- Email: [email protected]
Zn- and Cu-substituted Co2Y hexagonal ferrites: Sintering behavior and permeability
Journal of Magnetism and Magnetic Materials 324 (2012) 1804–1808
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Zn- and Cu-substituted Co2Y hexagonal ferrites: Sintering behavior and permeability n ¨ S. Bierlich, J. Topfer
Department of SciTec, University of Applied Sciences Jena, C.-Zeiss-Promenade 2, 07745 Jena, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 August 2011 Received in revised form 5 January 2012 Available online 16 January 2012
Y-type polycrystalline hexagonal ferrites Ba2Co2 x yZnxCuyFe12O22 with 0 r x r 2 and 0 r y r0.8 were prepared by the mixed-oxide route. Single phase Y-type ferrite powders were obtained after calcinations at 1000 1C. Samples sintered at 1200 1C show a permeability that increases with the substitution of Zn for Co and display maximum permeability of m0 ¼35 at 1 MHz for x ¼ 1.6 and y ¼0.4. A resonance frequency fr ¼500 MHz is observed for Zn-rich ferrites with y¼ 0 and 0.4. The saturation magnetization increases with substitution of Zn for Co. Addition of Bi2O3 shifts the temperature of maximum shrinkage down to T r 950 1C. Moreover, an increase of the Cu-concentration further lowers the sintering temperature to T r900 1C, enabling co-firing of the ferrites with Ag metallization for multilayer technologies. However, low-temperature firing reduces the permeability to m0 ¼ 10 and the resonance frequency is shifted to 1 GHz. Thus substituted hexagonal Y-type ferrites can be used as soft magnetic materials for multilayer inductors for high frequency applications. & 2012 Elsevier B.V. All rights reserved.
Keywords: Hexagonal ferrite Y-type ferrite Multilayer inductor Permeability
1. Introduction Y-type barium hexaferrites Ba2Me2Fe12O22 (Me2Y) are members of the family of hexagonal ferrites which includes a variety of compositions (e.g. M-, Y-, Z-, W-, and X-types). The crystal structures of these oxides are based on various stacking sequences of hexagonal dense O/Ba layers [1,2]. The Y-type ferrite structure has been described for the first time by Braun [3] for Ba2Zn2Fe12O22 (Zn2Y) and defined somewhat more precisely some years later by Townes and Fang [4]. The space group is R-3m (No. 166) with one formula unit in the rhombohedral cell; in the hexagonal setting (with three formula units per unit cell) the unit ˚ From the crystalcell dimensions are a0 E5.9 A˚ and c0 E43.5 A. lographic point of view, the Y-structure can be considered as a stacking of two S- [Me2Fe4O8] and T- [Ba2Fe8O14] building blocks along the c axis. The S-block consists of two oxygen layers and the T-block has four oxygen layers with two of them containing barium. The magnetic structures of several Y-type Ba2Me2Fe12O22 ferrites were investigated by several authors. Co2Y has a planar magneto-crystalline anisotropy [1]. The cation distribution at low temperature was deduced from magnetization measurements; the cations form a collinear magnetic structure and the Co2 þ ions occupy the octahedral spin-down and spin-up sites [5]. Neutron
n
Corresponding author. Tel.: þ49 3641 205350; fax: þ 49 3641 205451. ¨ E-mail address: [email protected] (J. Topfer).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2012.01.006
diffraction experiments performed at 480 1C revealed a somewhat different result: the Co2 þ ions were shown to be mainly located at tetrahedral 6cIV sites (27%) of the S-block and on octahedral 18h positions (60%) in the T-block [6]. Zn2Y has a large specific saturation magnetization (Ms) at low temperature of Ms ¼ 78 emu/g. This is in agreement with the collinear Gorter model, assuming the Zn ions to be located at the tetrahedral sites [7]. From single-crystal XRD measurements Collomb et al. [8] reported a non-even distribution of the Zn2 þ ions on the Me1 6cIV (26%) and Me2 6cIV (69%) tetrahedral positions in the T- and S-blocks, respectively. The magnetic properties of the solid solution series Co2 xZnxY were studied by Lee and Kwon [9]. They reported an almost linear increase of Ms (extrapolated to T¼0 K) with x and a maximum of Ms at room temperature at x ¼1.50. Cu2Y exhibits a smaller Ms ¼25 emu/g (being almost the same at 0 K and 298 K) which was attributed to 1.3Cu in spin-up and 0.7Cu in spin-down sites [7]. Moreover, in the series Zn2 yCuyY the magneto-crystalline anisotropy at room temperature changes from planar to uniaxial at around y¼ 1.6 [7]. Hexagonal ferrites were suggested as potential candidates for multilayer ferrite inductors (MLFIs) operating at high frequencies [10]. For applications in such passive components the ferrites should meet the following criteria: (i) single-phase powder material after calcinations, (ii) good densification behavior at the sintering temperature, and (iii) sufficient permeability (e.g. mZ10) in a wide frequency range up to 3 GHz. It is desirable to use silver as an internal winding material; hence the temperature of co-firing the multilayer is restricted to Tr950 1C, i.e. typical
S. Bierlich, J. T¨ opfer / Journal of Magnetism and Magnetic Materials 324 (2012) 1804–1808
processing temperatures of the low-temperature ceramic. So far, mainly Z-type ferrites were studied as materials for MLFI [10]. However, their high synthesis temperature of 1300 1C and instability at 900 1C pose severe drawbacks for application in multilayer inductors [11]. Y-type ferrites require lower synthesis temperatures and are therefore potential candidates for MLFIs. Among the Y-type hexagonal ferrites, Zn2Y has drawn some attention for its large initial permeability in the MHz range. Co2Y has a large planar anisotropy [1] and hence a high resonance frequency. Therefore, substituted Co2 xZnxY ferrites seem to be suitable materials for high-frequency application. Furthermore, Cu addition supports the low-temperature firing ability at Tr950 1C as shown for Co2 wCuwZ ferrites [12]. The magnetic properties of some Cu- and Zn-substituted Co2Y ferrites sintered at 1000–1100 1C were already studied [13]. This paper reports on a systematic investigation of substituted Y-type hexaferrites Ba2Co2 x yZnxCuyFe12O22 with 0 rxr2 and 0 ryr0.8. The phase formation at 1000 1C and the sintering behavior at 1200 1C were studied. With the addition of 2 wt% Bi2O3, low-temperature sintering at 950 1C was investigated. The shrinkage behavior, microstructure formation, and magnetic properties of substituted Y-type ferrites sintered at 1200, 1100, and 950 1C are compared. Their potential for multilayer inductor applications is discussed.
