þ

where a is the Gilbert damping constant and DH0 describes thelong-range inhomogeneity-caused line broadening. Fig. 5 showsthe fitting of the curves using Eqs. (1) and (2), and the fittedparameters of DH0, |c| and a are listed in the Table 1. It can beseen that the value of |c| for YIG-CTAB-1050 is a smaller thanthat of the other two samples. With increasing the sintering tem-perature, the |c| value increases, and is closer to the standardvalue of 2.80 MHz/Oe. The a value is much smaller for the YIG

Fig. 4. (a) FMR spectra of YIG ceramics samples measured at frequency of 8 GHz. (b) FMR spectra of YIG-CTAB-1100 measured at various frequencies. The determination ofthe peak-to-peak line width from the FMR spectra is shown by the dashed lines.

122 F. Xue et al. / Journal of Magnetism and Magnetic Materials 446 (2018) 118–124

prepared with CTAB than that of YIG without CTAB. The smallesta of 0.00136 for YIG-CTAB-1100 is comparable to the value of YIGfilm epitaxially grown on the GGG substrate (0.00103) [17] and Bisubstituted YIG ceramics (from 0.00031 to 0.00855) [20]. Gener-ally, the Gilbert damping constant is due to the exchange interac-tion and the spin–orbit coupling [21]. From Fig. 1, YIG-1200shows no significant difference from the YIG samples preparedwith addition of CTAB. Furthermore, the grain size of YIG-1200is much larger than that of YIG-CTAB-1050 and YIG-CTAB-1100.Thus, the crystalline quality should not be the origin of the signif-icantly decreased a for YIG with addition of CTAB. The main roleof CTAB on the decreased a can be understood by the uniformdistribution of ions, since the surfactant can modify the particlesurface and prohibit the agglomeration of particles, which notonly effectively decrease the sintering temperature, but also leadto the more uniform distribution of each element due to themuch short diffusion length.

4. Conclusion

A modified solid state reaction method has been applied toeffectively lower the sintering temperature of YIG. By adding CTABduring the ball milling processes, the sintering temperature hasbeen effectively decreased to 1050 �C from 1200 �C for YIG pre-pared without the addition of CTAB. No significant difference hasbeen observed for the grain size of YIG with and without additionof CTAB, and the grain size increases with increasing the sinteringtemperature, excluding the possible contribution of CTAB on graingrowth. The prepared YIG ceramic samples show soft ferromag-netic properties, with coercivity of only 21.2 Oe for the sample pre-pared with CTAB at 1050 �C, which decreases with increasingsintering temperature. The main role of adding CTAB is preventingthe agglomeration of ball milled ultrafine source particles, whichmay facilitate the interdiffusion among them and promote thereaction at lower temperatures. Furthermore, the Gilbert damping

Fig. 5. (a), (c), (e) are dependence of frequency on the resonance field HFMR, and (b), (d), (f) are FMR line width vs frequency, for YIG-1200, YIG-CTAB-1050, YIG-CTAB-1100,respectively. The fitting using Eqs. (1) and (2) are shown as blue lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the webversion of this article.)

Table 1The fitted values of DH0, jcj and a for YIG-1200, YIG-CTAB-1050 and YIG-CTAB-1100.

YIG-CTAB-1050 YIG-CTAB-1100 YIG-1200

DH0 (Oe) 378.6 347.7 272.2jcj (MHz/Oe) 2.30 � 106 2.56 � 106 2.56 � 106

a 0.00513 0.00136 0.01825

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constant is significantly reduced for YIG prepared by adding CTAB,and one order smaller than that without CTAB, which is due to themore uniform distribution of ions in YIG prepared with addition ofCTAB.

Acknowledgements

This work is supported by the National Natural Science Founda-tion of China (51471085), the Natural Science Foundation ofJiangsu Province of China (BK20151400), and the open researchfund of Key Laboratory of MEMS of Ministry of Education, South-east University.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jmmm.2017.08.076.

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