On the thermal stability of Co2Z hexagonal ferrites for low-temperature ceramic cofiring technologies

Article abstract of DOI:10.1016/j.jmmm.2007.11.016

Co₂Z hexaferrite Ba₃Co₂Fe₂₄O₄₁ was prepared by a mixed oxalate co-precipitation route and the standard ceramic technology. XRD studies show that at T<1300 °C different ferrite phases coexist with the M-type hexaferrite as majority phase between 1000 and 1100 °C and the Y-type ferrite at 1230 °C. The Z-type material has its stability interval between 1300 and 1350 °C. Both synthesis routes result in almost single-phase Z-type ferrites after calcination at 1330 °C, intermediate grinding and sintering at 1330 °C. The permeability of Co₂Z-type ferrite of about μ=20 is stable up to several 100 MHz, with maximum losses μ′′ around 700 MHz. Addition of 3 wt% Bi₂O₃ as sintering aid shifts the temperature of maximum shrinkage down to 950 °C and enables sintering of Z-type ferrite powders at 950 °C. However, the permeability is reduced to μ=3. It is shown here for the first time that Co₂Z ferrite is not stable under these conditions; partial thermal decomposition into other hexagonal ferrites is found by XRD studies. This is accompanied by a significant decrease of permeability. This shows that Co₂Z hexagonal ferrite is not suitable for the fabrication of multilayer inductors for high-frequency applications via the low-temperature ceramic cofiring technology since the material is not compatible with the typical process cofiring temperature of 950 °C.

Full text of DOI:10.1016/j.jmmm.2007.11.016

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Journal of Magnetism and Magnetic Materials 320 (2008) 1370–1376
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On the thermal stability of Co Z hexagonal ferrites for low-temperature
2
ceramic cofiring technologies
Ã
¨
S. Kracunovska, J. Topfer
ˇ
Department of SciTec, University of Applied Sciences Jena, Carl-Zeiss-Promenade 2, 07745 Jena, Germany
Received 13 April 2007; received in revised form 7 November 2007
Available online 21 November 2007
Abstract
Co Z hexaferrite Ba Co Fe O was prepared by a mixed oxalate co-precipitation route and the standard ceramic technology. XRD
2
3
2
24 41
studies show that at To1300 1C different ferrite phases coexist with the M-type hexaferrite as majority phase between 1000 and 1100 1C
and the Y-type ferrite at 1230 1C. The Z-type material has its stability interval between 1300 and 1350 1C. Both synthesis routes result in
almost single-phase Z-type ferrites after calcination at 1330 1C, intermediate grinding and sintering at 1330 1C. The permeability of Co
2
0
0
Z-type ferrite of about m ¼ 20 is stable up to several 100 MHz, with maximum losses m around 700 MHz. Addition of 3 wt% Bi O as
2
3
sintering aid shifts the temperature of maximum shrinkage down to 950 1C and enables sintering of Z-type ferrite powders at 950 1C.
However, the permeability is reduced to m ¼ 3. It is shown here for the first time that Co Z ferrite is not stable under these conditions;
partial thermal decomposition into other hexagonal ferrites is found by XRD studies. This is accompanied by a significant decrease of
permeability. This shows that Co Z hexagonal ferrite is not suitable for the fabrication of multilayer inductors for high-frequency
2
2
applications via the low-temperature ceramic cofiring technology since the material is not compatible with the typical process cofiring
temperature of 950 1C.
r 2007 Elsevier B.V. All rights reserved.
PACS: 75.50.Gg; 81.20.Ev; 81.40.Rs
2
Keywords: Soft ferrite; Co Z hexagonal ferrite; Permeability
1
. Introduction
Multilayer ferrite inductors (MLFI) are a major
to the lower GHz range, has been proposed several
years ago [2].
The structure of Z-type hexagonal ferrites may be
described as a stacking of close-packed oxygen/barium
layers along the hexagonal c-axis with an ordering sequence
of S-, R- and T-blocks, where S is a two-layer building
class of passive components. For standard ML-inductors
Ni–Cu–Zn ferrites are used as soft magnetic materials
because their low sintering temperature qualifies them for
co-firing with internal Ag conductors [1]. The melting point
of silver limits the maximum sintering temperature to
Tp950 1C. Ni–Cu–Zn ferrites show relatively high perme-
abilities (m ¼ 300–600) in the kHz range; however, their
resonance frequency at about 10–50 MHz limits their
application in high-frequency inductive devices. The use
of hexagonal ferrites, especially those of Z-type structure
for the fabrication of MLFI with operating frequencies up
2
+
unit (Fe O ) , R is a three-layer block of composition
6
8
2
ꢀ
(BaFe O )
and T consists of the four-layer unit Ba2
Fe O . Combination of RS results in M-type ferrite
6
11
8
14
BaFe O and of TS in Y-type hexagonal ferrite Ba Me
2
1
2
19
2
Fe O , whereas a stacking of RSTS (or MY) gives the
1
2 22
typical structural motif of the Z-type structure [3,4].
