Full text of DOI:10.1557/JMR.2000.0235

Synthesis of copper and lithium copper ferrites as high
magnetization materials
K.E. Kuehn, D. Sriram, S.S. Bayya, J.J. Simmins, and R.L. Snyder
New York State College of Ceramics at Alfred University, Alfred, New York 14802
(Received 26 April 2000; accepted 2 May 2000)
The ferrite with composition Cu0.5Fe2.5O4 was heat treated in air and in reducing
atmospheres to different temperatures within the solid solution region confirmed by
dynamic high-temperautre x-ray characterization. The samples were quenched in oil
and air, and lattice parameter, Curie temperature, and saturation magnetization
measurements were completed. The magnetization measurements for these samples
showed a maximum 4␲Ms of 0.7729 and 0.5426 T at 10 and 300 K, respectively.
The cationic distribution based on the low-temperature 4␲Ms measurements is
(Cu+0.24Fe3+0.76)A[Cu+0.26Fe3+1.74]BO4 → 4.9 ␮B. X-ray-pure Cu0.5Fe2.5O4 samples
were also synthesized by slow cooling from the formation temperature to 900 °C in a
reducing atmosphere. A temperature–PO diagram for the stability of Cu0.5Fe2.5O4
2
under the conditions of the experiment was determined. Low-temperature 4␲Ms
measurements did not indicate an increase in the Cu+ A site occupancy for the samples
cooled to 900 °C in a reducing environment above those samples that were quenched
from high temperature. Curie temperatures for all Cu0.5Fe2.5O4 samples ranged
from 348 to 369 °C. Lithium additions (0.1 mol/unit formula) to copper ferrite
Li0.1Cu0.4Fe2.5O4 decreased the room-temperature 4␲Ms values to 0.5234 T
with a corresponding decrease in the 10 K measurements to 0.7047 T. From
the low-temperature magnetization measurements, the distribution was
(Cu+0.15Fe3+0.85)A[Cu+0.25Li+0.1Fe3+1.65]BO4 → 4.48 ␮B.
Dionne2 postulated that the most favorable spinel com-
I. INTRODUCTION
Investigations of a wide variety of spinel ferrite com-
positions have taken place in the past few decades
with the goal of determining the composition, which
would increase the saturation magnetization of the fer-
rite crystal structure. The benefits in obtaining a higher
saturation magnetization in the cubic spinel ferrits are
well known, the most exciting being the possible in-
crease in microwave communication frequencies further
into the gigahertz range. However, there has been no
reported success in obtaining usable saturation mag-
netizations in excess of the 0.55 T found in zinc-
containing ferrites.
The problem with existing ferrites containing zinc is
the spin-canting phenomenon, observed for high con-
centrations of Zn2+. The zinc ion has a tetrahedral site
preference within the spinel structure and therefore
weakens or dilutes the tetrahedral sublattice moment.
This causes an initial increase in the net moment;
however, for higher Zn concentrations the magnetic
coupling between the octahedral and tetrahedral lat-
tice weakens causing a rapid decrease in the net magne-
tization due to spin–lattice canting. This was illustrated
by Gorter.1
positions capable of exceeding existing saturation mag-
netizations are the generic monovalent cation ferrites.
A monovalent cation with tetrahedral site preference
would allow dilution of the tetrahedral site moment while
keeping a significant tetrahedral site Fe3+ concentration
to maintain the magnetic coupling between the two
sublattices. Theoretically, this would allow 2 mol of Fe3+
on the octahedral sublattice while maintaining the cou-
pling with a partial dilution of the tetrahedral sublat-
tice moment.
