Size effects on multiferroic Behavior of BiFeo3 nanoparticles

<strong>Size</strong> <strong>effects</strong> on multiferroic
Behavior of BiFeo 3 nanoparticles
Alina Manzoor and S. K. Hasanain
Magnetic Nanostructures Laboratories.
Physics Department
Quaid-I-Azam University Islamabad.
1

Multiferroics
‣ Multiferroics are materials having coexistence of
ferroelectricity and ferromagnetism.
‣ Very few such materials exist.
‣ BiFeO 3 is one of these few.
‣ Ferroelectricity: The spontaneous alignment of electric
dipoles below a critical temperature T c .
T>T c
T

‣ Ferroelectricity requires the presence of an electric
dipole moment in a unit cell.
‣ only possible if the centers of positive and negative
charge do not coincide.
‣ Only certain lattice symmetries allow for FE to occur.
However,
‣ Those that allow FE, do not allow FM.
‣ Hence FM & FE do not coexist in general.

‣ However, AFM & FE can and do coexist. Many examples
of such systems exist.
‣ In some cases a weak FM is seen to coexist with FE.
BiFeO 3 is one of the few known, in which FE with a FM
behavior as well.
Because of the many important applications of MF in
read-write technologies, much effort to understand and
improve the multiferroics characteristics.
4

Introduction to BiFeO 3
‣ Rhombohedrally distorted perovskite structure
with lattice parameters, a = b ≠ c.
‣ Canted, spiral G-type, G
AFM order.
Structure of R3c BiFeO 3 . Notice the position of the oxygen octahedra
relative to the Bi frame work; in the ideal cubic perovskite structure
the oxygen ions would occupy the face centered sites.

Spin structure AFM (in plane; spiral)
Because of spin-lattice interactions the AFM spins are
pulled out of perfect antiparallel alignments;
A small net moment results; Weak ferromagnetism.
Evidence for magneto-electric coupling. The electric and
magnetic moments are coupled via the lattice.
Weak FM & FE
(a) The magnetic
moments, MFe1 and MFe2, of the two
iron atoms in the unit
cell are oriented antiferromagnetically and
collinearly in the [111]
plane, allowing weak ferromagnetism by
symmetry. (b) Calculated
magnetic structure including the spin-orbit
interaction: The two iron
magnetic moments rotate in the [111]
plane so that there is a resulting
spontaneous magnetization, M.

‣Since Ferroelectric behavior known to be
strongly sensitive to strains, we may expect the
multiferroic behavior to be strongly affected in
nanoparticles.
Why
‣Nanoparticles
→ small sizes, large surface area → strains are
enhanced → can affect FE &FM.
→ large number of O vacancies → Affect the FM.
7

Different studies on BiFeO 3 nanoparticles do
show evidence of Magneto-electric electric coupling.
Enhanced <strong>effects</strong> in small particles.
But reason not well understood.
<strong>Size</strong> and strains or
<strong>Size</strong> and O vacancies.
We investigate with this in mind.
8

Results:
‣ Particles made by soft chemical route.
‣ <strong>Size</strong> range; 11nm to 29nm.
‣ Two annealing enviorments.
‣ Annealed in air (more O vacancies).
‣ Annealed in O (less O vacancies).
‣ Measurements of M(H), ε’(T),
ε”(T),
ρ.
9

Added under
uniform stirring
Tartaric acid
(C 4 H 6 O 6 )
(3.0018 g)
Chelating
2N solution of
HNO 3
in100ml of
distilled
Water
Fluffy
green
agent powder
MMM Evaporation
Constant stirring
and heating at
150 0 C
Mixed solution
of metal
nitrates +
tartaric acid
Nitrates removed from BiFeO 3 powder by
annealing at high temperature (450 0 C-600 0 C)
: Block diagram for the preparation of BiFeO 3
nanoparticles by evaporation method

Crystal structure
X-RD data
Intensity(a.u)
---450-O
---500-O
---550-O
---600-O
(012)
(104)
(110)
(202)
(024)
(122)
(018)
(300)
10 20 30 40 50 60
2θ (degree)
X-ray diffraction pattern of BiFeO 3 samples annealed
in oxygen for 2 hours at various temperature
11

M-H H LOOP
0.2
11 nm
13 nm
21 nm
0.1
M (emu/g)
0.0
0.03
-0.1
M (emu/g)
0.00
-0.2
-0.03
-2000 0 2000
H (Oe)
-10000 -5000 0 5000 10000
H (Oe)
Magnetic hysteresis loops for air annealed samples at 300 K
12

