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Study of pulsed laser deposited magnetite thin film
Murtaza Bohraa, N. Venkataramanib, Shiva Prasada,, N. Kumara, D.S. Misraa,
S.C. Sahooa, R. Krishnanc
a
Department of Physics, Indian Institute of Technology Bombay, Mumbai 400076, India
Department of Metallurgical Engineering and Material Science, Indian Institute of Technology Bombay, Mumbai 400076, India
c
Groupe d’étude de la matière condensée ,CNRS/Universite de Versailles-St-Quentin, 45, avenue des Etats-Unis, 78035 Versailles Cedex, France
b
Abstract
Magnetite thin film with a predominant (1 1 0) texture has been deposited by pulsed laser ablation of a-Fe2O3 target onto fused quartz
substrate. Spontaneous magnetization of 5400 G and room temperature electrical resistivity of 4.2 103 O cm were measured for an
annealed magnetite thin film. Zero-field-cooled magnetization data clearly show the Verwey transition near 120 K through an abrupt
change, and is consistent with the resistivity measurement.
PACS: 75.70.i; 75.60.Nt
Keywords: Magnetite; Pulsed laser ablation; Thin film; Magnetization; Verwey transition
1. Introduction
Recently, there have been several papers on the deposition
of magnetite thin films on amorphous substrates using a
variety of deposition techniques. Most of these films do not
show any orientation, unless a buffer crystalline layer is used
in between. Diverse magnetic and electrical properties of the
films have been reported in them. Even the Verwey transition,
which is characteristic of magnetite, is not discernible from
most reported resistivity measurements [1,2]. In this work, we
report the preparation and characterization of magnetite thin
films on amorphous quartz substrate, which show an
orientation without any buffer layer and also show the
Verwey transition. The magnetization of these films is found
to be 92% of the single crystal value.
2. Experimental details
Magnetite films were grown on fused quartz substrate by
pulsed laser deposition, with the substrate kept at room
temperature. The third harmonic (355 nm) of Nd:YAG
laser, with 10 Hz repetition rate and 5–6 ns pulse width was
used to ablate laboratory sintered a-Fe2O3 (Aldrich99.99%) target. Deposition was carried out in a vacuum
of 1.5 105 mbar with laser energy density of 2.5 J/cm2.
The as-deposited film was subsequently annealed in
vacuum of 4 106 mbar at 500 1C for 1 h. Structural
characterization was performed using X-ray diffractometer
(XRD). The M–H and M–T data were measured with a
quantum design PPMS. Resistivity of the film was
measured by the four-probe method in the temperature
range of 40–300 K.
3. Results and discussion
The XRD of the as-deposited and annealed thin films are
shown in Fig. 1. No a-Fe2O3 peak is observed in these
films. The as-deposited film shows two broad (2 2 0) and
(4 4 0) peaks of magnetite. These peaks grow in intensity
upon annealing. The annealed film shows an additional
weak (3 1 1) peak. The most intense peak for the film is
(4 4 0), while that for bulk cubic magnetite is (3 1 1), as
reported in JCPDS (File no.19-0629). The intensities of
(2 2 0) and (4 4 0) peaks in the bulk are 30% and 40% of the
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(440)
6
Intensity (a.u.)
(220)
3
0
(311)
4πM (kG)
-3
20
30
40
50
60
70
2
annealed
1
as-deposited
0
80
90
annealed
-6
-1
2θ (degree)
as-deposited
-2
Fig. 1. The XRD pattern of as-deposited and annealed magnetite thin
film.
-8
-6
-4
-2
0
2
4
6
8
H (kOe)
Fig. 2. The M–H loops of as-deposited and annealed magnetite thin film.
104
5.44
103
H=2T
ZFC
102
4πM (kG)
5.36
ρ (Ω-cm)
(3 1 1) peak, respectively. This indicates that our annealed
magnetite thin film is textured in the (1 1 0) plane.
There have been reports of deposition of oriented thin
films on quartz substrates and also by using buffer layers
[3–6]. However, most of them grow in a fashion such that
the plane of the closest packing is along the plane of the
film. Thus, in the case of spinel films, we expect the film to
be textured in the (1 1 1) plane. Hence, it is interesting to see
that in the present case we are able to grow a (1 1 0)
textured film on an amorphous substrate. Such behavior
has also been seen in Cu ferrite films deposited under
specific conditions. The texture of 800 1C quenched
sputtered Cu ferrite could be changed from (1 1 1) to
(1 1 0). This change was achieved by varying O2XAr gas
ratio from 16% to 100% at same RF power of 50 W [7].
In Fig. 2, the in-plane M–H loops of the as-deposited
and the annealed film are shown. The loop of the films do
not saturate even at a field of 7 kOe. The spontaneous
magnetization (4pMS) of the films was evaluated by
linearly extrapolating the high-field M–H curve to the zero
fields. The 4pMS and the coercivity values of the annealed
film are about 40% and four times higher than the asdeposited film, respectively. This is attributed to the grain
growth when the film was vacuum annealed at 500 1C. The
4pMS value of the annealed film is 5400 G, and is 92% of
the single crystal value of 5900 G.
The temperature dependence of the resistivity for the
annealed film is shown in Fig. 3. A change in the slope
near about 120 K can be seen in the data and is attributed
to Verwey transition. The room-temperature resistivity of
the film is close to the single crystal value of 4 mO cm.
Above the transition temperature (Tv), a thermally
activated hopping process describes the electronic conduction. The activation energy Ea determined by fitting the
101
Tv
100
5.28
10-1
10-2
10-3
5.20
0
50
100
150
200
250
300
T (K)
Fig. 3. Temperature dependence of magnetization in a field of 2 T and the
resistivity of the annealed magnetite thin film.
data to loge(r/r0) ¼ Ea/kT is found to be around
20 meV, which is comparable to that of a single crystal
[8]. In the M–T curves an abrupt change is observed
around 120 K.
In conclusion, magnetite thin film has been pulsed laser
ablated with a predominant (1 1 0) texture on to amorphous quartz substrate. A magnetization value of 5400 G
with a coercivity of 480Oe was obtained for the magnetite
thin film. The magnetite thin film shows Verwey transition
with a room temperature resistivity value of 4.2 103 O cm.
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