The magnetic properties of Co Z ferrite as a member of
2
the ferroxplana family of ferrites have been reviewed in the
1950s [5]. At room temperature the preferred plane of
magnetization is perpendicular to the c-axis; the material is
a soft magnet since the magnetization may easily rotate
within that plane. It has been demonstrated that the
Ã
Corresponding author. Tel.: +49 3641 205450; fax: +49 3641 205451.
¨
E-mail address: joerg.toepfer@fh-jena.de (J. Topfer).
0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2007.11.016

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1371
2
S=1.3 m /g and BaCO (Merck, Germany, standard
resonance frequency of this compound is at f ¼ 1–4 GHz
2,5]. Recently, the magnetic and dielectric properties of
3
2
grade) with S=6.2 m /g were used as starting materials.
[
other hexagonal ferrites, e.g. M-type ferrites, have also
been studied in detail [6].
For the preparation of Co Z ferrite a variety of different
techniques have been reported, e.g. the standard ceramic
oxide route [7]; typical calcination temperatures are
The raw materials were wet mixed for 12 h in a
polyethylene container. The dried powders were calcined
at 900–1350 1C for 4 h. For both synthesis routes, after
calcination the powders were wet milled in a planetary ball
mill with zirconia balls of 3 mm diameter. The powders
were uniaxially compacted using polyvinyl-alcohol as
binder to give pellets for sintering studies or toroids for
permeability measurements. The specimens were sintered
at 1330 1C for 4 h.
2
1250–1300 1C. The beneficial influence of fine milling of
the calcined powders on the completeness of Z-type phase
formation has been demonstrated [8]. In addition, several
alternative routes for the preparation of Co Z were
2
explored. A citrate precursor route was reported to give a
Co Z at around 1200 1C [9]. Sol–gel processes were used to
synthesize Z-type ferrite, but different temperatures of
phase formation were reported, e.g. 1350 1C [10] or 1250 1C
The phase formation was evaluated with X-ray powder
diffractometry (Siemens D5000). The powder particle size
was measured using a laser diffraction system (Malvern
Mastersizer 2000). The bulk density of sintered samples
was determined from the dimensions and weight. Shrink-
age measurements were made with a NETSCH DIL402
dilatometer on cylindrical compacts during heating to
1000–1200 1C with 4 K/min heating rate. The microstruc-
ture of the samples was studied with a scanning electron
microscope (Zeiss DSM940A). The permeability of toroids
(13 mm outer diameter, 6.5 mm inner diameter, 3 mm
thickness) was measured at room temperature from
1 MHz up to 1 GHz with an Agilent E4991A impedance/
materials analyzer in combination with a 16455A test
fixture for magnetic materials measurements. The measure-
ment concept is based on the inductance of the toroid in an
ideal single turn inductor without flux leakage. The
accuracy of the permeability is greatly reduced when the
frequency approaches 1 GHz [16]; therefore the interpreta-
tion of results in the frequency range of f40.8 GHz is
difficult. The temperature dependence of magnetization
was measured on powdered samples with a VSM magnet-
ometer. The M vs. T curves were recorded in a field of
5 kOe.
2
[
11]. A self-propagating synthesis method was reported to
give a single-phase Co Z already at 1200 1C [12]. However,
2
the synthesis of single-phase Co Z ferrite is not straightfor-
2
ward due to the structural complexity, and many of the
synthesized materials shown in the literature (and often
claimed to be single phase) contain various amounts of
additional phases.
The fabrication of MLFI typically is based on a low-
temperature ceramic cofiring (LTCC) process: a stack of
ferrite tapes with screen printed Ag metallization is cofired
at To950 1C. To guarantee the LTCC processability of the
Co Z-type ferrite sintering, additives are used to reduce the
2
temperature of substantial shrinkage down to 950 1C;
Bi O [13] or lead-containing glasses [14,15] were proposed
2
3
as additives. More or less dense samples were obtained
after sintering at 900–950 1C; however, the permeability
usually is around m=2–4 in the MHz range and decays at
around 1 GHz.
