ARTICLE
pubs.acs.org/JPCC
€ ssbauer, Raman, and Magnetoresistance Study
Mo
of Aluminum-Based Iron Oxide Thin Films
S. S. Shinde,† Sher Singh Meena,‡ S. M. Yusuf,‡ and K. Y. Rajpure†,*
†
‡
Electrochemical Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, India
Solid State Physics Division, Bhabha Atomic Research Center, Mumbai
ABSTRACT: Thin films of Al-based hematite iron oxide were
synthesized by spray pyrolysis in aqueous medium onto the
glass microslides. The compact and homogeneous distribution
of grains (spindle-shaped hematite nanostructures) with varying
sizes has been observed in surface morphological studies. The
room temperature M€ossbauer study has been carried out to
monitor the local environment around Fe cations and valence
state of Fe ions. M€ossbauer and micro-Raman (low temperature) results suggest that oxygen vacancies cause cation redistribution between the interstitial sites resulting in magnetic
ordering. The variation of magnetoresistance in low magnetic field (<3 kOe) is also reported.
’ INTRODUCTION
In recent years, strongly correlated materials that simultaneously
show electric and magnetic orderings have attracted much attention
due to range of applications in multifunctional devices and the
underlying Physics. A number of examples of such materials have
been found in oxide systems including transition metals.1,2 Iron
oxide (Fe2O3) thin films are indeed excellent candidates for the
production of catalysts, sensors, nonlinear optical and magnetic
devices.3 Fe2O3 shows an important application in cancer treatment by elimination of cancerous cells in bones; by means of
hyperthermia. When this material is placed in the region of the
tumor and is subjected to an alternating magnetic field, heat is
generated by hysteretic losses.4 Since the magnetic properties of the
material depend on the environment of Fe, therefore the knowledge of structure and oxidation states of iron ions is beneficial for
ossbauer spectroscopy
synthesis of magnetic materials. 57Fe M€
(MS) offers several advantages for studies of iron-containing
compounds. The interaction of x-rays with materials is a powerful
tool to investigate the structural and electronic properties of
condensed matter. Several outstanding methods in the field of
X-ray spectroscopy are based on the M€ossbauer effect, which finds
its origin in the resonant and recoil-free absorption and emission of
photons by the nuclear levels of atoms bound in a solid. A
M€ossbauer spectrum described by the number, position, shape
and relative intensity of the absorption lines is governed by the
nature of hyperfine interactions like isomer shift (d), quadrupole
splitting (D) and the magnetic hyperfine field (Heff). These
parameters give valuable information regarding the symmetry of
the bonding environment and the local structure around Fe atoms.
It is specific for iron and the results (i.e., the various M€ossbauer
parameters) are extremely sensitive to the electronic, magnetic and
structural features of the involved Fe-bearing phases, thus generally
allowing phase identification and quantitative phase analysis of
r 2011 American Chemical Society
mixtures of iron oxides that are difficult to distinguish from each
other in the respective XRD patterns. M€ossbauer studies have been
made to evaluate the temperature dependence of lattice distortion
in two aerogel-synthesized iron-molybdenum oxides having different atomic ratios Fe:Mo = 1:1 and 2:3, but both with a βFeMoO4 structure by Hamdeh et al.5 Planckaert et al.6 presented a
critical comparison between conventional M€ossbauer spectroscopy
and energy and time-resolved nuclear resonant scattering. The
three M€ossbauer techniques are evaluated by the characterization
of the complex magnetic structure of Fe3Al alloy. Blanchard et al7
studied iron isotope fractionation between pyrite (FeS2), hematite
(Fe2O3), and siderite (FeCO3) with the help of a first-principle
density functional theory. M€ossbauer study at room temperature
and 120 K was used to identify the hyperfine parameters of the
pulsed laser deposited polycrystalline magnetic films.8 Zic et al.9
studied the effect of temperature on the crystallization of R-Fe2O3
particles from dense β-FeOOH suspensions by 57Fe M€ossbauer
spectroscopy. Crystallization at 120 °C between 18 and 72 h
yielded monodisperse R-Fe2O3 particles of a shape close to that of
double spheres with ring. Sun et al10 studied preparation of
mesoporous R-Fe2O3 materials in large quantity by the soft
template synthesis method using triblock copolymer surfactant
F127 as the template. Nitrogen adsorption-desorption isothermal
measurements and transmission electron microscope observation
revealed that the as-prepared mesoporous R-Fe2O3 nanostructures
have large mesopores in a wide size range of 5-30 nm.