1.5
1.0
17 nm
20 nm
29 nm
0.5
M (emu/g)
0.0
-0.5
M (emu/g)
0.00
20 nm
29 nm
-1.0
-1.5
0
H (Oe)
-10000 -5000 0 5000 10000
H (Oe)
Magnetic Hysteresis loops for oxygen annealed samples at 300K

Derived room temperature magnetic parameters for air annealed
samples
Particle size
(d, nm)
Ms at 10
KOe
(emu/g)
Ms
(
Mr/Ms
11
13
21
0.2
0.1
0.076
0.01
0.0056
0.0043
0.003
0.007
0.009
Derived room temperature magnetic parameter for oxygen
annealed samples
Particle size
(d, nm)
Ms at 10
KOe
(emu/g)
Ms
(
Mr/Ms
17
20
29
1.1
0.15
0.089
0.062
0.0085
0.0049
0.005
0.009
0.005

Dielectric measurements
36
d = 21nm
18
17
34
16
ε
32
15
30
T N
~560K
(ε')
14
28
13
12
26
Frequency=10KHz
11
Frequency=10kHz
24
300 350 400 450 500 550 600 650
T(K)
300 350 400 450 500 550 600 650
T(k)
Temperature dependence of real part of
dielectric constant for 21nm air annealed sample
Temperature dependence of real part of dielectric
constant for 29nm oxygen annealed sample

7.0
6.5
Frequency=10kHz
2.4
6.0
2.2
(ε")
5.5
5.0
T=490k
(ε")
2.0
4.5
ΔT=70k
T N
=560K
1.8
T=565k
4.0
1.6
3.5
Frequency 10kHz
3.0
300 350 400 450 500 550 600 650
T(K)
1.4
300 350 400 450 500 550 600 650
T(k)
Temperature dependence of imaginary part of
dielectric constant for 21nm air annealed
sample
Temperature dependence of imaginary part of dielectric
constant for 29nm oxygen annealed sample

Variation of dielectric constant with different sizes
80
75
70
65
11nm
13nm
21nm
16
14
12
11nm
13nm
21nm
60
10
ε'
55
50
45
ε″
8
6
40
4
35
2
30
25
300 350 400 450 500 550 600 650
T(K)
Freq=10kHz
0
300 350 400 450 500 550 600 650
T(K)
Temperature dependence of real part
of dielectric constant for air annealed
samples of various sizes
Temperature dependence of imaginary part
of dielectric constant for air annealed
samples of various sizes

55
50
17nm
20nm
29nm
16
14
17nm
20nm
29nm
45
12
40
10
35
ε''
8
ε'
30
6
25
4
20
2
15
0
10
300 350 400 450 500 550 600 650
T(K)
300 350 400 450 500 550 600 650
T(K)
Temperature dependence of real part
of dielectric constant for oxygen
annealed samples of various sizes.
Temperature dependence of imaginary
part of dielectric constant for oxygen
annealed samples of various sizes

Coincidence of anomalies in ε˝(T) with the magnetic
ordering temperatures. Evidence of M-E coupling.
Larger FM in smaller particles
Enhanced <strong>effects</strong> of M-E coupling in O- annealed
particles, particularly in smaller particles.
Strong M-E <strong>effects</strong> in smaller particles
Could the enhanced <strong>effects</strong> be due to larger amount of
O-vacancies in smaller particles
No- because <strong>effects</strong> are enhanced in O-annealed
samples where lesser number of O vacancies are
present.

Resistivity measurements
7.8
7.6
d=29 nm
ΔΕ=(0.028±0.00084)ev
8.0
d= 21nm
ΔΕ=(0.026±0.0004)ev
7.4
7.9
log ρ
7.2
7.0
6.8
ΔΕ=(0.37±0.0085)ev
Log ρ
7.8
7.7
6.6
7.6
6.4
6.2
0.0014 0.0016 0.0018 0.0020 0.0022 0.0024 0.0026
1/T
7.5
0.0014 0.0016 0.0018 0.0020 0.0022 0.0024 0.0026
1/T
log ρ vs. 1/T plot for oxygen annealed sample
log ρ vs. 1/T plot for air annealed sample

Resistivity Studies
Resistivity studies across the magnetic transition
region.
Activated behavior of ρ.
Decrease in activation energies for T

Conclusions
Preliminary results suggest that Multiferroic <strong>effects</strong> are
enhanced in BiFeO 3 nanoparticles with decreasing size
and with removal of O vacancies.
The <strong>effects</strong> are probably due to the enhanced strains in
smaller size particles and not due to O vacancies.
Suggested that the small size of NPs makes the
stabilization of the AFM spiral more difficult. Hence FM
tilting of spins becomes easier.
Future work:
Need to obtain quantification of strains in these particles.
Need to study a larger range of particles and study of
ε(T), (T) over wider range.