In this contribution we demonstrate that Co Z-type
2
hexaferrite is formed at 1330 1C using an oxalate precursor
process or the standard oxide route. These samples show
large permeabilities of m=20 up to several 100 MHz. With
the addition of 3 wt% Bi O the ferrite can be sintered at
3. Results and discussion
2
3
9
50 1C to sufficient density, but the permeability is reduced
3.1. Phase formation
to m=3. This is caused by thermal decomposition of the
Z-type material at 950 1C.
The thermal decomposition of the oxalate precursor was
studied with TG/DTA (Fig. 1). Two mass loss steps occur
between 100 and 250 1C, respectively, which are accom-
panied by an endothermic and exothermic heat effect. This
is the typical signature of the thermal decomposition
reaction of co-precipitated mixed oxalates. The thermal
effects are attributed to the removal of chemically bond
water at 100 1C and the transformation of the oxalate into
oxides at 200–250 1C. An additional small exothermic peak
at 400 1C indicates crystallization of the oxide products. An
oxalate precursor which has been heated at 400 1C for 4 h
in air gives a nanosize oxide powder with a specific surface
2
. Experimental
Co Z ferrite of composition Ba Co Fe O was pre-
24 41
2
3
2
pared by an oxalate precursor route. Metallic iron powder
was first dissolved in an acetic acid solution under Ar
atmosphere to give a clear solution of ferrous acetate.
Next, cobalt acetate and barium acetate were dissolved in
2
+
water and added to the Fe
solution; hence a clear
solution of the metal ions was obtained. Then an oxalic
acid solution was added dropwise and a yellow-colored
precipitate was formed immediately. After drying, this
oxalate precursor was calcined at various temperatures
2
of 83 m /g corresponding to a mean particle size of 16 nm.
A SEM micrograph of this powder (Fig. 2a) confirms the
nanosize of the primary particles and the micron-size of
aggregates, but points at the multi-phase nature of the
product. XRD shows that the powder obtained at 400 1C
for 4 h. Alternatively, Co Z was synthesized by the
2
standard ceramic route. Iron oxide (TKS Germany, grade
2
HP) with a specific surface S=4.3 m /g, Co O with
3
4

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1372
Fig. 1. TG/DTA curve of the Ba/Co/Fe-oxalate precursor in air (heating
rate 6 K/min).
consists of a mixture of iron oxide, barium monoferrite and
cobalt ferrite (Fig. 3a). The resulting series of phase
transformations with increasing temperature is shown in
Figs. 3a and b. At 900 1C barium monoferrite represents
the majority component with hematite, CoFe O4 and
M-type ferrite as minor constituents. At 1100 1C M-type
2
ferrite (BaFe O ) is formed as main phase, whereas
1
2
19
Y-type ferrite is the dominant phase in oxide mixture
obtained at 1230 1C. At a calcination temperature of
1
of Y-type are detectable as minority component. The Co Z
330 1C finally Co Z is formed as main phase, only traces
2
2
phase formation sequence might be described by the
idealized reaction scheme:
Ba3Co2Fe24ðC2O4Þ29
3
4
9
1
1
00 C
ꢀ
! 3BaFe2O4 þ 7Fe2O3 þ 2CoFe2O4 þ xCO þ yCO2
3
00 C
ꢀ
! BaFe12O19 þ 2BaFe2O4 þ 2CoFe2O4 þ 2Fe2O3
3
100ꢀ1300 C
ꢀ
!
Ba Co Fe O þ BaFe O
2
2
12 22
12 19
3
300 C
ꢀ! Ba3Co2Fe24O41:
These phase transformation reactions were frequently
observed for different precursors, e.g. in Refs. [9,11,12].
The thermal transformations starting from a mixed oxalate
precursor seem to fit into the same reaction sequence.
However, calcination at 1350 1C leads to the appearance
of W-type ferrite as minor component besides Z-type
Fig. 2. SEM micrographs of an oxalate precursor heated at 400 1C in air
a) and of the mixed-oxide route powder, calcined at 1330 1C and fine-
milled (b).
(
(
Fig. 3b), indicating that the stability region of the Co Z
2
compound is between 1300 and 1350 1C.
In order to prepare single-phase Z-type materials the
powders obtained after calcination at 1330 1C were wet
milled down to a particle size of d50 ¼ 0.8 mm measured
with laser diffraction. Pellets made from these powders
were sintered at 1330 1C for 4 h in air, showing an almost
single-phase Z-type product with small traces of Y-type.