Raman spectroscopy is a powerful method for investigation of
the structural properties (surface modes) of samples because the
variations in Raman spectra with change in particle size can be
Received: December 15, 2010
Revised:
January 20, 2011
Published: February 16, 2011
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Figure 1. FE-SEM images of (a) pure Fe2O3 and (b) typical 10 at. % Al:
Fe2O3 thin films.
easily detected. Although there is still some controversy as to
what effects variation in particle size has on phonon modes and
on the electron-phonon interaction. Raman studies of these
materials are useful in following way. It is possible to determine
the size of nanoparticles from a measurement of the maximum of
the low frequency Raman band.11 The frequency ν (in cm-1) of
the lowest-energy spherical mode of a free particle, corresponding to angular momentum l = 0, is given by12
ν¼
0:7νL
dc
ð1Þ
where νL is the speed of the longitudinal sound waves, c is the
vacuum light velocity of sound, and d is the particle diameter.
Among the metallic oxides, magnetite (Fe3O4) is one of the
oxides, which exhibits the high room temperature magnetoresistance (MR). Tunneling magnetoresistance in sintered Fe3O4
samples diluted with Fe and R-Fe2O3 was studied by Kim and
Moon.13 The enhanced MR ratios of Fe3O4-10 Fe and Fe3O4-RFe2O3 samples were explained by the increased interparticle
contact sites and the appropriate thickness of R-Fe2O3, respectively. In this study, we have investigated the surface morphology,
M€ossbauer and Raman analysis of Al-based iron oxide thin films.
Also we reported the effect of low magnetic field on to the
magnetoresistance (MR).
’ EXPERIMENTAL SECTION
Aluminum-based Fe2O3 thin films were deposited onto ultrasonically cleaned glass substrates (microslides) using chemical
spray pyrolysis technique. The ferric trichloride and aluminum
nitrate (99.99%, A.R. grade, Aldrich) were used as the source
materials for deposition of films in aqueous media. The
[Al]/[Fe] ratios calculated on at. %, used in the starting solution
were 5, 10, 15, and 20%. The resulting solution was sprayed onto
preheated substrates held at optimized substrate temperature of
623 K with compressed air as a carrier gas. Other preparative
parameters—viz. spray rate, 5 cc min-1; nozzle to substrate
distance, 32 cm; nozzle diameter, 0.05 cm—were kept constant
for all experiments.
The morphological characterization of the thin films was
studied by using field emission scanning electron microscopy
(FESEM, Model: JSM-6701F, Japan). M€ossbauer spectra have
been obtained using a spectrometer (Nucleonix Systems Pvt.
Ltd.) operated in constant acceleration mode (triangular wave)
in transmission geometry. The source employed was Co-57 in Rh
matrix of strength 50 mCi. The calibration of the velocity scale
was done by using a R-Fe metal foil. The outer line width of
calibration spectra was 0.29 mm/s. M€ossbauer spectra were fitted
by a least-squares fit (MOSFIT) program assuming Lorentzian
line shapes. Raman-scattering experiments were performed in air
at room temperature with micro Raman system from Jobin Yvon
Horibra LABRAM-HR visible within 100-1400 cm-1. The low
temperature Raman spectra were measured with (LN2) cooling
up to 100 K with spectral resolution of the order of 1 cm-1. The
Raman spectra were excited with the Argon 488 nm laser source.