The monovalent copper ferrite system is one case for
a monovalent cation with tetrahedral site preference. The
composition Cu0.5Fe2.5O4, where the copper is monova-
lent and occupies the tetrahedral site, should ideally ex-
hibit a net magnetic moment of 7.5 ␮B at 0 K. However,
the site distribution of the copper ions over the available
sites in the spinel is complicated by the multivalent na-
ture of the copper ion. Studies involving the CuFe2.5O4
system show that the divalent copper cation prefers the
octahedral site almost exclusively.3,4 In addition, the syn-
thesis of the ideal Cu0.5Fe2.5O4 ferrite is further compli-
cated by the desire of the Fe3+ cation to oxidize any
nearby octahedral Cu+.5 The studies of this material have
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K.E. Kuehn et al.: Synthesis of copper and lithium copper ferrites as high magnetization materials
caused further disagreement about the distribution and
material from the desired temperature rapidly enough to
prevent the decomposition to CuFe2O4 and Fe2O3. The
dynamic x-ray system was also used to determine more
precisely the quench rate needed to “lock-in” single-
phase Cu0.5Fe2.5O4 ferrite. Since the thermal mass of the
stage and sample is very small, controlled quenches of up
to 20 °C/s are possible. The two-phase material was
heated to 1250 °C and checked for completeness of re-
action. It was then quenched at different rates to deter-
mine the minimum rate needed to maintain the
Cu0.5Fe2.5O4 ferrite phase purity. The minimum quench
rate needed was determined to be 7 °C/s in air. Note that
this was for a powder film roughly 1–3 microns thick.
The temperature stability of Cu0.5Fe2.5O4 was also de-
termined for the purpose of synthesizing bulk samples.
With the dynamic XRD unit the precursor composition
was heated into the phase-pure region and verified as to
its phase purity. The sample was quenched on the hot-
stage and then reheated to temperatures up to 500 °C.
Continuous scanning of the diffraction pattern while
holding the sample at the temperature in question al-
lowed the time for decomposition into Fe2O3 and a cubic
copper containing ferrite to be determined. The time–
temperature decomposition plot is shown as Fig. 1.
valence state of the copper ion. Previous research involv-
ing Cu0.5Fe2.5O4 has provided lattice parameter values
ranging from 8.39 to 8.414 Å.6 Likewise, the measured
moment has varied from 4.05 to 5.4 ␮ near 0 K.6,8
B
With the addition of another monovalent cation, such
as lithium (which has octahedral site preference), to the
system Cu0.5Fe2.5O4, it is thought that the lithium cations
will decrease the probability of ferrous ion formation in
the spinel structure, thus, increasing the resistivity of the
material. The lithium addition will also increase the
room-temperature saturation magnetization by reducing
the spin-canting phenomena if it is active. Therefore, an
optimum between the lithium addition and saturation
magnetization can be expected. Calculated magnetic mo-
ments per unit formula, assuming ideal placement of the
cations, lie in the range 7.5 (pure Cu0.5Fe2.5O4) to 5
(Li0.25Cu0.25Fe2.5O4) Bohr magnetons at 0 K. The objec-
tive of this research is the realization of a ferrimagnet
with a room-temperature saturation magnetization in ex-
cess of the current practical value of about 0.55 T with
sufficiently high resistivity and acceptable dielectric
properties for soft ferrite applications.
II. EXPERIMENTAL PROCEDURE
Sample preparation and measurement
The initial oxides CuO and Fe2O3 were rotary calcined
at 950 °C in the correct ratio to obtain the final stoichi-
ometry for Cu0.5Fe2.5O4. The calcination resulted in an
initial phase composition CuFe2O4 and Fe2O3 as deter-
mined by x-ray diffraction (XRD). Some properties of
the bulk starting powder are shown in Table I.
The two-phase Fe2O3 and CuFe2O4 powder was iso-
statically pressed into rods at 103 MPa (15,000 psi). The
rods were sintered according to a sintering curve to near
full density. The final rod was wafered with a diamond
wafering blade into 0.4 × 1.5 cm disks. The disks were
then heated in a tube furnace in flowing air or air-CO2
The stability region for Fe2O3 was determined via dy-
namic x-ray analysis.7 With a locally automated Philips
diffractometer equipped with a scanning position sensi-
tive detector (PSD) and iron radiation (K␣ ␭ ס 1.938 Å;
(for reduced PO atmospheres) to temperatures within the
2
solid solution region, as determined by the hot-stage x-
ray data, and quenched into oil. Accurate lattice param-
K␤ ␭ ס 1.757 Å) it is possible to observ1e,2 the formation
1
of the Cu0.5Fe2.5O4 spinel structure at elevated tempera-
tures. The sample is mounted onto a platinum strip,
which is heated electrically under computer control. The
low- and high-temperature phase boundaries, in air, as
determined by this in situ method, are 1200 and 1350 °C,
respectively. These values are in good agreement with
those found in the literature.8,9
The instability of the Cu0.5Fe2.5O4 compound was
noted previously.9 To maintain a single-phase material
from high temperature in air it is necessary to quench the
TABLE I. Starting powder information.