In the standard ceramic route the mixed raw materials
were calcined at 1330 1C for 4 h. The formation of the
ferrite phase was evaluated by XRD (Fig. 4a); traces of
Y-type were detectable. This powder was also fine-milled
down to a d50 ¼ 0.8 mm. A SEM micrograph (Fig. 2b)
demonstrates that the primary particle size is below
200 nm. The particle size of 0.8 mm as determined by laser
diffraction therefore corresponds to an aggregate size;
these aggregates are also visible in Fig. 2b. The specific
surface of the milled powder is 19 m /g, which corresponds
to a particle size of dBET ¼ 58 nm. Pellets made from
this powder were sintered at 1330 1C for 4 h; almost
2

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1373
single-phase Z-type ferrite with very small traces of Y-type
was obtained (Fig. 4b). The X-ray diffractograms of both
ferrite powders (calcined and calcined+sintered) are
compared to JCPDS files of the Z- and Y-type ferrites in
Fig. 4. It can be concluded that the powder which has been
calcined and sintered at 1330 1C is almost single-phase
Z-type. Small differences of peak intensities between
measured and standard XRD data are caused by the
preferred orientation of individual particles in the sample
holder during XRD experiments. This observation has
already been reported in Refs. [8,17]. The lattice constants
˚
˚
of the Co Z ferrite are a =5.8755(2)A and c =52.227(2)A
2
0
0
for a sample synthesized via the oxalate route and
˚
a =5.8817(2)A and c =52.297(2)A via the mixed-oxide
˚
0
0
route, respectively.
The reported procedure of calcination and sintering at
1330 1C tends to be the optimum synthesis condition for
single-phase Co Z material. The effect of a substantial
reduction of the synthesis temperature of Co Z down to
2
2
1250 or even 1150 1C when using chemically prepared
precursors (as frequently reported in the literature) could
not be confirmed in our study. Powders prepared via the
oxalate route also require a synthesis temperature of
1330 1C for the complete formation of Z-phase.
The decomposition of Co Z at T413301 as indicated
2
from XRD results was also monitored by thermal analysis.
A Z-type sample was heated to 1500 1C in air; a significant
thermal effect at 1350 1C signals the decomposition of
Co Z (Fig. 5). A second endothermic effect at 1460 1C
2
might indicate the formation of a liquid phase.
ꢁ
0
00
The complex relative permeability m ¼ m ꢀ jm of Co Z
2
prepared by calcination and sintering at 1330 1C is shown
as a function of frequency in Fig. 6. The real part of the
0
permeability has a value of m ¼ 20 in the MHz range
Fig. 3. XRD plots of oxalate precursors calcined at 400, 900 and 1100 1C
a) and 1230, 1330 and 1350 1C (b).
which is one of the largest values reported in the literature.
Nakamura et al. [18] reported on the permeability of Zn-
substituted Co Z ferrites and measured a low-frequency
(
2
permeability of m ¼ 15. At frequencies of some hundreds
2
Fig. 4. XRD plots of Co Z samples prepared by a standard ceramic route:
after calcination at 1330 1C for 4 h (a) and after additional fine-milling and
sintering at 1330 1C for 4 h (b).
2
Fig. 5. TG/DTA curve of Co Z ferrite heated in air (heating rate
4 K/min).

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1374
0
00
Fig. 6. Permeability m and m as a function of frequency for Co
2
Z ferrite
calcined at 1330 1C and sintered at 1330 1C without sintering additive and
at 950 1C with the addition of 3 wt% Bi
2
3
O .
0
MHz the real part of the permeability m shows a significant
decay. Simultaneously, a broad maximum in the imaginary
00
part of the permeability m is seen. The resonance frequency
00
0
(
i.e. frequency where m is maximum or where m is reduced
to its half maximum value) is at around 700 MHz. This is in
good agreement with the results of permeabilty measure-
ments of other groups [9,12,18].
3
.2. Sintering behavior
In the following section we report on studies of the
densification and shrinkage behavior of Co Z hexagonal
2
ferrite powders at 950 1C; this is the maximum co-firing
temperature applicable in the LTCC process for the
fabrication of MLFIs based on Ag as metallization
material. Ferrite powders obtained by calcination at
Fig. 7. Relative shrinkage (a) and shrinkage rate (b) of compacts made
from ferrite powders calcined at 1330 1C and fine-milled with various
additions of sinter additives (heating rate 4 K/min).