The 600 and 1800 lines/mm gratings (detector: CCD detector)
were used. Magnetotransport measurements were performed by
an ac four terminal method with an excitation frequency of 10 Hz
in the field parallel to the current direction.
’ RESULTS AND DISCUSSION
Morphological Study. The surface morphology of Al-based
iron oxide thin films was studied by using high-resolution
FESEM and a few representative micrographs of the films
are shown in Figure 1. The micrographs show that the films
are uniform, compact and homogeneous (spindle-shaped hematite nanostructures) grains grown onto the substrate surface. The
pure iron (Figure 1a) oxide thin film shows the agglomerated
highly dense, compact, smooth, adherent and homogeneous
structure. The observed higher values of grain size may be due
to the tendency of small grains to aggregate to big grains on
surface of the films. After Al doping (Figure 1b), the compactness
of grains continues to decrease, i.e., it loses its densification and
the grain size decreases and number of grains increases.
€ ssbauer Spectroscopy. M€
Mo
ossbauer spectroscopy is a
specifically useful spectroscopic technique in the investigations
of iron oxyhydroxides and oxides.14 Each M€ossbauer spectrum as
obtained from the resonant and recoil-free emission and absorption of γ rays by Fe nuclei can have three basic components. An
isomer shift (IS) of nuclear energy levels, which depends strongly
on the electronic valence state. A quadrupole splitting (QS),
reflects the asymmetry of the Fe site and is, therefore, structure
sensitive. Finally, a hyperfine magnetic field at the Fe nucleus
would yield a sextet spectrum with Lorentzian line shapes, but it
generally prevails only in magnetically ordered materials. The
room temperature M€ossbauer spectra of pure and Al-based iron
oxide films (Figure 2a-e) exhibits an asymmetrical sextet with
broadened lines and a quadrupolar component: such hyperfine
structure suggests the presence of static magnetic ordering and
superparamagnetic relaxation phenomena originating from a
distribution of size and/or distances between particles giving
rise to an assembly of weakly interacting particles. The hyperfine
structure of the M€ossbauer spectra having the highest saturation
magnetization consists of broad line sextet, which has described
by at least three magnetic components with large line widths. The
asymmetry of outermost lines has decomposed into at least two
magnetic components, the hyperfine parameters of which are
listed in Table 1. Also it shifts toward the lower velocity side due
to doping. Peak intensity increases up to 10 at. % Al doping and
then decreases for higher doping percentages. The isomer shift
and line width parameters, assuming a line width of the inner
lines Γ and a line broadening parameter ΔΓ were coupled. One
observes two different isomer shift and hyperfine field values
rather consistent with two different environments for Fe ions
corresponding to tetrahedral (A) and octahedral (B) sites in the
lattice. In relation to the identification of the spectra due to
octahedral and tetrahedral iron ions, it is well-known that each
A-site ion has in its immediate surrounding 12 B-site neighbors
and each B-site ion has 6 A-site nearest neighbors. Fe3þ(A)O2--Fe3þ(B) exchange interaction between iron ions is known
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Figure 2. (a-e) M€ossbauer spectra of Al:Fe2O3 samples deposited for various doping concentrations from 0 to 20 at. %.
