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Crystallographic and magnetic properties of Cu2U-type hexaferrite
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Crystallographic and magnetic properties of Cu2U-type hexaferrite K. Kamishima a, R. Tajima a, K. Watanabe a, K. Kakizaki a, A. Fujimori a, M. Sakai a, K. Watanabe b, H. Abe c a
Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Saitama 338-8570, Japan Global Research Cluster, Collaboration Promotion Unit, RIKEN, 2-1 Wako, Saitama 351-0198, Japan c Advanced Electronic Materials Center, National Institute of Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan b
art ic l e i nf o
a b s t r a c t
Article history: Received 9 September 2014 Received in revised form 27 September 2014
We have investigated the synthesis conditions, and the magnetic properties of the Cu2U-type hexagonal ferrite, Ba4Cu2Fe36O60. The Cu2U-type hexaferrite was synthesized at the sintering temperature of 1050 °C with the initial composition of Ba4.4Cu2Fe37.6O62.8 (Cu2Uþ 0.2T-block). The saturation magnetizations at 300 K and 5 K are 46.8 A m2/kg and 65.0 A m2/kg, respectively. The Curie temperature is 420 °C which is lower than that of the M-type ferrite (450 °C) but higher than that of the Cu2Y-type ferrite (320 °C). The amount of the nonmagnetic impurity in this sample is estimated to be about 10 wt% by the electron probe micro analysis. We estimated the expected saturation magnetization to be 65.2 A m2/kg, by assuming the model of a Néel-type ferrimagnetic structure and the reduction of magnetization by the 10 wt% nonmagnetic impurity. This is consistent with the observed magnetization of 65.0 A m2/kg at 5 K. & 2014 Published by Elsevier B.V.
Keywords: U-type hexaferrite Crystal structure
1. Introduction Ferrimagnetic iron oxides (ferrites) have attracted great attention for decades because of their practical applications and low cost performance. Most of them are electrical insulators which are suitable for applications at high frequencies [1]. An oscillating magnetic field induces a voltage and then produces eddy currents which give resistive heating as energy loss. Electrical insulating properties reduce the eddy currents and the energy loss. Thus, ferrites are useful when a material with a spontaneous magnetization is desired to work even at high frequencies. The high frequency applications include antennas and transformers that need high permeability and low energy loss. The crystal structure of ferrites is a close-packed framework of large ions with intervening small ions. The large ions are oxygen anions (O2 ions) and heavy alkaline earth metal cations (Ba2 þ and Sr2 þ ions), which have the radii of 0.13–0.14 nm. The small ions are 3d transition metal cations (Fe3 þ and Me2 þ ions; Me ¼Zn, Mn, Fe, Co, Ni, or Cu), which have the radii of 0.06–0.08 nm. This close-packed framework has an oxygen layer where each O2 ion is surrounded by six other O2 ions. Another close-packed large ion layer has a heavy alkaline earth metal cation by which every fourth O2 ion is replaced. This Ba–O type layer gives an ordered 1:3 ratio of alkaline earth metal cation to oxygen anion. The hexagonal close-packed structure can be regarded as the pile of three kinds of blocks: S-block, R-block, and T-block. The basic S-block consists of two oxygen layers with a chemical formula of 2MeFe2O4 (spinel) where Me is a divalent transition
metal cation. So, the cubic spinel crystal can be formed if only the S-blocks are piled up. There are three transition metal cations in the S-block. One is in an octahedral coordination with six oxygen ligands, and the others are in a tetrahedral coordination with four oxygen ligands. The R-block (BaFe6O11) has a central Ba–O type layer that is sandwiched between two oxygen layers. There are three transition metal cations in the R-block. One of the transition metal cations just on the Ba–O type layer has a trigonal bipyramidal site with five oxygen ligands, which can cause magnetic anisotropy in the crystal. The other two cations are situated at octahedral sites. The T-block (Ba2Fe8O14) is made up from four layers, where two adjacent Ba–O type layers are sandwiched between two oxygen layers. There are five transition metal cations in the T-block. Two of them are in a tetrahedral coordination, and the others are in an octahedral coordination. Three transition metal cations among these S-, R-, and T-blocks form octahedral sites so as to magnetically connect each other blocks. The S-, Rand T-blocks are stacked up and form a hexagonal ferrite, whose structure depends on the combination of S-, R-, and T-blocks [2]. Kohn et al. pointed out the great pliability of the freedom of this stacking [3]. The major hexagonal ferrite structures are summarized as M-type, Y-type, W-type, Z-type, X-type, and U-type, as shown in Table 1. The U-type ferrite with a chemical formula of Ba4Me2Fe36O60 was discovered 50 years ago. Savage and Tauber found the U-type phase for Me¼Zn, Ni, and Co by surveying the system of NaFeO2–Fe2O3–BaO--MeO in platinum crucibles [4]. However, reproducible synthesis conditions were not identified because
http://dx.doi.org/10.1016/j.jmmm.2014.09.055 0304-8853/& 2014 Published by Elsevier B.V.