1330 1C for 4 h and subsequent fine milling were used for
sintering studies. Compacts made from these powders were
fired at 950 1C for 2 h. The density of the sintered pellets is
940 1C (Fig. 7b). Further increase of the concentration of
3
3
.5270.02 g/cm (66% of the theoretical density). The
addition of Bi O as sintering aid promotes densification.
Bi O alone does not seem to reduce the temperature of
maximum shrinkage. On the other hand, addition of 10%
of a Bi O /CuO mixture further reduces the maximum
2
3
2
3
Compacts with 2 and 3 wt% additions of bismuth oxide
3
have densities of 4.0570.06 and 4.6170.02 g/cm (76%
2
3
shrinkage temperature down to 860 1C. However, the large
amount of additives does not make such a composition
favorable for obtaining large permeabilities.
and 86%) after firing at 950 1C, respectively. In order to
obtain dense compacts after sintering at 950 1C one batch
of samples was prepared by the addition of 10 wt% of a
Ferrites with additions of 3 wt% Bi O appear to be
2
3
3
mixture of Bi O and CuO; a density of 5.1970.02 g/cm
promising materials for LTCC-type processes with a
sintering temperature of Tp950 1C. A SEM micrograph
of a sample fired at 950 1C is shown in Fig. 8, an
inhomogeneous microstructure with grains up to 10 mm in
length besides the majority of grains of 1–2 mm in size is
found. However, the permeability of these samples (Fig. 6)
only has values of m ¼ 3 over the whole frequency range up
2
3
(
98%) was observed.
Dilatometric measurements exhibit an interesting varia-
tion of the shrinkage and shrinkage rate for different Bi O
2
3
additions (Fig. 7). A sample without additive does not
show any significant shrinkage below 1000 1C; the main
shrinkage sets in at 1050 1C and is completed at 1200 1C.
The maximum shrinkage rate (Fig. 7b) is observed at about
0
0
to 1 GHz; the maximum of m is at 41 GHz. These low
values of permeability are not due to the low sample
density since dense toroids (sintered at 950 1C with 10 wt%
additives) have similar permeability curves.
1
towards lower temperatures. For 3 wt% bismuth oxide the
120 1C. With the addition of Bi O shrinkage is shifted
2
3
temperature of maximum shrinkage rate is reduced to

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1375
Fig. 10. Magnetization as function of temperature of Co Z ferrite
synthesized at 1330 1C for 4 h and subsequently at 950 1C for 4 h.
2
in the intensities of the diffraction peaks of the Y- and
M-type phases.
The decomposition reaction of the Z-type phase at
Fig. 8. SEM micrograph of Co
2
Z sintered at 950 1C for 4 h (powder
).
calcined at 1330 1C, fine-milled with 3 wt% Bi
2
O
3
950 1C has a dramatic effect on the permeability. This is
demonstrated for toroids which were prepared from
powders calcined at 1330 1C (fine-milled to 0.8 mm with
3
wt% Bi O addition) and subsequently sintered at 950 1C.
2
3
Whereas the sample sintered at 1330 1C has a m ¼ 20, the
permeability of the sample sintered at 950 1C is reduced to
m ¼ 3 (Fig. 6). Measurements of magnetization vs.
temperature also resemble the decomposition reaction of
the Co Z compound at lower temperature (Fig. 10). A
2
powdered sample synthesized at 1330 1C displays a regular
variation of M vs. T with a T ¼ 419 1C, which is in good
c
agreement with Curie temperatures of Co Z reported in
2
Refs. [4,9]. On the other hand, after additional firing at
950 1C one can clearly detect multiple Curie temperatures
signaling that the powder had transformed into a multi-
phase mixture. Besides the T =419 1C of Co Z a
c
2
T =442 1C of M-type ferrite appears; the T of Y-type
c
c
ferrite (330 1C) is not well resolved. These data are in good
agreement with results from the literature: for BaFe O
a
19
1
2
Fig. 9. XRD plots of Co
2
Z powder prepared at 1330 1C for 4 h and fired
T =450 1C [5] and for Ba Co Fe O a T ¼ 330ꢀ340 1C
c
2
2
12 22
c
at 950 1C for 15 min, 1 and 4 h.
were reported [4,5,9].
