Structural, magnetic and electrical properties of ho and ni co-Doped bifeo3 materials

 
BFO 5.584 13.867 60 
x = 0 5.589 13.875 60 
x = 0.025 5.588 13.873 50 
x = 0.050 5.588 13.868 42 
x = 0.075 5.586 13.863 37 
x = 0.1 5.582 13.857 33 
Figure 3. SEM images of BFO and Bi1-xHoxFe0.97Ni0.03O3 samples: (a) BFO; (b) x = 0; and (c) x = 0.1. 
Figure 3 shows scanning electron microscope (SEM) images of BiFeO3 and Bi1-
xHoxFe0.97Ni0.03O3 (x = 0, and 0.1) samples. BFO sample consists of granular particles with a 
fairly large size of about a few micrometers (Fig. 3a) and clear boundary. When doping Ni
2+
 into 
Fe
3+
 site, the particle size decreases obviously and becomes more inhomogeneous (Fig. 3b). The 
particle size decreases further to less than 1 micrometer when co-doping Ho
3+
 into Bi
3+
 site (Fig. 
3c). Furthermore, particles size and morphology are poorly homogeneous. In particular, the 
grain boundary becomes less clear. This reveals that Ni
2+ 
doping and (Ni
2+
, Ho
3+
) co-doping 
affect strongly on the particle size and morphology of BiFeO3 which also can be attributed to the 
Dao Viet Thang, Nguyen Manh Hung, Bui Thi Thu, Le Thi Mai Oanh, Bui Dinh Tu, Nguyen Van Minh 
156 
differences of doping ions (Ni
2+
, Ho
3+
) and host lattice ions (Fe
3+
, Bi
3+
). The substitution of these 
doping ions in BFO lattice leads to a certain disorder in crystal structure, resulting to restrict the 
crystalline growth. 
Figure 4. (a) Raman scattering spectra of BiFeO3 and Bi1-xHoxFe0.97Ni0.03O3 (x = 0, 0.025, 0.05, 0.075, and 
0.1) materials; (b) Fitted Raman peaks in the wavenumber region of 100 to 800 cm
-1
. 
Raman scattering spectra of BFO and Bi1-xHoxFe0.97Ni0.03O3 are shown in Fig. 4a. According 
to the group theory, 13 Raman active modes could be desirable for the rhombohedral BFO 
structure with R3C space group (Γ = 4A1 + 9E) [20, 21]. However, not all modes could be clearly 
observed at room temperature [22]. Raman peaks of all samples are fitted by Gaussian function, 
as are shown in Fig. 4b and Table 2. 
As seen in Table 2, for the x = 0 sample, positions of A1-1, A1-2, A1-3, A1-4, E-6, E-7 modes 
change a little, position of E-2, E-3, E-4 and E-5 modes shift toward higher frequency while E-8 
100 200 300 400 500 600 700 800
(a)
E
-8
E
-5
E
-4
E
-2 E
-3
A
1
-3
A
1
-2
x= 0.1
x=0.075
x=0.05
x=0.025
x= 0 
In
te
n
s
it
y
 (
a
.u
.)
Wavenumber (cm-1)
BFO
A
1
-1
E
-7
E
-6
E
-9
100 200 300 400 500 600 700 800
Wavenumber (cm-1)
In
te
n
s
it
y
 (
a
.u
.)
x = 0
x = 0.1
(b)
Table 2. The Raman modes positions of BiFeO3 and Bi1-xHoxFe0.97Ni0.03O3 (x = 0, 0.025, 0.05, 0.075, 
and 0.1) materials (cm
-1
). 
Modes BFO x = 0 x = 0.025 x = 0.05 x = 0.075 x = 0.1 
A1-1 139 140 138 143 141 142 
A1-2 173 171 172 171 172 171 
A1-3 225 223 222 228 227 228 
E-2 252 258 257 258 258 258 
E-3 262 278 296 280 279 272 
E-4 285 344 344 340 334 334 
E-5 336 369 372 370 365 371 
E-6 433 434 435 437 424 437 
E-7 474 470 477 472 475 473 
A1-4 521 517 518 530 571 527 
E-8 555 547 539 605 607 607 
E-9 706 604 602 655 668 689 
Structural, magnetic and electrical properties of Ho and Ni co-doped BiFeO3 materials 
157 
and E-9 modes shift toward lower frequency in comparison with those of BFO. For the (Ho, Ni) 
co-doped samples, positions of A1-1, A1-3, A1-4, E-3, E-6, and E-7 modes tend to shift slightly 
toward higher frequency, positions of E-2, E-4, and E-5 modes change a little, while positions of 
E-8 and E-9 modes tend to shift strongly toward higher frequency in comparison with those of 
the x = 0 sample. Previous studies have also showed that the A1 modes and E modes at low 
frequency characterize for Bi-O covalent bonds, other E modes at high frequency characterize 
for Fe-O bonds [23]. The A1-1, A1-3, A1-4, E-3, E-6, and E-7 modes characterize for Bi-O 
covalent bonds [14, 24], while the E-8, E-9 modes characterize for Fe-O bonds [11, 25]. So the 
changes of E-8, E-9 modes confirmed Ni
2+
 ions substituted into Fe-sites, and the changes of A1-
1, A1-2, A1-3, A1-4, E-3, E-6 and E-7 confirmed Ho
3+
 ions substituted into Bi-sites. These results 
are conformable to the XRD results, which confirmed that Ho
3+
 and Ni
2+
 ions substituted into 
Bi-sites and Fe-sites, respectively. 
Figure 5. (a) Magnetic hysteresis loops of BiFeO3 and Bi1-xHoxFe0.97Ni0.03O3 materials; 
(b) Dependence of saturation magnetization and remnant magnetization on concentration of Ho. 
Figure 5a shows magnetic hysteresis loops of BFO and Bi1-xHoxFe0.97Ni0.03O3 materials at 