Please cite this article as: K. Kamishima, et al., Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.09.055i
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Table 1 Chemical formula and unit cell structure of hexagonal ferrite. Type Chemical formulaa
Unit cell structureb Number of molecules in a unit cell
M
BaO·6Fe2 O3
2BaO ·2MeO ·6Fe2O3
RSR⁎S⁎ (TS)3
2
Y W
BaO ·2MeO ·8Fe2O3
2
Z
3BaO ·2MeO ·12Fe2O3
RSSR⁎S⁎S⁎ RSTSRnSnTnSn
X
n n 2BaO ·2MeO ·14Fe2O3 (RSSR S )3 n n 4BaO ·2MeO ·18Fe2O3 (RSTSR S )3
U
3 2 3 3
a
Me is basically a 3d transition metal element. The S, R and T blocks are shown in Fig. 1. An asterisk denotes that the layer is turned for 180° around the c-axis. b
the most pieces of U-type single crystal were intergrown with Z-type or Y-type phases [5]. Decades later, in the middle of 2000s, Lisjak et al. reported reliable crystallographic X-ray diffraction data of polycrystalline samples of the U-type hexagonal ferrites [6,7]. They prepared polycrystalline samples of the U-type ferrite for Me¼ Co, Ni, and Zn by solid-state reaction, high energy milling, and chemical coprecipitation methods. It seems that the investigation of the U-type hexaferrites started again from this study. Okumura et al. reported on structural, magnetic, and magnetoelectric properties of one of the Sr-based U-type hexaferrite Sr4Co2Fe36O60 prepared by solid state reaction [8]. Honda et al. refined the crystal structure of this U-type hexaferrite from synchrotron X-ray diffraction measurements [9]. They summarized the refined atomic positions and bond valence sum on each Fe site. The synthesis and magnetic properties of the U-type ferrite for Me¼ Cu (Ba4Cu2Fe36O60) appeared in recent publications although this Cu2U-type hexaferrite had not been reported. Chandra Dimri et al. set out that the single-phase Cu2U-type hexaferrite had been synthesized and characterized by magnetization measurements and zero-field 57Fe nuclear magnetic resonance (NMR) spectroscopy [10]. Shannigrahi et al. described that polycrystalline samples of the Cu2U-type hexaferrite were synthesized using a modified solid state reaction technique [11]. The X-ray diffraction patterns of the Cu2U-type hexaferrite in these studies, however, are not consistent with each other. Characteristic Bragg peaks of each hexaferrite appear at relatively lower angles because of the long-range stacking structures as shown in Table 1. Fig. 1 shows the long unit cell of the U-type ferrite [9]. This complex structure of the U-type hexaferrite may cause difficulties to its synthesis. The aim of this work is to investigate the synthetic condition and the crystal structure of the Cu2U-type ferrite. Detailed X-ray diffraction and magnetic measurements on the prepared polycrystals of the U-type ferrite prepared have elucidated that the ferrite has a long unit cell along the c-axis.
2. Experimental procedure Samples of the Cu2U-type ferrite (Ba4Cu2Fe36O60) were prepared by a conventional ceramic method. We used BaCO3, CuO, and α -Fe2O3 as starting materials. They were mixed in a desired proportion, which is based on the above-mentioned chemical formula, in a conventional ball-milling pot for 24 h. The powder was ground for 10 min at the rates of 750 or 1000 rpm in a varioplanetary mill (Fritsch, Premium-line P-7). The processed powder was dried in air and then pressed into disks. The disks were sintered in air at 1000–1250 °C for 5 h. The crystal structure was examined by powder X-ray diffraction experiments using Cu-Kα radiation. Magnetization measurements were performed with a
Fig. 1. Unit cell of U-type ferrite prepared by the use of CRYSTAL MAKER software and the atomic co-ordinates in the unit cell [9].
vibrating sample magnetometer (Tamakawa TM-VSM2130HGC) and a commercial superconducting quantum interference device magnetometer (Quantum Design MPMS-XL). The chemical composition was examined by the use of a commercial electron probe micro analyzer – EPMA (JEOL, JXA-8200).
3. Experimental results Fig. 2 shows experimental X-ray diffraction patterns of the products sintered at 1150, 1200 and 1250 °C, whose starting powder was mixed at the Cu2U chemical composition (Ba:Cu: Fe¼ 2:1:18), and ground at 750 rpm. The diffraction peaks are in agreement of those of the M- and Y-type ferrites [12,13]. The peaks indexed with the Y-type ferrite are very weak in contrast with those of the M-type ferrite. We also observed that part of these sintered samples encroached on the sample holder in the furnace. The melting parts are possibly Cu2Y-type ferrite ((TS)3 stacking structure). It is consistent that Cu2Y-type ferrite can be synthesized below 1100 °C as shown in Fig. 3. Decomposition of the Cu2Y sample started at 1100 °C because of the observation of the M-type diffraction peak. Pieces of molten slag appeared on the surface of the Cu2Y disk sintered at 1200 °C as shown in the inset of Fig. 3. So, the sintering temperatures are likely to be too high to synthesize the Cu2U-type hexaferrite.
Please cite this article as: K. Kamishima, et al., Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.09.055i
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Fig. 2. X-ray diffraction patterns of the products sintered at 1150, 1200 and 1250 °C, whose starting powder was mixed at the Cu2U chemical composition (Ba:Cu: Fe¼ 2:1:18).
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Fig. 4. X-ray diffraction patterns of the products sintered at 1000, 1050 and 1100 °C, whose starting powder was mixed at the Cu2U chemical composition (Ba:Cu: Fe¼ 2:1:18).
Fig. 5. Saturation magnetization and coercivity of the products sintered at 1000– 1250 °C, whose starting powder was mixed at the Cu2U chemical composition (Ba: Cu:Fe¼ 2:1:18).
Fig. 3. X-ray diffraction patterns of the products sintered up to 1200 °C, whose starting powder was mixed at the Cu2Y chemical composition (Ba:Cu:Fe¼ 1:1:6). The sample sintered at 1200 °C is partly molten as shown in the inset of this figure.
Fig. 4 shows the X-ray diffraction patterns of the Cu2U composition samples (Ba:Cu:Fe¼2:1:18) sintered at 1000, 1050 and 1100 °C. The low angle diffraction peak of the U-type structure at Q¼1.66(5) Å 1, corresponding to the expected (0 0 30) reflection, appears for these samples although the M-type and Y-type diffraction peaks also coexist. Fig. 5 shows the variations in the room-temperature saturation magnetization and coercive force of the Cu2U composition samples (Ba:Cu:Fe¼2:1:18) sintered at 1000–1250 °C. The sintering temperature dependences of saturation magnetization and coercivity are slight in contrast with the difference of the crystallographic phase in Figs. 2 and 4. The saturation magnetization
Please cite this article as: K. Kamishima, et al., Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.09.055i
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Fig. 6. X-ray diffraction patterns of the Cu2Uþ 0.2T-block Ba4 þ 2xCu2Fe36 þ 8xO60 þ 14x with x¼ 0.2 sintered at 1050 °C.
