Low field magnetoconductance studies in aggregated Al nano-clusters

Article abstract of DOI:10.1016/S0925-8388(01)01285-3

Nano-aggregates of Al have been formed by 70 keV He⁺ irradiation on spin cast films of aluminum acetyl acetonate. Three fold increase in conductivity was observed for a fluence range of 1×10¹⁶-5×10¹⁷ ions/cm². Electrical transport of the charge carriers was observed to follow Mott's variable range hopping (VRH) mechanism in 3D nano-aggregates of Al in the temperature regime of 110-300 K. Low field (≤1 T) magnetoresistance (MR) studies in these samples showed change in sign of MR from positive for a sample with low zero field resistance [R(H = 0)] to negative for samples with high R(H = 0) owing to quantum interference phenomena of hopping paths.

Full text of DOI:10.1016/S0925-8388(01)01285-3

Journal of Alloys and Compounds 326 (2001) 284–287
L
www.elsevier.com/locate/jallcom
Low field magnetoconductance studies in aggregated Al nano-clusters
*
S. Dhara , B.K. Panigrahi, P. Magudapathy, K.G.M. Nair
Materials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, India
Received 3 July 2000; received in revised form 13 November 2000; accepted 20 December 2000
Abstract
1
Nano-aggregates of Al have been formed by 70 keV He irradiation on spin cast films of aluminum acetyl acetonate. Three fold
1
6
17
2
increase in conductivity was observed for a fluence range of 1310 –5310 ions/cm . Electrical transport of the charge carriers was
observed to follow Mott’s variable range hopping (VRH) mechanism in 3D nano-aggregates of Al in the temperature regime of 110–300
K. Low field (#1 T) magnetoresistance (MR) studies in these samples showed change in sign of MR from positive for a sample with low
zero field resistance [R(H50)] to negative for samples with high R(H50) owing to quantum interference phenomena of hopping paths.

2001 Elsevier Science B .V . All rights reserved.
Keywords: Magnetoresistance; Variable range hopping; Nano-aggregates; Ion irradiation; Spin cast
1
. Introduction
system in the weak localization regime showed spin–orbit
and spin–flip scattering phenomena and their interplay as
dominant mechanisms on the interference [5] process in
such a system. However, detailed description of magneto-
transport in the strongly localized system has been formu-
lated for the first time by Nguen, Spivak and Shklovskii
(NSS) [8] and later on refined by Sivan, Entin-Wohlman
and Imry (SEWI) [9]. They have considered interference
of orbital electrons in the presence of a magnetic field
resulting in negative magnetoconductance (MC) [i.e.,
positive MR] in the low field regime with a low value of
zero field resistance [R(H50)] along with an indication of
change of sign for high value of R(H50). No experimental
evidence has been reported for this prediction. They also
predicted a steady positive MC in the high field regime for
both the samples with low and high values of R(H50).
Refined SEWI theory, using the CPM [7] model as a basic
consideration of electrical transport and subsequent
generalization to include the interference effect in a
Nonmetallic conduction in thin metal films at low
temperatures have been both theoretically predicted [1,2]
and experimentally observed [3,4] as a result of localiza-
tion of the carriers in the one-dimensional (1D) or two-
dimensional (2D) system. In 2D, there is no true metallic
behaviour, the conductance smoothly varies from logarith-
mic to exponential decrease with the length of the wire
owing to either the scaling theory of weak localization [2]
or electron–electron interaction [5] in the system. The
presence of strong localization has also been predicted by
Mott in disordered 3D systems as an effect of electron–
phonon interactions at very low temperature, better known
as variable range hoping (VRH) [6]. The hopping con-
dition in disordered materials follows a temperature depen-
a
dence of s ~exp 2 (T0/T) where a [ 5 1/d 1 1, d being
the dimension] is a constant ranging from 1/4 to 1/2. The
value of a51/4 [1/3 in 2D] has been justified in terms of
the VRH process proposed by Mott [6] considering density
2
magnetic field, showed justification of H dependence on
of states (DOS) around the Fermi energy (E ) to be slowly
varying or constant and derived by Ambegaokar et al. [7]
using the critical path model (CPM).
MC unlike the NSS prediction of uHu dependence at small
fields. All these calculations were performed in the strong-
ly localized VRH regime.
F
The study of magnetoresistance (MR) in the disordered
Percolating 3D nano-aggregates of Al used for these
studies where grown using 70 keV He irradiation on spin
1
*
Corresponding author. Fax: 191-41-1480-381.
cast
films
of13 aluminum
acetyl
acetonate
E-mail address: dhara@igcar.ernet.in (S. Dhara).
[((CH CO) CH) Al : Al(acac) ] at fluences in the range
3
2
3
3
0
925-8388/01/$ – see front matter
 2001 Elsevier Science B .V . All rights reserved.
PII: S0925-8388(01)01285-3