Mean
size
(␮m)
Standard
deviation
(␮m)
Specific surface
area (m2/g)
Density
(g/cm3)
Ferrite
FIG. 1. Time for decomposition of Cu0.5Fe2.5O4 into Fe2O3 and cop-
per ferrite at a given temperature as determined by dynamic x-ray
characterization.
Cu0.5Fe2.5O4
Li0.1Cu0.4Fe2.5O4
2.51
2.03
1.26
1.15
4.93
5.05
5.37
5.13
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K.E. Kuehn et al.: Synthesis of copper and lithium copper ferrites as high magnetization materials
eters were determined for each of the samples. The lattice
composition of the sample was determined by XRD. An
parameters were determined by the zero background
example of the experimental temperature and PO pro-
2
holder external standard method.10
files is shown as Fig. 3. The final temperature and oxy-
gen partial pressures, including phase compositions, for
all samples involved in the reduced PO study are shown
The sample diffraction peak locations were deter-
mined by profile fitting the sample peaks with a split
Pearson VII profile11 and correcting the d-value with an
external standard calibration curve. The lattice parameter
was then calculated on the basis of a least-squares re-
finement of the d-values. The lattice parameter for the
Cu0.5Fe2.5O4 heated in air, versus the quench tempera-
ture, is tabulated in Table II.
in Fig. 4. These curves show how the2 PO needs to be
2
changed during cooling as a function of temperature to
stabilize the Cu0.5Fe2.5O4 phase for the conditions of the
study.
To characterize the valence of the iron cations, Mo¨ss-
bauer studies were performed on the samples quenched,
into oil, from the Cu0.5Fe2.5O4 stability region in air.
Resistivity measurements were also made on quenched
samples, crushed and dry pressed into pellets. In order to
maintain consistency in testing of different samples, all
of the powder samples were dry pressed into pellets un-
der identical conditions. This meant that in making each
of the pellets the same amount of powder was weighed
out and the uniaxial pressing pressures used were exactly
the same (51 MPa or 7500 psi). All pellets obtained had
the same dimensions (1.7-cm diameter and 10 mm
thick). The opposite faces of each of the pellets (cross
section) were electroded with silver paste. Resistivity
Saturation magnetization measurements were per-
formed on the superconducting quantum interference de-
vice (SQUID) at the State University of New York at
Buffalo. SQUID measurements were taken at 10 and
300 K on powdered samples quenched from differ-
ent temperatures and on the phase-pure samples from
the related oxygen pressures and temperatures in the
Cu0.5Fe2.5O4 and Li0.1Cu0.4Fe2.5O4 systems. Part of
the hysteresis curve, for different samples of the
Cu0.5Fe2.5O4 phase, used for calculating the saturation
magnetization data, is shown in Fig. 2. Curie temperature
measurements were performed by Trans-Tech, Inc., Ad-
amstown, MD, and are tabulated in Table III. Bulk
samples for saturation magnetization measurements were
also produced. The samples were quenched in air to re-
duce the possibility of thermal shock. Samples on both
sides of the transition temperature including the two-
phase samples above and below the solid solution region
were prepared. XRD of the samples was carried out in
order to verify the phase purity of the material. The phase
stability region of Cu0.5Fe2.5O4 was traced as a function
of temperature and oxygen partial pressure (PO ). The
2
partial pressure of oxygen in the tube furnace was con-
trolled with a CO2 air mixture. A zirconia sensor (oxygen
sensor model OX manufactured by AACC/Zircoa, Solon,
OH) positioned adjacent to the sample was used to moni-
tor the PO . Following heat treatment in air within the
2
solid solution region the samples were subject to a 5 °C/
min furnace cool with simultaneous decrease in oxygen
pressure. When the designated PO and final temperature
were reached, the samples were 2quenched in oil. The
FIG. 2. SQUID magnetization measurements at 10 K for Cu0.5Fe2.5O4
samples quenched from various temperatures.
TABLE II. Lattice parameter for Cu0.5Fe2.5O4 heated in air mixture
followed by oil quenching.