1805
1100 1C. All substituted compositions Ba2Co2 x yZnxCuyFe12O22 with 0 oxr2 and 0 oyr0.8 were calcined at 1000 1C for 4 h. The single-phase character of the calcined powders was corroborated via X-ray diffraction and Rietveld analyses. As a typical example, the refined diffraction pattern of the sample with x ¼1 and y¼0.4 is shown in Fig. 1, demonstrating that a single-phase Y-type ferrite is formed at 1000 1C. The refined unit cell parameters for Co2Y (x ¼y¼0) a0 ¼5.862(7) A˚ and c0 ¼43.518(7) A˚ agree well with those reported in the literature [1,6,14]. Very recently, the lattice parameters of Co2Y (prepared by a coprecipitation route) were reported as a function of synthesis temperature [15]; the values found in our study agree well with the lattice parameters obtained at synthesis temperatures of TZ1000 1C in Ref. [15] and referred to as bulk values. The variation of lattice parameters as functions of composition x for the ferrites with y ¼0.4 is shown in Fig. 2. A slight increase of the parameter a0 from 5.861 to 5.870 A˚ with increasing of x is observed, whereas c0 remains almost constant. This small increase of the unit cell might be interpreted by the substitution of divalent Zn2 þ (r¼ 0.60 A˚ for Zn2 þ in tetrahedral sites [16]) for Co2 þ (r ¼0.74 A˚ for Co2 þ in octahedral sites [16]). Simultaneously, some Fe3 þ ions are transferred from tetrahedral to octahedral sites (r ¼0.49 A˚ and r ¼0.64 A˚ for Fe3 þ on tetrahedral and octahedral sites, respectively [16]), which
2. Experimental The Y-type ferrites were prepared by the mixed-oxide route. The analytic grade raw materials Fe2O3, BaCO3, ZnO, CuO, and Co3O4 were homogenized for 6 h. The mixture was calcined at 1000 1C for 6 h in air. Fine milling was performed in an aqueous suspension in a planetary rotation mill (Pulverisette 6, Fritzsch, Germany) with zirconia grinding media for 6 h. The mean particle size of the powder is d50 ¼0.8 mm. For some of the samples, Bi2O3 sintering aid was added during fine milling. Next, pellets (d¼ 10 mm) for sintering studies or toroids (fA ¼23 mm; fI ¼ 8 mm; h ¼5 mm) for permeability measurements were prepared by uniaxial pressing. Sintering was performed in air at 1200 1C, 1100 1C, and 950 1C. Phase formation of the calcined powders and sintered ceramics was evaluated by X-ray diffraction (Cu-Ka radiation, Bruker D8) for 201r2y r601 with a step size of 0.0151 and a step time of 8 s. Lattice parameters were refined using the TOPAS R software package (Bruker, Germany). The powder particle size was measured using laser diffraction (Malvern Mastersizer 2000). Shrinkage measurements were made on cylindrical compacts with a Netzsch DIL402 dilatometer with 4 K/min heating rate. The bulk density of sintered samples was determined from the dimensions and weight. The microstructure of sintered samples was studied with scanning electron microscopy (SEM, Zeiss DSM940A). The permeability of toroids was measured in the range from 1 MHz to 2 GHz with an Agilent E4991A impedance/materials analyzer with a magnetic materials test fixture (16454A). Due to the sample size and permittivity, measurements at f41 GHz might be complicated through the appearance of parasitic resonances. The saturation magnetization was measured at T¼5 K on powdered samples with a SQUID magnetometer.
Fig. 1. Rietveld refinement of XRD diffraction pattern of Ba2Co0.6Zn1Cu0.4Fe12O22; measured data and best fit profile (upper part), difference plot (middle section), and vertical tick marks indicating Bragg positions for CuKa1 radiation (bottom).
3. Results and discussion 3.1. Phase formation Stoichiometric Co2Y ferrite Ba2Co2Fe12O22 was synthesized through calcinations of the mixture of starting materials at
Fig. 2. Lattice parameters a0 and c0 of Ba2Co1.6 xZnxCu0.4Fe12O22 as function of Zn-concentration x.
1806
S. Bierlich, J. T¨ opfer / Journal of Magnetism and Magnetic Materials 324 (2012) 1804–1808
Table 1 Lattice parameters (a0 and c0), theoretical density, temperature of maximum shrinkage rate (TMSR), density (r), initial permeability at 1 MHz (m), and resonance frequency (fr) in the system Ba2Co2 x yZnxCuyFe12O22 for samples sintered at 1200 (1100) 1C and 950 1C. y
x
˚ /c0 (A) ˚ a0 (A)
Theoretical density (g/cm3)
TMSR (1C)
r (g/cm3)
a/b
Ts (1C) 1200a/950b
m Ts (1C) 1200a/950b
fg (MHz) Ts (1C) 1200a/950b
0
0 0.5 1 1.5 2
5.862 (7)/43.518 5.863(2)/43.520 5.866 (2)/43.520 5.869 (2)/43.520 5,870 (2)/43.536
(7) (1) (1) (2) (2)
5.441 5.439 5.434 5.428 5.407
1180/950 1200/950 1200/950 1200/950 1180/950
5.12 5.31 5.18 5.14 5.09
(6)/4.64 (2)/4.89 (1)/5.19 (3)/5.07 (5)/4.91
(2) (7) (4) (2) (8)
5/2.5 7.5/3 10/4 13/5 32.5/10
2100/2500 2000/2200 1500/2000 1100/800 700/1500
0.4
0 0.5 1 1.6
5.861 5.864 5.868 5.870
(6) (7) (8) (2)
5.443 5.436 5.429 5.425
1140/920 1130/910 1120/890 1125/900
5.19 5.18 5.12 5.20
(2)/4,95 (2)/5.00 (4)/5.07 (5)/5.11
(3) (3) (2) (2)
5/3 7.5/4 10/5 36/10
2000/2500 1600/2100 1200/1900 700/1300
(7)/43.507 (8)/43.519 (9)/43.520 (2)/43.520
r (g/cm3)
0.8
Ts (1C) 1100a/950b 0.6 0.9 1.1 1.2 a b
5.867 5.868 5.870 5.870
(8)/43.519 (9)/43.520 (6)/43.520 (9)/43.520
(7) (8) (5) (8)
5.432 5.429 5.426 5.426
1100/810 1090/820 1100/820 1090/800
5.04 5.17 5.23 5.02
(3)/5.06 (7)/5.05 (4)/5.07 (3)/5.08
(4) (8) (6) (2)
m(1 MHz) Ts (1C) 1100a/950b
fg (MHz) Ts (1C) 1100a/950b
7.5/5 12.5/6.5 16/7.5 18/10
1200/1900 1200/1700 1100/1600 1100/1500
Without Bi2O3. With 2 wt% Bi2O3.