These results for the first time demonstrate that Co Z
2
hexagonal ferrite is not suitable for the fabrication of
multilayer inductors for high-frequency operations via a
LTCC technology. Unfortunately, this ferrite material is
not compatible with the LTCC processing temperature, i.e.
with T ¼ 950 1C for co-firing of ferrite with internal Ag
metallizations. The reported findings help to understand
the large scattering of reported permeability values of
Z-type ferrites in the literature, since most of the papers
devoted to LTCC firing conditions report permeabilities
between m ¼ 1–3 whereas those studies that rely on higher
sintering temperatures usually find higher permeabilities of
m ¼ 10–20.
3
.3. Thermal stability of Co Z ferrite at low temperatures
2
We suspected that the dramatic reduction of the
permeability after firing at 950 1C is due to chemical
transformations in the sample. Fig. 9 shows the results of
an XRD study of Co Z powder synthesized at 1330 1C and
subsequently annealed at 950 1C for 15 min, 1 and 4 h: the
2
single-phase Co Z sample rapidly decomposes into a
2
mixture of Y- and M-types. Even after 15 min considerable
amounts of Y-type are clearly detectable. With increasing
annealing time at 950 1C we observe a significant increase

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1376
4
. Conclusions
References
[
[
[
1] T. Nomura, A. Nakano, in: Proceedings of the Sixth International
We have shown that the synthesis of Co Z hexaferrite
2
Conference on Ferrites (ICF6), Tokyo, 1992, p. 1198.
2] M. Endo, A. Nakano, in: Proceedings of the Eighth International
Conference on Ferrites (ICF8), Kyoto, 2000.
Ba Co Fe O via the ceramic mixed oxide route and an
2
3
24 41
oxalate precursor route lead to almost single-phase
products after calcinations and sintering at 1330 1C. The
3] P.B. Brown, Philips Res. Rep. 12 (1957) 491.
0
0
permeability of Co Z is m ¼ 20 at 10 MHz; the m has its
[4] M. Sugimoto, Ferromagn. Mater., vol. 3, in: E.P. Wohlfahrt, (Ed.),
North-Holland Publishing Co., Amsterdam, 1982, pp. 393–439.
2
maximum at around 700 MHz. However, sintering at a
temperature of 950 1C, which is the characteristic tempera-
ture for a LTCC-type process, does not result in dense
samples. Addition of 3 wt% Bi O reduces the temperature
[
[
[
[
[
5] J. Smit, H.P.J. Wijn, Ferrites, Philips Technical Library, Eindhoven,
959.
6] K.K. Mallick, P. Shepherd, R.J. Green, J. Magn. Magn. Mater. 312
2007) 418.
1
2
3
(
of maximum shrinkage to 940 1C and dense samples were
sintered at 950 1C. The permeability of these samples is
significantly reduced to m ¼ 3. XRD studies have shown
7] X. Wang, L. Li, S. Su, Z. Yue, J. Magn. Magn. Mater. 280
(2004) 10.
8] J. Temuujin, M. Aoyama, M. Senna, T. Masuko, C. Ando, H. Kishi,
J. Mater. Res. 20 (2005) 1939.
that Co Z is not stable at that temperature. Firing at
2
9] X.H. Wang, L. Li, Z. Gui, S. Shu, J. Zhou, Mater. Chem. Phys. 77
950 1C leads to a decomposition into Y- and M-type
hexagonal ferrites, which is accompanied by a substantial
(
2002) 248.
[
10] D.W. Hahn, Y.H. Han, Mater. Chem. Phys. 95 (2006) 248.
reduction of permeability. Therefore, Co Z hexagonal
2
[11] R.C. Pullar, M.H. Stacey, M.D. Taylor, A.K. Bhattacharya, Acta
Mater. 49 (2001) 4241.
ferrite is not the preferred high-permeability material for
the LTCC-compatible fabrication of MLFI for operating
frequencies of up to 1–3 GHz.
[
[
[
12] H. Zhang, L. Li, J. Zhou, Z. Yue, Z. Ma, Z. Gui, J. Eur. Ceram. Soc.
1 (2001) 149.
13] H. Zhang, L. Li, J. Zhou, Z. Yue, Z. Gui, J. Am. Ceram. Soc. 84
2001) 2889.
14] H. Hang, C.S. Hsi, T.C. Lee, C.H. Chang, J. Magn. Magn. Mater.
68 (2004) 186.
2
(
Acknowledgments
2
The authors thank the Bundesministerium fu
und Forschung (Germany) for financial support (Grant
3WKF22E). We acknowledge the contributions of Mrs.
M. Friedrich (FHJ) for SEM, Mr. J. Bierlich for TG/DTA
measurements and Dr. R. Muller (IPHT Jena) for high-
temperature VSM measurements.
¨
r Bildung
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[
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0
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