to be the strongest, whereas A-A and B-B interactions between
iron ions via oxygen ions are relatively weaker. It follows that
A-site iron ions are coupled to a large number of Fe magnetic
bonds as compared to iron ions at B-sites. One therefore expects
a larger hyperfine field at the A-site iron nuclei than that at the
iron ions at B-sites. Identification of sextets on the basis of
hyperfine field offers difficulty at times because in some studies
hyperfine field at A-site iron nuclei has been reported15 to be
larger than that at B-site iron nuclei and vice versa.16 Hence, in
the present study, assignment of sextets corresponding to A- and
B-sites has been done on the basis of isomer shift. Since the bond
separation between Fe3þ-O2- is larger for octahedral ions when
compared to that for tetrahedral ions, smaller overlapping of the
orbitals of Fe3þ ions and oxygen anions and the smaller
covalency lead to larger isomer shift at the octahedral site. So
the sextet with a lower isomer shift corresponds to the A-site, and
the sextet having a higher isomer shift represents the B-site. The
M€ossbauer parameters are very close to the parameters of
AlFeO3 phase and iron is present in high spin Fe3þ state.17
The increase of QS is attributed to an increase in the asymmetry
around the iron ions. Magnetic hyperfine field and isomer shift
values decrease with Al-doping concentration.
The nucleus has magnetic moment (μ), when the spin quantum number (I) to be greater than zero. Its energy is then affected
by presence of magnetic field and interaction of μ with magnetic
flux density of B is formally expressed by Hamiltonian
H ¼ - μB ¼ - gμN IB
ð2Þ
where μN is nuclear quantum magneton (eh/4πmp) and g is the
nuclear g-factor. Solving this Hamiltonian it gives energy levels of
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Table 1. Results of Hyperfine Field, Quadrupole Splitting, Isomer Shift, Outer Line Width, and Relative Intensity of Al:Fe2O3
Thin Films
sample
phases
pure iron oxide
5 at. % Al:Fe2O3
outer line width,
relative intensity,
IS (mm/s)
Γ (mm/s)
RI (%)
0.0773
0.014
0.4508
0.3224
0.3150
0.5710
51.01
28.25
sextet 3 (blue)
458.75
0.0227
0.6872
0.5157
13.35
doublet (cyan)
-
0.0211
0.7979
3.4435
7.39
sextet 1 (red)
515.1673
0.0819
0.4496
0.3150
44.28
sextet 2 (green)
493.7967
0.0202
0.2986
0.4710
27.74
sextet 3 (blue)
461.0564
0.0062
0.6602
1.0981
22.74
0.4934
0.8521
1.7877
-
5.2361
sextet 1 (red)
sextet 2 (green)
513.9719
496.5372
0.0800
-0.0416
0.4483
0.2336
0.3150
0.4710
22.13
29.19
sextet 3 (blue)
489.1152
0.0564
0.4005
0.8080
41.22
0.0583
0.6138
4.6242
513.9763
0.0726
0.4392
0.3150
16.20
sextet 2 (green)
495.6120
-0.0259
0.2567
0.4710
30.06
sextet 3 (blue)
485.7328
0.0546
0.4069
0.9815
44.19
0.0034
0.5120
4.0428
sextet 1 (red)
doublet (cyan)
20 at. % Al:Fe2O3
isomer shift,
QS (mm/s)
516.79
500.11
doublet (cyan)
15 at. % Al:Fe2O3
quadrupole splitting,
Hint (kG)
sextet 1 (red)
sextet 2 (green)
doublet (cyan)
10 at. % Al:Fe2O3
hyperfine field,
-
-
7.4617
9.5558
sextet 1 (red)
sextet 2 (green)
512.0463
494.4803
0.0754
-0.0442
0.4455
0.2401
0.3150
0.4710
15.74
28.61
sextet 3 (blue)
483.4537
0.0476
0.3754
0.9160
47.85
0.0070
0.4018
3.4562
doublet (cyan)
-
7.7941
Table 2. Energy Positions of the Possible Transitions of Iron
Oxide Thin Films
transitions
/2 f 1/2
Δm
energy position
3
þI
E0þ((3)/(2)geμnHnþ(1)/(2)ggμnHn)
-3/2 f -1/2
-I
E0þ(-(3)/(2)geμnHn-(1)/(2)ggμnHn)
1
0
E0þ((1)/(2)geμnHnþ(1)/(2)ggμnHn)
-1/2 f -1/2
0
E0þ(-(1)/(2)geμnHn-(1)/(2)ggμnHn)
-1/2 f 1/2
-I
E0þ(-(1)/(2)geμnHnþ(1)/(2)ggμnHn)
/2 f -1/2
þI
E0þ((1)/(2)geμnHn-(1)/(2)ggμnHn)
/2 f 1/2
1
the nucleus
Em ¼ -
μB
mz ¼ - gμN Bmz
I
ð3Þ
where mz is the magnetic quantum number and can take the
values I, I -1, ..., -I. In effect magnetic field splits energy level
into 2Iþ1 nondegenerate equi-spaced sublevel with a separation
of (μB)/(I). In a M€ossbauer experiment there may be a transition
from a ground state with a spin quantum number Ig and magnetic moment μg to an excited state with spin Ie and magnetic
moment μe. In a magnetic field, both states will split according to
selection rule Δmz = 0, ( 1. The resultant M€ossbauer spectrum
contains a number of resonance lines, but is nevertheless symmetrical about the centroid.