samples
of
suggests that the samples sintered above 1150 °C contain components with small magnetization that are possibly Cu2Y-type ferrite or amorphous BaFe2O4. Low-melting-point polyanionic materials such as phosphates, tungstates and/or borates, when melted at elevated temperatures, efficiently work as solvent and/or a flux to synthesize thermally unstable oxides [14] . BaFe4O7, which is identical to the chemical formula of the T-block, can be synthesized as an individual compound by the hydrothermal synthesis method below 300 °C [15–17]. This T-block composition powder mixture tends to become a glass-like product when it is sintered by conventional solid state reaction. In order to promote formation of the desired Cu2U-type hexaferrite phase, an excess of BaFe4O7 (T-block phase) was added to the reaction system, because the T-block phase is likely to have a lower melting point than the Cu2U-type phase, acting as an efficient flux. Fig. 6 shows the X-ray diffraction patterns of the samples of Ba 4 + 2x Cu2 Fe36 + 8x O60 + 14x with x ¼0.2 (Cu2Uþ 0.2T-block) sintered at 1050 °C. The diffraction peaks from the M-type ferrite depend on the initial milling conditions. The M-type peaks disappear for the sample without the use of the planetary ball milling, showing that we have successfully obtained the targeted U-type phase containing no other ferrites. This result seems to contradict with our intuition that the small grains of starting materials would help the formation of the desired compounds in solid state reaction. However, the sintering temperature of 1050 °C is close to the above-mentioned decomposing point of the Cu2Y-type ferrite around 1100 °C. This proximity might lead Cu2Y oxides to flow out from the sample when the starting materials were crushed into too fine powder. The experimental peak positions for the sample (Cu2U þ0.2T-block; no milling; sintered at 1050 °C) permit us to calculate the lattice constants as a ¼5.883 Å and c¼ 113.2 Å by the use of Cohen's method of least squares [18]. The EPMA analysis for 100 points on the sample (Cu2Uþ 0.2T-block; no milling; sintered at 1050 °C) demonstrated the average atomic
Fig. 7. Magnetization curves of the Cu2Uþ 0.2T-block and Cu2Y samples at T ¼ 5 and 300 K. The inset shows the M–T curves of the Cu2Uþ 0.2T-block sample and the Cu2Y sample at μ0 H = 0.1 T . The Cu2Y sample was prepared at 1000 °C (see Fig. 3).
ratio as Cu:Ba:Fe¼2:5.5:38.6, although the starting composition was Cu:Ba:Fe¼2:4.4:37.6. The excess of Ba and Fe is likely to be due to the amorphous form of BaFe2O4. This result suggests that the amount of this nonmagnetic impurity is about 10 wt% in this sample. Fig. 7 shows the magnetization curves of the synthesized Cu2U-type and Cu2Y-type ferrites at T ¼5 and 300 K. The saturation magnetization MS of the Cu2U-type ferrite at 300 K is 46.8 A m2/kg. This is much smaller than that of the M-type ferrite (70 A m2/kg) but larger than that of the Cu2Y-type ferrite. We can also see this difference at T ¼5 K, where MS of the Cu2U-type ferrite is 65.0 A m2/kg. The M-type ferrite should show the saturation magnetic moment of 20 μ B/f.u. at low temperatures that corresponds to 100 A m2/kg. On the other hand, the magnetization curve of the Cu2Y-type ferrite at 5 K shows a relatively large high-field-susceptibility above 2 T, suggesting that the saturation field of the Cu2Y-type ferrite is higher than that of the Cu2U-type ferrite. These facts demonstrate that the alignment of magnetic moments in the Cu2U-type ferrite is much different from those in the M-type and the Cu2Y-type ferrite. The inset of Fig. 7 shows the temperature dependence of magnetization at high temperatures. The Curie temperature is 420 °C which is lower than that of the M-type ferrite (450 °C) but higher than that of the Cu2Y ferrite (320 °C).
4. Discussion First, we would like to untangle the discrepancy between the experimental diffraction patterns for Cu2U-type ferrite reported previously. Fig. 8 shows the X-ray diffraction patterns of the (a) M-type [12], (b) U-type [9], (c) W-type [19], (d) X-type [20], (e) Y-type [13], and (f) Z-type [21] hexagonal ferrites. The diffraction patterns (b) and (d) were reproduced by the use of CRYSTAL MAKER and CRYSTAL DIFFRACT software. These patterns reflect the stacking
Please cite this article as: K. Kamishima, et al., Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.09.055i
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Fig. 8. X-ray diffraction patterns of the (a) M-type, (b) U-type, (c) W-type, (d) X-type, (e) Y-type, and (f) Z-type hexagonal ferrites.
combination of the S-, R-, and T-blocks, which enables us to perform qualitative identification of hexagonal ferrites. The peaks −1 at relatively lower Q ( < 2.0 A˚ ) (lower diffraction angle) are good indicators of these hexagonal ferrite phases. The stacking period of the blocks along the c-axis is long and so the characteristic peaks tend to appear at lower Q. Furthermore, the formation of hexagonal-basal-shaped grains is apt to enhance these peaks in experiments. The phase identification with the M-type hexaferrite is indispensable because the formation of the M-type hexaferrite is very easy in contrast with those of other hexaferrites. The peak at Q¼1.62(4) Å 1 (2θ ¼ 23.0° for Cu-Kα radiation) is the (0 0 6) peak of M-type hexaferrite at room temperature. On the other hand, the U-type (0 0 30) peak appears at Q¼1.66(5) Å 1 (2θ ¼23.6° for CuKα radiation). Fig. 9(a) shows the reference pattern of the typical M-type hexaferrite, BaFe12O19. This pattern is consistent with (b) the X-ray diffraction pattern of Cu2U hexaferrite reported by Chandra Dimri et al. [10]. The saturation magnetization at 300 K and the Curie temperature of their Cu2U hexaferrite are 70 A m2/kg and 452 °C, respectively. These values are almost same as those of the M-type ferrite (MS (R. T. ) ≃ 70 A m2/kg and TC = 450 °C for BaFe12O19). Their Cu2U hexaferrite had a relatively low coercivity of 300 Oe (30.0 mT) for the M-type hexaferrite, which led to a conclusion that their material consisted of the Cu2U-type phase. The coercivity, however, does not depend only on the chemical composition but also on the state of preparation of the specimen [22–24]. The magnetic dipolar interactions in ferromagnets are changed by various factors resulted from the sintering process, such as porosity, the size and shape of the pores/grains. Fig. 9(c) and (d) shows the reference patterns of the U-type and the Z-type hexaferrites, respectively. The Z-type hexaferrite can be formed at relatively high temperatures because the block-stacking
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Fig. 9. X-ray diffraction patterns of the (a) M-type, (b) “Cu2U” in Ref. [10], (c) U-type, (d) Z-type, and (e) “Cu2U” in Ref. [11].