If the substituted Co2Y ferrites are to be used for the multilayer inductors with Ag metallization the temperature of co-firing should be at around 900 1C. Since the densification of the additive-free samples is sufficient at temperatures of Z1100 1C only, the addition of 2 wt% Bi2O3 as liquid phase sintering additive was evaluated. To demonstrate the effect of Bi2O3 addition, the shrinkage behavior of Ba2Co0.6Zn1Cu0.4Fe12O22 with addition of 2% Bi2O3 is compared to that of the additive-free sample in Fig. 3. It can be noted that the shrinkage in the Bi-containing sample sets in at 800 1C and the maximum shrinkage rate is at TMSR ¼890 1C. Substitution of Cu for Co in combination with the addition of 2 wt% Bi2O3 reduces TMSR to 890–920 1C and 800–820 1C for y¼0.4 and 0.8, respectively (Table 1). Moreover, samples sintered at 950 1C show excellent densification for yZ0.4. 3.3. Magnetic properties Fig. 3. Shrinkage and shrinkage rate of Ba2Co0.6Zn1Cu0.4Fe12O22 ferrite powder without and with addition of 2 wt% Bi2O3.
seems to compensate for the substitution of smaller Zn2 þ for Co2 þ . The lattice parameters of the systems with y¼0 and 0.8 show similar trends (Table 1). 3.2. Sintering behavior The shrinkage of the substituted Co2Y ferrites was studied by dilatometry. A representative shrinkage curve of Ba2Co0.6Zn1Cu0.4Fe12O22 is shown in Fig. 3. This substituted Co2Y ferrite does not show any significant shrinkage below 1000 1C and the shrinkage rate peaks at about 1120 1C. The shrinkage behavior does not show significant changes upon substitution of Zn for Co. The temperature of maximum shrinkage rate TMSR of all samples with the same Cu-concentration y is almost identical (Table 1). However, introduction of Cu shifts the shrinkage down to lower temperatures. From Table 1 we can note that for y¼0 the TMSR is in the range of 1180–1200 1C. For y¼0.4 a TMSR ¼1120–1140 1C is observed, whereas for y¼0.8 the maximum shrinkage rate appears at TMSR ¼1090–1100 1C. Samples were sintered at 1200 1C for y¼0 and 0.4, or at 1100 1C for y¼0.8. The sintered density is 45 g/cm3 (95%) for all samples (Table 1).
The effect of substitution of Zn and Cu for Co on the initial permeability is summarized in Table 1. The permeability of samples sintered at 1200 1C (or 1100 1C for y¼0.8) without additive as well as those sintered at 950 1C with 2 wt% Bi2O3 increases with the Zn concentration x. Fig. 4 shows the permeability spectra of Y-type ferrites with different Zn-substitutions x, but constant Cu-content y¼0.4, sintered at 1200 1C (Fig. 4a) and 9501 (Fig. 4b). The initial permeability of samples without Bi2O3 addition increases from m¼5 for x¼0 to a maximum permeability of m¼35 for x¼1.6. Simultaneously, the resonance frequency (taken as the maximum in the m00 vs. frequency curve) decreases from 2 GHz for x¼ 0 to several hundred MHz for x ¼1.6. This is in agreement with Snoek’s law, i.e. the lower the initial permeability the higher the cut-off frequency, and vice versa. Closer inspection of the m00 curves reveals a continuous shift of the m00 -peak with increasing x starting from 2 GHz down to lower frequencies, but for x ¼1.6 (i.e. the Zn-rich and Co-free sample) a second peak appears at around 100 MHz. This double resonance behavior is observed for all substituted Co2Y samples with large Zn and small (or zero) Co-concentrations. The permeability spectrum is caused by two contributions, i.e. spin rotation and domain wall motion [17]. Since all samples are sintered at the same temperature and have the same grain size, all samples are assumed to consist of
S. Bierlich, J. T¨ opfer / Journal of Magnetism and Magnetic Materials 324 (2012) 1804–1808
Fig. 4. The real (left axis, open symbols) and imaginary (right axis, filled symbols) parts of the permeability vs. frequency for Ba2Co1.6 xZnxCu0.4Fe12O22 sintered at 1200 1C without Bi2O3 (a) and at 950 1C with 2 wt% Bi2O3 (b).
multi-domain particles. The observation of a decrease of m0 at f¼50 MHz and simultaneously a peak in m00 around 70 MHz in the permeability spectrum of the sample with x¼1.6 indicates that one of the permeability mechanisms is becoming dominant at low frequency in these materials. Similar effects were observed in Cosubstituted Zn2Y ferrites [18]. The authors attribute this behavior as to a rather low planar magneto-crystalline anisotropy in Znrich Y-type ferrites, which causes the domain wall motion to predominate at low frequency, whereas in Co-containing Y-type ferrites the planar anisotropy of Co2 þ favours spin rotation to be prevailing [18]. The permeability of the sample set with Bi2O3 addition and sintered at 950 1C shows similar trends (Table 1). At the first view it can be seen that the permeability m is somewhat smaller compared to substituted Co2Y-samples sintered at higher temperatures. The reduced permeability values of samples sintered at 950 1C can be induced through several effects: (i) the smaller grain size and larger number of the grain boundaries, (ii) the presence of nonmagnetic Bi2O3 in the sample, and (iii) thermodynamic instability of the Co2Y phase at Tr 950 1C. The last topic will be discussed in the next paragraph. However, the permeability also increases with increasing Zn-content x and exhibits an initial permeability m¼ 10 at x ¼1.6 and y¼0.4. The permeability is constant over the whole frequency range (except for x ¼1.6) with a cut-off frequency above 1 GHz. To interpret the variation of initial permeability with composition we also measured the saturation magnetization at T¼5 K.
1807
Fig. 5. Variation of magnetization vs. magnetic field at T¼5 K (a) and saturation magnetization Ms at T¼ 5 K vs. composition x (b) for Ba2Co2 xZnxFe12O22 ferrite powders calcined at 1000 1C.
The M vs. H plots (Fig. 5a) show that the samples are saturated at 10 kOe. The magnetization at 50 kOe was taken as the saturation magnetization Ms. Fig. 5b shows the effect of the Zn-concentration x on Ms at 5 K for Ba2Co2 x yZnxCuyFe12O22 with 0rx r2 and y¼0. The saturation magnetization of Co2Y (x ¼0) is Ms ¼35.3 emu/g and continuously increases to Ms ¼69.6 emu/g for Zn2Y (x¼2). These findings are in a good agreement with the parameters reported by Lee and Kwon [9] (Fig. 5b). Included are calculated values of Ms for x¼0 and x¼2 based on the collinear Gorter concept and the Co- and Zn-distributions on the six metal sites as reported by Collomb et al. [6,8]. For Co2Y a calculated Ms ¼32.1 emu/g is in fair agreement with the measured value. For Zn2Y the calculated Ms ¼77.1 emu/g is larger than the measured value. This might be caused by a somewhat non-collinear spin arrangement due to weakened magnetic interactions. However, the measured saturation magnetizations of Co2 xZnxY ferrites confirm the reported trend of increasing Ms with Zn-concentration x [9]. The observed increase of permeability of the substituted Ba2Co2 x yZnxCuyFe12O22 ferrites with Zn-concentration x might be caused by the increased magnetization. The grain size and the anisotropy field also affect the permeability. The microstructures of ferrites sintered at 1200 1C are rather non-homogeneous with a broad spectrum of grain sizes (inset Fig. 6). The microstructures of all samples with different compositions are similar, however, in samples with high Zn content, e.g.with y¼0.4 and x¼1 or 1.6, there might be an increased concentration
1808
S. Bierlich, J. T¨ opfer / Journal of Magnetism and Magnetic Materials 324 (2012) 1804–1808
4. Conclusions
Fig. 6. XRD patterns of Ba2Co0.6Zn1Cu0.4Fe12O22 sintered at 1200 1C without additive and at 950 1C for 4 h with 2 wt% Bi2O3 (S – spinel, P – perowskite); inset: SEM micrographs of the sintered samples.