S. Dhara et al. / Journal of Alloys and Compounds 326 (2001) 284–287
285
1
6
17
2
of 1310 –5310
ions/cm . Transverse low field
(
#1T) MR studies have been performed in the tempera-
ture range of 110–300 K to explore the possible crossover
of sign in magnetoresistance in the 3D nano-aggregates of
Al.
2
. Experimental
Al(acac) in the powder form was prepared by precipi-
3
tating AlCl3 and acetyl acetone in an acidic medium.
Crystals of Al(acac) were obtained by slow sublimation of
3
o
22
the powder at 200 C and at a pressure of 10
mbar.
Al(acac) dissolves easily in chloroform (CHCl ) and this
3
3
property was used for growing thin films of Al(acac) by
3
the spin casting technique. Thin films of 0.25–1.25 mm
thickness were grown using a solution of Al(acac) (0.02–
3
0
.10 gm/cc) in CHCl on fused SiO and single crystal
3 2
NaCl substrates at a chuck speed of 500 r.p.m. for 15 s.
Films coated on SiO and NaCl were chosen mainly for
2
electrical conductivity measurements and TEM studies1,
respectively. The samples were irradiated with 70 keV He
1
1
6
17
Fig. 1. Bright field image of 70 keV He irradiated Al(acac)3 film at a
to various fluences in the range of 1310 –5310 ions/
17 2
2
fluence of 1310 ions/cm showing inhomogeneous aggregated clusters
cm . Resistivity measurements were carried out with a
in the nework like configuration.
Time Electonics 9616 current/voltage calibrator for cur-
rent source and Keithley 196 DMM for voltage measure-
ments in the four probe configuration at room temperature
with resistance at a maximum of 1 MV, across which the
voltage was measured. Resistance measurements at low
temperature were performed in a two probe configuration
using a HP 4329D high resistance meter (maximum 10
V) where resistance at lowest temperature was 100 MV,
which is comparable to the upper limit for the Keithley
However, the low values of conductivity in the irradiated
films might have originated from the inhomogeneities in
the networked structure (Fig. 1) with thin or no contacts
(observed in the magnified image, not shown in the figure)
1
6
between Al aggregates.
We conducted transport measurements with two samples
17
17
2
irradiated at 5310 and 1310 ions/cm with zero field
resistances [R(H50)] values of 200 and 1 MV, respective-
ly, in the two probe configuration. The lower R(H50)
value in the sample irradiated with higher fluence was
understood from their microstructure as the network
structure grew further with more availability of the metal
1
96 meter (maximum 200 MV).
3
. Results and discussion
2
1 / 4
clusters with increasing fluence. A ln R versus T
plot
The conductivity measured in van der Pauw four probe
for the temperature range of 110–300 K for the sample
1
7
2
configuration at room temperature increases as a function
irradiated with 5310 ions/cm is shown. This shows
that electrical transport of charge carriers follow Mott’s
VRH mechanism in 3D structure of Al nano-aggregates.
We also performed the low field (#1 T) transverse
magnetoresistance (MR) studies on these samples in the
temperature range of 110–300 K (Fig. 2). MR was found
to be positive (i.e., MC negative) suggesting a constructive
quantum interference of hopping paths amounting to a
higher resistive path for the hopping carriers in the sample
with low zero field resistance (200 kV for samples
1
of increasing He fluence (f) with an onset of measurable
1
6
2
conductivity at 1310 ions/cm . Three orders of change
2
9
26
in conductivity (4.4310 –2.4310 S/cm) have been
1
6
17
observed over a fluence range of 1310 –5310 ions/
2
cm . The values of conductivity measured by us is similar
to that reported for granular Al in the insulating limit [10].
In exploring the origin of electrical conductivity, micro-
structural studies were carried out using a Philips CM200
transmission electron microscope (TEM). Bright field
image (Fig. 1) of the sample irradiated to a fluence of
1
7
2
irradiated at 5310 ions/cm ). However, for the sample
1
7
2
1
17
2
1
310 ions/cm with 70 keV He irradiation showed
irradiated at 1310 ions/cm with high R(H50) of 1
MV showed negative MR (i.e., MC positive) with increas-
ing field as shown in Fig. 3. We observed this fact, for the
first time, where a changeover of sign of MR from positive
in the VRH regime to negative with high R(H50) was
formation of a network-like structure of inhomogeneous
aggregates of Al clusters. Networked structures of aggre-
gated Al clusters might have been formed due to overlap-
ping of ion tracks at relatively higher fluences [11].