The transition probabilities of various possible transitions are
given by Clebsch-Gordon coefficients along with the energy
positions as shown in Table 2. Figure 3 shows the allowed
transitions and resultant spectrum for magnetic hyperfine splitting of I = 1/2 to I = 3/2 transition for optimized 10 at. % Al-doped
iron oxide. There are two parameters which give information
about magnetic properties: (1) The position of the six fingers
Figure 3. Energy level scheme and resultant spectrum for magnetic
hyperfine splitting of an I = 1/2 to I = 3/2 transition for optimized 10 at.
% Al-doped iron oxide.
giving information on the splitting of excited state μ1Hn and that
of ground state μ0Hn; (2) the relative intensity of the lines giving
information on the polarization.
Raman Analysis. Raman spectrum of the pure and Al-based
iron oxide thin films obtained at room temperature is shown in
Figure 4, where the peaks at ∼224, 291, 408, 495, 607, and 686
cm-1 are clearly observed in the low wavenumber region having
acoustic combinations. On the other hand, several peaks located,
respectively, at 865, 984, 1096, and 1312 cm-1 are found in the
high wavenumber region from 700 to 1400 cm-1 having acoustic,
optical combinations and overtones are shown in Table 3. Raman
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The Journal of Physical Chemistry C
ARTICLE
Figure 4. Raman spectra of Al:Fe2O3 samples deposited for various
doping concentrations from 0 to 20 at. % excited by 488 nm Arþ laser.
Figure 5. Typical Raman spectra of the 10 at. % Al:Fe2O3 sample for
different temperatures in the range of 100-300 K.
Table 3. Wave Number (in cm-1) and Symmetries of the
Modes Found in the Raman Spectrum of Iron Oxide Thin
Films and Their Assignments
Brillouin zone
points/lines
our data
frequency (cm-1)
symmetry
224
A1
2TA
L, M, H, Γ
291
A1
-
Γ
408
E1
E1(TO)
Γ
495
A1
2LA
M-K
607
A1
TA þ TO
H, M
686
865
A1
A1
LA þ TO
LA þ LO
M
L, M
984
A1
2TO
L-M-K-H
1096
A1
2LO
H, K
process
intensity increases up to 10 at. % Al doping and then decreases for
higher doping concentrations. The peak observed at 224 cm-1 is
assigned to 2TA transverse acoustic mode. In addition to the
2TA mode, a new peak appears at 291 cm-1 which has A1
symmetry. An additional peak emerges at 408 cm-1 in the
spectrum (E1 symmetry), which is assigned to the E1(TO)
mode. The peaks observed at 495, 607, and 686 cm-1 can be
assigned to A1 symmetry with M - K points and longitudinal
acoustic (LA) modes, transverse acoustic plus transverse optical
(TA þ TO) combinations with H, M points and longitudinal
acoustic plus transverse optical (LAþTO) combinations with M
point, respectively. Figure 5 shows the low temperature variation
of Raman spectra from 100 to 300 K. The Raman intensity
increases with decrease in temperature from room temperature
toward 100 K. This shows that collective vibrations of Al and Fe
cations occur from the same site. It has been reported that the
changes in peak width arise from the electron phonon (e-p)
coupling strength (λ) associated with the decay of phonon into
electron-hole pair. The value of λ is estimated from the average
phonon line width over all “q” wave vectors. Raman modes are
calculated using Allen’s formula.18 For an mth phonon, the line
width (Γ) and frequency (ω) are related by the following
equation:
!