of very large grains. The samples sintered at 950 1C (see inset of Fig. 6) exhibit similar microstructures. Therefore we conclude that the influence of the grain size on permeability in the studied samples is not significant. On the other hand, a decrease of the anisotropy field with x is likely: this also contributes to the increase of permeability for Zn-rich ferrites. 3.4. Microstructure and phase stability XRD and SEM studies reveal the phase compositions of samples sintered at 1200 1C and 950 1C. We present a representative example (sample with x¼1 and y¼0.4) in Fig. 6. The XRD pattern of the sample sintered at 1200 1C exhibits reflections of a single Y-type ferrite only. For the sample fired at 950 1C additional peaks of lower intensity were observed, which were attributed to belong to a spineland a perovskite-type compound. SEM micrographs confirm this finding: the sample sintered at 1200 1C consists of a large-grained, but homogeneous and single-phase microstructure (inset of Fig. 6). The microstructure of the sample sintered at 950 1C is composed of smaller grains. The formation of coexisting phases (dark gray grains and small white spots) embedded in a matrix Co2Y phase is clearly visible. EDX analysis suggests that the small white spots are rich in Bi (sintering additive) and the dark gray regions are composed of Co, Zn, and Fe cations mainly (corresponding to the observed spinel phase). This observation suggests that the substituted Co2Y phase is thermodynamically unstable at temperatures Tr950 1C. This partial decomposition for the ferrite samples sintered at 950 1C is an additional reason of the significant reduction of permeability. Similar behavior was already reported for Z-type ferrites, which were sintered to single-phase material in the temperature window of 1300–1330 1C; at the sintering temperature of 900 or 950 1C the Co2Z ferrites decompose into a mixture of Y- and M-type ferrites, which is accompanied by a reduction of permeability [11]. On the other hand, the SEM micrograph (Fig. 6) shows that ferrite grains grow larger as the sintering temperature increases. This enables the magnetic domains to move more easily and as a result, the permeability increases and the cut-off frequency shifts toward the MHz range.
Substituted Co2Y ferrites of composition Ba2Zn2 x yCuyFe12O22 (0rxr2 and y¼0, 0.4, and 0.8) are synthesized by the mixed-oxide route at 1000 1C. Sintering at 1200 1C gives dense samples with large grains and a permeability increasing with the Zn-concentration x. This effect was shown to be originated by the saturation magnetization increasing with x. Dense and fine-grained ferrites were sintered at 950 1C using an appropriate concentration of Bi2O3 sintering aid. Substitution of Zn for Co increases the initial permeability. It was shown that Y-type ferrites can be used for the fabrication of highfrequency multilayer inductors. However, Y-type ferrites show a tendency of phase transformation during sintering at 950 1C. As a consequence, a lower permeability is measured. Nevertheless, these novel substituted low-temperature sintered Co2Y ferrites exhibit interesting high-frequency properties, e.g. permeability of up to m¼10 and high cut-off frequency above 1 GHz. Integration of these ferrites into LTCC multilayer modules requires further investigation and adaptation of the ferrite materials. One crucial point is that the firing temperature has to be further reduced down to 900 1C in order to limit the diffusivity of the Ag metallization. A study of the fabrication, co-firing, and design of multilayer modules based on substituted Co2Y ferrites is in progress.
Acknowledgments ¨ The authors thank Mrs. R. Lohnert for help in the preparation of ferrite samples. This work was supported by the Bundesminis¨ Bildung und Forschung (Germany) in the research terium fur project HF-Multifer (1762X05) and the State of Thuringia through the Pro-Exzellenz network (Kerfunmat PE214). References [1] J. Smit, H.P.J. Wijn, Ferrites, Philips Technical Library, Eindhoven, 1959. p. 285. [2] M. Sugimoto, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials, vol. 3, 1982, p. 394. [3] P.B. Braun, Philips Research Reports 12 (1957) 491. [4] W.D. Townes, J.H. Fang, Zeitschrift Fur Kristallographie 131 (1970) 196–205. [5] G. Albanese, M. Carbucicicchio, A. Deriu, G. Astri, S. Rinaldi, Applied Physics 7 (1975) 227–238. [6] A. Collomb, M.A. Hadj Farhat, J.C. Joubert, Materials Research Bulletin 24 (1989) 453–458. [7] A. Deriu, F. Licci, S. Rinaldi, T. Besagni, Journal of Magnetism and Magnetic Materials 22 (1981) 257–262. [8] A. Collomb, J. Muller, J.C. Guitel, J.M. Desvignes, Journal of Magnetism and Magnetic Materials 78 (1989) 77–84. [9] S.G. Lee, S.J. Kwon, Journal of Magnetism and Magnetic Materials 153 (1996) 279. [10] M. Endo, A. Nakano, in: Proceedings of the Eighth International Conference on Ferrites, ICF8, Kyoto and Tokyo, Japan, 2000, pp. 1168–1170. ¨ [11] S. Kracˇunovska´, J. Topfer, Journal of Magnetism and Magnetic Materials 320 (2008) 1370–1376. ˇ ´ ¨ [12] S. Kracunovska, J. Topfer, Journal of Materials Science: Materials in Electronics 22 (2011) 467–473. [13] Y. Bai, J. Zhou, Z. Gui, L. Li, Journal of Magnetism and Magnetic Materials 246 (2002) 140–144. [14] H.S. Shin, S.J. Kwon, Powder Diffraction 8 (1993) 98–101. [15] A. Daigle, E. DuPre, A. Geiler, Y. Chen, P.V. Parimi, C. Vittoria, V.G. Harris, Journal of the American Ceramic Society 93 (2010) 2994–2997. [16] R.D. Shannon, Acta Crystallographica A32 (1976) 751. [17] T. Nakamura, K. Hatakeyama, IEEE Transactions on Magnetics 36 (2000) 3415–3417. [18] Y. Bai, J. Zhou, Z. Gui, L. Li, Materials Letters 58 (2004) 1602–1606.