room temperature. As seen in Fig. 5a, all samples present weak ferromagnetic behavior. BFO 
sample has saturation magnetization of Ms = 0.047 emu/g and remanent magnetization of Mr = 
0.008 emu/g. When doping with Ni
2+
 ions in Fe
3+
 site, Ms and Mr values increase to 0.702 emu/g 
and 0.169 emu/g, respectively. However, when co-doping Ho
3+
 in Bi
3+
 site they decrease slightly 
as observed in Fig. 5b. This reveals that Ni
2+
 doping enhances ferromagnetization in BFO that 
can be attributed to some following reasons: (i) the appearance of ferrimagnetic order Ni
2+
-O
2-
-
Fe
3+
 besides antiparallel indirect interaction between Fe
3+
 and neighbor Fe
3+ 
[26, 27]; (ii) Ni
2+
replacement in Fe
3+
 site causes the charge shortage, in order to neutralize the charge, some Fe
3+
ions transform to Fe
4+
, resulting the ferromagnetic double exchange interaction Fe
3+
-O
2-
-
Fe
4+
; 
(iii) the enhancement of magnetization due to the appearance of oxygen vacancies and lattice 
defects when doping Ni
2+
 in BFO lattice [28]. The slight decrease of saturation magnetization 
and remanent magnetization in (Ho, Ni) co-doped samples in Fig. 5b could be explained by the 
reduction of oxygen vacancies due to Bi
3+
 volatilization [29, 30]. However, the Ms and Mr values 
in (Ho, Ni) co-doped samples are still larger 12 and 15 times in comparison with those of BFO. 
Figure 6 shows complex impedance spectra of BiFeO3 and Bi1-xHoxFe0.97Ni0.03O3 (x = 0, 
0.05, and 0.1) which can indicate the contribution of grain, grain boundaries, and electrode 
interface into impedance of materials. As can be seen, simulated curves fit well with the 
experimental curves in all samples. Fig. 6a presents that the impedance spectrum of BFO 
-4000 -2000 0 2000 4000
-0.5
0.0
0.5
-200 -100 0 100 200
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
M
 (
e
m
u
/g
)
H (Oe)
M
 (
e
m
u
/g
)
H (Oe)
 BFO
 x=0
 x=0.025
 x=0.05
 x=0.075
 x=0.1
(a)
0.000 0.025 0.050 0.075 0.100
0.0
0.2
0.4
0.6
0.8
M
 (
e
m
u
/g
)
Ho content
 Ms
 Mr
(b)
BFO
Dao Viet Thang, Nguyen Manh Hung, Bui Thi Thu, Le Thi Mai Oanh, Bui Dinh Tu, Nguyen Van Minh 
158 
consists of one semicircle at high frequency and one semicircle at intermediate frequency, which 
indicate the contribution of grains and grain boundaries, respectively. The resistance of grain 
boundaries is much larger than that of grains. Figs. 6b, c, d exhibit only one semicircle that 
corresponds to the contribution of grain. As observed in XRD and SEM results, particle size was 
large for BFO and then decreased obviously when doping Ni
2+
 and co-doping Ho
3+
 into BFO. 
Furthermore, the grain boundary became less clear after doping Ni
2+
 and co-doping Ho
3+
. This 
may be the reason for the contribution of grains and grain boundaries into impedance of 
samples. Since the limitation of frequency range, the contribution of electrode interface to 
impedance can’t be detected in all samples. 
Figure 6. Impedance spectra of BiFeO3 (a) and Bi1-xHoxFe0.97Ni0.03O3 materials with 
x = 0 (b), x = 0.05 (c), and x = 0.1 (d). 
4. CONCLUSION 
In summary, BiFeO3 and Bi1-xHoxFe0.97Ni0.03O3 (x = 0, 0.025, 0.05, 0.075, and 0.1) materials 
have been successfully fabricated by sol-gel method. All samples crystallize in rhombohedral 
structure of BFO materials. Crystal lattice parameters and average crystallite size increased with 
doping Ni
2+
 and decreased gradually with co-doping Ho
3+
. X-ray diffraction patterns and Raman 
scattering spectra confirmed the successful substitution of Ho
3+
 and Ni
2+
 ions into Bi-sites and 
Fe-sites, respectively, which affected obviously to magnetic and electrical properties of BFO. 
Ni
2+
 replacement kept an important role in enhancing ferromagnetization of BFO. Complex 
0.0 30.0M 60.0M 90.0M 120.0M
0.0
3.0M
6.0M
9.0M
-Z
"
 (

)
Z' ()
 Fit
 Fit
 Experiment
(a)
0.0 20.0M 40.0M 60.0M
0.0
6.0M
12.0M
18.0M
24.0M
-Z
''
 (

)
Z' ()
 Fit
 Experiment
(b)
0.0 2.0M 4.0M 6.0M
0
1M
2M
3M
- 
Z
''
 (

)
Z' ()
 Fit
 Experiment
(c)
0.0 6.0M 12.0M 18.0M
0.0
2.0M
4.0M
6.0M
- 
Z
''
 (

)
Z' ()
 Fit
 Experiment
(d)
Structural, magnetic and electrical properties of Ho and Ni co-doped BiFeO3 materials 
159 
impedance spectra showed the main contribution of grains and grain boundaries into impedance 
of samples. 
Acknowledgement. This work has been supported by the Ministry of Education and Training of Vietnam 
(Code B2018-MDA-02-CtrVL). 
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