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Modification of Co/Cu nanoferrites properties via Gd3+/Er3+doping
Ebtesam E. Ateia1 • Fatma S. Soliman1
Received: 27 January 2017 / Accepted: 31 March 2017
� Springer-Verlag Berlin Heidelberg 2017
Abstract Pure nanoparticles of the rare earth-substituted
cobalt and copper ferrites with general formula Me Gd0.025Er0.05 Fe1.925 O4 (Me = Co, Cu) were prepared by the
chemical citrate method. X-ray diffraction, field emission
scanning electron microscopy, BET analysis are utilized to
study the effect of rare earth substitution and its impact on
the physical properties of the investigated samples. Rare
earth-doped cobalt shows type IV isotherm suggesting
mesopore structure with its hysteresis loop. The estimated
crystallite sizes are found in the range of 21.49 and
36.11 nm for the doped Co and Cu samples, respectively.
The magnetic properties of rare earth-substituted cobalt
and copper ferrites showed a definite hysteresis loop at
room temperature. An increase in coercivity and a decrease
in saturation magnetization were detected. This can be
explained in view of weaker nature of the Re3?–Fe3?
interaction compared to Fe3?–Fe3? interaction. Greater
than 1.13-fold increase in coercivity (Hc = 2184 Oe) was
observed in doped cobalt nanoferrite samples compared to
copper (Hc = 1936 Oe). It was found that the decreasing in
temperature leads to great improvement in the magnetic
properties of the investigated samples. As the magnetic
recording performance of the magnetic samples is
improved for well-crystallized samples with nano-struc-
tural, the effect of rare earth substitution seems to be par-
ticularly valuable in this regard.
1 Introduction
Study of the various categories of magnetic spinel ferrites
(MSFs) [1], doped ferrites [2] and nanomaterials [3] has a
great importance because of their variant applications in
many electronic and microwave devices [4]. The research
interest lies on cobalt ferrite-based material because of
their potential applications in high-density information
storage and magneto-optical devices [5, 6]. The ferro-
magnetic samples of Cu-ferrites may exist in modifications
with cubic or tetragonal crystal structure [7]. Copper-based
ferrites have been commercially used in high-frequency
devices as radiofrequency coils and magnetic cores of
read–write heads for high-speed digital tapes [8]. The
substitution of rare earth elements in cobalt and copper
ferrite are promising for their magneto-optical recording
application as they are helpful in reducing the grain size of
the materials and also alter the saturation magnetization
and coercivity as compared to the pure ferrite [9, 10]. It is
known that the magnetic behavior of the ferrimagnetic
oxide compounds is largely governed by the Fe3?–Fe3?
interaction (the coupling of the spin of the 3d electrons).
By introducing rare earth metal ions (Re3?) in spinel lat-
tice, Re3?–Fe3? interactions appear too, which can lead to
changes in the magnetization and curie temperature [11].
Moreover, lanthanide ions can be isotropic or anisotropic
due to the variation in the f electron orbital contribution to
the magnetic interactions. Therefore, the diverse properties
of the RE3? (lanthanide) ions make them interesting can-
didates for doping CoFe2O4 and CuFe2O4 nanoparticles to
modulate the magnetic characteristics [12, 13]. The sub-
stitution of rare earth ions with large ionic radii in spinel
ferrites is expected to induce strain and to significantly
modify the structural and magnetic properties.
& Fatma S. Soliman
fsamy@sci.cu.edu.eg
1 Physics Department, Faculty of Science, Cairo University,
Giza, Egypt
123
Appl. Phys. A (2017) 123:312
DOI 10.1007/s00339-017-0948-8
Rare earth-doped ferrites with modified properties find
applications in enhanced magnetic storage, electronic and
microwave devices and catalysis [14–16].
In this work we have studied the doping effects on
structural, magnetic and morphological properties of the
rare earth-doped samples. The samples were prepared with
nominal composition CoGd0.025Er0.05Fe1.925O4 and
CuGd0.025Er0.05Fe1.925O4 where the rare earth ions (Er3?/
Gd3?) were inserted in the Co/Cu sites.
2 Experimental work
Nanoparticles of doped cobalt and copper ferrite were
prepared using citrate combustion method [17]. In this
method, the stoichiometric quantities of Fe(NO3)3�9H2O,
Co(NO3)2�6H2O or Cu(NO3)2�3H2O, Gd(NO3)3�6H2O and
Er(NO3)3�5H2O were dissolved in double-distilled water
and stirred well using a magnetic stirrer for about 1 h at
80 �C, followed by drying at 200 �C. The structure and
crystallite sizes were tested by X-ray diffractometer (XRD)
using Diano corporation of target Cu-Ka (k = 1.5424 A).
