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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