TABLE III. Curie temperature measurements for air Cu0.5Fe2.5O4
samples cooled at 20 °C/min from peak temperature.
Lattice parameter
(Å)
Standard deviation
Temperature (°C)
␴ (Å)
Final
temperature
(°C)
Lattice
parameter
(Å)
Standard
deviation
␴ (Å)
Figure
of merit
(7)
Curie
temperature
(°C)
1226
1246
1286
1326
1348
1366
1372
1404
8.4154
8.4152
8.4162
8.4187
8.4189
8.4200
8.4191
8.4184
0.0003
0.0002
0.0004
0.0007
0.0007
0.0005
0.0004
0.0006
1326
1348
1366
1372
1246
1226
8.4181
8.4185
8.4192
8.4167
8.4152
8.4162
0.0007
0.0005
0.0007
0.0004
0.0002
0.0003
137
142
131
251
354
319
350
369
364
348
353
356
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K.E. Kuehn et al.: Synthesis of copper and lithium copper ferrites as high magnetization materials
data for the samples was obtained using an ac impedance
Cu0.5Fe2.5O4. A clear drop in the FWHM of the outer
Mo¨ssbauer peak and increase in the resistivity of the
samples at higher temperature were observed.
analyzer (RF impedence analyzer, model 4191A, Hew-
lett Packard, Palo Alto, CA). The conductance of the
pellet sample was obtained at three different frequencies:
0.1, 1, and 10 kHz. From these data, the average conduc-
tivity was then extracted and the resistivity values
calculated.
The full width at half-maximum (FWHM) of the out-
ermost peak of the Mo¨ssbauer spectra and the resistivity
measurements obtained for samples from different
quench temperatures are tabulated in Table IV. Figure 5
shows a sample of the Mo¨ssbauer data obtained for
III. RESULTS AND DISCUSSION
The results of the lattice parameter measurements for
the final heat treatment in air followed by oil quenching
are tabulated in Table II and shown in Fig. 6.
TABLE IV. FWHM of the outermost Mo¨ssbauer peak and resistivity
for Cu0.5Fe2.5O4 heated in air mixture followed by oil quenching.
Temperature (°C)
FWHM (mm/s)
Resistivity (ohm/cm)
1226
1246
1286
1326
1348
1.108
1.082
0.985
0.906
0.804
5.08
5.28
5.59
8.78
11.93
FIG. 3. Example of Cu0.5Fe2.5O4 samples temperature–oxygen partial
pressure processing path.
FIG. 5. Mo¨ssbauer data for Cu0.5Fe2.5O4 in air at Alfred University.
FIG. 4. Final temperatures and oxygen partial pressures for
Cu0.5Fe2.5O4 samples showing a region of stability (pure-phase region)
for the conditions of the experiment. The horizontal line represents the
partial oxygen pressure of air.
FIG. 6. Lattice parameter versus final heat-treatment temperature for
oil-quenched Cu0.5Fe2.5O4.
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K.E. Kuehn et al.: Synthesis of copper and lithium copper ferrites as high magnetization materials
Three sigma error bars are shown indicating a 99.87%
confidence level, and Fig. 6 shows that there is a signifi-
cant difference in the lattice parameter of the material
depending on the temperature from which the sample
was quenched. The trend toward larger lattice parameter
at high temperature may be explained by the increase in
the formation of Cu+, which migrates to the preferred
tetrahedral position. Qualitatively, Cu+ has a larger ionic
radius than the Cu2+ ion and Cu+ prefers the smaller
four-coordinated site in the spinel structure. Therefore,
the expansion is intuitively explainable by the increase in
Cu+ on the tetrahedral sites which displaces the smaller
Fe3+ cation to the octahedral sublattice resulting in an
overall expansion of the lattice. The lattice then begins to
contract as the copper is lost from the ferrite lattice in the
form of CuFeO2.