The average nanoparticle sizes were estimated using
Scherrer’s relationship [18]. The morphology of the sam-
ples was studied by field emission scanning electron
microscopy (FESEM) attached with EDX unit (energy-
dispersive X-ray analyses). The magnetization M (emu/g)
was measured at room temperature using a vibrating
sample magnetometer (VSM) Model Lake Shore 7410. The
synthesized powder of Co and Cu samples was calcined at
400 and 800 �C, respectively, for 4 h with heating rate of
4 �C/min. The specific surface area (SBET) was deter-
mined by Brunauer–Emmet–Teller (BET) method [19]
based on adsorption/desorption isotherms of nitrogen at
77 K obtained with NOVA 2200, USA, Automated gas
sorption system. The magnetization M (emu/g) was mea-
sured at room temperature and at 77 K using a vibrating
sample magnetometer (VSM) Model Lake Shore 7410.
3 Results and discussion
The formation of rare earth-substituted cobalt/copper fer-
rites is established by their characteristic powder X-ray
diffraction pattern as shown in Fig. 1a, b. The diffraction
peaks for the investigated samples correspond to spinel
lattice with a cubic structure for CoGd0.025Er0.05Fe1.925O4
and tetragonal crystal structure for CuGd0.025Er0.05Fe1.925-O4. No extra reflection peaks are observed in the X-ray
diffraction patterns corresponding to rare earth ions. The
rare earth-substituted Co samples have peaks at positions
30.09� and 62.73� with higher intensity compared to the
pure cobalt. On the other hand, for Cu samples the peaks
appear at positions 29.84�, 63.01� and 74.5� with higher
intensity compared to the pure Cu samples. The broader
diffraction peaks indicate the nanocrystalline nature of the
investigated samples.
The theoretical lattice parameter for rare earth-substi-
tuted cobalt/copper is calculated as mentioned before [20]
and the obtained data are tabulated in Table 1.
On rare earth substitution, the lattice constant is found to
appreciably increase than the pure Co/Cu nanoferrite
samples [21]. This increase can be attributed to complete
incorporation of large ionic radii Gd3? (0.938 A)/Er3?
(0.89 A) ions instead of smaller Fe3? (0.645 A) in Co/Cu
lattice leading to increase in lattice parameter [22]. The
difference between the theoretical and the experimental
lattice parameters for cobalt samples may be ascribed to
the migration of small ratio of Co2? (3d7) ions from the
octahedral to the tetrahedral sites with a magnetic moment
aligned anti-parallel to those of rare earth (RE3?) ions in
the spinel lattice [23].
The tolerance factor (T) [24] for the spinel structured
materials is tabulated in Table 1. For an ideal spinel
structure tolerance factor (T) values are close to unity. It is
found that for the investigated samples, value of T is close
to unity suggesting defect-free formation of spinel
structure.
Figure 2a, b shows the energy-dispersive X- ray spec-
troscopy (EDAX) analysis for the investigated samples.
The characteristic peaks in the spectrum comprise Co, Cu,
Fe, Gd, Er and O.
The atomic percentage (at%) and weight percentage
(wt%) of constituent elements, (Co, Cu, Fe, Gd, Er and O)
are calculated theoretically from the given formula Co
Gd0.025 Er0.05 Fe 1.925 O4 and CuGd0.025Er0.05Fe1.925O4.
The obtained data from EDAX elemental analysis are
shown as inset of the figure. EDAX analyses indicate that
Gd3?/Er3? ions are successfully incorporated into the Co/
Cu ferrite samples. The variation between the estimated
weight percentage and the starting stoichiometric ratio of
the investigated samples can be attributed to many factors.
The most important are the time constant (Tc), acceleration
voltage (AV), dead time (DT), acquisition time (AT),
magnification and work distance (WD) which have direct
effect on the energy resolution, peak intensity and natural
width of characteristic X-ray lines [25].
Figure 3a, b illustrates the FESEM micrographs of
CoGd0.025Er0.05Fe1.925O4 and CuGd0.025Er0.05Fe1.925O4.
The grain shape of doped cobalt sample is highly
agglomerated, but the doped copper sample has a lower
agglomeration with a fine capsule nanostructure. The
growth of the crystalline grains is restricted leading to the
relative small grains. The average grain sizes are 29.55 and
48.73 nm for doped Co and Cu samples, respectively, as
detected from the inset of the figure.