structures, (RSTSRnSn)3 for the U-type and RSTSRnSnTnSn for the Z-type, are similar to each other. Fig. 9(e) shows the X-ray diffraction pattern of Cu2U hexaferrite prepared at 1275 °C by Shannigrahi et al. [11]. There is no low angle peaks of the U-type phase. This seems to be the mixed phase of Z-type hexaferrite and others such as X-type hexaferrite because of the existence of the X-type characteristic peaks at Q¼ 2.24 and 2.30 Å 1 [25]. Next, we would like to discuss the magnetic structure of the U-type ferrite which consists of the R-, T- and S-blocks as shown in Table 1. Table 2 shows the distribution of transition metal sites and the expected directions of magnetic moments in the R-, T- and S-blocks [2,21,26]. The magnetic moments at two octahedral sites in the T-block, all octahedral sites in the R-block, and all tetrahedral sites in the S- and T-blocks are in the downward direction. The others are in the upward direction, including the central octahedral site in the T-block, and the trigonal bipyramidal site in the R-block. Therefore, the number of the up-spin sites is expected to be 24(¼ 3(octa) 6(block-borders)þ 1(octa) 1(T-block)þ1(tribipyramid) 2(R-blocks) þ 1(octa) 3(S-blocks)) and the number of the down-spin sites is 14( ¼2(octa) 2(R-blocks) þ2(octa) 1 (T-block)þ 2(tetra) 1(T-block)þ2(tetra) 3(S-blocks)) with Table 2 Crystallographic and magnetic characteristics of the metallic sublattices in the U-type Ba4M2Fe36O60 hexagonal ferrite. Block
Coordination
Number per block
Expected spin direction
R R R–S S S S–S T T T–S
Octahedral Trigonal bipyramidal Octahedral Octahedral Tetrahedral Octahedral Octahedral Tetrahedral Octahedral
2 1 3 1 2 3 3 2 3
Down Up Up Up Down Up Up 1, down 2 Down Up
Please cite this article as: K. Kamishima, et al., Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.09.055i
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(tetra)) 1(T-block)þ(1(tri-bipyramid)-2(octa)) 2(R-blocks)]. Two copper cations are distributed in the three S-blocks of RSTSRnSn where the number of sites is 18 (¼ 3(octa) 3(blockborders) þ1(octa) 3(S-blocks) þ 2(tetra) 3(S-blocks)). The average moment in the three S-blocks is 4.55 μ B (16 5 μB (Fe3 þ ) and 2 1 μB (Cu2 þ )). Therefore, the contribution of the three S-blocks to the total moment is 27.3 μB ¼4.55 [3(octa) 3(blockborders) þ(1(octal)-2(tetra)) 3(S-blocks)]. The magnetic moment of 47.3 μB/f.u. corresponds to 72.5 A m2/kg. The magnetization of the synthesized U-type ferrite is reduced to 65.2 A m2/kg because the material contained 10 wt% of non-magnetic BaFeO. This estimated magnetization is consistent with the observed magnetization of 65.0 A m2/kg at 5 K.
5. Conclusions
Fig. 10. Spin alignment in the one-third of the unit cell of the U-type ferrite.
respect to the block stacking of the U-type ferrite (RSTSRnSn). The spin alignment in the one-third of the unit cell of the U-type ferrite is summarized in Fig. 10. The spin alignment in the Cu2Y-type ferrite (Ba2Cu2Fe12O22) can be discussed in the similar way. The number of the up-spin sites is expected to be 8( ¼3(octa) 2(block-borders)þ1(octa) 1 (T-block)þ 1(octa) 1(S-block)) and the number of the down-spin sites is 6( ¼2(octa) 1(T-block)þ 2(tetra) 1(T-block)þ2(tetra) 1(S-block)) with respect to the block stacking of the Y-type ferrite (TS). If y of the two Cu2 þ cations are located at down spin sites in the Y-type structure, the total magnetic moment of the Cu2Y-type ferrite becomes
MSCu2Y = 5 × (8 − (2 − y)) + 1 × (2 − y) − 5 × (6 − y) − 1 × y = 8y + 2 (μ B/f. u. ).
(1)
The magnetic moment of the Cu2Y-type ferrite is 7.06 μ B/f. u. (27.7 A m2/kg) at 5 K and 7 T. We can estimate from this value that Cu2 þ ions are located at down spin sites for 31.6% (y¼ 0.633) in the Cu2Y-type ferrite, assuming that the Cu2Y-type ferrite nearly has a collinear magnetic structure at 7 T. This ratio suggests that the copper cations are distributed randomly over the spin-up and the spin-down sites in the S-blocks (containing the block borders) in the Y-type structure. This random distribution is consistent with our previous study of the Cu2X-type ferrite because the ratio of the spin is down/(upþdown)¼1/3 both in the X-type structure (RSSRnSn) and in the S-blocks (containing the block borders) [25]. We can estimate the magnetic moment of Cu2U-type hexaferrite to be 47.3 μ B/f. u. by assuming the random distribution of two copper cations in the three S-blocks in the U-type ferrite structure (RSTSRnSn). The contribution of two R-blocks and one T-block in the U-type structure to the total magnetic moment is 20 μB ¼ 5(Fe3 þ ) [3(octa) 3(block-borders) þ(1(octa)-2(octa)-2
We have investigated the synthesis conditions and the magnetic properties of the Cu2U-type hexagonal ferrite, Ba4Cu2Fe36O60. The Cu2U-type hexaferrite was synthesized at the sintering temperature of 1050 °C with the initial composition of Ba 4 + 2x Cu2 Fe36 + 8x O60 + 14x with x ¼0.2 (Cu2Uþ 0.2T-block) without milling starting powder. The X-ray diffraction pattern of this Cu2U þ0.2T-block sample is similar to the pattern for the Sr-based U-type hexaferrite Sr4Co2Fe36O60 [9]. The saturation magnetization MS at 300 K is 46.8 A m2/kg, and MS at 5 K is 65.0 A m2/kg. The Curie temperature is 420 °C which is lower than that of the M-type ferrite (450 °C) but higher than that of the Cu2Y-type ferrite (320 °C). The EPMA analysis on this sample shows the average atomic ratio of Cu:Ba:Fe¼2:5.5:38.6, suggesting that the sample contained 10 wt% of non-magnetic impurity, BaFeO. We estimated the saturation magnetization to be 65.2 A m2/kg, by assuming a possible arrangement of the magnetic moments of Fe3 þ and Cu2 þ cations. This is consistent with the observed magnetization of 65.0 A m2/kg at 5 K.
Acknowledgment This work was supported by JSPS KAKENHI Grant number 24560005.