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Zn- and Cu-substituted Co2Y hexagonal ferrites: Sintering behavior and permeability n ¨ S. Bierlich, J. Topfer
Department of SciTec, University of Applied Sciences Jena, C.-Zeiss-Promenade 2, 07745 Jena, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 August 2011 Received in revised form 5 January 2012 Available online 16 January 2012
Y-type polycrystalline hexagonal ferrites Ba2Co2 x yZnxCuyFe12O22 with 0 r x r 2 and 0 r y r0.8 were prepared by the mixed-oxide route. Single phase Y-type ferrite powders were obtained after calcinations at 1000 1C. Samples sintered at 1200 1C show a permeability that increases with the substitution of Zn for Co and display maximum permeability of m0 ¼35 at 1 MHz for x ¼ 1.6 and y ¼0.4. A resonance frequency fr ¼500 MHz is observed for Zn-rich ferrites with y¼ 0 and 0.4. The saturation magnetization increases with substitution of Zn for Co. Addition of Bi2O3 shifts the temperature of maximum shrinkage down to T r 950 1C. Moreover, an increase of the Cu-concentration further lowers the sintering temperature to T r900 1C, enabling co-firing of the ferrites with Ag metallization for multilayer technologies. However, low-temperature firing reduces the permeability to m0 ¼ 10 and the resonance frequency is shifted to 1 GHz. Thus substituted hexagonal Y-type ferrites can be used as soft magnetic materials for multilayer inductors for high frequency applications. & 2012 Elsevier B.V. All rights reserved.
Keywords: Hexagonal ferrite Y-type ferrite Multilayer inductor Permeability
1. Introduction Y-type barium hexaferrites Ba2Me2Fe12O22 (Me2Y) are members of the family of hexagonal ferrites which includes a variety of compositions (e.g. M-, Y-, Z-, W-, and X-types). The crystal structures of these oxides are based on various stacking sequences of hexagonal dense O/Ba layers [1,2]. The Y-type ferrite structure has been described for the first time by Braun [3] for Ba2Zn2Fe12O22 (Zn2Y) and defined somewhat more precisely some years later by Townes and Fang [4]. The space group is R-3m (No. 166) with one formula unit in the rhombohedral cell; in the hexagonal setting (with three formula units per unit cell) the unit ˚ From the crystalcell dimensions are a0 E5.9 A˚ and c0 E43.5 A. lographic point of view, the Y-structure can be considered as a stacking of two S- [Me2Fe4O8] and T- [Ba2Fe8O14] building blocks along the c axis. The S-block consists of two oxygen layers and the T-block has four oxygen layers with two of them containing barium. The magnetic structures of several Y-type Ba2Me2Fe12O22 ferrites were investigated by several authors. Co2Y has a planar magneto-crystalline anisotropy [1]. The cation distribution at low temperature was deduced from magnetization measurements; the cations form a collinear magnetic structure and the Co2 þ ions occupy the octahedral spin-down and spin-up sites [5]. Neutron
n
Corresponding author. Tel.: þ49 3641 205350; fax: þ 49 3641 205451. ¨ E-mail address: [email protected] (J. Topfer).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2012.01.006
diffraction experiments performed at 480 1C revealed a somewhat different result: the Co2 þ ions were shown to be mainly located at tetrahedral 6cIV sites (27%) of the S-block and on octahedral 18h positions (60%) in the T-block [6]. Zn2Y has a large specific saturation magnetization (Ms) at low temperature of Ms ¼ 78 emu/g. This is in agreement with the collinear Gorter model, assuming the Zn ions to be located at the tetrahedral sites [7]. From single-crystal XRD measurements Collomb et al. [8] reported a non-even distribution of the Zn2 þ ions on the Me1 6cIV (26%) and Me2 6cIV (69%) tetrahedral positions in the T- and S-blocks, respectively. The magnetic properties of the solid solution series Co2 xZnxY were studied by Lee and Kwon [9]. They reported an almost linear increase of Ms (extrapolated to T¼0 K) with x and a maximum of Ms at room temperature at x ¼1.50. Cu2Y exhibits a smaller Ms ¼25 emu/g (being almost the same at 0 K and 298 K) which was attributed to 1.3Cu in spin-up and 0.7Cu in spin-down sites [7]. Moreover, in the series Zn2 yCuyY the magneto-crystalline anisotropy at room temperature changes from planar to uniaxial at around y¼ 1.6 [7]. Hexagonal ferrites were suggested as potential candidates for multilayer ferrite inductors (MLFIs) operating at high frequencies [10]. For applications in such passive components the ferrites should meet the following criteria: (i) single-phase powder material after calcinations, (ii) good densification behavior at the sintering temperature, and (iii) sufficient permeability (e.g. mZ10) in a wide frequency range up to 3 GHz. It is desirable to use silver as an internal winding material; hence the temperature of co-firing the multilayer is restricted to Tr950 1C, i.e. typical
S. Bierlich, J. T¨ opfer / Journal of Magnetism and Magnetic Materials 324 (2012) 1804–1808
processing temperatures of the low-temperature ceramic. So far, mainly Z-type ferrites were studied as materials for MLFI [10]. However, their high synthesis temperature of 1300 1C and instability at 900 1C pose severe drawbacks for application in multilayer inductors [11]. Y-type ferrites require lower synthesis temperatures and are therefore potential candidates for MLFIs. Among the Y-type hexagonal ferrites, Zn2Y has drawn some attention for its large initial permeability in the MHz range. Co2Y has a large planar anisotropy [1] and hence a high resonance frequency. Therefore, substituted Co2 xZnxY ferrites seem to be suitable materials for high-frequency application. Furthermore, Cu addition supports the low-temperature firing ability at Tr950 1C as shown for Co2 wCuwZ ferrites [12]. The magnetic properties of some Cu- and Zn-substituted Co2Y ferrites sintered at 1000–1100 1C were already studied [13]. This paper reports on a systematic investigation of substituted Y-type hexaferrites Ba2Co2 x yZnxCuyFe12O22 with 0 rxr2 and 0 ryr0.8. The phase formation at 1000 1C and the sintering behavior at 1200 1C were studied. With the addition of 2 wt% Bi2O3, low-temperature sintering at 950 1C was investigated. The shrinkage behavior, microstructure formation, and magnetic properties of substituted Y-type ferrites sintered at 1200, 1100, and 950 1C are compared. Their potential for multilayer inductor applications is discussed.