312 Page 2 of 9 E. E. Ateia, F. S. Soliman
123
Figure 4 a, b illustrates the adsorption/desorption iso-
therm of nitrogen at 77 K for CoGd0.025Er0.05Fe1.925O4 and
CuGd0.025Er0.05Fe1.925O4, the inset figures show the dis-
tribution of pore size versus pore volume of the investi-
gated samples. The specific surface area (SBET) with pore
volume and pore width are calculated and tabulated in
Table 2.
The isotherm of adsorption/desorption of the doped Co
sample exhibits type IV behavior with H3-type hysteresis
loop according to the IUPAC classifications [26–28]. The
IV hysteresis loop is the characteristic of mesopore struc-
ture which have pore sizes intermediate between 2.0 and
50.0 nm [29]. While the Cu-doped sample has pore width
up to 2.0 nm it exhibits type I behavior. This type (H1) is
related to micropores with nonlinearity of the adsorption
isotherm [30].
Figure 5a–c shows the hysteresis loops of the investi-
gated samples using a vibrating sample magnetometer
(VSM) at room temperature and 100 K. From the figure, it
is clear that the magnetization increases with the applied
magnetic field until reaching saturation behavior.
From the hysteresis loops, the coercivity (HC), satura-
tion magnetization (MS), remnant magnetization (Mr),
squareness (Mr/Ms) and magnetic moment (nB) are cal-
culated and tabulated in Table 3. Generally, the magnetic
moment nB is calculated from the saturation magnetization
(MS) value at room temperature by the following equation
[31].
nB ¼ Mw �MS=5585 ð1Þ
where Mw is the molecular weight of a particular ferrite
composition. The observed nB values obtained by this
0
50
100
150
200
250
300
350
400
Inte
nsity
(C
ount
s/Se
c)
2 (º)
Card no. (04-006-4148)
Pure Co Ferrite
Doped Co Ferrite
0
50
100
150
200
250
300
350
400
10 20 30 40 50 60 70 80
10 20 30 40 50 60 70 80
Inte
nsity
(C
ount
s/Se
c)
2 (º)
Card no. (00-034-0425)
Pure Cu Ferrite
Doped Cu Ferrite
(a)
(b)
Fig. 1 The X-ray diffraction
pattern for a CoFe2O4 and
CoGd0.025Er0.05Fe1.925O4 and
b CuFe2O4 and
CuGd0.025Er0.05Fe1.925O4
Table 1 Values of theoretical lattice parameter (atheo.), experimental lattice parameter (aexp.), tolerance factor and crystallite size for
CoGd0.025Er0.05Fe1.925O4 and CuGd0.025Er0.05Fe1.925O4
Cation distribution atheo. (A) aexp. (A) Tolerance factor (T) Crystallite size (nm)
Fe3þ� �
Co2þGd3þ0:025Er3þ0:05Fe
3þ0:925
� �O4 8.438 8.373 0.986 21.49
ðCu2þ0:08Fe2þ0:9Fe3þ0:02Þ a = 5.844 a = 5.813 1.02 36.11
Cu2þ0:92Gd3þ0:025Er
3þ0:05Fe
3þ1:005
� �O4 c = 8.613 c = 8.683
Modification of Co/Cu nanoferrite properties via Gd3?/Er3?doping Page 3 of 9 312
123
equation are tabulated in Table 3. It is clear from the
table that the magnetization values decrease with the sub-
stitution of rare earth ions. This behavior can be explained
on the basis of site occupancy of the cations and the mod-
ifications in the exchange effects due to the doping of rare
earth ions. The main contribution of magnetic properties
arises from Fe3? on B-sites of spinal structure. Actually, the
effect of RE3? substitution on the magnetic properties of
spinel ferrites is complicated, and it is a comprehensive
influence of numerous factors, such as morphology, struc-
ture, cation redistribution, particle size, etc.
It is clear that the coercivity of Gd/Er-doped cobalt is
very high as compared to the copper-doped samples. This
is a typical behavior of hard ferrites. Therefore, it can be
concluded that doped Co is a hard ferrite while doped Cu
sample is a soft ferrite. The variation of coercivity with
grain size is discussed by Stoner–Wohlfarth theory [32]. In
multi-domain particles, the magnetization reversal arises
due to the domain wall movement. As the domain walls
move through a particle, they are pinned at grain bound-
aries. The additional energy is required for domain walls to
continue the wall movement.