Γm
2π
¼
ð4Þ
λm NðEF Þ
gm
ω2
Figure 6. Magnetoresistance of pure iron oxide and 10 at. % Al:Fe2O3
samples in a low fields.
where, gm is the degeneracy of the mth mode and N(EF) is the
density of states at Fermi level. We have used the calculated
value19 of N(EF) = 3 states/eV per Fe at RT, for estimating λ.
Raman spectra of Al:Fe2O3 shows an increase in peak intensity
and a decrease in Γ for all modes, resulting in a low λ value. This
clearly indicates a reduction in the disorder at the interstitial sites
due to cation redistribution that favors normal spinel structure.
These observations further corroborate the findings from
M€ossbauer spectroscopy.
The phonon lifetime (τ) can be derived from the Raman
spectra via the energy-time uncertainty relation,20
1
ΔE
¼
¼ 2πcΓ
τ
p
ð5Þ
where ΔE is the uncertainty in the energy of the phonon mode, p
is Planck’s constant, and Γ is the full width at half-maximum
(fwhm) of the Raman peak in units of cm-1. Phonon lifetime is
mainly limited by two mechanisms: (i) anharmonic decay of the
phonon into two or more phonons so that energy and momentum are conserved, with a characteristic decay time τA and (ii)
perturbation of the translational symmetry of the crystal by the
presence of impurities, defects and isotopic fluctuations, with a
characteristic decay time τI. The phonon lifetime deduced from
the Raman measurements are observed in the range of 0.38 to
0.45 ps.
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Magnetoresistance. As previously reported21 an extremum
of the magnetoresistance (MR) is observed in the Fe2O3 thin
film at the Verwey transition temperature. The cause for the
appearance of the extremum is possibly associated with the
partial condensation of magnon mode. MR was estimated by
using following equation:22
MR ð%Þ ¼
½RðHÞ - R0 100
R0
ð6Þ
MR variation of pure and 10 at. % Al-doped Fe2O3 thin films in
low fields at 300 K are shown in Figure 6. The low field MR
increases with doping concentration of Al accompanied by the
change in the shape of the profile. The MR increases from 0.31 to
0.43% from pure to 10 at. % Al doping. The low field MR
gradually increases and shows no saturation even at 3kOe in spite
of magnetic saturation around 1kOe. Since the MR change
occurs at the percolation threshold of R-Fe2O3, it is most likely
that the increase in the MR below the threshold is closely
associated with the conduction on connecting Fe2O3 path.
’ CONCLUSIONS
The spindle-shaped hematite nanostructured grains are observed in FESEM images of spray deposited iron oxide thin films.
The room temperature M€ossbauer spectra of Al-based iron oxide
films exhibit an asymmetrical sextet with broadened lines and a
quadrupole component. Seven optical modes of even symmetry
are observed in the Raman spectra. The magnetoresistance
increases from 0.31 to 0.43% after doping.
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’ AUTHOR INFORMATION
Corresponding Author
*Telephone: þ91-231-2609435. Fax: þ91-231-2691533. E-mail:
[email protected].
’ ACKNOWLEDGMENT
The authors are very much thankful to Defense Research and
Development Organization (DRDO), New Delhi, for the financial support through its Project ERIP/ER/0503504/M/01/
1007.
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