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[19] The Powder Diffraction File (PDF00-052-1868) by the International Center for Diffraction Data (ICDD); L. Fang, H. Zhang, R. Yuan, State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, PR China. ICDD Grant-in-Aid, 2001. [20] P.B. Braun, Philips Res. Rep. 12 (1957) 491. [21] S. Kawano, T. Shimada, T.A. Yamamoto, K. Izumi, Y. Takada, T. Nakagawa, T. Tachibana, J. Magn. Magn. Mater. 262 (2003) 248. [22] J. Smit, H.P. Wijn, Ferrites, vol. 301, John Wiley and Sons, 1959. [23] N.W. Ashcroft, H.D. Mermin, Solid State Physics, vol. 722, Saunders College, Philadelphia, 1976.
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Crystallographic and magnetic properties of Cu2U-type hexaferrite K. Kamishima a, R. Tajima a, K. Watanabe a, K. Kakizaki a, A. Fujimori a, M. Sakai a, K. Watanabe b, H. Abe c a
Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Saitama 338-8570, Japan Global Research Cluster, Collaboration Promotion Unit, RIKEN, 2-1 Wako, Saitama 351-0198, Japan c Advanced Electronic Materials Center, National Institute of Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan b
art ic l e i nf o
a b s t r a c t
Article history: Received 9 September 2014 Received in revised form 27 September 2014
We have investigated the synthesis conditions, and the magnetic properties of the Cu2U-type hexagonal ferrite, Ba4Cu2Fe36O60. The Cu2U-type hexaferrite was synthesized at the sintering temperature of 1050 °C with the initial composition of Ba4.4Cu2Fe37.6O62.8 (Cu2Uþ 0.2T-block). The saturation magnetizations at 300 K and 5 K are 46.8 A m2/kg and 65.0 A m2/kg, respectively. The Curie temperature is 420 °C which is lower than that of the M-type ferrite (450 °C) but higher than that of the Cu2Y-type ferrite (320 °C). The amount of the nonmagnetic impurity in this sample is estimated to be about 10 wt% by the electron probe micro analysis. We estimated the expected saturation magnetization to be 65.2 A m2/kg, by assuming the model of a Néel-type ferrimagnetic structure and the reduction of magnetization by the 10 wt% nonmagnetic impurity. This is consistent with the observed magnetization of 65.0 A m2/kg at 5 K. & 2014 Published by Elsevier B.V.
Keywords: U-type hexaferrite Crystal structure
1. Introduction Ferrimagnetic iron oxides (ferrites) have attracted great attention for decades because of their practical applications and low cost performance. Most of them are electrical insulators which are suitable for applications at high frequencies [1]. An oscillating magnetic field induces a voltage and then produces eddy currents which give resistive heating as energy loss. Electrical insulating properties reduce the eddy currents and the energy loss. Thus, ferrites are useful when a material with a spontaneous magnetization is desired to work even at high frequencies. The high frequency applications include antennas and transformers that need high permeability and low energy loss. The crystal structure of ferrites is a close-packed framework of large ions with intervening small ions. The large ions are oxygen anions (O2 ions) and heavy alkaline earth metal cations (Ba2 þ and Sr2 þ ions), which have the radii of 0.13–0.14 nm. The small ions are 3d transition metal cations (Fe3 þ and Me2 þ ions; Me ¼Zn, Mn, Fe, Co, Ni, or Cu), which have the radii of 0.06–0.08 nm. This close-packed framework has an oxygen layer where each O2 ion is surrounded by six other O2 ions. Another close-packed large ion layer has a heavy alkaline earth metal cation by which every fourth O2 ion is replaced. This Ba–O type layer gives an ordered 1:3 ratio of alkaline earth metal cation to oxygen anion. The hexagonal close-packed structure can be regarded as the pile of three kinds of blocks: S-block, R-block, and T-block. The basic S-block consists of two oxygen layers with a chemical formula of 2MeFe2O4 (spinel) where Me is a divalent transition
metal cation. So, the cubic spinel crystal can be formed if only the S-blocks are piled up. There are three transition metal cations in the S-block. One is in an octahedral coordination with six oxygen ligands, and the others are in a tetrahedral coordination with four oxygen ligands. The R-block (BaFe6O11) has a central Ba–O type layer that is sandwiched between two oxygen layers. There are three transition metal cations in the R-block. One of the transition metal cations just on the Ba–O type layer has a trigonal bipyramidal site with five oxygen ligands, which can cause magnetic anisotropy in the crystal. The other two cations are situated at octahedral sites. The T-block (Ba2Fe8O14) is made up from four layers, where two adjacent Ba–O type layers are sandwiched between two oxygen layers. There are five transition metal cations in the T-block. Two of them are in a tetrahedral coordination, and the others are in an octahedral coordination. Three transition metal cations among these S-, R-, and T-blocks form octahedral sites so as to magnetically connect each other blocks. The S-, Rand T-blocks are stacked up and form a hexagonal ferrite, whose structure depends on the combination of S-, R-, and T-blocks [2]. Kohn et al. pointed out the great pliability of the freedom of this stacking [3]. The major hexagonal ferrite structures are summarized as M-type, Y-type, W-type, Z-type, X-type, and U-type, as shown in Table 1. The U-type ferrite with a chemical formula of Ba4Me2Fe36O60 was discovered 50 years ago. Savage and Tauber found the U-type phase for Me¼Zn, Ni, and Co by surveying the system of NaFeO2–Fe2O3–BaO--MeO in platinum crucibles [4]. However, reproducible synthesis conditions were not identified because
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Table 1 Chemical formula and unit cell structure of hexagonal ferrite. Type Chemical formulaa
Unit cell structureb Number of molecules in a unit cell
M
BaO·6Fe2 O3
2BaO ·2MeO ·6Fe2O3
RSR⁎S⁎ (TS)3
2
Y W
BaO ·2MeO ·8Fe2O3
2
Z
3BaO ·2MeO ·12Fe2O3
RSSR⁎S⁎S⁎ RSTSRnSnTnSn
X
n n 2BaO ·2MeO ·14Fe2O3 (RSSR S )3 n n 4BaO ·2MeO ·18Fe2O3 (RSTSR S )3
U
3 2 3 3
a
Me is basically a 3d transition metal element. The S, R and T blocks are shown in Fig. 1. An asterisk denotes that the layer is turned for 180° around the c-axis. b
the most pieces of U-type single crystal were intergrown with Z-type or Y-type phases [5]. Decades later, in the middle of 2000s, Lisjak et al. reported reliable crystallographic X-ray diffraction data of polycrystalline samples of the U-type hexagonal ferrites [6,7]. They prepared polycrystalline samples of the U-type ferrite for Me¼ Co, Ni, and Zn by solid-state reaction, high energy milling, and chemical coprecipitation methods. It seems that the investigation of the U-type hexaferrites started again from this study. Okumura et al. reported on structural, magnetic, and magnetoelectric properties of one of the Sr-based U-type hexaferrite Sr4Co2Fe36O60 prepared by solid state reaction [8]. Honda et al. refined the crystal structure of this U-type hexaferrite from synchrotron X-ray diffraction measurements [9]. They summarized the refined atomic positions and bond valence sum on each Fe site. The synthesis and magnetic properties of the U-type ferrite for Me¼ Cu (Ba4Cu2Fe36O60) appeared in recent publications although this Cu2U-type hexaferrite had not been reported. Chandra Dimri et al. set out that the single-phase Cu2U-type hexaferrite had been synthesized and characterized by magnetization measurements and zero-field 57Fe nuclear magnetic resonance (NMR) spectroscopy [10]. Shannigrahi et al. described that polycrystalline samples of the Cu2U-type hexaferrite were synthesized using a modified solid state reaction technique [11]. The X-ray diffraction patterns of the Cu2U-type hexaferrite in these studies, however, are not consistent with each other. Characteristic Bragg peaks of each hexaferrite appear at relatively lower angles because of the long-range stacking structures as shown in Table 1. Fig. 1 shows the long unit cell of the U-type ferrite [9]. This complex structure of the U-type hexaferrite may cause difficulties to its synthesis. The aim of this work is to investigate the synthetic condition and the crystal structure of the Cu2U-type ferrite. Detailed X-ray diffraction and magnetic measurements on the prepared polycrystals of the U-type ferrite prepared have elucidated that the ferrite has a long unit cell along the c-axis.