1805
1100 1C. All substituted compositions Ba2Co2 x yZnxCuyFe12O22 with 0 oxr2 and 0 oyr0.8 were calcined at 1000 1C for 4 h. The single-phase character of the calcined powders was corroborated via X-ray diffraction and Rietveld analyses. As a typical example, the refined diffraction pattern of the sample with x ¼1 and y¼0.4 is shown in Fig. 1, demonstrating that a single-phase Y-type ferrite is formed at 1000 1C. The refined unit cell parameters for Co2Y (x ¼y¼0) a0 ¼5.862(7) A˚ and c0 ¼43.518(7) A˚ agree well with those reported in the literature [1,6,14]. Very recently, the lattice parameters of Co2Y (prepared by a coprecipitation route) were reported as a function of synthesis temperature [15]; the values found in our study agree well with the lattice parameters obtained at synthesis temperatures of TZ1000 1C in Ref. [15] and referred to as bulk values. The variation of lattice parameters as functions of composition x for the ferrites with y ¼0.4 is shown in Fig. 2. A slight increase of the parameter a0 from 5.861 to 5.870 A˚ with increasing of x is observed, whereas c0 remains almost constant. This small increase of the unit cell might be interpreted by the substitution of divalent Zn2 þ (r¼ 0.60 A˚ for Zn2 þ in tetrahedral sites [16]) for Co2 þ (r ¼0.74 A˚ for Co2 þ in octahedral sites [16]). Simultaneously, some Fe3 þ ions are transferred from tetrahedral to octahedral sites (r ¼0.49 A˚ and r ¼0.64 A˚ for Fe3 þ on tetrahedral and octahedral sites, respectively [16]), which
2. Experimental The Y-type ferrites were prepared by the mixed-oxide route. The analytic grade raw materials Fe2O3, BaCO3, ZnO, CuO, and Co3O4 were homogenized for 6 h. The mixture was calcined at 1000 1C for 6 h in air. Fine milling was performed in an aqueous suspension in a planetary rotation mill (Pulverisette 6, Fritzsch, Germany) with zirconia grinding media for 6 h. The mean particle size of the powder is d50 ¼0.8 mm. For some of the samples, Bi2O3 sintering aid was added during fine milling. Next, pellets (d¼ 10 mm) for sintering studies or toroids (fA ¼23 mm; fI ¼ 8 mm; h ¼5 mm) for permeability measurements were prepared by uniaxial pressing. Sintering was performed in air at 1200 1C, 1100 1C, and 950 1C. Phase formation of the calcined powders and sintered ceramics was evaluated by X-ray diffraction (Cu-Ka radiation, Bruker D8) for 201r2y r601 with a step size of 0.0151 and a step time of 8 s. Lattice parameters were refined using the TOPAS R software package (Bruker, Germany). The powder particle size was measured using laser diffraction (Malvern Mastersizer 2000). Shrinkage measurements were made on cylindrical compacts with a Netzsch DIL402 dilatometer with 4 K/min heating rate. The bulk density of sintered samples was determined from the dimensions and weight. The microstructure of sintered samples was studied with scanning electron microscopy (SEM, Zeiss DSM940A). The permeability of toroids was measured in the range from 1 MHz to 2 GHz with an Agilent E4991A impedance/materials analyzer with a magnetic materials test fixture (16454A). Due to the sample size and permittivity, measurements at f41 GHz might be complicated through the appearance of parasitic resonances. The saturation magnetization was measured at T¼5 K on powdered samples with a SQUID magnetometer.
Fig. 1. Rietveld refinement of XRD diffraction pattern of Ba2Co0.6Zn1Cu0.4Fe12O22; measured data and best fit profile (upper part), difference plot (middle section), and vertical tick marks indicating Bragg positions for CuKa1 radiation (bottom).
3. Results and discussion 3.1. Phase formation Stoichiometric Co2Y ferrite Ba2Co2Fe12O22 was synthesized through calcinations of the mixture of starting materials at
Fig. 2. Lattice parameters a0 and c0 of Ba2Co1.6 xZnxCu0.4Fe12O22 as function of Zn-concentration x.
1806
S. Bierlich, J. T¨ opfer / Journal of Magnetism and Magnetic Materials 324 (2012) 1804–1808
Table 1 Lattice parameters (a0 and c0), theoretical density, temperature of maximum shrinkage rate (TMSR), density (r), initial permeability at 1 MHz (m), and resonance frequency (fr) in the system Ba2Co2 x yZnxCuyFe12O22 for samples sintered at 1200 (1100) 1C and 950 1C. y
x
˚ /c0 (A) ˚ a0 (A)
Theoretical density (g/cm3)
TMSR (1C)
r (g/cm3)
a/b
Ts (1C) 1200a/950b
m Ts (1C) 1200a/950b
fg (MHz) Ts (1C) 1200a/950b
0
0 0.5 1 1.5 2
5.862 (7)/43.518 5.863(2)/43.520 5.866 (2)/43.520 5.869 (2)/43.520 5,870 (2)/43.536
(7) (1) (1) (2) (2)
5.441 5.439 5.434 5.428 5.407
1180/950 1200/950 1200/950 1200/950 1180/950
5.12 5.31 5.18 5.14 5.09
(6)/4.64 (2)/4.89 (1)/5.19 (3)/5.07 (5)/4.91
(2) (7) (4) (2) (8)
5/2.5 7.5/3 10/4 13/5 32.5/10
2100/2500 2000/2200 1500/2000 1100/800 700/1500
0.4
0 0.5 1 1.6
5.861 5.864 5.868 5.870
(6) (7) (8) (2)
5.443 5.436 5.429 5.425
1140/920 1130/910 1120/890 1125/900
5.19 5.18 5.12 5.20
(2)/4,95 (2)/5.00 (4)/5.07 (5)/5.11
(3) (3) (2) (2)
5/3 7.5/4 10/5 36/10
2000/2500 1600/2100 1200/1900 700/1300
(7)/43.507 (8)/43.519 (9)/43.520 (2)/43.520
r (g/cm3)
0.8
Ts (1C) 1100a/950b 0.6 0.9 1.1 1.2 a b
5.867 5.868 5.870 5.870
(8)/43.519 (9)/43.520 (6)/43.520 (9)/43.520
(7) (8) (5) (8)
5.432 5.429 5.426 5.426
1100/810 1090/820 1100/820 1090/800
5.04 5.17 5.23 5.02
(3)/5.06 (7)/5.05 (4)/5.07 (3)/5.08
(4) (8) (6) (2)
m(1 MHz) Ts (1C) 1100a/950b
fg (MHz) Ts (1C) 1100a/950b
7.5/5 12.5/6.5 16/7.5 18/10
1200/1900 1200/1700 1100/1600 1100/1500
Without Bi2O3. With 2 wt% Bi2O3.