The lattice parameter of the bulk air-quenched samples
was determined in the same manner as the oil-quenched
samples. The results of the 4␲Ms at room temperature
and density measurements are tabulated in Table V. The
4␲Ms values given have been scaled to the theoretical
density of 5.25 g/cm3. The samples heated at 1360 °C
and the furnace-cooled samples are not phase pure but
were scaled for comparison. The 3␴ error bars in all
cases are larger than for the oil quench samples; this
indicates that there is a gradient in lattice parameter
through the sample thickness. Since the outer surface of
the samples cools more rapidly than the interior, it is
suspected that this outer surface is composed of the larger
lattice parameter material. Unfortunately the “skin” of
the samples is removed when the sample is ground into
a sphere for 4␲Ms measurements so that the results of the
measurements may not be indicative of the lattice param-
eters measured. Figure 7 illustrates the mean air-
quenched sample lattice parameters indicated by the
horizontal line and the corresponding 4␲Ms measure-
ments relative to the lattice parameter measurements of
the oil-quenched samples.
FIG. 7. Air-quenched bulk samples lattice parameters relative to oil-
quenched samples including 4␲Ms measurements. The mean lattice
parameter is indicated by a horizontal line with corresponding 4␲Ms.
for the quenched samples seem to confirm the SQUID
magnetization results showing a mixture of both Fe2+ and
Fe3+, hence the redox of Cu (and Fe), at low tempera-
tures, and the formation of pure or close to pure mono-
valent but disordered Cu0.5Fe2.5O4 phase at higher
temperatures.
The maximum value of 4␲Ms determined from the
SQUID measurements for Cu0.5Fe2.5O4 was found to be
0.7729 and 0.5426 T at 10 and 300 K, respectively, for
the sample quenched into oil from a temperature of
1348 °C and PO of 0.21 atm. The cation distribution
2
based on the low-temperature 0.7729 T 4␲Ms measure-
ment, in light of the Mo¨ssbauer, lattice parameter, and
resistivity data is
(Cu+0.24Fe3+0.76)A[Cu+0.26Fe3+1.74]BO4 → 4.9 ␮
.
B
However, the presence of small amounts of Cu2+ in the
sample cannot be completely ruled out. Consistent with
small amounts of Cu2+ that may be present, the spinel
structure can be written as
The SQUID magnetization measurement on the
samples quenched into oil, from the air mixture, seen
from Table VI, shows an increasing trend as the tempera-
ture of the quench is increased. The lattice parameter
results along with the Mo¨ssbauer and resistivity studies
(Cu+0.24Fe3+0.76)A[(Cu+0.26−xCu2+x)(Fe3+1.76−xFe2+x)]BO4
→ 4.9 ␮
.
B
TABLE V. Bulk samples lattice parameter and magnetization as de-
termined for Cu0.5Fe2.5O4 heated in air/CO2 mixture followed by
air quenching.
The corresponding cation distribution in the case of
Li0.1Cu0.4Fe2.5O4 based on the maximum value of 4␲Ms
of 0.7047 T (4.48 ␮B) is
Lattice
parameter
(Å)
Standard
deviation
␴ (Å)
(Cu+0.15Fe3+0.85)A[(Cu+0.25Li+0.1Fe3+1.65]BO4
Temperature
(°C)
Density (%)
theoretical
Corrected
4␲Ms (T)
→ 4.48 ␮
.
B
(1) 1265
(2) 1350
(3) 1360
furnace cool
8.4160
8.4182
8.4197
8.4111
0.0024
0.0012
0.0012
0.0009
94
92
93
96
0.4948
0.5173
0.5070
0.3539
The lattice parameter and magnetization of the
Cu0.5Fe2.5O4 and Li0.1Cu0.4Fe2.5O4 phases are tabulated
in Table VII. A decrease in magnetization value on the
addition of Li suggests that the low values of saturation
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K.E. Kuehn et al.: Synthesis of copper and lithium copper ferrites as high magnetization materials
TABLE VI. SQUID magnetization measurements of powder samples including lattice parameters at 10 K as determined for Cu0.5Fe2.5O4 under
conditions similar to those in Figure 3.