2. Experimental procedure Samples of the Cu2U-type ferrite (Ba4Cu2Fe36O60) were prepared by a conventional ceramic method. We used BaCO3, CuO, and α -Fe2O3 as starting materials. They were mixed in a desired proportion, which is based on the above-mentioned chemical formula, in a conventional ball-milling pot for 24 h. The powder was ground for 10 min at the rates of 750 or 1000 rpm in a varioplanetary mill (Fritsch, Premium-line P-7). The processed powder was dried in air and then pressed into disks. The disks were sintered in air at 1000–1250 °C for 5 h. The crystal structure was examined by powder X-ray diffraction experiments using Cu-Kα radiation. Magnetization measurements were performed with a
Fig. 1. Unit cell of U-type ferrite prepared by the use of CRYSTAL MAKER software and the atomic co-ordinates in the unit cell [9].
vibrating sample magnetometer (Tamakawa TM-VSM2130HGC) and a commercial superconducting quantum interference device magnetometer (Quantum Design MPMS-XL). The chemical composition was examined by the use of a commercial electron probe micro analyzer – EPMA (JEOL, JXA-8200).
3. Experimental results Fig. 2 shows experimental X-ray diffraction patterns of the products sintered at 1150, 1200 and 1250 °C, whose starting powder was mixed at the Cu2U chemical composition (Ba:Cu: Fe¼ 2:1:18), and ground at 750 rpm. The diffraction peaks are in agreement of those of the M- and Y-type ferrites [12,13]. The peaks indexed with the Y-type ferrite are very weak in contrast with those of the M-type ferrite. We also observed that part of these sintered samples encroached on the sample holder in the furnace. The melting parts are possibly Cu2Y-type ferrite ((TS)3 stacking structure). It is consistent that Cu2Y-type ferrite can be synthesized below 1100 °C as shown in Fig. 3. Decomposition of the Cu2Y sample started at 1100 °C because of the observation of the M-type diffraction peak. Pieces of molten slag appeared on the surface of the Cu2Y disk sintered at 1200 °C as shown in the inset of Fig. 3. So, the sintering temperatures are likely to be too high to synthesize the Cu2U-type hexaferrite.
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Fig. 2. X-ray diffraction patterns of the products sintered at 1150, 1200 and 1250 °C, whose starting powder was mixed at the Cu2U chemical composition (Ba:Cu: Fe¼ 2:1:18).
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Fig. 4. X-ray diffraction patterns of the products sintered at 1000, 1050 and 1100 °C, whose starting powder was mixed at the Cu2U chemical composition (Ba:Cu: Fe¼ 2:1:18).
Fig. 5. Saturation magnetization and coercivity of the products sintered at 1000– 1250 °C, whose starting powder was mixed at the Cu2U chemical composition (Ba: Cu:Fe¼ 2:1:18).
Fig. 3. X-ray diffraction patterns of the products sintered up to 1200 °C, whose starting powder was mixed at the Cu2Y chemical composition (Ba:Cu:Fe¼ 1:1:6). The sample sintered at 1200 °C is partly molten as shown in the inset of this figure.
Fig. 4 shows the X-ray diffraction patterns of the Cu2U composition samples (Ba:Cu:Fe¼2:1:18) sintered at 1000, 1050 and 1100 °C. The low angle diffraction peak of the U-type structure at Q¼1.66(5) Å 1, corresponding to the expected (0 0 30) reflection, appears for these samples although the M-type and Y-type diffraction peaks also coexist. Fig. 5 shows the variations in the room-temperature saturation magnetization and coercive force of the Cu2U composition samples (Ba:Cu:Fe¼2:1:18) sintered at 1000–1250 °C. The sintering temperature dependences of saturation magnetization and coercivity are slight in contrast with the difference of the crystallographic phase in Figs. 2 and 4. The saturation magnetization
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Fig. 6. X-ray diffraction patterns of the Cu2Uþ 0.2T-block Ba4 þ 2xCu2Fe36 þ 8xO60 þ 14x with x¼ 0.2 sintered at 1050 °C.