If the substituted Co2Y ferrites are to be used for the multilayer inductors with Ag metallization the temperature of co-firing should be at around 900 1C. Since the densification of the additive-free samples is sufficient at temperatures of Z1100 1C only, the addition of 2 wt% Bi2O3 as liquid phase sintering additive was evaluated. To demonstrate the effect of Bi2O3 addition, the shrinkage behavior of Ba2Co0.6Zn1Cu0.4Fe12O22 with addition of 2% Bi2O3 is compared to that of the additive-free sample in Fig. 3. It can be noted that the shrinkage in the Bi-containing sample sets in at 800 1C and the maximum shrinkage rate is at TMSR ¼890 1C. Substitution of Cu for Co in combination with the addition of 2 wt% Bi2O3 reduces TMSR to 890–920 1C and 800–820 1C for y¼0.4 and 0.8, respectively (Table 1). Moreover, samples sintered at 950 1C show excellent densification for yZ0.4. 3.3. Magnetic properties Fig. 3. Shrinkage and shrinkage rate of Ba2Co0.6Zn1Cu0.4Fe12O22 ferrite powder without and with addition of 2 wt% Bi2O3.
seems to compensate for the substitution of smaller Zn2 þ for Co2 þ . The lattice parameters of the systems with y¼0 and 0.8 show similar trends (Table 1). 3.2. Sintering behavior The shrinkage of the substituted Co2Y ferrites was studied by dilatometry. A representative shrinkage curve of Ba2Co0.6Zn1Cu0.4Fe12O22 is shown in Fig. 3. This substituted Co2Y ferrite does not show any significant shrinkage below 1000 1C and the shrinkage rate peaks at about 1120 1C. The shrinkage behavior does not show significant changes upon substitution of Zn for Co. The temperature of maximum shrinkage rate TMSR of all samples with the same Cu-concentration y is almost identical (Table 1). However, introduction of Cu shifts the shrinkage down to lower temperatures. From Table 1 we can note that for y¼0 the TMSR is in the range of 1180–1200 1C. For y¼0.4 a TMSR ¼1120–1140 1C is observed, whereas for y¼0.8 the maximum shrinkage rate appears at TMSR ¼1090–1100 1C. Samples were sintered at 1200 1C for y¼0 and 0.4, or at 1100 1C for y¼0.8. The sintered density is 45 g/cm3 (95%) for all samples (Table 1).
The effect of substitution of Zn and Cu for Co on the initial permeability is summarized in Table 1. The permeability of samples sintered at 1200 1C (or 1100 1C for y¼0.8) without additive as well as those sintered at 950 1C with 2 wt% Bi2O3 increases with the Zn concentration x. Fig. 4 shows the permeability spectra of Y-type ferrites with different Zn-substitutions x, but constant Cu-content y¼0.4, sintered at 1200 1C (Fig. 4a) and 9501 (Fig. 4b). The initial permeability of samples without Bi2O3 addition increases from m¼5 for x¼0 to a maximum permeability of m¼35 for x¼1.6. Simultaneously, the resonance frequency (taken as the maximum in the m00 vs. frequency curve) decreases from 2 GHz for x¼ 0 to several hundred MHz for x ¼1.6. This is in agreement with Snoek’s law, i.e. the lower the initial permeability the higher the cut-off frequency, and vice versa. Closer inspection of the m00 curves reveals a continuous shift of the m00 -peak with increasing x starting from 2 GHz down to lower frequencies, but for x ¼1.6 (i.e. the Zn-rich and Co-free sample) a second peak appears at around 100 MHz. This double resonance behavior is observed for all substituted Co2Y samples with large Zn and small (or zero) Co-concentrations. The permeability spectrum is caused by two contributions, i.e. spin rotation and domain wall motion [17]. Since all samples are sintered at the same temperature and have the same grain size, all samples are assumed to consist of
S. Bierlich, J. T¨ opfer / Journal of Magnetism and Magnetic Materials 324 (2012) 1804–1808
Fig. 4. The real (left axis, open symbols) and imaginary (right axis, filled symbols) parts of the permeability vs. frequency for Ba2Co1.6 xZnxCu0.4Fe12O22 sintered at 1200 1C without Bi2O3 (a) and at 950 1C with 2 wt% Bi2O3 (b).
multi-domain particles. The observation of a decrease of m0 at f¼50 MHz and simultaneously a peak in m00 around 70 MHz in the permeability spectrum of the sample with x¼1.6 indicates that one of the permeability mechanisms is becoming dominant at low frequency in these materials. Similar effects were observed in Cosubstituted Zn2Y ferrites [18]. The authors attribute this behavior as to a rather low planar magneto-crystalline anisotropy in Znrich Y-type ferrites, which causes the domain wall motion to predominate at low frequency, whereas in Co-containing Y-type ferrites the planar anisotropy of Co2 þ favours spin rotation to be prevailing [18]. The permeability of the sample set with Bi2O3 addition and sintered at 950 1C shows similar trends (Table 1). At the first view it can be seen that the permeability m is somewhat smaller compared to substituted Co2Y-samples sintered at higher temperatures. The reduced permeability values of samples sintered at 950 1C can be induced through several effects: (i) the smaller grain size and larger number of the grain boundaries, (ii) the presence of nonmagnetic Bi2O3 in the sample, and (iii) thermodynamic instability of the Co2Y phase at Tr 950 1C. The last topic will be discussed in the next paragraph. However, the permeability also increases with increasing Zn-content x and exhibits an initial permeability m¼ 10 at x ¼1.6 and y¼0.4. The permeability is constant over the whole frequency range (except for x ¼1.6) with a cut-off frequency above 1 GHz. To interpret the variation of initial permeability with composition we also measured the saturation magnetization at T¼5 K.
1807
Fig. 5. Variation of magnetization vs. magnetic field at T¼5 K (a) and saturation magnetization Ms at T¼ 5 K vs. composition x (b) for Ba2Co2 xZnxFe12O22 ferrite powders calcined at 1000 1C.
The M vs. H plots (Fig. 5a) show that the samples are saturated at 10 kOe. The magnetization at 50 kOe was taken as the saturation magnetization Ms. Fig. 5b shows the effect of the Zn-concentration x on Ms at 5 K for Ba2Co2 x yZnxCuyFe12O22 with 0rx r2 and y¼0. The saturation magnetization of Co2Y (x ¼0) is Ms ¼35.3 emu/g and continuously increases to Ms ¼69.6 emu/g for Zn2Y (x¼2). These findings are in a good agreement with the parameters reported by Lee and Kwon [9] (Fig. 5b). Included are calculated values of Ms for x¼0 and x¼2 based on the collinear Gorter concept and the Co- and Zn-distributions on the six metal sites as reported by Collomb et al. [6,8]. For Co2Y a calculated Ms ¼32.1 emu/g is in fair agreement with the measured value. For Zn2Y the calculated Ms ¼77.1 emu/g is larger than the measured value. This might be caused by a somewhat non-collinear spin arrangement due to weakened magnetic interactions. However, the measured saturation magnetizations of Co2 xZnxY ferrites confirm the reported trend of increasing Ms with Zn-concentration x [9]. The observed increase of permeability of the substituted Ba2Co2 x yZnxCuyFe12O22 ferrites with Zn-concentration x might be caused by the increased magnetization. The grain size and the anisotropy field also affect the permeability. The microstructures of ferrites sintered at 1200 1C are rather non-homogeneous with a broad spectrum of grain sizes (inset Fig. 6). The microstructures of all samples with different compositions are similar, however, in samples with high Zn content, e.g.with y¼0.4 and x¼1 or 1.6, there might be an increased concentration
1808
S. Bierlich, J. T¨ opfer / Journal of Magnetism and Magnetic Materials 324 (2012) 1804–1808
4. Conclusions
Fig. 6. XRD patterns of Ba2Co0.6Zn1Cu0.4Fe12O22 sintered at 1200 1C without additive and at 950 1C for 4 h with 2 wt% Bi2O3 (S – spinel, P – perowskite); inset: SEM micrographs of the sintered samples.