2
86
S. Dhara et al. / Journal of Alloys and Compounds 326 (2001) 284–287
Fig. 4. Quadratic dependence of magnetoconductance [Ds/s ; (s(H) 2
2
1 / 4
0
Fig. 2. ln R vs. T
5
plot for the sample irradiated at a fluence of
s(H 5 0))/s(H 5 0)] with field at different temperatures for the sample
1
7
2
310 ions/cm at different transverse magnetic fields showing positive
magnetoresistance for samples with low R(H50).
17
2
irradiated at a fluence of 1310 ions/cm .
paths in the VRH regime where R(H50) is high and will
measured. According to the SEWI model [9], the positive
MR is accounted for by a constructive quantum interfer-
ence of forward scattered hopping paths, which is favoured
with the application of a small field where R(H50) is low.
The magnetic field will act to improve the matrix element,
hence resulting in a destructive interference of hopping
result in a negative MR. A quadratic dependence of
magnetoconductance [DS/S ], which is defined as [s(H) 2
0
s(H 5 0)]/s(H 5 0) where s(H) is conductivity at field H
and s(H 5 0) is zero field conductivity on field, is shown
1
7
2
in Fig. 4 for the sample irradiated at 1310 ions/cm at
different temperatures. This is also in agreement with the
prediction of the SEWI [9] theory.
4
. Conclusions
Electrical transport of the charge carriers was observed
to follow Mott’s variable range hopping (VRH) mecha-
nism in 3D nano-aggregates of Al. Change of sign of
transverse magentoresistance (MR) studies was observed
from positive for samples with low zero field resistance
[R(H50)] to negative for samples with high R(H50).
Quantum interference of hopping paths, which is construc-
tive for samples with low R(H50) and destructive for high
R(H50), was designated for positive and negative MR,
respectively. Magnetoconductance was found to have
quadratic field dependence. These results are in agreement
with the predictions of the Sivan, Entin-Wohlman and Imry
(
SEWI) [9] model.
Acknowledgements
2
1 / 4
Fig. 3. ln R vs. T
1
plot for the sample irradiated at a fluence of
310 ions/cm at different transverse magnetic fields showing nega-
tive magnetoresistance for samples with high R(H50).
1
7
2
We acknowledge Saroja Saibaba of the Physical Metal-
lurgy Section, IGCAR, Kalpakkam for the TEM studies.

S. Dhara et al. / Journal of Alloys and Compounds 326 (2001) 284–287
287
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