samples
of
suggests that the samples sintered above 1150 °C contain components with small magnetization that are possibly Cu2Y-type ferrite or amorphous BaFe2O4. Low-melting-point polyanionic materials such as phosphates, tungstates and/or borates, when melted at elevated temperatures, efficiently work as solvent and/or a flux to synthesize thermally unstable oxides [14] . BaFe4O7, which is identical to the chemical formula of the T-block, can be synthesized as an individual compound by the hydrothermal synthesis method below 300 °C [15–17]. This T-block composition powder mixture tends to become a glass-like product when it is sintered by conventional solid state reaction. In order to promote formation of the desired Cu2U-type hexaferrite phase, an excess of BaFe4O7 (T-block phase) was added to the reaction system, because the T-block phase is likely to have a lower melting point than the Cu2U-type phase, acting as an efficient flux. Fig. 6 shows the X-ray diffraction patterns of the samples of Ba 4 + 2x Cu2 Fe36 + 8x O60 + 14x with x ¼0.2 (Cu2Uþ 0.2T-block) sintered at 1050 °C. The diffraction peaks from the M-type ferrite depend on the initial milling conditions. The M-type peaks disappear for the sample without the use of the planetary ball milling, showing that we have successfully obtained the targeted U-type phase containing no other ferrites. This result seems to contradict with our intuition that the small grains of starting materials would help the formation of the desired compounds in solid state reaction. However, the sintering temperature of 1050 °C is close to the above-mentioned decomposing point of the Cu2Y-type ferrite around 1100 °C. This proximity might lead Cu2Y oxides to flow out from the sample when the starting materials were crushed into too fine powder. The experimental peak positions for the sample (Cu2U þ0.2T-block; no milling; sintered at 1050 °C) permit us to calculate the lattice constants as a ¼5.883 Å and c¼ 113.2 Å by the use of Cohen's method of least squares [18]. The EPMA analysis for 100 points on the sample (Cu2Uþ 0.2T-block; no milling; sintered at 1050 °C) demonstrated the average atomic
Fig. 7. Magnetization curves of the Cu2Uþ 0.2T-block and Cu2Y samples at T ¼ 5 and 300 K. The inset shows the M–T curves of the Cu2Uþ 0.2T-block sample and the Cu2Y sample at μ0 H = 0.1 T . The Cu2Y sample was prepared at 1000 °C (see Fig. 3).
ratio as Cu:Ba:Fe¼2:5.5:38.6, although the starting composition was Cu:Ba:Fe¼2:4.4:37.6. The excess of Ba and Fe is likely to be due to the amorphous form of BaFe2O4. This result suggests that the amount of this nonmagnetic impurity is about 10 wt% in this sample. Fig. 7 shows the magnetization curves of the synthesized Cu2U-type and Cu2Y-type ferrites at T ¼5 and 300 K. The saturation magnetization MS of the Cu2U-type ferrite at 300 K is 46.8 A m2/kg. This is much smaller than that of the M-type ferrite (70 A m2/kg) but larger than that of the Cu2Y-type ferrite. We can also see this difference at T ¼5 K, where MS of the Cu2U-type ferrite is 65.0 A m2/kg. The M-type ferrite should show the saturation magnetic moment of 20 μ B/f.u. at low temperatures that corresponds to 100 A m2/kg. On the other hand, the magnetization curve of the Cu2Y-type ferrite at 5 K shows a relatively large high-field-susceptibility above 2 T, suggesting that the saturation field of the Cu2Y-type ferrite is higher than that of the Cu2U-type ferrite. These facts demonstrate that the alignment of magnetic moments in the Cu2U-type ferrite is much different from those in the M-type and the Cu2Y-type ferrite. The inset of Fig. 7 shows the temperature dependence of magnetization at high temperatures. The Curie temperature is 420 °C which is lower than that of the M-type ferrite (450 °C) but higher than that of the Cu2Y ferrite (320 °C).
4. Discussion First, we would like to untangle the discrepancy between the experimental diffraction patterns for Cu2U-type ferrite reported previously. Fig. 8 shows the X-ray diffraction patterns of the (a) M-type [12], (b) U-type [9], (c) W-type [19], (d) X-type [20], (e) Y-type [13], and (f) Z-type [21] hexagonal ferrites. The diffraction patterns (b) and (d) were reproduced by the use of CRYSTAL MAKER and CRYSTAL DIFFRACT software. These patterns reflect the stacking
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Fig. 8. X-ray diffraction patterns of the (a) M-type, (b) U-type, (c) W-type, (d) X-type, (e) Y-type, and (f) Z-type hexagonal ferrites.
combination of the S-, R-, and T-blocks, which enables us to perform qualitative identification of hexagonal ferrites. The peaks −1 at relatively lower Q ( < 2.0 A˚ ) (lower diffraction angle) are good indicators of these hexagonal ferrite phases. The stacking period of the blocks along the c-axis is long and so the characteristic peaks tend to appear at lower Q. Furthermore, the formation of hexagonal-basal-shaped grains is apt to enhance these peaks in experiments. The phase identification with the M-type hexaferrite is indispensable because the formation of the M-type hexaferrite is very easy in contrast with those of other hexaferrites. The peak at Q¼1.62(4) Å 1 (2θ ¼ 23.0° for Cu-Kα radiation) is the (0 0 6) peak of M-type hexaferrite at room temperature. On the other hand, the U-type (0 0 30) peak appears at Q¼1.66(5) Å 1 (2θ ¼23.6° for CuKα radiation). Fig. 9(a) shows the reference pattern of the typical M-type hexaferrite, BaFe12O19. This pattern is consistent with (b) the X-ray diffraction pattern of Cu2U hexaferrite reported by Chandra Dimri et al. [10]. The saturation magnetization at 300 K and the Curie temperature of their Cu2U hexaferrite are 70 A m2/kg and 452 °C, respectively. These values are almost same as those of the M-type ferrite (MS (R. T. ) ≃ 70 A m2/kg and TC = 450 °C for BaFe12O19). Their Cu2U hexaferrite had a relatively low coercivity of 300 Oe (30.0 mT) for the M-type hexaferrite, which led to a conclusion that their material consisted of the Cu2U-type phase. The coercivity, however, does not depend only on the chemical composition but also on the state of preparation of the specimen [22–24]. The magnetic dipolar interactions in ferromagnets are changed by various factors resulted from the sintering process, such as porosity, the size and shape of the pores/grains. Fig. 9(c) and (d) shows the reference patterns of the U-type and the Z-type hexaferrites, respectively. The Z-type hexaferrite can be formed at relatively high temperatures because the block-stacking
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Fig. 9. X-ray diffraction patterns of the (a) M-type, (b) “Cu2U” in Ref. [10], (c) U-type, (d) Z-type, and (e) “Cu2U” in Ref. [11].