of very large grains. The samples sintered at 950 1C (see inset of Fig. 6) exhibit similar microstructures. Therefore we conclude that the influence of the grain size on permeability in the studied samples is not significant. On the other hand, a decrease of the anisotropy field with x is likely: this also contributes to the increase of permeability for Zn-rich ferrites. 3.4. Microstructure and phase stability XRD and SEM studies reveal the phase compositions of samples sintered at 1200 1C and 950 1C. We present a representative example (sample with x¼1 and y¼0.4) in Fig. 6. The XRD pattern of the sample sintered at 1200 1C exhibits reflections of a single Y-type ferrite only. For the sample fired at 950 1C additional peaks of lower intensity were observed, which were attributed to belong to a spineland a perovskite-type compound. SEM micrographs confirm this finding: the sample sintered at 1200 1C consists of a large-grained, but homogeneous and single-phase microstructure (inset of Fig. 6). The microstructure of the sample sintered at 950 1C is composed of smaller grains. The formation of coexisting phases (dark gray grains and small white spots) embedded in a matrix Co2Y phase is clearly visible. EDX analysis suggests that the small white spots are rich in Bi (sintering additive) and the dark gray regions are composed of Co, Zn, and Fe cations mainly (corresponding to the observed spinel phase). This observation suggests that the substituted Co2Y phase is thermodynamically unstable at temperatures Tr950 1C. This partial decomposition for the ferrite samples sintered at 950 1C is an additional reason of the significant reduction of permeability. Similar behavior was already reported for Z-type ferrites, which were sintered to single-phase material in the temperature window of 1300–1330 1C; at the sintering temperature of 900 or 950 1C the Co2Z ferrites decompose into a mixture of Y- and M-type ferrites, which is accompanied by a reduction of permeability [11]. On the other hand, the SEM micrograph (Fig. 6) shows that ferrite grains grow larger as the sintering temperature increases. This enables the magnetic domains to move more easily and as a result, the permeability increases and the cut-off frequency shifts toward the MHz range.
Substituted Co2Y ferrites of composition Ba2Zn2 x yCuyFe12O22 (0rxr2 and y¼0, 0.4, and 0.8) are synthesized by the mixed-oxide route at 1000 1C. Sintering at 1200 1C gives dense samples with large grains and a permeability increasing with the Zn-concentration x. This effect was shown to be originated by the saturation magnetization increasing with x. Dense and fine-grained ferrites were sintered at 950 1C using an appropriate concentration of Bi2O3 sintering aid. Substitution of Zn for Co increases the initial permeability. It was shown that Y-type ferrites can be used for the fabrication of highfrequency multilayer inductors. However, Y-type ferrites show a tendency of phase transformation during sintering at 950 1C. As a consequence, a lower permeability is measured. Nevertheless, these novel substituted low-temperature sintered Co2Y ferrites exhibit interesting high-frequency properties, e.g. permeability of up to m¼10 and high cut-off frequency above 1 GHz. Integration of these ferrites into LTCC multilayer modules requires further investigation and adaptation of the ferrite materials. One crucial point is that the firing temperature has to be further reduced down to 900 1C in order to limit the diffusivity of the Ag metallization. A study of the fabrication, co-firing, and design of multilayer modules based on substituted Co2Y ferrites is in progress.
Acknowledgments ¨ The authors thank Mrs. R. Lohnert for help in the preparation of ferrite samples. This work was supported by the Bundesminis¨ Bildung und Forschung (Germany) in the research terium fur project HF-Multifer (1762X05) and the State of Thuringia through the Pro-Exzellenz network (Kerfunmat PE214). References [1] J. Smit, H.P.J. Wijn, Ferrites, Philips Technical Library, Eindhoven, 1959. p. 285. [2] M. Sugimoto, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials, vol. 3, 1982, p. 394. [3] P.B. Braun, Philips Research Reports 12 (1957) 491. [4] W.D. Townes, J.H. Fang, Zeitschrift Fur Kristallographie 131 (1970) 196–205. [5] G. Albanese, M. Carbucicicchio, A. Deriu, G. Astri, S. Rinaldi, Applied Physics 7 (1975) 227–238. [6] A. Collomb, M.A. Hadj Farhat, J.C. Joubert, Materials Research Bulletin 24 (1989) 453–458. [7] A. Deriu, F. Licci, S. Rinaldi, T. Besagni, Journal of Magnetism and Magnetic Materials 22 (1981) 257–262. [8] A. Collomb, J. Muller, J.C. Guitel, J.M. Desvignes, Journal of Magnetism and Magnetic Materials 78 (1989) 77–84. [9] S.G. Lee, S.J. Kwon, Journal of Magnetism and Magnetic Materials 153 (1996) 279. [10] M. Endo, A. Nakano, in: Proceedings of the Eighth International Conference on Ferrites, ICF8, Kyoto and Tokyo, Japan, 2000, pp. 1168–1170. ¨ [11] S. Kracˇunovska´, J. Topfer, Journal of Magnetism and Magnetic Materials 320 (2008) 1370–1376. ˇ ´ ¨ [12] S. Kracunovska, J. Topfer, Journal of Materials Science: Materials in Electronics 22 (2011) 467–473. [13] Y. Bai, J. Zhou, Z. Gui, L. Li, Journal of Magnetism and Magnetic Materials 246 (2002) 140–144. [14] H.S. Shin, S.J. Kwon, Powder Diffraction 8 (1993) 98–101. [15] A. Daigle, E. DuPre, A. Geiler, Y. Chen, P.V. Parimi, C. Vittoria, V.G. Harris, Journal of the American Ceramic Society 93 (2010) 2994–2997. [16] R.D. Shannon, Acta Crystallographica A32 (1976) 751. [17] T. Nakamura, K. Hatakeyama, IEEE Transactions on Magnetics 36 (2000) 3415–3417. [18] Y. Bai, J. Zhou, Z. Gui, L. Li, Materials Letters 58 (2004) 1602–1606.