Final temperature
(°C)
Final
Lattice parameter
(Å)
Standard deviation ␴
Figure of merit
(7)
PO
(Å)
4␲Ms (T)
2
Cu0.5Fe2.5O4
1226
1286
1326
1366
1348
1341
1205
1201
1091
992
889
789
0.21
0.21
0.21
0.21
0.21
8.4162
8.4164
8.4181
8.4192
8.4185
8.4172
8.4141
8.4196
8.4154
8.4141
8.4162
8.4158
0.0003
0.0004
0.0007
0.0007
0.0005
0.0008
0.0006
0.0009
0.0003
0.0004
0.0004
0.0002
319
251
137
131
66
0.7474
0.7640
0.7718
0.7569
0.7729
0.7610
0.7523
0.7472
0.7528
0.7293
0.7648
0.6733
0.17
91
4.3 × 10−2
8.9 × 10−3
1.5 × 10−2
1.2 × 10−2
2.5 × 10−3
1.4 × 10−3
145
103
268
208
146
397
Li0.1Cu0.4Fe2.5O4 Samples
1226
1323
889
0.21
0.21
8.4000
8.4014
8.3939
8.3957
0.0009
0.0009
0.0009
0.0009
69
77
91
75
0.7047
0.6913
0.6809
0.6618
2.5 × 10−3
1.4 × 10−4
789
TABLE VII. SQUID magnetization measurements of powder samples including lattice parameters at 300 K as determined for Cu0.5Fe2.5O4 and
Li0.1Cu0.4Fe2.5O4 under conditions similar to those in Figure 3.
Final temperature
(°C)
Final
Lattice parameter
(Å)
Standard deviation
Figure of merit
(7)
PO
␴ (Å)
4␲Ms (T)
2
Cu0.5Fe2.5O4
1348
1226
0.21
0.21
8.4185
8.4162
8.4153
0.0005
0.0003
0.0002
66
319
397
0.5426
0.5071
0.4612
992
1.2 × 10−2
Li0.1Cu0.4Fe2.5O4
1226
0.21
8.400
0.0009
69
0.5234
magnetization (from speculated 1.1 T) observed in
Cu0.5Fe2.5O4 system is not due to spin–lattice canting but
is due to the distribution of Cu+ on A and B sites.
temperature and at the same time maintain a highly
reducing atmosphere to stop redox from occurring.
This optimization of temperature and PO demands
2
further work on dynamic in situ XRD with atmosphere
control.
The solid solution region for the composition studied
was determined to be from 1200 to 1350 °C. This region
was determined via dynamic x-ray characterization. This
characterization method was also used to determine that
the minimum quench rate necessary to maintain the
phase purity of a thin film is 7 °C/s. The rate of decom-
position of the Cu0.5Fe2.5O4 structure into Fe2O3 and
CuFe2O4 air was also determined.
Cu0.5Fe2.5O4 may be cooled to temperatures under
1200 °C by decreasing the oxygen partial pressure. In
this study, phase-pure Cu0.5Fe2.5O4 ferrite was main-
tained by slow cooling to 900 °C, 300 °C below the low
temperature boundary for this material in air. However,
the slow-cooled samples do not show increased satura-
tion magnetization over the samples quenched from the
high-temperature formation region.
IV. CONCLUSIONS
The nature of Cu0.5Fe2.5O4 with the presence of
multivalent copper and iron ions with differing site pref-
erences makes it a difficult material to study. Ideally,
this material is capable of extending the saturation
magnetization limit beyond the present day maximum.
The expansion of the lattice for high-temperature heat
treatments followed by quenching is thought to be due
to the redox of Cu2+ ion into Cu+ ion, which in turn
migrates to the tetrahedral positions. However, since
the temperature at this point is very high, all Cu+ formed
is not able to occupy the preferred tetrahedral site but
is disordered between the tetrahedral A-site and octa-
hedral B-site. The key to the ordering of the Cu+ ions
to the tetrahedral A-site, it now seems, is to lower the
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K.E. Kuehn et al.: Synthesis of copper and lithium copper ferrites as high magnetization materials
4. X. Tang, A. Manthiram, and J.B. Goodenough, Solid State Chem.
The maximum saturation magnetization for the
79, 250 (1989).
Cu0.5Fe2.5O4 samples was 0.7729 T at 10 K and 0.5426 T
at 300 K. The 300 K saturation magnetization surpasses
that of the present zinc-containing ferrite compositions,
and due to the mixed nature of the copper distribution
over the A and B sites, the composition Cu0.5Fe2.5O4 has
great potential to exceed, by far, the present magnetiza-
tion maximum in the ferrite crystal structure.
5. E.Ya. Sapozhnikova, A.G. Davidovich, E.M. Roizenblat, M.A.
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