Therefore, the doping of rare earth ions creates more
pinning sites and increases the coercivity of the samples. In
the present investigation, the detected behavior in the
doped Co/Cu ferrite nanoparticles attributes to spin canting
Fig. 2 The energy-dispersive
X-ray spectroscopy (EDAX) for
a CoGd0.025Er0.05Fe1.925O4 and
b CuGd0.025Er0.05Fe1.925O4
samples
312 Page 4 of 9 E. E. Ateia, F. S. Soliman
123
Fig. 3 FESEM images of
a CoGd0.025Er0.05Fe1.925O4 and
b CuGd0.025Er0.05Fe1.925O4; the
inset figures show the grain size
distributions for the investigated
samples
(a)
(b)
0
10
20
30
40
50
60
70
80
90
0.00E+00 2.00E-01 4.00E-01 6.00E-01 8.00E-01 1.00E+00
Vol
ume
(cc/
g)
P/Po
adsorption
desorption
0
5
10
15
20
25
30
35
0.00E+00 2.00E-01 4.00E-01 6.00E-01 8.00E-01 1.00E+00
Vol
ume
(cc/
g)
P/Po
adsorption
desorption
Fig. 4 The adsorption/
desorption isotherm of nitrogen
at 77 K for
a CoGd0.025Er0.05Fe1.925O4 and
b CuGd0.025Er0.05Fe1.925O4; the
inset figures show the
distribution of pore size versus
pore volume of the investigated
samples
Modification of Co/Cu nanoferrite properties via Gd3?/Er3?doping Page 5 of 9 312
123
-30
-20
-10
0
10
20
30
Mag
netiz
atio
n (e
mu/
g)
Magnetic Field (Oe)
Room100 K
(b) CuRE
-60
-40
-20
0
20
40
60
Mag
netiz
atio
n (e
mu/
g)
Magnetic Field (Oe)
CoRECuRE
(c) 100 K
-60
-40
-20
0
20
40
60
-40000 -20000 0 20000 40000
-40000 -20000 0 20000 40000
-30000 -20000 -10000 0 10000 20000 30000
Mag
netiz
atio
n (e
mu/
g)
Magnetic Field (Oe)
Room100 K
(a) CoRE
Fig. 5 Magnetic hysteresis loops for a CoGd0.025Er0.05Fe1.925O4, b CuGd0.025Er0.05Fe1.925O4 at 300 and 100 K, c Doped Co and Cu samples at
100 K
Table 3 Saturation magnetization, remnant magnetization, coercive field, squareness, energy loss, exp. magnetic moment, anisotropy constant,
and (BH)max for the investigated samples
Sample Ms
(emu/g)
Mr
(emu/g)
Hc (Oe) Mr/Ms Energy loss
(erg/g)105nBExp Anisotropy
Const. 9 104 (emu.Oe/g)
Co samples
CoFe2O4 [20] 66.85 31.11 1641 0.465 3.37 2.8 11.43
CoGd0.025Er0.05Fe1.925O4
300 K 53.74 25.14 2184 0.467 3.59 2.3 11.97
100 K 42.24 29.85 3343 0.706 8.11 1.8 14.41
Cu samples
CuFe2O4 [20] 22.06 11.65 1041 0.528 0.72 0.94 2.39
CuGd0.025Er0.05Fe1.925O4
300 K 20.64 10.92 1936 0.529 1.08 0.91 4.08
100 K 25.88 13.43 1492 0.518 1.17 1.14 3.94
Table 2 The calculated surface
area, pore size and pore volume
for the investigated samples
Sample Surface area (m2/g) Pore volume (cc/g) Pore width (nm)
CoGd0.025Er0.05Fe1.925O4 75.51 0.013 2.226
CuGd0.025Er0.05Fe1.925O4 21.48 0.048 1.386
312 Page 6 of 9 E. E. Ateia, F. S. Soliman
123
and surface spin disorder which occurred in the studied
samples [33]. The obtained results for Ms and Mr of Gd/Er-
doped nanoferrite samples suggest their suitability in
applications like magnetic targeting and separators.
The existence of Cu2? ion as a Jahn–Teller ion in the
octahedral sites of the copper-doped samples causes a lat-
tice distortion, which has the effect of removing the orbital
degeneracies of Cu2? cations. It is expected that this effect
in turn generates large strains in the copper ferrite lattice,
and as a result improved magnetic properties. In other
words, this distortion changes A–B distance (Fe–Cu dis-
tance) and hence A–B interaction.
As shown from Table 3 the magnetization value at a
given temperature (100 K) is significantly higher for cop-
per ferrite with tetragonal structure compared to cobalt
ferrite with a cubic spinel structure. The obtained data are
explained on the basis of the competition between the
thermal energy and the magnetocrystalline anisotropy
energy in response to the applied magnetic field [34]. As
temperature decreases from 300 to 100 K, the applied
magnetic field increasingly became more effective in
aligning the magnetic moments in its direction. This is the
main reason for the observed increase of saturation mag-
netization as shown in copper sample.