structures, (RSTSRnSn)3 for the U-type and RSTSRnSnTnSn for the Z-type, are similar to each other. Fig. 9(e) shows the X-ray diffraction pattern of Cu2U hexaferrite prepared at 1275 °C by Shannigrahi et al. [11]. There is no low angle peaks of the U-type phase. This seems to be the mixed phase of Z-type hexaferrite and others such as X-type hexaferrite because of the existence of the X-type characteristic peaks at Q¼ 2.24 and 2.30 Å 1 [25]. Next, we would like to discuss the magnetic structure of the U-type ferrite which consists of the R-, T- and S-blocks as shown in Table 1. Table 2 shows the distribution of transition metal sites and the expected directions of magnetic moments in the R-, T- and S-blocks [2,21,26]. The magnetic moments at two octahedral sites in the T-block, all octahedral sites in the R-block, and all tetrahedral sites in the S- and T-blocks are in the downward direction. The others are in the upward direction, including the central octahedral site in the T-block, and the trigonal bipyramidal site in the R-block. Therefore, the number of the up-spin sites is expected to be 24(¼ 3(octa) 6(block-borders)þ 1(octa) 1(T-block)þ1(tribipyramid) 2(R-blocks) þ 1(octa) 3(S-blocks)) and the number of the down-spin sites is 14( ¼2(octa) 2(R-blocks) þ2(octa) 1 (T-block)þ 2(tetra) 1(T-block)þ2(tetra) 3(S-blocks)) with Table 2 Crystallographic and magnetic characteristics of the metallic sublattices in the U-type Ba4M2Fe36O60 hexagonal ferrite. Block
Coordination
Number per block
Expected spin direction
R R R–S S S S–S T T T–S
Octahedral Trigonal bipyramidal Octahedral Octahedral Tetrahedral Octahedral Octahedral Tetrahedral Octahedral
2 1 3 1 2 3 3 2 3
Down Up Up Up Down Up Up 1, down 2 Down Up
Please cite this article as: K. Kamishima, et al., Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.09.055i
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(tetra)) 1(T-block)þ(1(tri-bipyramid)-2(octa)) 2(R-blocks)]. Two copper cations are distributed in the three S-blocks of RSTSRnSn where the number of sites is 18 (¼ 3(octa) 3(blockborders) þ1(octa) 3(S-blocks) þ 2(tetra) 3(S-blocks)). The average moment in the three S-blocks is 4.55 μ B (16 5 μB (Fe3 þ ) and 2 1 μB (Cu2 þ )). Therefore, the contribution of the three S-blocks to the total moment is 27.3 μB ¼4.55 [3(octa) 3(blockborders) þ(1(octal)-2(tetra)) 3(S-blocks)]. The magnetic moment of 47.3 μB/f.u. corresponds to 72.5 A m2/kg. The magnetization of the synthesized U-type ferrite is reduced to 65.2 A m2/kg because the material contained 10 wt% of non-magnetic BaFeO. This estimated magnetization is consistent with the observed magnetization of 65.0 A m2/kg at 5 K.
5. Conclusions
Fig. 10. Spin alignment in the one-third of the unit cell of the U-type ferrite.
respect to the block stacking of the U-type ferrite (RSTSRnSn). The spin alignment in the one-third of the unit cell of the U-type ferrite is summarized in Fig. 10. The spin alignment in the Cu2Y-type ferrite (Ba2Cu2Fe12O22) can be discussed in the similar way. The number of the up-spin sites is expected to be 8( ¼3(octa) 2(block-borders)þ1(octa) 1 (T-block)þ 1(octa) 1(S-block)) and the number of the down-spin sites is 6( ¼2(octa) 1(T-block)þ 2(tetra) 1(T-block)þ2(tetra) 1(S-block)) with respect to the block stacking of the Y-type ferrite (TS). If y of the two Cu2 þ cations are located at down spin sites in the Y-type structure, the total magnetic moment of the Cu2Y-type ferrite becomes
MSCu2Y = 5 × (8 − (2 − y)) + 1 × (2 − y) − 5 × (6 − y) − 1 × y = 8y + 2 (μ B/f. u. ).
(1)
The magnetic moment of the Cu2Y-type ferrite is 7.06 μ B/f. u. (27.7 A m2/kg) at 5 K and 7 T. We can estimate from this value that Cu2 þ ions are located at down spin sites for 31.6% (y¼ 0.633) in the Cu2Y-type ferrite, assuming that the Cu2Y-type ferrite nearly has a collinear magnetic structure at 7 T. This ratio suggests that the copper cations are distributed randomly over the spin-up and the spin-down sites in the S-blocks (containing the block borders) in the Y-type structure. This random distribution is consistent with our previous study of the Cu2X-type ferrite because the ratio of the spin is down/(upþdown)¼1/3 both in the X-type structure (RSSRnSn) and in the S-blocks (containing the block borders) [25]. We can estimate the magnetic moment of Cu2U-type hexaferrite to be 47.3 μ B/f. u. by assuming the random distribution of two copper cations in the three S-blocks in the U-type ferrite structure (RSTSRnSn). The contribution of two R-blocks and one T-block in the U-type structure to the total magnetic moment is 20 μB ¼ 5(Fe3 þ ) [3(octa) 3(block-borders) þ(1(octa)-2(octa)-2
We have investigated the synthesis conditions and the magnetic properties of the Cu2U-type hexagonal ferrite, Ba4Cu2Fe36O60. The Cu2U-type hexaferrite was synthesized at the sintering temperature of 1050 °C with the initial composition of Ba 4 + 2x Cu2 Fe36 + 8x O60 + 14x with x ¼0.2 (Cu2Uþ 0.2T-block) without milling starting powder. The X-ray diffraction pattern of this Cu2U þ0.2T-block sample is similar to the pattern for the Sr-based U-type hexaferrite Sr4Co2Fe36O60 [9]. The saturation magnetization MS at 300 K is 46.8 A m2/kg, and MS at 5 K is 65.0 A m2/kg. The Curie temperature is 420 °C which is lower than that of the M-type ferrite (450 °C) but higher than that of the Cu2Y-type ferrite (320 °C). The EPMA analysis on this sample shows the average atomic ratio of Cu:Ba:Fe¼2:5.5:38.6, suggesting that the sample contained 10 wt% of non-magnetic impurity, BaFeO. We estimated the saturation magnetization to be 65.2 A m2/kg, by assuming a possible arrangement of the magnetic moments of Fe3 þ and Cu2 þ cations. This is consistent with the observed magnetization of 65.0 A m2/kg at 5 K.
Acknowledgment This work was supported by JSPS KAKENHI Grant number 24560005.
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Please cite this article as: K. Kamishima, et al., Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.09.055i
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