An important parameter for hard magnetic materials is
the (BH)max value, which is the largest area of the rectangle
that can fit in the demagnetizing M versus H curve at the
second quadrant, see Fig. 5. CoGd0.025Er0.05Fe1.925O4
nanoparticles exhibit unusual magnetic properties with a
(BH)max equal to 0.382 kJ/m3. This result is in agreement
with the large value of remanence magnetization and high
anisotropy constant for cobalt samples.
Squareness ratio (Mr/Ms) is calculated and tabulated in
Table 3. According to the Stoner–Wohlfarth model [32], the
investigated samples can be considered as uniformly mag-
netized and isotropically distributed without intergrain inter-
actions. If the squareness (Mr/Ms) value ismore than 0.5, then
the exchange coupling between adjacent grainswill take place
as in the case of CoGd0.025Er0.05Fe1.925O4 at 100 K.
The analysis of optical absorption spectra is a powerful
tool for understanding the band structure and band gap of
the nanoferrite particles. The optical properties of the fer-
rite samples are characterized by UV–Vis DRS with the
help of optical reflection data. Figure 6a shows the UV–
visible diffuse reflectance spectra of CoGd0.025Er0.05-Fe1.925O4 and CuGd0.025Er0.05Fe1.925O4 nanoparticles
recorded in the wavelength range from 200 to 2000 nm.
Generally, the ferrites can absorb a significant quantity of
visible light due to electron excitation from the O–2p level
(valence band) to the Fe-3d level (conduction band) [35].
Table 4 The experimental optical band gap values for doped Co/Cu
samples
Samples Energy gap (eV)
Indirect Direct
CoGd0.025Er0.05Fe1.925O4 0.9 1.4
CuGd0.025Er0.05Fe1.925O4 1.37 1.86
0
20
40
60
80
100
120(F
(R)h
)2 (eV
)2
h (eV)
doped-Co sampledoped-Cu sample
Direct
0
10
20
30
40
50
0.5 1 1.5 2 2.5 3
200 600 1000 1400 1800
Ref
lect
ance
(%
)
Wavelength (nm)
doped-Co sampledoped-Cu sample
(a)
(b)
Fig. 6 a Optical reflectance
spectrum and b optical band gap
energy from plot of (F(R?)hm)2
versus (hm) forCoGd0.025Er0.05Fe1.925O4 and
CuGd0.025Er0.05Fe1.925O4
samples
Modification of Co/Cu nanoferrite properties via Gd3?/Er3?doping Page 7 of 9 312
123
In addition to this, the synthesized nanoparticles also
exhibit good magnetic behavior at room temperature,
which can find a major role in recovery and recycling of
photocatalysts. Thus, the combination of these two prop-
erties makes doped Co/Cu nanoferrite a superior candidate
for visible light photo catalysis.
The band gap, Eg, is determined from optical reflectance
spectra by extrapolating the straight line plot of (F(R?)
hm) n versus (ht) as shown in Fig. 6 b and according to the
following Kubelka–Munk equation [36].
FðR1Þ:hmð Þn¼ Aðhv - Eg) ð2Þ
where h is the Plank’s constant, m is the frequency of
vibration, A is a constant and Eg is the band gap [37].
Exponent n depends on the type of transition. Table 4
shows the values of optical band gap for CoGd0.025Er0.05-Fe1.925O4 and CuGd0.025Er0.05Fe1.925O4, respectively.
These values are closer to the well-known energy gap that
has been extensively reported for the semiconductor
materials [38]. Similar results were obtained for three types
of compound semiconductor materials (CuInSe2, CuIn0.5-Ga0.5Se2, CuGaSe2), and band gaps were found in the
range of 0.99 B Eg B 1.64 [39].
4 Conclusion
1. The substitution of rare earth elements in cobalt and
copper ferrite is promising for their magneto-optical
recording application as they are helpful in reducing
the grain size of the materials.
2. The successful introduction of Gd3?/Er3? into Co/Cu
ferrite is confirmed by EDAX analysis.
3. The presence of Jahn–Teller ions in copper ferrite
enhances the magnetic properties.
4. The calculated band gap for doped Co/Cu is close to
the band gap for semiconducting material.
5. The obtained magnetic data of Gd/Er-doped nanofer-
rite samples suggest their suitability in applications
like magnetic targeting and separators.
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